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Grant Negotiation and Authorization Protocol
draft-ietf-gnap-core-protocol-13

Document Type Active Internet-Draft (gnap WG)
Authors Justin Richer , Fabien Imbault
Last updated 2023-02-27 (Latest revision 2022-11-29)
Replaces draft-richer-transactional-authz
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draft-ietf-gnap-core-protocol-13
GNAP                                                      J. Richer, Ed.
Internet-Draft                                       Bespoke Engineering
Intended status: Standards Track                              F. Imbault
Expires: 31 August 2023                                         acert.io
                                                        27 February 2023

              Grant Negotiation and Authorization Protocol
                    draft-ietf-gnap-core-protocol-13

Abstract

   GNAP defines a mechanism for delegating authorization to a piece of
   software, and conveying the results and artifacts of that delegation
   to the software.  This delegation can include access to a set of APIs
   as well as subject information passed directly to the software.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 31 August 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.2.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .  10
     1.4.  Trust relationships . . . . . . . . . . . . . . . . . . .  11
     1.5.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . .  13
     1.6.  Sequences . . . . . . . . . . . . . . . . . . . . . . . .  16
       1.6.1.  Overall Protocol Sequence . . . . . . . . . . . . . .  16
       1.6.2.  Redirect-based Interaction  . . . . . . . . . . . . .  19
       1.6.3.  User-code Interaction . . . . . . . . . . . . . . . .  22
       1.6.4.  Asynchronous Authorization  . . . . . . . . . . . . .  25
       1.6.5.  Software-only Authorization . . . . . . . . . . . . .  27
       1.6.6.  Refreshing an Expired Access Token  . . . . . . . . .  28
       1.6.7.  Requesting Subject Information Only . . . . . . . . .  29
       1.6.8.  Cross-User Authentication . . . . . . . . . . . . . .  31
   2.  Requesting Access . . . . . . . . . . . . . . . . . . . . . .  33
     2.1.  Requesting Access to Resources  . . . . . . . . . . . . .  35
       2.1.1.  Requesting a Single Access Token  . . . . . . . . . .  35
       2.1.2.  Requesting Multiple Access Tokens . . . . . . . . . .  38
     2.2.  Requesting Subject Information  . . . . . . . . . . . . .  40
     2.3.  Identifying the Client Instance . . . . . . . . . . . . .  41
       2.3.1.  Identifying the Client Instance by Reference  . . . .  43
       2.3.2.  Providing Displayable Client Instance Information . .  44
       2.3.3.  Authenticating the Client Instance  . . . . . . . . .  45
     2.4.  Identifying the User  . . . . . . . . . . . . . . . . . .  45
       2.4.1.  Identifying the User by Reference . . . . . . . . . .  47
     2.5.  Interacting with the User . . . . . . . . . . . . . . . .  47
       2.5.1.  Start Mode Definitions  . . . . . . . . . . . . . . .  49
       2.5.2.  Interaction Finish Methods  . . . . . . . . . . . . .  52
       2.5.3.  Hints . . . . . . . . . . . . . . . . . . . . . . . .  54
   3.  Grant Response  . . . . . . . . . . . . . . . . . . . . . . .  55
     3.1.  Request Continuation  . . . . . . . . . . . . . . . . . .  57
     3.2.  Access Tokens . . . . . . . . . . . . . . . . . . . . . .  58
       3.2.1.  Single Access Token . . . . . . . . . . . . . . . . .  58
       3.2.2.  Multiple Access Tokens  . . . . . . . . . . . . . . .  62
     3.3.  Interaction Modes . . . . . . . . . . . . . . . . . . . .  63
       3.3.1.  Redirection to an arbitrary URI . . . . . . . . . . .  64
       3.3.2.  Launch of an application URI  . . . . . . . . . . . .  65
       3.3.3.  Display of a Short User Code  . . . . . . . . . . . .  65
       3.3.4.  Display of a Short User Code and URI  . . . . . . . .  66
       3.3.5.  Interaction Finish  . . . . . . . . . . . . . . . . .  67
     3.4.  Returning Subject Information . . . . . . . . . . . . . .  68
     3.5.  Returning a Dynamically-bound Client Instance
           Identifier  . . . . . . . . . . . . . . . . . . . . . . .  70
     3.6.  Error Response  . . . . . . . . . . . . . . . . . . . . .  71
   4.  Determining Authorization and Consent . . . . . . . . . . . .  73

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     4.1.  Starting Interaction With the End User  . . . . . . . . .  77
       4.1.1.  Interaction at a Redirected URI . . . . . . . . . . .  77
       4.1.2.  Interaction at the Static User Code URI . . . . . . .  78
       4.1.3.  Interaction at a Dynamic User Code URI  . . . . . . .  79
       4.1.4.  Interaction through an Application URI  . . . . . . .  80
     4.2.  Post-Interaction Completion . . . . . . . . . . . . . . .  80
       4.2.1.  Completing Interaction with a Browser Redirect to the
               Callback URI  . . . . . . . . . . . . . . . . . . . .  81
       4.2.2.  Completing Interaction with a Direct HTTP Request
               Callback  . . . . . . . . . . . . . . . . . . . . . .  82
       4.2.3.  Calculating the interaction hash  . . . . . . . . . .  83
   5.  Continuing a Grant Request  . . . . . . . . . . . . . . . . .  84
     5.1.  Continuing After a Completed Interaction  . . . . . . . .  87
     5.2.  Continuing During Pending Interaction (Polling) . . . . .  89
     5.3.  Modifying an Existing Request . . . . . . . . . . . . . .  90
     5.4.  Revoking a Grant Request  . . . . . . . . . . . . . . . .  97
   6.  Token Management  . . . . . . . . . . . . . . . . . . . . . .  97
     6.1.  Rotating the Access Token . . . . . . . . . . . . . . . .  98
       6.1.1.  Binding a New Key to the Rotated Access Token . . . . 100
     6.2.  Revoking the Access Token . . . . . . . . . . . . . . . . 101
   7.  Securing Requests from the Client Instance  . . . . . . . . . 102
     7.1.  Key Formats . . . . . . . . . . . . . . . . . . . . . . . 102
       7.1.1.  Key References  . . . . . . . . . . . . . . . . . . . 104
       7.1.2.  Key Protection  . . . . . . . . . . . . . . . . . . . 104
     7.2.  Presenting Access Tokens  . . . . . . . . . . . . . . . . 105
     7.3.  Proving Possession of a Key with a Request  . . . . . . . 106
       7.3.1.  HTTP Message Signatures . . . . . . . . . . . . . . . 109
       7.3.2.  Mutual TLS  . . . . . . . . . . . . . . . . . . . . . 115
       7.3.3.  Detached JWS  . . . . . . . . . . . . . . . . . . . . 117
       7.3.4.  Attached JWS  . . . . . . . . . . . . . . . . . . . . 121
   8.  Resource Access Rights  . . . . . . . . . . . . . . . . . . . 126
     8.1.  Requesting Resources By Reference . . . . . . . . . . . . 130
   9.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 133
     9.1.  RS-first Method of AS Discovery . . . . . . . . . . . . . 134
     9.2.  Dynamic grant endpoint discovery  . . . . . . . . . . . . 136
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 136
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 137
     11.1.  Grant Request Parameters . . . . . . . . . . . . . . . . 137
       11.1.1.  Registration Template  . . . . . . . . . . . . . . . 138
       11.1.2.  Initial Contents . . . . . . . . . . . . . . . . . . 138
     11.2.  Access Token Flags . . . . . . . . . . . . . . . . . . . 139
       11.2.1.  Registration Template  . . . . . . . . . . . . . . . 139
       11.2.2.  Initial Contents . . . . . . . . . . . . . . . . . . 139
     11.3.  Subject Information Request Fields . . . . . . . . . . . 140
       11.3.1.  Registration Template  . . . . . . . . . . . . . . . 140
       11.3.2.  Initial Contents . . . . . . . . . . . . . . . . . . 140
     11.4.  Assertion Formats  . . . . . . . . . . . . . . . . . . . 141
       11.4.1.  Registration Template  . . . . . . . . . . . . . . . 141

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       11.4.2.  Initial Contents . . . . . . . . . . . . . . . . . . 141
     11.5.  Client Instance Fields . . . . . . . . . . . . . . . . . 141
       11.5.1.  Registration Template  . . . . . . . . . . . . . . . 142
       11.5.2.  Initial Contents . . . . . . . . . . . . . . . . . . 142
     11.6.  Client Instance Display Fields . . . . . . . . . . . . . 142
       11.6.1.  Registration Template  . . . . . . . . . . . . . . . 143
       11.6.2.  Initial Contents . . . . . . . . . . . . . . . . . . 143
     11.7.  Interaction Start Modes  . . . . . . . . . . . . . . . . 143
       11.7.1.  Registration Template  . . . . . . . . . . . . . . . 144
       11.7.2.  Initial Contents . . . . . . . . . . . . . . . . . . 144
     11.8.  Interaction Finish Methods . . . . . . . . . . . . . . . 145
       11.8.1.  Registration Template  . . . . . . . . . . . . . . . 145
       11.8.2.  Initial Contents . . . . . . . . . . . . . . . . . . 145
     11.9.  Interaction Hints  . . . . . . . . . . . . . . . . . . . 146
       11.9.1.  Registration Template  . . . . . . . . . . . . . . . 146
       11.9.2.  Initial Contents . . . . . . . . . . . . . . . . . . 146
     11.10. Grant Response Parameters  . . . . . . . . . . . . . . . 146
       11.10.1.  Registration Template . . . . . . . . . . . . . . . 147
       11.10.2.  Initial Contents  . . . . . . . . . . . . . . . . . 147
     11.11. Interaction Mode Responses . . . . . . . . . . . . . . . 148
       11.11.1.  Registration Template . . . . . . . . . . . . . . . 148
       11.11.2.  Initial Contents  . . . . . . . . . . . . . . . . . 148
     11.12. Subject Information Response Fields  . . . . . . . . . . 149
       11.12.1.  Registration Template . . . . . . . . . . . . . . . 149
       11.12.2.  Initial Contents  . . . . . . . . . . . . . . . . . 149
     11.13. Error Codes  . . . . . . . . . . . . . . . . . . . . . . 150
       11.13.1.  Registration Template . . . . . . . . . . . . . . . 150
       11.13.2.  Initial Contents  . . . . . . . . . . . . . . . . . 150
     11.14. Key Proofing Methods . . . . . . . . . . . . . . . . . . 151
       11.14.1.  Registration Template . . . . . . . . . . . . . . . 151
       11.14.2.  Initial Contents  . . . . . . . . . . . . . . . . . 152
     11.15. Key Formats  . . . . . . . . . . . . . . . . . . . . . . 152
       11.15.1.  Registration Template . . . . . . . . . . . . . . . 152
       11.15.2.  Initial Contents  . . . . . . . . . . . . . . . . . 153
     11.16. Authorization Server Discovery Fields  . . . . . . . . . 153
       11.16.1.  Registration Template . . . . . . . . . . . . . . . 153
       11.16.2.  Initial Contents  . . . . . . . . . . . . . . . . . 154
   12. Implementation Status . . . . . . . . . . . . . . . . . . . . 154
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 155
     13.1.  TLS Protection in Transit  . . . . . . . . . . . . . . . 155
     13.2.  Signing Requests from the Client Software  . . . . . . . 156
     13.3.  Protection of Client Instance Key Material . . . . . . . 157
     13.4.  Protection of Authorization Server . . . . . . . . . . . 159
     13.5.  Symmetric and Asymmetric Client Instance Keys  . . . . . 159
     13.6.  Generation of Access Tokens  . . . . . . . . . . . . . . 161
     13.7.  Bearer Access Tokens . . . . . . . . . . . . . . . . . . 161
     13.8.  Key-Bound Access Tokens  . . . . . . . . . . . . . . . . 162
     13.9.  Exposure of End-user Credentials to Client Instance  . . 163

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     13.10. Mixing Up Authorization Servers  . . . . . . . . . . . . 163
     13.11. Processing of Client-Presented User Information  . . . . 164
     13.12. Client Instance Pre-registration . . . . . . . . . . . . 165
     13.13. Client Instance Impersonation  . . . . . . . . . . . . . 167
     13.14. Interception of Information in the Browser . . . . . . . 167
     13.15. Callback URI Manipulation  . . . . . . . . . . . . . . . 168
     13.16. Redirection Status Codes . . . . . . . . . . . . . . . . 168
     13.17. MTLS Message Integrity . . . . . . . . . . . . . . . . . 169
     13.18. MTLS Deployment Patterns . . . . . . . . . . . . . . . . 170
     13.19. Interception of Responses from the AS  . . . . . . . . . 170
     13.20. Key Distribution . . . . . . . . . . . . . . . . . . . . 171
     13.21. Key Rotation Policy  . . . . . . . . . . . . . . . . . . 172
     13.22. Interaction Finish Modes and Polling . . . . . . . . . . 172
     13.23. Session Management for Interaction Finish Methods  . . . 173
     13.24. Calculating Interaction Hash . . . . . . . . . . . . . . 175
     13.25. Storage of Information During Interaction and
             Continuation  . . . . . . . . . . . . . . . . . . . . . 176
     13.26. Denial of Service (DoS) through Grant Continuation . . . 177
     13.27. Exhaustion of Random Value Space . . . . . . . . . . . . 178
     13.28. Front-channel URIs . . . . . . . . . . . . . . . . . . . 178
     13.29. Processing Assertions  . . . . . . . . . . . . . . . . . 179
     13.30. Stolen Token Replay  . . . . . . . . . . . . . . . . . . 180
     13.31. Self-contained Stateless Access Tokens . . . . . . . . . 181
     13.32. Network Problems and Token and Grant Management  . . . . 182
     13.33. Server-side Request Forgery (SSRF) . . . . . . . . . . . 183
     13.34. Multiple Key Formats . . . . . . . . . . . . . . . . . . 184
     13.35. Asynchronous Interactions  . . . . . . . . . . . . . . . 185
     13.36. Compromised RS . . . . . . . . . . . . . . . . . . . . . 186
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . . 186
     14.1.  Surveillance . . . . . . . . . . . . . . . . . . . . . . 186
       14.1.1.  Surveillance by the Client . . . . . . . . . . . . . 187
       14.1.2.  Surveillance by the Authorization Server . . . . . . 187
     14.2.  Stored Data  . . . . . . . . . . . . . . . . . . . . . . 187
     14.3.  Intrusion  . . . . . . . . . . . . . . . . . . . . . . . 188
     14.4.  Correlation  . . . . . . . . . . . . . . . . . . . . . . 188
       14.4.1.  Correlation by Clients . . . . . . . . . . . . . . . 189
       14.4.2.  Correlation by Resource Servers  . . . . . . . . . . 189
       14.4.3.  Correlation by Authorization Servers . . . . . . . . 190
     14.5.  Disclosure in Shared References  . . . . . . . . . . . . 190
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . . 190
     15.1.  Normative References . . . . . . . . . . . . . . . . . . 190
     15.2.  Informative References . . . . . . . . . . . . . . . . . 193
   Appendix A.  Document History . . . . . . . . . . . . . . . . . . 195
   Appendix B.  Compared to OAuth 2.0  . . . . . . . . . . . . . . . 200
   Appendix C.  Example Protocol Flows . . . . . . . . . . . . . . . 202
     C.1.  Redirect-Based User Interaction . . . . . . . . . . . . . 203
     C.2.  Secondary Device Interaction  . . . . . . . . . . . . . . 206
     C.3.  No User Involvement . . . . . . . . . . . . . . . . . . . 209

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     C.4.  Asynchronous Authorization  . . . . . . . . . . . . . . . 210
     C.5.  Applying OAuth 2.0 Scopes and Client IDs  . . . . . . . . 214
   Appendix D.  Interoperability Profiles  . . . . . . . . . . . . . 215
     D.1.  Web-based Redirection . . . . . . . . . . . . . . . . . . 216
     D.2.  Secondary Device  . . . . . . . . . . . . . . . . . . . . 216
   Appendix E.  Guidance for Extensions  . . . . . . . . . . . . . . 217
   Appendix F.  JSON Structures and Polymorphism . . . . . . . . . . 218
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 219

1.  Introduction

   This protocol allows a piece of software, the client instance, to
   request delegated authorization to resource servers and subject
   information.  This delegation is facilitated by an authorization
   server usually on behalf of a resource owner.  The end user operating
   the software can interact with the authorization server to
   authenticate, provide consent, and authorize the request as a
   resource owner.

   The process by which the delegation happens is known as a grant, and
   GNAP allows for the negotiation of the grant process over time by
   multiple parties acting in distinct roles.

   This specification focuses on the portions of the delegation process
   facing the client instance.  In particular, this specification
   defines interoperable methods for a client instance to request,
   negotiate, and receive access to information facilitated by the
   authorization server.  This specification additionally defines
   methods for the client instance to access protected resources at a
   resource server.  This specification also discusses discovery
   mechanisms for the client instance to configure itself dynamically.
   The means for an authorization server and resource server to
   interoperate are discussed in the companion document,
   [I-D.ietf-gnap-resource-servers].

   The focus of this protocol is to provide interoperability between the
   different parties acting in each role, and is not to specify
   implementation details of each.  Where appropriate, GNAP may make
   recommendations about internal implementation details, but these
   recommendations are to ensure the security of the overall deployment
   rather than to be prescriptive in the implementation.

   This protocol solves many of the same use cases as OAuth 2.0
   [RFC6749], OpenID Connect [OIDC], and the family of protocols that
   have grown up around that ecosystem.  However, GNAP is not an
   extension of OAuth 2.0 and is not intended to be directly compatible
   with OAuth 2.0.  GNAP seeks to provide functionality and solve use
   cases that OAuth 2.0 cannot easily or cleanly address.  Appendix B

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   further details the protocol rationale compared to OAuth 2.0.  GNAP
   and OAuth 2.0 will likely exist in parallel for many deployments, and
   considerations have been taken to facilitate the mapping and
   transition from legacy systems to GNAP.  Some examples of these can
   be found in Appendix C.5.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document contains non-normative examples of partial and complete
   HTTP messages, JSON structures, URIs, query components, keys, and
   other elements.  Whenever possible, the document uses URI as a
   generic term, since it aligns with [RFC3986] recommendations and
   matches better with the intent that the identifier may be reachable
   through various/generic means (compared to URLs).  Some examples use
   a single trailing backslash \ to indicate line wrapping for long
   values, as per [RFC8792].  The \ character and leading spaces on
   wrapped lines are not part of the value.

1.2.  Roles

   The parties in GNAP perform actions under different roles.  Roles are
   defined by the actions taken and the expectations leveraged on the
   role by the overall protocol.

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   +-------------+            +------------+
   |             |            |            |
   |Authorization|            |  Resource  |
   |   Server    |            |   Server   |
   |             |<--+   +--->|            |
   +-----+-------+   |   |    +------------+
         ║           |   |
         ║        +--+---+---+
         ║        |  Client  |
         ║        | Instance |
         ║        +----+-----+
         ║             ║
    .----+----.        ║      .----------.
   |           |       +=====+            |
   |  Resource |             |    End     |
   |   Owner   | ~ ~ ~ ~ ~ ~ |    User    |
   |           |             |            |
    `---------`               `----------`

   Legend

   ===== indicates interaction between a human and computer
   ----- indicates interaction between two pieces of software
   ~ ~ ~ indicates a potential equivalence or out-of-band
             communication between roles

   Authorization Server (AS)  server that grants delegated privileges to
      a particular instance of client software in the form of access
      tokens or other information (such as subject information).  The AS
      is uniquely defined by the _grant endpoint URI_, which the
      absolute URI where grant requests are started by clients.

   Client  application that consumes resources from one or several RSs,
      possibly requiring access privileges from one or several ASs.  The
      client is operated by the end user or it runs autonomously on
      behalf of a resource owner.

      Example: a client can be a mobile application, a web application,
      etc.

      Note: this specification differentiates between a specific
      instance (the client instance, identified by its unique key) and
      the software running the instance (the client software).  For some
      kinds of client software, there could be many instances of that
      software, each instance with a different key.

   Resource Server (RS)  server that provides operations on protected

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      resources, where operations require a valid access token issued by
      an AS.

   Resource Owner (RO)  subject entity that may grant or deny operations
      on resources it has authority upon.

      Note: the act of granting or denying an operation may be manual
      (i.e. through an interaction with a physical person) or automatic
      (i.e. through predefined organizational rules).

   End user  natural person that operates a client instance.

      Note: that natural person may or may not be the same entity as the
      RO.

   The design of GNAP does not assume any one deployment architecture,
   but instead attempts to define roles that can be fulfilled in a
   number of different ways for different use cases.  As long as a given
   role fulfills all of its obligations and behaviors as defined by the
   protocol, GNAP does not make additional requirements on its structure
   or setup.

   Multiple roles can be fulfilled by the same party, and a given party
   can switch roles in different instances of the protocol.  For
   example, the RO and end user in many instances are the same person,
   where a user is authorizing the client instance to act on their own
   behalf at the RS.  In this case, one party fulfills both of the RO
   and end-user roles, but the roles themselves are still defined
   separately from each other to allow for other use cases where they
   are fulfilled by different parties.

   For another example, in some complex scenarios, an RS receiving
   requests from one client instance can act as a client instance for a
   downstream secondary RS in order to fulfill the original request.  In
   this case, one piece of software is both an RS and a client instance
   from different perspectives, and it fulfills these roles separately
   as far as the overall protocol is concerned.

   A single role need not be deployed as a monolithic service.  For
   example, a client instance could have components that are installed
   on the end user's device as well as a back-end system that it
   communicates with.  If both of these components participate in the
   delegation protocol, they are both considered part of the client
   instance.  If there are several copies of the client software that
   run separately but all share the same key material, such as a
   deployed cluster, then this cluster is considered a single client
   instance.  In these cases, the distinct components of what is
   considered a GNAP client instance may use any number of different

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   communication mechanisms between them, all of which would be
   considered an implementation detail of the client instances and out
   of scope of GNAP.

   For another example, an AS could likewise be built out of many
   constituent components in a distributed architecture.  The component
   that the client instance calls directly could be different from the
   component that the RO interacts with to drive consent, since API
   calls and user interaction have different security considerations in
   many environments.  Furthermore, the AS could need to collect
   identity claims about the RO from one system that deals with user
   attributes while generating access tokens at another system that
   deals with security rights.  From the perspective of GNAP, all of
   these are pieces of the AS and together fulfill the role of the AS as
   defined by the protocol.  These pieces may have their own internal
   communications mechanisms which are considered out of scope of GNAP.

1.3.  Elements

   In addition to the roles above, the protocol also involves several
   elements that are acted upon by the roles throughout the process.

   Attribute  characteristics related to a subject.

   Access Token  a data artifact representing a set of rights and/or
      attributes.

      Note: an access token can be first issued to a client instance
      (requiring authorization by the RO) and subsequently rotated.

   Grant  (verb): to permit an instance of client software to receive
      some attributes at a specific time and valid for a specific
      duration and/or to exercise some set of delegated rights to access
      a protected resource;

      (noun): the act of granting permission to a client instance.

   Privilege  right or attribute associated with a subject.

      Note: the RO defines and maintains the rights and attributes
      associated to the protected resource, and might temporarily
      delegate some set of those privileges to an end user.  This
      process is refered to as privilege delegation.

   Protected Resource  protected API (Application Programming Interface)
      served by an RS and that can be accessed by a client, if and only
      if a valid and sufficient access token is provided.

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      Note: to avoid complex sentences, the specification document may
      simply refer to "resource" instead of "protected resource".

   Right  ability given to a subject to perform a given operation on a
      resource under the control of an RS.

   Subject  person, organization or device.  The subject decides whether
      and under which conditions its attributes can be disclosed to
      other parties.

   Subject Information  set of statements asserted by an AS about a
      subject.

1.4.  Trust relationships

   GNAP defines its trust objective as: "the RO trusts the AS to ensure
   access validation and delegation of protected resources to end users,
   through third party clients."

   This trust objective can be decomposed into trust relationships
   between software elements and roles, especially the pairs end user/
   RO, end user/client, client/AS, RS/RO, AS/RO, AS/RS.  Trust of an
   agent by its pair can exist if the pair is informed that the agent
   has made a promise to follow the protocol in the past (e.g. pre-
   registration, uncompromised cryptographic components) or if the pair
   is able to infer by indirect means that the agent has made such a
   promise (e.g. a compliant client request).  Each agent defines its
   own valuation function of promises given or received.  Examples of
   such valuations can be the benefits from interacting with other
   agents (e.g. safety in client access, interoperability with identity
   standards), the cost of following the protocol (including its
   security and privacy requirements and recommendations), a ranking of
   promise importance (e.g. a policy decision made by the AS), the
   assessment of one's vulnerability or risk of not being able to defend
   against threats, etc.  Those valuations may depend on the context of
   the request.  For instance, the AS may decide to either take into
   account or discard hints provided by the client, the RS may refuse
   bearer tokens, etc. depending on the specific case in which GNAP is
   used.  Some promises can be conditional of some previous interactions
   (e.g. repeated requests).

   Looking back on each trust relationship:

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   *  end user/RO: this relationship exists only when the end user and
      the RO are different, in which case the end user needs some out of
      band mechanism of getting the RO consent (see Section 4).  GNAP
      generally assumes that humans can be authenticated thanks to
      identity protocols (for instance, through an id_token assertion in
      Section 2.2).

   *  end user/client: the client acts as a user agent.  Depending on
      the technology used (browser, SPA, mobile application, IoT device,
      etc.), some interactions may or may not be possible (as described
      in Section 2.5.1).  Client developers promise to implement
      requirements and generally some recommendations or best practices,
      so that the end users may confidently use their software.
      However, end users might also be facing some attacker's client
      software, without even realizing it.

   *  end user/AS: when the client supports it (see Section 3.3), the
      end user gets to interact with front-channel URIs provided by the
      AS.  See Section 13.28 for some considerations in trusting these
      interactions.

   *  client/AS: An honest AS may be facing an attacker's client (as
      discussed just above), or the reverse, and GNAP aims at making
      common attacks impractical.  The core specification makes access
      tokens opaque to the client and defines the request/response
      scheme in detail, therefore avoiding extra trust hypotheses from
      this critical piece of software.  Yet the AS may further define
      cryptographic attestations or optional rules to simplify the
      access of clients it already trusts, due to past behavior or
      organizational policies (see Section 2.3).

   *  RS/RO: the RS promises it protects its resources from unauthorized
      access, and only accepts valid access tokens issued by a trusted
      AS.  In case tokens are key bound, proper validation is expected
      from the RS.

   *  AS/RO: the AS is expected to follow the decisions made by the RO,
      either through interactive consent requests, repeated
      interactions, or automated rules (as described in Section 1.6).
      Privacy considerations aim to reduce the risk of an honest but
      too-curious AS, or the consequences of an unexpected user data
      exposure.

   *  AS/RS: the AS promises to issue valid access tokens to legitimate
      client requests (i.e. after carrying out appropriate due
      diligence, as defined in the GNAP protocol).  Some optional
      configurations are covered by [I-D.ietf-gnap-resource-servers].

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   A global assumption made by GNAP is that authorization requests are
   security and privacy sensitive, and appropriate measures are
   respectively detailed in Section 13 and Section 14.

   A formal trust model is out of scope of this specification, but one
   could be developed using techniques such as [promise-theory].

1.5.  Protocol Flow

   GNAP is fundamentally designed to allow delegated access to APIs and
   other information, such as subject information, using a multi-stage,
   stateful process.  This process allows different parties to provide
   information into the system to alter and augment the state of the
   delegated access and its artifacts.

   The underlying requested grant moves through several states as
   different actions take place during the protocol:

                                                       .-----.
                                                      |       |
                                               +------+--+    | Continue
                      .---Need Interaction---->|         |    |
                     /                         | Pending |<--`
                    /   .--Finish Interaction--+         |
                   /   /     (approve/deny)    +----+----+
                  /   /                             |
                 /   /                              | Cancel
                /   v                               v
             +-+----------+                   +===========+
             |            |                   ║           ║
---Request-->| Processing +------Finalize---->║ Finalized ║
             |            |                   ║           ║
             +-+----------+                   +===========+
                \    ^                              ^
                 \    \                             | Revoke or
                  \    \                            | Finalize
                   \    \                     +-----+----+
                    \    `-----Update---------+          |
                     \                        | Approved |<--.
                      `-----No Interaction--->|          |    |
                                              +-------+--+    | Continue
                                                      |       |
                                                       `-----`

   _Processing_  When a request for access (Section 2) is received by
      the AS, a new grant request is created and placed in the
      _processing_ state by the AS.  This state is also entered when an
      existing grant request is updated by the client instance and when

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      interaction is completed.  In this state, the AS processes the
      context of the grant request to determine whether interaction with
      the end user or RO is required for approval of the request.  The
      grant request has to exit this state before a response can be
      returned to the client instance.  If approval is required, the
      request moves to the _pending_ state and the AS returns a continue
      response (Section 3.1) along with any appropriate interaction
      responses (Section 3.3).  If no such approval is required, such as
      when the client instance is acting on its own behalf or the AS can
      determine that access has been fulfilled, the request moves to the
      _approved_ state where access tokens for API access (Section 3.2)
      and subject information (Section 3.4) can be issued to the client
      instance.  If the AS determines that no additional processing can
      occur (such as a timeout or an unrecoverable error), the grant
      request is moved to the _finalized_ state and is terminated.

   _Pending_  When a request needs to be approved by a RO, or
      interaction with the end user is required, the grant request
      enters a state of _pending_. In this state, no access tokens can
      be granted and no subject information can be released to the
      client instance.  While a grant request is in this state, the AS
      seeks to gather the required consent and authorization (Section 4)
      for the requested access.  A grant request in this state is always
      associated with a _continuation access token_ bound to the client
      instance's key.  If no interaction finish method (Section 2.5.2)
      is associated with this request, the client instance can send a
      polling continue request (Section 5.2) to the AS.  This returns a
      continue response (Section 3.1) while the grant request remains in
      this state, allowing the client instance to continue to check the
      state of the pending grant request.  If an interaction finish
      method (Section 2.5.2) is specified in the grant request, the
      client instance can continue the request after interaction
      (Section 5.1) to the AS to move this request to the _processing_
      state to be re-evaluated by the AS.  Note that this occurs whether
      the grant request has been approved or denied by the RO, since the
      AS needs to take into account the full context of the request
      before determining the next step for the grant request.  When
      other information is made available in the context of the grant
      request, such as through the asynchronous actions of the RO, the
      AS moves this request to the _processing_ state to be re-
      evaluated.  If the AS determines that no additional interaction
      can occur, such as all the interaction methods have timed out or a
      revocation request (Section 5.4) is received from the client
      instance, the grant request can be moved to the _finalized_ state.

   _Approved_  When a request has been approved by an RO and no further

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      interaction with the end user is required, the grant request
      enters a state of _approved_. In this state, responses to the
      client instance can include access tokens for API access
      (Section 3.2) and subject information (Section 3.4).  If
      continuation and updates are allowed for this grant request, the
      AS can include the continuation response (Section 3.1).  In this
      state, post-interaction continuation requests (Section 5.1) are
      not allowed and will result in an error, since all interaction is
      assumed to have been completed.  If the client instance sends a
      polling continue request (Section 5.2) while the request is in
      this state, new access tokens (Section 3.2) can be issued in the
      response.  Note that this always creates a new access token, but
      any existing access tokens could be rotated and revoked using the
      token management API (Section 6).  The client instance can send an
      update continuation request (Section 5.3) to modify the requested
      access, causing the AS to move the request back to the
      _processing_ state for re-evaluation.  If the AS determines that
      no additional tokens can be issued, and that no additional updates
      are to be accepted (such as the continuation access tokens have
      expired), the grant is moved to the _finalized_ state.

   _Finalized_  After the access tokens are issued, if the AS does not
      allow any additional updates on the grant request, the grant
      request enters the _finalized_ state.  This state is also entered
      when an existing grant request is revoked by the client instance
      (Section 5.4) or otherwise revoked by the AS (such as through out-
      of-band action by the RO).  This state can also be entered if the
      AS determines that no additional processing is possible, for
      example if the RO has denied the requested access or if
      interaction is required but no compatible interaction methods are
      available.  Once in this state, no new access tokens can be
      issued, no subject information can be returned, and no
      interactions can take place.  Once in this state, the grant
      request is dead and cannot be revived.  If future access is
      desired by the client instance, a new grant request can be
      created, unrelated to this grant request.

   While it is possible to deploy an AS in a stateless environment, GNAP
   is a stateful protocol and such deployments will need a way to manage
   the current state of the grant request in a secure and deterministic
   fashion without relying on other components, such as the client
   software, to keep track of the current state.

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1.6.  Sequences

   GNAP can be used in a variety of ways to allow the core delegation
   process to take place.  Many portions of this process are
   conditionally present depending on the context of the deployments,
   and not every step in this overview will happen in all circumstances.

   Note that a connection between roles in this process does not
   necessarily indicate that a specific protocol message is sent across
   the wire between the components fulfilling the roles in question, or
   that a particular step is required every time.  For example, for a
   client instance interested in only getting subject information
   directly, and not calling an RS, all steps involving the RS below do
   not apply.

   In some circumstances, the information needed at a given stage is
   communicated out of band or is preconfigured between the components
   or entities performing the roles.  For example, one entity can
   fulfill multiple roles, and so explicit communication between the
   roles is not necessary within the protocol flow.  Additionally some
   components may not be involved in all use cases.  For example, a
   client instance could be calling the AS just to get direct user
   information and have no need to get an access token to call an RS.

1.6.1.  Overall Protocol Sequence

   The following diagram provides a general overview of GNAP, including
   many different optional phases and connections.  The diagrams in the
   following sections provide views of GNAP under more specific
   circumstances.

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    .----------.           .----------.
   |  End user  | ~ ~ ~ ~ |  Resource  |
   |            |         | Owner (RO) |
    `----+-----`           `-----+----`
         ║                       ║
         ║                       ║
        (A)                     (B)
         ║                       ║
         ║                       ║
   +-----+--+                    ║           +------------+
   | Client | (1)                ║           |  Resource  |
   |Instance|                    ║           |   Server   |
   |        |        +-----------+---+       |    (RS)    |
   |        +--(2)-->| Authorization |       |            |
   |        |<-(3)---+     Server    |       |            |
   |        |        |      (AS)     |       |            |
   |        +--(4)-->|               |       |            |
   |        |<-(5)---+               |       |            |
   |        |        |               |       |            |
   |        +---------------(6)------------->|            |
   |        |        |               |   (7) |            |
   |        |<--------------(8)------------->|            |
   |        |        |               |       |            |
   |        +--(9)-->|               |       |            |
   |        |<-(10)--+               |       |            |
   |        |        |               |       |            |
   |        +---------------(11)------------>|            |
   |        |        |               |  (12) |            |
   |        +--(13)->|               |       |            |
   |        |        |               |       |            |
   +--------+        +---------------+       +------------+

   Legend
   ===== indicates a possible interaction with a human
   ----- indicates an interaction between protocol roles
   ~ ~ ~ indicates a potential equivalence or out-of-band
           communication between roles

   *  (A) The end user interacts with the client instance to indicate a
      need for resources on behalf of the RO.  This could identify the
      RS the client instance needs to call, the resources needed, or the
      RO that is needed to approve the request.  Note that the RO and
      end user are often the same entity in practice, but GNAP makes no
      general assumption that they are.

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   *  (1) The client instance determines what access is needed and which
      AS to approach for access.  Note that for most situations, the
      client instance is pre-configured with which AS to talk to and
      which kinds of access it needs, but some more dynamic processes
      are discussed in Section 9.1.

   *  (2) The client instance requests access at the AS (Section 2).

   *  (3) The AS processes the request and determines what is needed to
      fulfill the request (See Section 4).  The AS sends its response to
      the client instance (Section 3).

   *  (B) If interaction is required, the AS interacts with the RO
      (Section 4) to gather authorization.  The interactive component of
      the AS can function using a variety of possible mechanisms
      including web page redirects, applications, challenge/response
      protocols, or other methods.  The RO approves the request for the
      client instance being operated by the end user.  Note that the RO
      and end user are often the same entity in practice, and many of
      GNAP's interaction methods allow the client instance to facilitate
      the end user interacting with the AS in order to fulfill the role
      of the RO.

   *  (4) The client instance continues the grant at the AS (Section 5).
      This action could occur in response to receiving a signal that
      interaction has finished (Section 4.2) or through a periodic
      polling mechanism, depending on the interaction capabilities of
      the client software and the options active in the grant request.

   *  (5) If the AS determines that access can be granted, it returns a
      response to the client instance (Section 3) including an access
      token (Section 3.2) for calling the RS and any directly returned
      information (Section 3.4) about the RO.

   *  (6) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (7) The RS determines if the token is sufficient for the request
      by examining the token.  The means of the RS determining this
      access are out of scope of this specification, but some options
      are discussed in [I-D.ietf-gnap-resource-servers].

   *  (8) The client instance calls the RS (Section 7.2) using the
      access token until the RS or client instance determine that the
      token is no longer valid.

   *  (9) When the token no longer works, the client instance rotates
      the access token (Section 6.1).

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   *  (10) The AS issues a new access token (Section 3.2) to the client
      instance with the same rights as the original access token
      returned in (5).

   *  (11) The client instance uses the new access token (Section 7.2)
      to call the RS.

   *  (12) The RS determines if the new token is sufficient for the
      request, as in (7).

   *  (13) The client instance disposes of the token (Section 6.2) once
      the client instance has completed its access of the RS and no
      longer needs the token.

   The following sections and Appendix C contain specific guidance on
   how to use GNAP in different situations and deployments.  For
   example, it is possible for the client instance to never request an
   access token and never call an RS, just as it is possible to have no
   end user involved in the delegation process.

1.6.2.  Redirect-based Interaction

   In this example flow, the client instance is a web application that
   wants access to resources on behalf of the current user, who acts as
   both the end user and the resource owner (RO).  Since the client
   instance is capable of directing the user to an arbitrary URI and
   receiving responses from the user's browser, interaction here is
   handled through front-channel redirects using the user's browser.
   The redirection URI used for interaction is a service hosted by the
   AS in this example.  The client instance uses a persistent session
   with the user to ensure the same user that is starting the
   interaction is the user that returns from the interaction.

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 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance|                                  |        |         | User |
 |        |<=(1)== Start Session ===============================+      |
 |        |                                  |        |         |      |
 |        +--(2)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(3)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(4)== Redirect for Interaction ===================>|      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |  AuthN  |  RO  |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(6)==>|      |
 |        |                                  |        |  AuthZ  +------+
 |        |                                  |        |         | End  |
 |        |<=(7)== Redirect for Continuation ===================+ User |
 |        |                                  |        |          `----`
 |        +--(8)--- Continue Request ------->|        |
 |        |                                  |        |
 |        |<-(9)----- Grant Access ----------+        |
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(10)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(11)-- API Response ---------------------------|        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

   1.   The client instance establishes a session with the user, in the
        role of the end user.

   2.   The client instance requests access to the resource (Section 2).
        The client instance indicates that it can redirect to an
        arbitrary URI (Section 2.5.1.1) and receive a redirect from the
        browser (Section 2.5.2.1).  The client instance stores
        verification information for its redirect in the session created
        in (1).

   3.   The AS determines that interaction is needed and responds
        (Section 3) with a URI to send the user to (Section 3.3.1) and
        information needed to verify the redirect (Section 3.3.5) in
        (7).  The AS also includes information the client instance will
        need to continue the request (Section 3.1) in (8).  The AS
        associates this continuation information with an ongoing request
        that will be referenced in (4), (6), and (8).

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   4.   The client instance stores the verification and continuation
        information from (3) in the session from (1).  The client
        instance then redirects the user to the URI (Section 4.1.1)
        given by the AS in (3).  The user's browser loads the
        interaction redirect URI.  The AS loads the pending request
        based on the incoming URI generated in (3).

   5.   The user authenticates at the AS, taking on the role of the RO.

   6.   As the RO, the user authorizes the pending request from the
        client instance.

   7.   When the AS is done interacting with the user, the AS redirects
        the user back (Section 4.2.1) to the client instance using the
        redirect URI provided in (2).  The redirect URI is augmented
        with an interaction reference that the AS associates with the
        ongoing request created in (2) and referenced in (4).  The
        redirect URI is also augmented with a hash of the security
        information provided in (2) and (3).  The client instance loads
        the verification information from (2) and (3) from the session
        created in (1).  The client instance calculates a hash
        (Section 4.2.3) based on this information and continues only if
        the hash validates.  Note that the client instance needs to
        ensure that the parameters for the incoming request match those
        that it is expecting from the session created in (1).  The
        client instance also needs to be prepared for the end user never
        being returned to the client instance and handle timeouts
        appropriately.

   8.   The client instance loads the continuation information from (3)
        and sends the interaction reference from (7) in a request to
        continue the request (Section 5.1).  The AS validates the
        interaction reference ensuring that the reference is associated
        with the request being continued.

   9.   If the request has been authorized, the AS grants access to the
        information in the form of access tokens (Section 3.2) and
        direct subject information (Section 3.4) to the client instance.

   10.  The client instance uses the access token (Section 7.2) to call
        the RS.

   11.  The RS validates the access token and returns an appropriate
        response for the API.

   An example set of protocol messages for this method can be found in
   Appendix C.1.

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1.6.3.  User-code Interaction

   In this example flow, the client instance is a device that is capable
   of presenting a short, human-readable code to the user and directing
   the user to enter that code at a known URI.  The user enters the code
   at a URI that is an interactive service hosted by the AS in this
   example.  The client instance is not capable of presenting an
   arbitrary URI to the user, nor is it capable of accepting incoming
   HTTP requests from the user's browser.  The client instance polls the
   AS while it is waiting for the RO to authorize the request.  The
   user's interaction is assumed to occur on a secondary device.  In
   this example it is assumed that the user is both the end user and RO.
   Note that since the user is not assumed to be interacting with the
   client instance through the same web browser used for interaction at
   the AS, the user is not shown as being connected to the client
   instance in this diagram.

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 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance+--(1)--- Request Access --------->|        |         | User |
 |        |                                  |        |         |      |
 |        |<-(2)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(3)==== Display User Code ========================>|      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(4)===+      |
 |        |                                  |        |Open URI |      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(5)==>|  RO  |
 |        |                                  |        |  AuthN  |      |
 |        +--(9)--- Continue Request (A) --->|        |         |      |
 |        |                                  |        |<==(6)==>|      |
 |        |<-(10)-- Not Yet Granted (Wait) --+        |  Code   |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(7)==>|      |
 |        |                                  |        |  AuthZ  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(8)==>|      |
 |        |                                  |        |Completed+------+
 |        |                                  |        |         | End  |
 |        +--(11)-- Continue Request (B) --->|        |         | User |
 |        |                                  |        |          `----`
 |        |<-(12)----- Grant Access ---------+        |
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(13)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(14)-- API Response ---------------------------+        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

   1.   The client instance requests access to the resource (Section 2).
        The client instance indicates that it can display a user code
        (Section 2.5.1.3).

   2.   The AS determines that interaction is needed and responds
        (Section 3) with a user code to communicate to the user
        (Section 3.3.3).  The AS also includes information the client
        instance will need to continue the request (Section 3.1) in (8)
        and (10).  The AS associates this continuation information with
        an ongoing request that will be referenced in (4), (6), (8), and
        (10).

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   3.   The client instance stores the continuation information from (2)
        for use in (8) and (10).  The client instance then communicates
        the code to the user (Section 4.1.2) given by the AS in (2).

   4.   The users directs their browser to the user code URI.  This URI
        is stable and can be communicated via the client software's
        documentation, the AS documentation, or the client software
        itself.  Since it is assumed that the RO will interact with the
        AS through a secondary device, the client instance does not
        provide a mechanism to launch the RO's browser at this URI.

   5.   The end user authenticates at the AS, taking on the role of the
        RO.

   6.   The RO enters the code communicated in (3) to the AS.  The AS
        validates this code against a current request in process.

   7.   As the RO, the user authorizes the pending request from the
        client instance.

   8.   When the AS is done interacting with the user, the AS indicates
        to the RO that the request has been completed.

   9.   Meanwhile, the client instance loads the continuation
        information stored at (3) and continues the request (Section 5).
        The AS determines which ongoing access request is referenced
        here and checks its state.

   10.  If the access request has not yet been authorized by the RO in
        (6), the AS responds to the client instance to continue the
        request (Section 3.1) at a future time through additional polled
        continuation requests.  This response can include updated
        continuation information as well as information regarding how
        long the client instance should wait before calling again.  The
        client instance replaces its stored continuation information
        from the previous response (2).  Note that the AS may need to
        determine that the RO has not approved the request in a
        sufficient amount of time and return an appropriate error to the
        client instance.

   11.  The client instance continues to poll the AS (Section 5.2) with
        the new continuation information in (9).

   12.  If the request has been authorized, the AS grants access to the
        information in the form of access tokens (Section 3.2) and
        direct subject information (Section 3.4) to the client instance.

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   13.  The client instance uses the access token (Section 7.2) to call
        the RS.

   14.  The RS validates the access token and returns an appropriate
        response for the API.

   An example set of protocol messages for this method can be found in
   Appendix C.2.

1.6.4.  Asynchronous Authorization

   In this example flow, the end user and RO roles are fulfilled by
   different parties, and the RO does not interact with the client
   instance.  The AS reaches out asynchronously to the RO during the
   request process to gather the RO's authorization for the client
   instance's request.  The client instance polls the AS while it is
   waiting for the RO to authorize the request.

 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         |  RO  |
 |Instance+--(1)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(2)-- Not Yet Granted (Wait) ---+        |         |      |
 |        |                                  |        |<==(3)==>|      |
 |        |                                  |        |  AuthN  |      |
 |        +--(6)--- Continue Request (A) --->|        |         |      |
 |        |                                  |        |<==(4)==>|      |
 |        |<-(7)-- Not Yet Granted (Wait) ---+        |  AuthZ  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |Completed|      |
 |        |                                  |        |         |      |
 |        +--(8)--- Continue Request (B) --->|        |          `----`
 |        |                                  |        |
 |        |<-(9)------ Grant Access ---------+        |
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(10)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(11)-- API Response ---------------------------+        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

   1.   The client instance requests access to the resource (Section 2).
        The client instance does not send any interaction modes to the
        server, indicating that it does not expect to interact with the
        RO.  The client instance can also signal which RO it requires
        authorization from, if known, by using the subject request

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        (Section 2.2) and user request (Section 2.4) sections.  It's
        also possible for the AS to determine which RO needs to be
        contacted by the nature of what access is being requested.

   2.   The AS determines that interaction is needed, but the client
        instance cannot interact with the RO.  The AS responds
        (Section 3) with the information the client instance will need
        to continue the request (Section 3.1) in (6) and (8), including
        a signal that the client instance should wait before checking
        the status of the request again.  The AS associates this
        continuation information with an ongoing request that will be
        referenced in (3), (4), (5), (6), and (8).

   3.   The AS determines which RO to contact based on the request in
        (1), through a combination of the user request (Section 2.4),
        the subject request (Section 2.2), the access request
        (Section 2.1), and other policy information.  The AS contacts
        the RO and authenticates them.

   4.   The RO authorizes the pending request from the client instance.

   5.   When the AS is done interacting with the RO, the AS indicates to
        the RO that the request has been completed.

   6.   Meanwhile, the client instance loads the continuation
        information stored at (2) and continues the request (Section 5).
        The AS determines which ongoing access request is referenced
        here and checks its state.

   7.   If the access request has not yet been authorized by the RO in
        (6), the AS responds to the client instance to continue the
        request (Section 3.1) at a future time through additional
        polling.  Note that this response is not an error message, since
        no error has yet occurred.  This response can include refreshed
        credentials as well as information regarding how long the client
        instance should wait before calling again.  The client instance
        replaces its stored continuation information from the previous
        response (2).  Note that the AS may need to determine that the
        RO has not approved the request in a sufficient amount of time
        and return an appropriate error to the client instance.

   8.   The client instance continues to poll the AS (Section 5.2) with
        the new continuation information from (7).

   9.   If the request has been authorized, the AS grants access to the
        information in the form of access tokens (Section 3.2) and
        direct subject information (Section 3.4) to the client instance.

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   10.  The client instance uses the access token (Section 7.2) to call
        the RS.

   11.  The RS validates the access token and returns an appropriate
        response for the API.

   An example set of protocol messages for this method can be found in
   Appendix C.4.

   Additional considerations for asynchronous interactions like this are
   discussed in Section 13.35.

1.6.5.  Software-only Authorization

   In this example flow, the AS policy allows the client instance to
   make a call on its own behalf, without the need for an RO to be
   involved at runtime to approve the decision.  Since there is no
   explicit RO, the client instance does not interact with an RO.

   +--------+                            +--------+
   | Client |                            |   AS   |
   |Instance+--(1)--- Request Access --->|        |
   |        |                            |        |
   |        |<-(2)---- Grant Access -----+        |
   |        |                            |        |  +--------+
   |        +--(3)--- Access API ------------------->|   RS   |
   |        |                            |        |  |        |
   |        |<-(4)--- API Response ------------------+        |
   |        |                            |        |  +--------+
   +--------+                            +--------+

   1.  The client instance requests access to the resource (Section 2).
       The client instance does not send any interaction modes to the
       server.

   2.  The AS determines that the request has been authorized based on
       the identity of the client instance making the request and the
       access requested (Section 2.1).  The AS grants access to the
       resource in the form of access tokens (Section 3.2) to the client
       instance.  Note that direct subject information (Section 3.4) is
       not generally applicable in this use case, as there is no user
       involved.

   3.  The client instance uses the access token (Section 7.2) to call
       the RS.

   4.  The RS validates the access token and returns an appropriate
       response for the API.

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   An example set of protocol messages for this method can be found in
   Appendix C.3.

1.6.6.  Refreshing an Expired Access Token

   In this example flow, the client instance receives an access token to
   access a resource server through some valid GNAP process.  The client
   instance uses that token at the RS for some time, but eventually the
   access token expires.  The client instance then gets a refreshed
   access token by rotating the expired access token at the AS using the
   token's management URI.

   +--------+                                          +--------+
   | Client |                                          |   AS   |
   |Instance+--(1)--- Request Access ----------------->|        |
   |        |                                          |        |
   |        |<-(2)--- Grant Access --------------------+        |
   |        |                                          |        |
   |        |                             +--------+   |        |
   |        +--(3)--- Access Resource --->|   RS   |   |        |
   |        |                             |        |   |        |
   |        |<-(4)--- Success Response ---+        |   |        |
   |        |                             |        |   |        |
   |        |       ( Time Passes )       |        |   |        |
   |        |                             |        |   |        |
   |        +--(5)--- Access Resource --->|        |   |        |
   |        |                             |        |   |        |
   |        |<-(6)--- Error Response -----+        |   |        |
   |        |                             +--------+   |        |
   |        |                                          |        |
   |        +--(7)--- Rotate Token ------------------->|        |
   |        |                                          |        |
   |        |<-(8)--- Rotated Token -------------------+        |
   |        |                                          |        |
   +--------+                                          +--------+

   1.  The client instance requests access to the resource (Section 2).

   2.  The AS grants access to the resource (Section 3) with an access
       token (Section 3.2) usable at the RS.  The access token response
       includes a token management URI.

   3.  The client instance uses the access token (Section 7.2) to call
       the RS.

   4.  The RS validates the access token and returns an appropriate
       response for the API.

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   5.  Time passes and the client instance uses the access token to call
       the RS again.

   6.  The RS validates the access token and determines that the access
       token is expired.  The RS responds to the client instance with an
       error.

   7.  The client instance calls the token management URI returned in
       (2) to rotate the access token (Section 6.1).  The client
       instance uses the access token (Section 7.2) in this call as well
       as the appropriate key, see the token rotation section for
       details.

   8.  The AS validates the rotation request including the signature and
       keys presented in (7) and refreshes the access token
       (Section 3.2.1).  The response includes a new version of the
       access token and can also include updated token management
       information, which the client instance will store in place of the
       values returned in (2).

1.6.7.  Requesting Subject Information Only

   In this scenario, the client instance does not call an RS and does
   not request an access token.  Instead, the client instance only
   requests and is returned direct subject information (Section 3.4).
   Many different interaction modes can be used in this scenario, so
   these are shown only in the abstract as functions of the AS here.

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 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance|                                  |        |         | User |
 |        +--(1)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(2)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(3)== Facilitate Interaction =====================>|      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(4)==>|  RO  |
 |        |                                  |        |  AuthN  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |  AuthZ  +------+
 |        |                                  |        |         | End  |
 |        |<=(6)== Signal Continuation =========================+ User |
 |        |                                  |        |          `----`
 |        +--(7)--- Continue Request ------->|        |
 |        |                                  |        |
 |        |<-(8)----- Grant Access ----------+        |
 |        |                                  |        |
 +--------+                                  +--------+

   1.  The client instance requests access to subject information
       (Section 2).

   2.  The AS determines that interaction is needed and responds
       (Section 3) with appropriate information for facilitating user
       interaction (Section 3.3).

   3.  The client instance facilitates the user interacting with the AS
       (Section 4) as directed in (2).

   4.  The user authenticates at the AS, taking on the role of the RO.

   5.  As the RO, the user authorizes the pending request from the
       client instance.

   6.  When the AS is done interacting with the user, the AS returns the
       user to the client instance and signals continuation.

   7.  The client instance loads the continuation information from (2)
       and calls the AS to continue the request (Section 5).

   8.  If the request has been authorized, the AS grants access to the
       requested direct subject information (Section 3.4) to the client
       instance.  At this stage, the user is generally considered
       "logged in" to the client instance based on the identifiers and

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       assertions provided by the AS.  Note that the AS can restrict the
       subject information returned and it might not match what the
       client instance requested, see the section on subject information
       for details.

1.6.8.  Cross-User Authentication

   In this scenario, the end user and resource owner are two different
   people.  Here, the client instance already knows who the end user is,
   likely through a separate authentication process.  The end user,
   operating the client instance, needs to get subject information about
   another person in the system, the RO.  The RO is given an opportunity
   to release this information using an asynchronous interaction method
   with the AS.  This scenario would apply, for instance, when the end
   user is an agent in a call-center and the resource owner is a
   customer authorizing the call center agent to access their account on
   their behalf.

  .----.                                                         .----.
 | End  |                                                       |  RO  |
 | User |<=================(1)== Identify RO ==================>|      |
 |      |                                                       |      |
 |      |        +--------+                  +--------+         |      |
 |      +==(2)==>| Client |                  |   AS   |         |      |
 |      | RO ID  |Instance|                  |        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        +--(3)-- Req. ---->|        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |<-(4)-- Res. -----+        |         |      |
 |      |        |        |                  |        |<==(5)==>|      |
 |      |        |        |                  |        |  AuthN  |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |                  |        |<==(6)==>|      |
 |      |        |        |                  |        |  AuthZ  |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |                  |        |<==(7)==>|      |
 |      |        |        |<-(8)--- Finish --+        |Completed|      |
 |      |        |        |                  |        |         |      |
 |      |        |        +--(9)--- Cont. -->|        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |<-(10)-- Subj. ---+        |         |      |
 |      |<=(11)==+        |         Info     |        |         |      |
 |      | Return |        |                  |        |         |      |
 |      | RO     |        |                  |        |         |      |
 |      | Info   |        |                  |        |         |      |
  `----`         +--------+                  +--------+          `----`

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   Precondition: The end user is authenticated to the client instance,
   and the client instance has an identifier representing the end user
   that it can present to the AS.  This identifier should be unique to
   the particular session with the client instance and the AS.  The
   client instance is also known to the AS and allowed to access this
   advanced functionality where the information of someone other than
   the end user is returned to the client instance.

   1.   The RO communicates a human-readable identifier to the end user,
        such as an email address or account number.  This communication
        happens out of band from the protocol, such as over the phone
        between parties.  Note that the RO is not interacting with the
        client instance.

   2.   The end user communicates the identifier to the client instance.
        The means by which the identifier is communicated to the client
        instance is out of scope for this specification.

   3.   The client instance requests access to subject information
        (Section 2).  The request includes the RO's identifier in the
        subject information request (Section 2.2) sub_ids field, and the
        end user's identifier in the user information field
        (Section 2.4) of the request.  The request includes no
        interaction start methods, since the end user is not expected to
        be the one interacting with the AS.  The request does include
        the push based interaction finish method (Section 2.5.2.2) to
        allow the AS to signal to the client instance when the
        interaction with the RO has concluded.

   4.   The AS sees that the identifier for the end user and subject
        being requested are different.  The AS determines that it can
        reach out to the RO asynchronously for approval.  While it is
        doing so, the AS returns a continuation response (Section 3.1)
        with a finish nonce to allow the client instance to continue the
        grant request after interaction with the RO has concluded.

   5.   The AS contacts the RO and has them authenticate to the system.
        The means for doing this are outside the scope of this
        specification, but the identity of the RO is known from the
        subject identifier sent in (3).

   6.   The RO is prompted to authorize the end user's request via the
        client instance.  Since the end user was identified in (3) via
        the user field, the AS can show this information to the RO
        during the authorization request.

   7.   The RO completes the authorization with the AS.  The AS marks
        the request as _approved_.

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   8.   The RO pushes the interaction finish message (Section 4.2.2) to
        the client instance.  Note that in the case the RO cannot be
        reached or the RO denies the request, the AS still sends the
        interaction finish message to the client instance, after which
        the client instance can negotiate next steps if possible.

   9.   The client instance validates the interaction finish message and
        continues the grant request (Section 5.1).

   10.  The AS returns the RO's subject information (Section 3.4) to the
        client instance.

   11.  The client instance can display or otherwise utilize the RO's
        user information in its session with the end user.  Note that
        since the client instance requested different sets of user
        information in (3), the client instance does not conflate the
        end user with the RO.

   Additional considerations for asynchronous interactions like this are
   discussed in Section 13.35.

2.  Requesting Access

   To start a request, the client instance sends an HTTP POST with a
   JSON [RFC8259] document to the grant endpoint of the AS.  The grant
   endpoint is a URI that uniquely identifies the AS to client instances
   and serves as the identifier for the AS.  The document is a JSON
   object where each field represents a different aspect of the client
   instance's request.  Each field is described in detail in a section
   below.

   access_token (object / array of objects):  Describes the rights and
      properties associated with the requested access token.  REQUIRED
      if requesting an access token.  See Section 2.1.

   subject (object):  Describes the information about the RO that the
      client instance is requesting to be returned directly in the
      response from the AS.  REQUIRED if requesting subject information.
      See Section 2.2.

   client (object / string):  Describes the client instance that is
      making this request, including the key that the client instance
      will use to protect this request and any continuation requests at
      the AS and any user-facing information about the client instance
      used in interactions.  REQUIRED.  See Section 2.3.

   user (object / string):  Identifies the end user to the AS in a

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      manner that the AS can verify, either directly or by interacting
      with the end user to determine their status as the RO.  OPTIONAL.
      See Section 2.4.

   interact (object):  Describes the modes that the client instance
      supports for allowing the RO to interact with the AS and modes for
      the client instance to receive updates when interaction is
      complete.  REQUIRED if interaction is supported.  See Section 2.5.

   Additional members of this request object can be defined by
   extensions using the Grant Request Parameters Registry
   (Section 11.1).

   A non-normative example of a grant request is below:

   {
       "access_token": {
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "dolphin-metadata"
           ]
       },
       "client": {
         "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
         },
         "key": {
           "proof": "httpsig",
           "jwk": {
             "kty": "RSA",
             "e": "AQAB",
             "kid": "xyz-1",

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             "alg": "RS256",
             "n": "kOB5rR4Jv0GMeL...."
           }
         }
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "subject": {
           "sub_id_formats": ["iss_sub", "opaque"],
           "assertion_formats": ["id_token"]
       }
   }

   Sending a request to the grant endpoint creates a grant request in
   the _processing_ state.  The AS processes this request to determine
   whether interaction or authorization are necessary (moving to the
   _pending_ state), or if access can be granted immediately (moving to
   the _approved_ state).

   The request MUST be sent as a JSON object in the body of the HTTP
   POST request with Content-Type application/json, unless otherwise
   specified by the signature mechanism.

2.1.  Requesting Access to Resources

   If the client instance is requesting one or more access tokens for
   the purpose of accessing an API, the client instance MUST include an
   access_token field.  This field MUST be an object (for a single
   access token (Section 2.1.1)) or an array of these objects (for
   multiple access tokens (Section 2.1.2)), as described in the
   following sections.

2.1.1.  Requesting a Single Access Token

   To request a single access token, the client instance sends an
   access_token object composed of the following fields.

   access (array of objects/strings):  Describes the rights that the
      client instance is requesting for one or more access tokens to be
      used at the RS.  REQUIRED.  See Section 8.

   label (string):  A unique name chosen by the client instance to refer

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      to the resulting access token.  The value of this field is opaque
      to the AS.  If this field is included in the request, the AS MUST
      include the same label in the token response (Section 3.2).
      REQUIRED if used as part of a multiple access token request
      (Section 2.1.2), OPTIONAL otherwise.

   flags (array of strings):  A set of flags that indicate desired
      attributes or behavior to be attached to the access token by the
      AS.  OPTIONAL.

   The values of the flags field defined by this specification are as
   follows:

   "bearer":  If this flag is included, the access token being requested
      is a bearer token.  If this flag is omitted, the access token is
      bound to the key used by the client instance in this request (or
      that key's most recent rotation) and the access token MUST be
      presented using the same key and proofing method.  Methods for
      presenting bound and bearer access tokens are described in
      Section 7.2.  See Section 13.7 for additional considerations on
      the use of bearer tokens.

   Flag values MUST NOT be included more than once.

   Additional flags can be defined by extensions using the Access Token
   Flags Registry (Section 11.2).

   In the following example, the client instance is requesting access to
   a complex resource described by a pair of access request object.

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   "access_token": {
       "access": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "delete"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           {
               "type": "walrus-access",
               "actions": [
                   "foo",
                   "bar"
               ],
               "locations": [
                   "https://resource.other/"
               ],
               "datatypes": [
                   "data",
                   "pictures",
                   "walrus whiskers"
               ]
           }
       ],
       "label": "token1-23"
   }

   If access is approved, the resulting access token is valid for the
   described resource.  Since the "bearer" flag is not provided in this
   example, the token is bound to the client instance's key (or its most
   recent rotation).  The token is labeled "token1-23".  The token
   response structure is described in Section 3.2.1.

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2.1.2.  Requesting Multiple Access Tokens

   To request multiple access tokens to be returned in a single
   response, the client instance sends an array of objects as the value
   of the access_token parameter.  Each object MUST conform to the
   request format for a single access token request, as specified in
   requesting a single access token (Section 2.1.1).  Additionally, each
   object in the array MUST include the label field, and all values of
   these fields MUST be unique within the request.  If the client
   instance does not include a label value for any entry in the array,
   or the values of the label field are not unique within the array, the
   AS MUST return an "invalid_request" error (Section 3.6).

   The following non-normative example shows a request for two separate
   access tokens, token1 and token2.

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   "access_token": [
       {
           "label": "token1",
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "dolphin-metadata"
           ]
       },
       {
           "label": "token2",
           "access": [
               {
                   "type": "walrus-access",
                   "actions": [
                       "foo",
                       "bar"
                   ],
                   "locations": [
                       "https://resource.other/"
                   ],
                   "datatypes": [
                       "data",
                       "pictures",
                       "walrus whiskers"
                   ]
               }
           ],
           "flags": [ "bearer" ]
       }
   ]

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   All approved access requests are returned in the multiple access
   token response (Section 3.2.2) structure using the values of the
   label fields in the request.

2.2.  Requesting Subject Information

   If the client instance is requesting information about the RO from
   the AS, it sends a subject field as a JSON object.  This object MAY
   contain the following fields.

   sub_id_formats (array of strings):  An array of subject identifier
      subject formats requested for the RO, as defined by
      [I-D.ietf-secevent-subject-identifiers].  REQUIRED if subject
      identifiers are requested.

   assertion_formats (array of strings):  An array of requested
      assertion formats.  Possible values include id_token for an OpenID
      Connect ID Token ([OIDC]) and saml2 for a SAML 2 assertion
      ([SAML2]).  Additional assertion formats are defined by the
      Assertion Formats Registry (Section 11.4).  REQUIRED if assertions
      are requested.

   sub_ids (array of objects):  An array of subject identifiers
      representing the subject that information is being requested for.
      Each object is a subject identifier as defined by
      [I-D.ietf-secevent-subject-identifiers].  All identifiers in the
      sub_ids array MUST identify the same subject.  If omitted, the AS
      SHOULD assume that subject information requests are about the
      current user and SHOULD require direct interaction or proof of
      presence before releasing information.  OPTIONAL.

   Additional fields are defined in the Subject Information Request
   Fields Registry (Section 11.3).

   "subject": {
     "sub_id_formats": [ "iss_sub", "opaque" ],
     "assertion_formats": [ "id_token", "saml2" ]
   }

   The AS can determine the RO's identity and permission for releasing
   this information through interaction with the RO (Section 4), AS
   policies, or assertions presented by the client instance
   (Section 2.4).  If this is determined positively, the AS MAY return
   the RO's information in its response (Section 3.4) as requested.

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   Subject identifier types requested by the client instance serve only
   to identify the RO in the context of the AS and can't be used as
   communication channels by the client instance, as discussed in
   Section 3.4.

2.3.  Identifying the Client Instance

   When sending new grant request to the AS, the client instance MUST
   identify itself by including its client information in the client
   field of the request and by signing the request with its unique key
   as described in Section 7.3.  Note that once a grant has been created
   and is in the _pending_ or _accepted_ states, the AS can determine
   which client is associated with the grant by dereferencing the
   continuation access token sent in the continuation request
   (Section 5).  As a consequence, the client field is not sent or
   accepted for continuation requests.

   Client information MUST either be sent by value as an object or by
   reference as a string (see Section 2.3.1).

   When client instance information is sent by value, the client field
   of the request consists of a JSON object with the following fields.

   key (object / string):  The public key of the client instance to be
      used in this request as described in Section 7.1 or a reference to
      a key as described in Section 7.1.1.  REQUIRED.

   class_id (string):  An identifier string that the AS can use to
      identify the client software comprising this client instance.  The
      contents and format of this field are up to the AS.  OPTIONAL.

   display (object):  An object containing additional information that
      the AS MAY display to the RO during interaction, authorization,
      and management.  OPTIONAL.  (Section 2.3.2)

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   "client": {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-1",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8..."
           }
       },
       "class_id": "web-server-1234",
       "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
       }
   }

   Additional fields are defined in the Client Instance Fields Registry
   (Section 11.5).

   Both the display and class_id are self-declarative and thus the AS
   SHOULD exercise caution in their interpretation, taking them as a
   hint but not as absolute truth.  The class_id field can be used in a
   variety of ways to help the a variety of ways to help the AS make
   sense of the particular context in which the client instance is
   operating.  In corporate environments, for example, different levels
   of trust might apply depending on security policies.  This field aims
   to help the AS adjust its own access decisions for different classes
   of client software.  It is possible to configure a set of values and
   rules during a pre-registration, and then have the client instances
   provide them later in runtime as a hint to the AS.  In other cases,
   the client runs with a specific AS in mind, so a single hardcoded
   value would acceptable (for instance, a set top box with a class_id
   claiming to be "FooBarTV version 4").  While the client instance may
   not have contacted the AS yet, the value of this class_id field can
   be evaluated by the AS according to a broader context of dynamic use,
   alongside other related information available elsewhere (for
   instance, corresponding fields in a certificate).  If the AS is not
   able to interpret the class_id field, it SHOULD return an
   invalid_client error (Section 3.6) or choose to return lesser levels
   of privileges.  See additional discussion of client instance
   impersonation in Section 13.13.

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   The client instance MUST prove possession of any presented key by the
   proof mechanism associated with the key in the request.  Key proofing
   methods are defined in the Key Proofing Methods Registry
   (Section 11.14) and an initial set of methods is described in
   Section 7.3.

   If the same public key is sent by value on different access requests,
   the AS MUST treat these requests as coming from the same client
   instance for purposes of identification, authentication, and policy
   application.  If the AS does not know the client instance's public
   key ahead of time, the AS MAY accept or reject the request based on
   AS policy, attestations within the client request, and other
   mechanisms.

   The client instance MUST NOT send a symmetric key by value in the
   request, as doing so would expose the key directly instead of simply
   proving possession of it.  See considerations on symmetric keys in
   Section 13.5.

   The client instance's key MAY be pre-registered with the AS ahead of
   time and associated with a set of policies and allowable actions
   pertaining to that client.  If this pre-registration includes other
   fields that can occur in the client request object described in this
   section, such as class_id or display, the pre-registered values MUST
   take precedence over any values given at runtime.  Additional fields
   sent during a request but not present in a pre-registered client
   instance record at the AS SHOULD NOT be added to the client's pre-
   registered record.  See additional considerations regarding client
   instance impersonation in Section 13.13.

   A client instance that is capable of talking to multiple AS's SHOULD
   use a different key for each AS to prevent a class of mix-up attacks
   as described in Section 13.30.

2.3.1.  Identifying the Client Instance by Reference

   If the client instance has an instance identifier that the AS can use
   to determine appropriate key information, the client instance can
   send this instance identifier as a direct reference value in lieu of
   the client object.  The instance identifier MAY be assigned to a
   client instance at runtime through a grant response (Section 3.5) or
   MAY be obtained in another fashion, such as a static registration
   process at the AS.

   "client": "client-541-ab"

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   When the AS receives a request with an instance identifier, the AS
   MUST ensure that the key used to sign the request (Section 7.3) is
   associated with the instance identifier.

   If the AS does not recognize the instance identifier, the request
   MUST be rejected with an invalid_client error (Section 3.6).

   If the client instance is identified in this manner, the registered
   key for the client instance MAY be a symmetric key known to the AS.
   See considerations on symmetric keys in Section 13.5.

2.3.2.  Providing Displayable Client Instance Information

   If the client instance has additional information to display to the
   RO during any interactions at the AS, it MAY send that information in
   the "display" field.  This field is a JSON object that declares
   information to present to the RO during any interactive sequences.

   name (string):  Display name of the client software.  RECOMMENDED.

   uri (string):  User-facing information about the client software,
      such as a web page.  This URI MUST be an absolute URI.  OPTIONAL.

   logo_uri (string)  Display image to represent the client software.
      This URI MUST be an absolute URI.  The logo MAY be passed by value
      by using a data: URI [RFC2397] referencing an image mediatype.
      OPTIONAL.

   "display": {
       "name": "My Client Display Name",
       "uri": "https://example.net/client",
       "logo_uri": "data:image/png;base64,Eeww...="
   }

   Additional display fields are defined by the Client Instance Display
   Fields Registry (Section 11.6).

   The AS SHOULD use these values during interaction with the RO.  The
   values are for informational purposes only and MUST NOT be taken as
   authentic proof of the client instance's identity or source.  The AS
   MAY restrict display values to specific client instances, as
   identified by their keys in Section 2.3.  See additional
   considerations for displayed client information in Section 13.13.

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2.3.3.  Authenticating the Client Instance

   If the presented key is known to the AS and is associated with a
   single instance of the client software, the process of presenting a
   key and proving possession of that key is sufficient to authenticate
   the client instance to the AS.  The AS MAY associate policies with
   the client instance identified by this key, such as limiting which
   resources can be requested and which interaction methods can be used.
   For example, only specific client instances with certain known keys
   might be trusted with access tokens without the AS interacting
   directly with the RO as in Appendix C.3.

   The presentation of a key allows the AS to strongly associate
   multiple successive requests from the same client instance with each
   other.  This is true when the AS knows the key ahead of time and can
   use the key to authenticate the client instance, but also if the key
   is ephemeral and created just for this series of requests.  As such
   the AS MAY allow for client instances to make requests with unknown
   keys.  This pattern allows for ephemeral client instances, such as
   single-page applications, and client software with many individual
   long-lived instances, such as mobile applications, to generate key
   pairs per instance and use the keys within the protocol without
   having to go through a separate registration step.  The AS MAY limit
   which capabilities are made available to client instances with
   unknown keys.  For example, the AS could have a policy saying that
   only previously-registered client instances can request particular
   resources, or that all client instances with unknown keys have to be
   interactively approved by an RO.

2.4.  Identifying the User

   If the client instance knows the identity of the end user through one
   or more identifiers or assertions, the client instance MAY send that
   information to the AS in the "user" field.  The client instance MAY
   pass this information by value or by reference (See Section 2.4.1).

   sub_ids (array of objects):  An array of subject identifiers for the
      end user, as defined by [I-D.ietf-secevent-subject-identifiers].
      OPTIONAL.

   assertions (array of objects)  An array containing assertions as
      objects each containing the assertion format and the assertion
      value as the JSON string serialization of the assertion.
      OPTIONAL.

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   "user": {
     "sub_ids": [ {
       "format": "opaque",
       "id": "J2G8G8O4AZ"
     } ],
     "assertions": [ {
       "format": "id_token",
       "value": "eyj..."
     } ]
   }

   Subject identifiers are hints to the AS in determining the RO and
   MUST NOT be taken as authoritative statements that a particular RO is
   present at the client instance and acting as the end user.

   Assertions presented by the client instance SHOULD be validated by
   the AS.  While the details of such validation are outside the scope
   of this specification, common validation steps include verifying the
   signature of the assertion against a trusted signing key, verifying
   the audience and issuer of the assertion map to expected values, and
   verifying the time window for the assertion itself.  However, note
   that in many use cases, some of these common steps are relaxed.  For
   example, an AS acting as an IdP could expect that assertions being
   presented using this mechanism were issued by the AS to the client
   software.  The AS would verify that the AS is the issuer of the
   assertion, not the audience, and that the client instance is instead
   the audience of the assertion.  Similarly, an AS might accept a
   recently-expired assertion in order to help bootstrap a new session
   with a specific end user.

   If the identified end user does not match the RO present at the AS
   during an interaction step, and the AS is not explicitly allowing a
   cross-user authorization, the AS SHOULD reject the request with an
   unknown_user error (Section 3.6).

   If the AS trusts the client instance to present verifiable assertions
   or known subject identifiers, such as an opaque identifier issued by
   the AS for this specific client instance, the AS MAY decide, based on
   its policy, to skip interaction with the RO, even if the client
   instance provides one or more interaction modes in its request.

   See Section 13.29 for considerations that the AS has to make when
   accepting and processing assertions from the client instance.

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2.4.1.  Identifying the User by Reference

   The AS can identify the current end user to the client instance with
   a reference which can be used by the client instance to refer to the
   end user across multiple requests.  If the client instance has a
   reference for the end user at this AS, the client instance MAY pass
   that reference as a string.  The format of this string is opaque to
   the client instance.

   "user": "XUT2MFM1XBIKJKSDU8QM"

   One means of dynamically obtaining such a user reference is from the
   AS returning an opaque subject identifier as described in
   Section 3.4.  Other means of configuring a client instance with a
   user identifier are out of scope of this specification.  The lifetime
   and validity of these user references is determined by the AS and
   this lifetime is not exposed to the client instance in GNAP.  As
   such, a client instance using such a user reference is likely to keep
   using that reference until such a time as it stops working.

   User reference identifiers are not intended to be human-readable user
   identifiers or structured assertions.  For the client instance to
   send either of these, the client can use the full user request object
   (Section 2.4) instead.

   If the AS does not recognize the user reference, it MUST return an
   unknown_user error (Section 3.6).

2.5.  Interacting with the User

   Often, the AS will require interaction with the RO (Section 4) in
   order to approve a requested delegation to the client instance for
   both access to resources and direct subject information.  Many times
   the end user using the client instance is the same person as the RO,
   and the client instance can directly drive interaction with the end
   user by facilitating the process through means such as redirection to
   a URI or launching an application.  Other times, the client instance
   can provide information to start the RO's interaction on a secondary
   device, or the client instance will wait for the RO to approve the
   request asynchronously.  The client instance could also be signaled
   that interaction has concluded through a callback mechanism.

   The client instance declares the parameters for interaction methods
   that it can support using the interact field.

   The interact field is a JSON object with three keys whose values
   declare how the client can initiate and complete the request, as well
   as provide hints to the AS about user preferences such as locale.  A

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   client instance MUST NOT declare an interaction mode it does not
   support.  The client instance MAY send multiple modes in the same
   request.  There is no preference order specified in this request.  An
   AS MAY respond to any, all, or none of the presented interaction
   modes (Section 3.3) in a request, depending on its capabilities and
   what is allowed to fulfill the request.

   start (array of objects/strings):  Indicates how the client instance
      can start an interaction.  REQUIRED.  (Section 2.5.1)

   finish (object):  Indicates how the client instance can receive an
      indication that interaction has finished at the AS.  OPTIONAL.
      (Section 2.5.2)

   hints (object):  Provides additional information to inform the
      interaction process at the AS.  OPTIONAL.  (Section 2.5.3)

   In this non-normative example, the client instance is indicating that
   it can redirect (Section 2.5.1.1) the end user to an arbitrary URI
   and can receive a redirect (Section 2.5.2.1) through a browser
   request.

   "interact": {
       "start": ["redirect"],
       "finish": {
           "method": "redirect",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   In this non-normative example, the client instance is indicating that
   it can display a user code (Section 2.5.1.3) and direct the end user
   to an arbitrary URI (Section 2.5.1.1) on a secondary device, but it
   cannot accept a redirect or push callback.

   "interact": {
       "start": ["redirect", "user_code"]
   }

   In this non-normative example, the client instance is indicating that
   it can not start any interaction with the end-user, but that the AS
   can push an interaction finish message (Section 2.5.2.2) when
   authorization from the RO is received asynchronously.

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   "interact": {
       "start": [],
       "finish": {
           "method": "push",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   If the client instance does not provide a suitable interaction
   mechanism, the AS cannot contact the RO asynchronously, and the AS
   determines that interaction is required, then the AS SHOULD return an
   invalid_interaction error (Section 3.6) since the client instance
   will be unable to complete the request without authorization.

2.5.1.  Start Mode Definitions

   If the client instance is capable of starting interaction with the
   end user, the client instance indicates this by sending an array of
   start modes under the start key.  Each interaction start modes has a
   unique identifying name.  Interaction start modes are specified in
   the array either by a string, which consists of the start mode name
   on its own, or by a JSON object with the required field mode:

   mode:  The interaction start mode.  REQUIRED.

   Interaction start modes defined as objects MAY define additional
   parameters to be required in the object.

   The start array can contain both string-type and object-type modes.

   This specification defines the following interaction start modes:

   "redirect" (string):  Indicates that the client instance can direct
      the end user to an arbitrary URI for interaction.  Section 2.5.1.1

   "app" (string):  Indicates that the client instance can launch an
      application on the end user's device for interaction.
      Section 2.5.1.2

   "user_code" (string):  Indicates that the client instance can
      communicate a human-readable short code to the end user for use
      with a stable URI.  Section 2.5.1.3

   "user_code_uri" (string):  Indicates that the client instance can
      communicate a human-readable short code to the end user for use
      with a short, dynamic URI.  Section 2.5.1.4

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   All interaction start method definitions MUST provide enough
   information to uniquely identify the grant request during the
   interaction.  In the redirect and app modes, this is done using a
   unique URI (including its parameters).  In the user_code and
   user_code_uri mode, this is done using the value of the user code.

   Additional start modes are defined in the Interaction Start Modes
   Registry (Section 11.7).

2.5.1.1.  Redirect to an Arbitrary URI

   If the client instance is capable of directing the end user to a URI
   defined by the AS at runtime, the client instance indicates this by
   including redirect in the array under the start key.  The means by
   which the client instance will activate this URI is out of scope of
   this specification, but common methods include an HTTP redirect,
   launching a browser on the end user's device, providing a scannable
   image encoding, and printing out a URI to an interactive console.
   While this URI is generally hosted at the AS, the client instance can
   make no assumptions about its contents, composition, or relationship
   to the grant endpoint URI.

   "interact": {
     "start": ["redirect"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a redirect interaction response
   Section 3.3.1.  The client instance manages this interaction method
   as described in Section 4.1.1.

   See Section 13.28 for more considerations regarding the use of front-
   channel communication techniques such as this.

2.5.1.2.  Open an Application-specific URI

   If the client instance can open a URI associated with an application
   on the end user's device, the client instance indicates this by
   including app in the array under the start key.  The means by which
   the client instance determines the application to open with this URI
   are out of scope of this specification.

   "interact": {
     "start": ["app"]
   }

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   If this interaction mode is supported for this client instance and
   request, the AS returns an app interaction response with an app URI
   payload (Section 3.3.2).  The client instance manages this
   interaction method as described in Section 4.1.4.

2.5.1.3.  Display a Short User Code

   If the client instance is capable of displaying or otherwise
   communicating a short, human-entered code to the RO, the client
   instance indicates this by including user_code in the array under the
   start key.  This code is to be entered at a static URI that does not
   change at runtime.  The client instance has no reasonable means to
   communicate a dynamic URI to the RO, and so this URI is usually
   communicated out of band to the RO through documentation or other
   messaging outside of GNAP.  While this URI is generally hosted at the
   AS, the client instance can make no assumptions about its contents,
   composition, or relationship to the grant endpoint URI.

   "interact": {
       "start": ["user_code"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a user code as specified in Section 3.3.3.
   The client instance manages this interaction method as described in
   Section 4.1.2.

2.5.1.4.  Display a Short User Code and URI

   If the client instance is capable of displaying or otherwise
   communicating a short, human-entered code along with a short, human-
   entered URI to the RO, the client instance indicates this by
   including user_code_uri in the array under the start key.  This code
   is to be entered at the dynamic URL given in the response.  While
   this URL is generally hosted at the AS, the client instance can make
   no assumptions about its contents, composition, or relationship to
   the grant endpoint URI.

   "interact": {
       "start": ["user_code_uri"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a user code and interaction URL as specified
   in Section 3.3.4.  The client instance manages this interaction
   method as described in Section 4.1.3.

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2.5.2.  Interaction Finish Methods

   If the client instance is capable of receiving a message from the AS
   indicating that the RO has completed their interaction, the client
   instance indicates this by sending the following members of an object
   under the finish key.

   method (string):  The callback method that the AS will use to contact
      the client instance.  REQUIRED.

   uri (string):  Indicates the URI that the AS will either send the RO
      to after interaction or send an HTTP POST request.  This URI MAY
      be unique per request and MUST be hosted by or accessible by the
      client instance.  This URI MUST be an absolute URI, and MUST NOT
      contain any fragment component.  If the client instance needs any
      state information to tie to the front channel interaction
      response, it MUST use a unique callback URI to link to that
      ongoing state.  The allowable URIs and URI patterns MAY be
      restricted by the AS based on the client instance's presented key
      information.  The callback URI SHOULD be presented to the RO
      during the interaction phase before redirect.  REQUIRED for
      redirect and push methods.

   nonce (string):  Unique ASCII string value to be used in the
      calculation of the "hash" query parameter sent to the callback
      URI, must be sufficiently random to be unguessable by an attacker.
      MUST be generated by the client instance as a unique value for
      this request.  REQUIRED.

   hash_method (string):  An identifier of a hash calculation mechanism
      to be used for the callback hash in Section 4.2.3, as defined in
      the IANA Named Information Hash Algorithm Registry
      (https://www.iana.org/assignments/named-information/named-
      information.xhtml#hash-alg).  If absent, the default value is sha-
      256.  OPTIONAL.

   This specification defines the following values for the method
   parameter, with other values defined by the Interaction Finish
   Methods Registry (Section 11.8):

   "redirect":  Indicates that the client instance can receive a
      redirect from the end user's device after interaction with the RO
      has concluded.  Section 2.5.2.1

   "push":  Indicates that the client instance can receive an HTTP POST
      request from the AS after interaction with the RO has concluded.
      Section 2.5.2.2

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   If interaction finishing is supported for this client instance and
   request, the AS will return a nonce (Section 3.3.5) used by the
   client instance to validate the callback.  All interaction finish
   methods MUST use this nonce to allow the client to verify the
   connection between the pending interaction request and the callback.
   GNAP does this through the use of the interaction hash, defined in
   Section 4.2.3.  All requests to the callback URI MUST be processed as
   described in Section 4.2.

   All interaction finish methods MUST require presentation of an
   interaction reference for continuing this grant request.  This means
   that the the interaction reference MUST be returned by the AS and
   MUST be presented by the client as described in Section 5.1.  The
   means by which the interaction reference is returned to the client
   instance is specific to the interaction finish method.

2.5.2.1.  Receive an HTTP Callback Through the Browser

   A finish method value of redirect indicates that the client instance
   will expect a request from the RO's browser using the HTTP method GET
   as described in Section 4.2.1.

   The client instance's URI MUST be protected by HTTPS, be hosted on a
   server local to the RO's browser ("localhost"), or use an
   application-specific URI scheme.

   "interact": {
       "finish": {
           "method": "redirect",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   Requests to the callback URI MUST be processed by the client instance
   as described in Section 4.2.1.

   Since the incoming request to the callback URI is from the RO's
   browser, this method is usually used when the RO and end user are the
   same entity.  See Section 13.23 for considerations on ensuring the
   incoming HTTP message matches the expected context of the request.
   See Section 13.28 for more considerations regarding the use of front-
   channel communication techniques such as this.

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2.5.2.2.  Receive an HTTP Direct Callback

   A finish method value of push indicates that the client instance will
   expect a request from the AS directly using the HTTP method POST as
   described in Section 4.2.2.

   The client instance's URI MUST be an HTTP URI and MUST be protected
   by HTTPS or equivalent.

   "interact": {
       "finish": {
           "method": "push",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   Requests to the callback URI MUST be processed by the client instance
   as described in Section 4.2.2.

   Since the incoming request to the callback URI is from the AS and not
   from the RO's browser, this request is not expected to have any
   shared session information from the start method.  See Section 13.23
   and Section 13.22 for more considerations regarding the use of back-
   channel and polling mechanisms like this.

2.5.3.  Hints

   The hints key is an object describing one or more suggestions from
   the client instance that the AS can use to help drive user
   interaction.

   This specification defines the following properties under the hints
   key:

   ui_locales (array of strings):  Indicates the end user's preferred
      locales that the AS can use during interaction, particularly
      before the RO has authenticated.  OPTIONAL.  Section 2.5.3.1

   The following sections detail requests for interaction hints.
   Additional interaction hints are defined in the Interaction Hints
   Registry (Section 11.9).

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2.5.3.1.  Indicate Desired Interaction Locales

   If the client instance knows the end user's locale and language
   preferences, the client instance can send this information to the AS
   using the ui_locales field with an array of locale strings as defined
   by [RFC5646].

   "interact": {
       "hints": {
           "ui_locales": ["en-US", "fr-CA"]
       }
   }

   If possible, the AS SHOULD use one of the locales in the array, with
   preference to the first item in the array supported by the AS.  If
   none of the given locales are supported, the AS MAY use a default
   locale.

3.  Grant Response

   In response to a client instance's request, the AS responds with a
   JSON object as the HTTP entity body.  Each possible field is detailed
   in the sections below.

   continue (object):  Indicates that the client instance can continue
      the request by making one or more continuation requests.  REQUIRED
      if continuation calls are allowed for this client instance on this
      grant request.  See Section 3.1.

   access_token (object / array of objects):  A single access token or
      set of access tokens that the client instance can use to call the
      RS on behalf of the RO.  REQUIRED if an access token is included.
      See Section 3.2.

   interact (object):  Indicates that interaction through some set of
      defined mechanisms needs to take place.  REQUIRED if interaction
      is expected.  See Section 3.3.

   subject (object):  Claims about the RO as known and declared by the
      AS.  REQUIRED if subject information is included.  See
      Section 3.4.

   instance_id (string):  An identifier this client instance can use to
      identify itself when making future requests.  OPTIONAL.  See
      Section 3.5.

   error (object or string):  An error code indicating that something

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      has gone wrong.  REQUIRED for an error condition.  See
      Section 3.6.

   Additional fields can be defined by extensions to GNAP in the Grant
   Response Parameters Registry (Section 11.10).

   In this example, the AS is returning an interaction URI
   (Section 3.3.1), a callback nonce (Section 3.3.5), and a continuation
   response (Section 3.1).

   NOTE: '\' line wrapping per RFC 8792

   {
       "interact": {
           "redirect": "https://server.example.com/interact/4CF492ML\
             VMSW9MKMXKHQ",
           "finish": "MBDOFXG4Y5CVJCX821LH"
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
           },
           "uri": "https://server.example.com/tx"
       }
   }

   In this example, the AS is returning a bearer access token
   (Section 3.2.1) with a management URI and a subject identifier
   (Section 3.4) in the form of an opaque identifier.

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "flags": ["bearer"],
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

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   In this example, the AS is returning set of subject identifiers
   (Section 3.4), simultaneously as an opaque identifier, an email
   address, and a decentralized identifier (DID).

   {
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           }, {
             "format": "email",
             "email": "user@example.com"
           }, {
             "format": "did",
             "url": "did:example:123456"
           } ]
       }
   }

   The response MUST be sent as a JSON object in the body of the HTTP
   response with Content-Type application/json, unless otherwise
   specified by the specific response (eg, an empty response with no
   Content-Type).

   The authorization server MUST include the HTTP Cache-Control response
   header field [RFC7234] with a value set to "no-store".

3.1.  Request Continuation

   If the AS determines that the grant request can be continued by the
   client instance, the AS responds with the continue field.  This field
   contains a JSON object with the following properties.

   uri (string):  The URI at which the client instance can make
      continuation requests.  This URI MAY vary per request, or MAY be
      stable at the AS.  This URI MUST be an an absolute URI.  The
      client instance MUST use this value exactly as given when making a
      continuation request (Section 5).  REQUIRED.

   wait (integer):  The amount of time in integer seconds the client
      instance MUST wait after receiving this request continuation
      response and calling the continuation URI.  The value SHOULD NOT
      be less than five seconds, and omission of the value MUST NOT be
      interpreted as zero (i.e., no delay between requests).
      RECOMMENDED.

   access_token (object):  A unique access token for continuing the

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      request, called the "continuation access token".  The value of
      this property MUST be an object in the format specified in
      Section 3.2.1.  This access token MUST be bound to the client
      instance's key used in the request and MUST NOT be a bearer token.
      As a consequence, the flags array of this access token MUST NOT
      contain the string bearer and the key field MUST be omitted.  The
      client instance MUST present the continuation access token in all
      requests to the continuation URI as described in Section 7.2.
      REQUIRED.

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   This field is REQUIRED if the grant request is in the _pending_
   state, as the field contains the information needed by the client
   request to continue the request as described in Section 5.  Note that
   the continuation access token is bound to the client instance's key,
   and therefore the client instance MUST sign all continuation requests
   with its key as described in Section 7.3 and MUST present the
   continuation access token in its continuation request.

3.2.  Access Tokens

   If the AS has successfully granted one or more access tokens to the
   client instance, the AS responds with the access_token field.  This
   field contains either a single access token as described in
   Section 3.2.1 or an array of access tokens as described in
   Section 3.2.2.

   The client instance uses any access tokens in this response to call
   the RS as described in Section 7.2.

   The grant request MUST be in the _approved_ state to include this
   field in the response.

3.2.1.  Single Access Token

   If the client instance has requested a single access token and the AS
   has granted that access token, the AS responds with the
   "access_token" field.  The value of this field is an object with the
   following properties.

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   value (string):  The value of the access token as a string.  The
      value is opaque to the client instance.  The value MUST be limited
      to the token68 character set defined in Section 11.2 of [RFC9110]
      to facilitate transmission over HTTP headers and within other
      protocols without requiring additional encoding.  REQUIRED.

   label (string):  The value of the label the client instance provided
      in the associated token request (Section 2.1), if present.
      REQUIRED for multiple access tokens or if a label was included in
      the single access token request, OPTIONAL for a single access
      token where no label was included in the request.

   manage (string):  The management URI for this access token.  This URI
      MUST be an absolute URI.  If provided, the client instance MAY
      manage its access token as described in Section 6.  This
      management URI is a function of the AS and is separate from the RS
      the client instance is requesting access to.  This URI MUST NOT
      include the access token value and SHOULD be different for each
      access token issued in a request.  OPTIONAL.

   access (array of objects/strings):  A description of the rights
      associated with this access token, as defined in Section 8.  If
      included, this MUST reflect the rights associated with the issued
      access token.  These rights MAY vary from what was requested by
      the client instance.  REQUIRED.

   expires_in (integer):  The number of seconds in which the access will
      expire.  The client instance MUST NOT use the access token past
      this time.  Note that the access token MAY be revoked by the AS or
      RS at any point prior to its expiration.  OPTIONAL.

   key (object / string):  The key that the token is bound to, if
      different from the client instance's presented key.  The key MUST
      be an object or string in a format described in Section 7.1.  The
      client instance MUST be able to dereference or process the key
      information in order to be able to sign subsequent requests using
      the access token (Section 7.2).  It is RECOMMENDED that keys
      returned for use with access tokens be key references as described
      in Section 7.1.1 that the client instance can correlate to its
      known keys.  OPTIONAL.

   flags (array of strings):  A set of flags that represent attributes
      or behaviors of the access token issued by the AS.  OPTIONAL.

   The values of the flags field defined by this specification are as
   follows:

   "bearer":  This flag indicates whether the token is a bearer token,

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      not bound to a key and proofing mechanism.  If the bearer flag is
      present, the access token is a bearer token, and the key field in
      this response MUST be omitted.  See Section 13.7 for additional
      considerations on the use of bearer tokens.

   "durable":  Flag indicating a hint of AS behavior on token rotation.
      If this flag is present, then the client instance can expect a
      previously-issued access token to continue to work after it has
      been rotated (Section 6.1) or the underlying grant request has
      been modified (Section 5.3), resulting in the issuance of new
      access tokens.  If this flag is omitted, the client instance can
      anticipate a given access token could stop working after token
      rotation or grant request modification.  Note that a token flagged
      as durable can still expire or be revoked through any normal
      means.

   Flag values MUST NOT be included more than once.

   Additional flags can be defined by extensions using the Access Token
   Fields Registry (Section 11.2).

   If the bearer flag and the key field in this response are omitted,
   the token is bound the key used by the client instance (Section 2.3)
   in its request for access.  If the bearer flag is omitted, and the
   key field is present, the token is bound to the key and proofing
   mechanism indicated in the key field.  The means by which the AS
   determines how to bind an access token to a key other than that
   presented by the client instance is out of scope for this
   specification, but common practices include pre-registering specific
   keys in a static fashion.

   The client software MUST reject any access token where the flags
   field contains the bearer flag and the key field is present with any
   value.

   The following non-normative example shows a single access token bound
   to the client instance's key used in the initial request, with a
   management URI, and that has access to three described resources (one
   using an object and two described by reference strings).

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   NOTE: '\' line wrapping per RFC 8792

   "access_token": {
       "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "manage": "https://server.example.com/token/PRY5NM33O\
           M4TB8N6BW7OZB8CDFONP219RP1L",
       "access": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "read", "dolphin-metadata"
       ]
   }

   The following non-normative example shows a single bearer access
   token with access to two described resources.

   "access_token": {
       "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "flags": ["bearer"],
       "access": [
           "finance", "medical"
       ]
   }

   If the client instance requested a single access token
   (Section 2.1.1), the AS MUST NOT respond with the multiple access
   token structure.

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3.2.2.  Multiple Access Tokens

   If the client instance has requested multiple access tokens and the
   AS has granted at least one of them, the AS responds with the
   "access_token" field.  The value of this field is a JSON array, the
   members of which are distinct access tokens as described in
   Section 3.2.1.  Each object MUST have a unique label field,
   corresponding to the token labels chosen by the client instance in
   the multiple access token request (Section 2.1.2).

   In this non-normative example, two tokens are issued under the names
   token1 and token2, and only the first token has a management URI
   associated with it.

   NOTE: '\' line wrapping per RFC 8792

   "access_token": [
       {
           "label": "token1",
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [ "finance" ]
       },
       {
           "label": "token2",
           "value": "UFGLO2FDAFG7VGZZPJ3IZEMN21EVU71FHCARP4J1",
           "access": [ "medical" ]
       }
   }

   Each access token corresponds to one of the objects in the
   access_token array of the client instance's request (Section 2.1.2).

   The AS MAY refuse to issue one or more of the requested access
   tokens, for any reason.  In such cases the refused token is omitted
   from the response and all of the other issued access tokens are
   included in the response under their respective requested labels.  If
   the client instance requested multiple access tokens (Section 2.1.2),
   the AS MUST NOT respond with a single access token structure, even if
   only a single access token is granted.  In such cases, the AS MUST
   respond with a multiple access token structure containing one access
   token.

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   "access_token": [
       {
           "label": "token2",
           "value": "8N6BW7OZB8CDFONP219-OS9M2PMHKUR64TBRP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [ "fruits" ]
       }
   ]

   The parameters of each access token are separate.  For example, each
   access token is expected to have a unique value and (if present)
   label, and likely has different access rights associated with it.
   Each access token could also be bound to different keys with
   different proofing mechanisms.

3.3.  Interaction Modes

   If the client instance has indicated a capability to interact with
   the RO in its request (Section 2.5), and the AS has determined that
   interaction is both supported and necessary, the AS responds to the
   client instance with any of the following values in the interact
   field of the response.  There is no preference order for interaction
   modes in the response, and it is up to the client instance to
   determine which ones to use.  All supported interaction methods are
   included in the same interact object.

   redirect (string):  Redirect to an arbitrary URI.  REQUIRED if the
      redirect interaction start mode is possible for this request.  See
      Section 3.3.1.

   app (string):  Launch of an application URI.  REQUIRED if the app
      interaction start mode is possible for this request.  See
      Section 3.3.2.

   user_code (string):  Display a short user code.  REQUIRED if the
      user_code interaction start mode is possible for this request.
      See Section 3.3.3.

   user_code_uri (object):  Display a short user code and URI.  REQUIRED
      if the user_code_uri interaction start mode is possible for this
      request.  Section 3.3.4

   finish (string):  A unique ASCII string value provided by the AS as a
      nonce.  This is used by the client instance to verify the callback
      after interaction is completed.  REQUIRED if the interaction
      finish method requested by the client instance is possible for
      this request.  See Section 3.3.5.

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   expires_in (integer):  The number of integer seconds after which this
      set of interaction responses will expire and no longer be usable
      by the client instance.  If the interaction methods expire, the
      client MAY re-start the interaction process for this grant request
      by sending an update (Section 5.3) with a new interaction request
      (Section 2.5) section.  OPTIONAL.  If omitted, the interaction
      response modes returned do not expire but MAY be invalidated by
      the AS at any time.

   Additional interaction mode responses can be defined in the
   Interaction Mode Responses Registry (Section 11.11).

   The AS MUST NOT respond with any interaction mode that the client
   instance did not indicate in its request.  The AS MUST NOT respond
   with any interaction mode that the AS does not support.  Since
   interaction responses include secret or unique information, the AS
   SHOULD respond to each interaction mode only once in an ongoing
   request, particularly if the client instance modifies its request
   (Section 5.3).

   The grant request MUST be in the _pending_ state to include this
   field in the response.

3.3.1.  Redirection to an arbitrary URI

   If the client instance indicates that it can redirect to an arbitrary
   URI (Section 2.5.1.1) and the AS supports this mode for the client
   instance's request, the AS responds with the "redirect" field, which
   is a string containing the URI to direct the end user to.  This URI
   MUST be unique for the request and MUST NOT contain any security-
   sensitive information such as user identifiers or access tokens.

   "interact": {
       "redirect": "https://interact.example.com/4CF492MLVMSW9MKMXKHQ"
   }

   The URI returned is a function of the AS, but the URI itself MAY be
   completely distinct from the grant endpoint URI that the client
   instance uses to request access (Section 2), allowing an AS to
   separate its user-interactive functionality from its back-end
   security functionality.  The AS will need to dereference the specific
   grant request and its information from the URI alone.  If the AS does
   not directly host the functionality accessed through the redirect
   URI, then the means for the interaction functionality to communicate
   with the rest of the AS are out of scope for this specification.

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   The client instance sends the end user to the URI to interact with
   the AS.  The client instance MUST NOT alter the URI in any way.  The
   means for the client instance to send the end user to this URI is out
   of scope of this specification, but common methods include an HTTP
   redirect, launching the system browser, displaying a scannable code,
   or printing out the URI in an interactive console.  See details of
   the interaction in Section 4.1.1.

3.3.2.  Launch of an application URI

   If the client instance indicates that it can launch an application
   URI (Section 2.5.1.2) and the AS supports this mode for the client
   instance's request, the AS responds with the "app" field, which is a
   string containing the URI for the client instance to launch.  This
   URI MUST be unique for the request and MUST NOT contain any security-
   sensitive information such as user identifiers or access tokens.

   "interact": {
       "app": "https://app.example.com/launch?tx=4CF492MLV"
   }

   The means for the launched application to communicate with the AS are
   out of scope for this specification.

   The client instance launches the URI as appropriate on its platform,
   and the means for the client instance to launch this URI is out of
   scope of this specification.  The client instance MUST NOT alter the
   URI in any way.  The client instance MAY attempt to detect if an
   installed application will service the URI being sent before
   attempting to launch the application URI.  See details of the
   interaction in Section 4.1.4.

3.3.3.  Display of a Short User Code

   If the client instance indicates that it can display a short
   user-typeable code (Section 2.5.1.3) and the AS supports this mode
   for the client instance's request, the AS responds with a "user_code"
   field.  This field is string containing a unique short code that the
   user can type into a web page.  To facilitate usability, this string
   MUST be case-insensitive, MUST consist of only easily typeable
   characters (such as letters or numbers).  The string MUST be randomly
   generated so as to be unguessable by an attacker within the time it
   is accepted.  The time in which this code will be accepted SHOULD be
   short lived, such as several minutes.  It is RECOMMENDED that this
   code be no more than eight characters in length.

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   "interact": {
       "user_code": "A1BC3DFF"
   }

   The client instance MUST communicate the "user_code" value to the end
   user in some fashion, such as displaying it on a screen or reading it
   out audibly.  This code is used by the interaction component of the
   AS as a means of identifying the pending grant request and does not
   function as an authentication factor for the RO.

   The URI that the end user is intended to enter the code into MUST be
   stable, since the client instance is expected to have no means of
   communicating a dynamic URI to the end user at runtime.

   As this interaction mode is designed to facilitate interaction via a
   secondary device, it is not expected that the client instance
   redirect the end user to the URI where the code is entered.  If the
   client instance is capable of communicating an short arbitrary URI to
   the end user for use with the user code, the client instance SHOULD
   instead use the "user_code_uri" (Section 2.5.1.4) mode.  If the
   client instance is capable of communicating a long arbitrary URI to
   the end user, such as through a scannable code, the client instance
   SHOULD use the "redirect" (Section 2.5.1.1) mode for this purpose
   instead of or in addition to the user code mode.

   See details of the interaction in Section 4.1.2.

3.3.4.  Display of a Short User Code and URI

   If the client instance indicates that it can display a short
   user-typeable code (Section 2.5.1.3) and the AS supports this mode
   for the client instance's request, the AS responds with a
   "user_code_uri" object that contains the following members.

   code (string):  A unique short code that the end user can type into a
      provided URI.  To facilitate usability, this string MUST be case-
      insensitive, MUST consist of only easily typeable characters (such
      as letters or numbers).  The string MUST be randomly generated so
      as to be unguessable by an attacker within the time it is
      accepted.  The time in which this code will be accepted SHOULD be
      short lived, such as several minutes.  It is RECOMMENDED that this
      code be no more than eight characters in length.  REQUIRED.

   uri (string):  The interaction URI that the client instance will

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      direct the RO to.  This URI MUST be short enough to be
      communicated to the end user by the client instance.  It is
      RECOMMENDED that this URI be short enough for an end user to type
      in manually.  The URI MUST NOT contain the code value.  This URI
      MUST be an absolute URI.  REQUIRED.

   "interact": {
       "user_code_uri": {
           "code": "A1BC3DFF",
           "uri": "https://srv.ex/device"
       }
   }

   The client instance MUST communicate the "code" to the end user in
   some fashion, such as displaying it on a screen or reading it out
   audibly.  This code is used by the interaction component of the AS as
   a means of identifying the pending grant request and does not
   function as an authentication factor for the RO.

   The client instance MUST also communicate the URI to the end user.
   Since it is expected that the end user will continue interaction on a
   secondary device, the URI needs to be short enough to allow the end
   user to type or copy it to a secondary device without mistakes.

   The URI returned is a function of the AS, but the URI itself MAY be
   completely distinct from the grant endpoint URI that the client
   instance uses to request access (Section 2), allowing an AS to
   separate its user-interactive functionality from its back-end
   security functionality.  If the AS does not directly host the
   functionality accessed through the given URI, then the means for the
   interaction functionality to communicate with the rest of the AS are
   out of scope for this specification.

   See details of the interaction in Section 4.1.2.

3.3.5.  Interaction Finish

   If the client instance indicates that it can receive a
   post-interaction redirect or push at a URI (Section 2.5.2) and the AS
   supports this mode for the client instance's request, the AS responds
   with a finish field containing a nonce that the client instance will
   use in validating the callback as defined in Section 4.2.

   "interact": {
       "finish": "MBDOFXG4Y5CVJCX821LH"
   }

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   When the interaction is completed, the interaction component of the
   AS MUST contact the client instance using the means defined by the
   finish method as described in Section 4.2.

   If the AS returns the finish field, the client instance MUST NOT
   continue a grant request before it receives the associated
   interaction reference on the callback URI.  See details in
   Section 4.2.

3.4.  Returning Subject Information

   If information about the RO is requested and the AS grants the client
   instance access to that data, the AS returns the approved information
   in the "subject" response field.  The AS MUST return the subject
   field only in cases where the AS is sure that the RO and the end user
   are the same party.  This can be accomplished through some forms of
   interaction with the RO (Section 4).

   This field is an object with the following properties.

   sub_ids (array of objects):  An array of subject identifiers for the
      RO, as defined by [I-D.ietf-secevent-subject-identifiers].
      REQUIRED if returning subject identifiers.

   assertions (array of objects):  An array containing assertions as
      objects each containing the assertion object described below.
      REQUIRED if returning assertions.

   updated_at (string):  Timestamp as an [RFC3339] date string,
      indicating when the identified account was last updated.  The
      client instance MAY use this value to determine if it needs to
      request updated profile information through an identity API.  The
      definition of such an identity API is out of scope for this
      specification.  RECOMMENDED.

   Assertion objects contain the following fields:

   format (string):  The assertion format.  Possible formats include
      id_token for an OpenID Connect ID Token ([OIDC]) and saml2 for a
      SAML 2 assertion ([SAML2]).  Additional assertion formats are
      defined by the Assertion Formats Registry (Section 11.4).
      REQUIRED.

   value (string):  The assertion value as the JSON string serialization
      of the assertion.  REQUIRED.

   The following non-normative example contains an opaque identifier and
   an OpenID Connect ID Token:

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   "subject": {
     "sub_ids": [ {
       "format": "opaque",
       "id": "XUT2MFM1XBIKJKSDU8QM"
     } ],
     "assertions": [ {
       "format": "id_token",
       "value": "eyj..."
     } ]
   }

   Subject identifiers returned by the AS SHOULD uniquely identify the
   RO at the AS.  Some forms of subject identifier are opaque to the
   client instance (such as the subject of an issuer and subject pair),
   while others forms (such as email address and phone number) are
   intended to allow the client instance to correlate the identifier
   with other account information at the client instance.  The client
   instance MUST NOT request or use any returned subject identifiers for
   communication purposes (see Section 2.2).  That is, a subject
   identifier returned in the format of an email address or a phone
   number only identifies the RO to the AS and does not indicate that
   the AS has validated that the represented email address or phone
   number in the identifier is suitable for communication with the
   current user.  To get such information, the client instance MUST use
   an identity protocol to request and receive additional identity
   claims.  The details of an identity protocol and associated schema
   are outside the scope of this specification.

   The AS MUST ensure that the returned subject information represents
   the RO.  In most cases, the AS will also ensure that the returned
   subject information represents the end user authenticated
   interactively at the AS.  The AS SHOULD NOT re-use subject
   identifiers for multiple different ROs.

   The "sub_ids" and "assertions" response fields are independent of
   each other.  That is, a returned assertion MAY use a different
   subject identifier than other assertions and subject identifiers in
   the response.  However, all subject identifiers and assertions
   returned MUST refer to the same party.

   The client instance MUST interpret all subject information in the
   context of the AS that the subject information is received from, as
   is discussed in Section 6 of [SP80063C].  For example, one AS could
   return an email identifier of "user@example.com" for one RO, and a
   different AS could return that same email identifier of
   "user@example.com" for a completely different RO.  A client instance
   talking to both AS's needs to differentiate between these two
   accounts by accounting for the AS source of each identifier.

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   Extensions to this specification MAY define additional response
   properties in the Subject Information Response Fields Registry
   (Section 11.12).

   The grant request MUST be in the _approved_ state to return this
   field in the response.

   See Section 13.29 for considerations that the client instance has to
   make when accepting and processing assertions from the AS.

3.5.  Returning a Dynamically-bound Client Instance Identifier

   Many parts of the client instance's request can be passed as either a
   value or a reference.  The use of a reference in place of a value
   allows for a client instance to optimize requests to the AS.

   Some references, such as for the client instance's identity
   (Section 2.3.1) or the requested resources (Section 8.1), can be
   managed statically through an admin console or developer portal
   provided by the AS or RS.  The developer of the client software can
   include these values in their code for a more efficient and compact
   request.

   If desired, the AS MAY also generate and return an instance
   identifier dynamically to the client instance in the response to
   facilitate multiple interactions with the same client instance over
   time.  The client instance SHOULD use this instance identifier in
   future requests in lieu of sending the associated data values in the
   client field.

   Dynamically generated client instance identifiers are string values
   that MUST be protected by the client instance as secrets.  Instance
   identifier values MUST be unguessable and MUST NOT contain any
   information that would compromise any party if revealed.  Instance
   identifier values are opaque to the client instance.

   instance_id (string):  A string value used to represent the
      information in the client object that the client instance can use
      in a future request, as described in Section 2.3.1.  OPTIONAL.

   This non-normative example shows an instance identifier along side an
   issued access token.

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   {
       "instance_id": "7C7C4AZ9KHRS6X63AJAO",
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
       }
   }

3.6.  Error Response

   If the AS determines that the request cannot be completed for any
   reason, it responds to the client instance with an error field in the
   response message.  This field is either an object or a string.

   When returned as an object, the object contains the following fields:

   code (string):  A single ASCII error code defining the error.
      REQUIRED.

   description (string):  A human-readable string description of the
      error intended for the developer of the client.  OPTIONAL.

   This specification defines the following code values:

   "invalid_request":  The request is missing a required parameter,
      includes an invalid parameter value or is otherwise malformed.

   "invalid_client":  The request was made from a client that was not
      recognized or allowed by the AS, or the client's signature
      validation failed.

   "invalid_interaction"  The client instance has provided an
      interaction reference that is incorrect for this request or the
      interaction modes in use have expired.

   "invalid_flag"  The flag configuration is not valid.

   "invalid_rotation"  The token rotation request is not valid.

   "key_rotation_not_supported"  The AS does not allow rotation of this
      access token's key.

   "invalid_continuation":  The continuation of the referenced grant
      could not be processed.

   "user_denied":  The RO denied the request.

   "request_denied":  The request was denied for an unspecified reason.

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   "unknown_user":  The user presented in the request is not known to
      the AS or does not match the user present during interaction.

   "unknown_interaction":  The interaction integrity could not be
      established.

   "too_fast":  The client instance did not respect the timeout in the
      wait response before the next call.

   "too_many_attempts":  A limit has been reached in the total number of
      reasonable attempts.  This number is either defined statically or
      adjusted based on runtime conditions by the AS.

   Additional error codes can be defined in the Error Code Registry
   (Section 11.13).

   For example, if the RO denied the request while interacting with the
   AS, the AS would return the following error when the client instance
   tries to continue the grant request:

   {
       "error": {
           "code": "user_denied",
           "description": "The RO denied the request"
       }
   }

   Alternatively, the AS MAY choose to only return the error as codes
   and provide the error as a string.  Since the description field is
   not intended to be machine-readable, the following response is
   considered functionally equivalent to the previous example for the
   purposes of the client software's understanding:

   {
       "error": "user_denied"
   }

   If an error state is reached but the grant is in the _pending_ state
   (and therefore the client instance can continue), the AS MAY include
   the continue field in the response along with the error, as defined
   Section 3.1.  This allows the client instance to modify its request
   for access, potentially leading to prompting the RO again.  Other
   fields MUST NOT be included in the response.

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4.  Determining Authorization and Consent

   When the client instance makes its initial request (Section 2) to the
   AS for delegated access, it is capable of asking for several
   different kinds of information in response:

   *  the access being requested, in the access_token request parameter

   *  the subject information being requested, in the subject request
      parameter

   *  any additional requested information defined by extensions of this
      protocol

   When the grant request is in the _processing_ state, the AS
   determines what authorizations and consents are required to fulfill
   this requested delegation.  The details of how the AS makes this
   determination are out of scope for this document.  However, there are
   several common patterns defined and supported by GNAP for fulfilling
   these requirements, including information sent by the client
   instance, information gathered through the interaction process, and
   information supplied by external parties.  An individual AS can
   define its own policies and processes for deciding when and how to
   gather the necessary authorizations and consent, and how those are
   applied to the grant request.

   To facilitate the AS fulfilling this request, the client instance
   sends information about the actions the client software can take,
   including:

   *  starting interaction with the end user, in the interact request
      parameter

   *  receiving notification that interaction with the RO has concluded,
      in the interact request parameter

   *  any additional capabilities defined by extensions of this protocol

   The client instance can also supply information directly to the AS in
   its request.  The client instance can send several kinds of things,
   including:

   *  the identity of the client instance, known from the keys or
      identifiers in the client request parameter

   *  the identity of the end user, in the user request parameter

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   *  any additional information presented by the client instance in the
      request defined by extensions of this protocol

   The AS will process this presented information in the context of the
   client instance's request and can only trust the information as much
   as it trusts the presentation and context of that request.  If the AS
   determines that the information presented in the initial request is
   sufficient for granting the requested access, the AS MAY move the
   grant request to the _approved_ state and return results immediately
   in its response (Section 3) with access tokens and subject
   information.

   If the AS determines that additional runtime authorization is
   required, the AS can either deny the request outright (if there is no
   possible recovery) or move the grant request to the _pending_ state
   and use a number of means at its disposal to gather that
   authorization from the appropriate ROs, including for example:

   *  starting interaction with the end user facilitated by the client
      software, such as a redirection or user code

   *  challenging the client instance through a challenge-response
      mechanism

   *  requesting that the client instance present specific additional
      information, such as a user's credential or an assertion

   *  contacting an RO through an out-of-band mechanism, such as a push
      notification

   *  executing an auxiliary software process through an out-of-band
      mechanism, such as querying a digital wallet

   The authorization and consent gathering process in GNAP is left
   deliberately flexible to allow for a wide variety of different
   deployments, interactions, and methodologies.  In this process, the
   AS can gather consent from the RO or apply the RO's policy as
   necessitated by the access that has been requested.  The AS can
   sometimes determine which RO needs to prompt for consent based on
   what has been requested by the client instance, such as a specific RS
   record, an identified subject, or a request requiring specific access
   such as approval by an administrator.  In other cases, the request is
   applied to whichever RO is present at the time of consent gathering.
   This pattern is especially prevalent when the end user is sent to the
   AS for an interactive session, during which the end user takes on the
   role of the RO.  In these cases, the end user is delegating their own
   access as RO to the client instance.

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   The client instance can indicate that it is capable of facilitating
   interaction with the end user, another party, or another piece of
   software through its interaction start (Section 2.5.1) request.
   Here, the AS usually needs to interact directly with the end user to
   determine their identity, determine their status as an RO, and
   collect their consent.  If the AS has determined that authorization
   is required and the AS can support one or more of the requested
   interaction start methods, the AS returns the associated interaction
   start responses (Section 3.3).  The client instance SHOULD initiate
   one or more of these interaction methods (Section 4.1) in order to
   facilitate the granting of the request.  If more than one interaction
   start method is available, the means by which the client chooses
   which methods to follow is out of scope of this specification.

   After starting interaction, the client instance can then make a
   continuation request (Section 5) either in response to a signal
   indicating the finish of the interaction (Section 4.2), after a time-
   based polling, or through some other method defined by an extension
   of this specification through the Interaction Mode Responses registry
   (Section 11.11).

   If the grant request is not in the _approved_ state, the client
   instance can repeat the interaction process by sending a grant update
   request (Section 5.3) with new interaction (Section 2.5) methods.

   The client instance MUST use each interaction method at most once.
   The AS SHOULD handle any interact request as a one-time-use mechanism
   and SHOULD apply suitable timeouts to any interaction start methods
   provided, including user codes and redirection URIs.  The client
   instance SHOULD apply suitable timeouts to any interaction finish
   method.

   If the AS instead has a means of contacting the RO directly, it could
   do so without involving the client instance in its consent gathering
   process.  For example, the AS could push a notification to a known RO
   and have the RO approve the pending request asynchronously.  These
   interactions can be through an interface of the AS itself (such as a
   hosted web page), through another application (such as something
   installed on the RO's device), through a messaging fabric, or any
   other means.

   When interacting with an RO, the AS can do anything it needs to
   determine the authorization of the requested grant, including:

   *  authenticate the RO, through a local account or some other means
      such as federated login

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   *  validate the RO through presentation of claims, attributes, or
      other information

   *  prompt the RO for consent for the requested delegation

   *  describe to the RO what information is being released, to whom,
      and for what purpose

   *  provide warnings to the RO about potential attacks or negative
      effects of allowing the information

   *  allow the RO to modify the client instance's requested access,
      including limiting or expanding that access

   *  provide the RO with artifacts such as receipts to facilitate an
      audit trail of authorizations

   *  allow the RO to deny the requested delegation

   The AS is also allowed to request authorization from more than one
   RO, if the AS deems fit.  For example, a medical record might need to
   be released by both an attending nurse and a physician, or both
   owners of a bank account need to sign off on a transfer request.
   Alternatively, the AS could require N of M possible RO's to approve a
   given request.  In some circumstances, the AS could even determine
   that the end user present during the interaction is not the
   appropriate RO for a given request and reach out to the appropriate
   RO asynchronously.

   The RO is also allowed to define an automated policy at the AS to
   determine which kind of end user can get access to the resource, and
   under which condition.  For instance, such a condition might require
   the end user login and the acceptance of the RO's legal provisions.
   Alternatively, client software could be acting without an end user,
   and the RO's policy allows issuance of access tokens to specific
   instances of that client software without human interaction.

   While all of these cases are supported by GNAP, the details of their
   implementation, and for determining which RO's or related policies
   are required for a given request, are out of scope for this
   specification.

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4.1.  Starting Interaction With the End User

   When a grant request is in the _pending_ state, the interaction start
   methods sent by the client instance can be used to facilitate
   interaction with the end user.  To initiate an interaction start
   method indicated by the interaction start responses (Section 3.3)
   from the AS, the client instance follows the steps defined by that
   interaction start mode.  The actions of the client instance required
   for the interaction start modes defined in this specification are
   described in the following sections.  Interaction start modes defined
   in extensions to this specification MUST define the expected actions
   of the client software when that interaction start mode is used.

   If the client instance does not start an interaction start mode
   within an AS-determined amount of time, the AS SHOULD reject attempts
   to use the interaction start modes.  If the client instance has
   already begun one interaction start mode, the AS SHOULD reject
   attempts to use other interaction start modes.  For example, if a
   user code has been successfully entered for a grant request, the AS
   will probably want to reject requests to an arbitrary redirect URI on
   the same grant request.

4.1.1.  Interaction at a Redirected URI

   When the end user is directed to an arbitrary URI through the
   "redirect" (Section 3.3.1) mode, the client instance facilitates
   opening the URI through the end user's web browser.  The client
   instance could launch the URI through the system browser, provide a
   clickable link, redirect the user through HTTP response codes, or
   display the URI in a form the end user can use to launch such as a
   multidimensional barcode.  In all cases, the URI is accessed with an
   HTTP GET request, and the resulting page is assumed to allow direct
   interaction with the end user through an HTTP user agent.  With this
   method, it is common (though not required) for the RO to be the same
   party as the end user, since the client instance has to communicate
   the redirection URI to the end user.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The URI value is used to identify the
   grant request being authorized.  If the URI cannot be associated with
   a currently active request, the AS MUST display an error to the RO
   and MUST NOT attempt to redirect the RO back to any client instance
   even if a redirect finish method is supplied (Section 2.5.2.1).  If
   the URI is not hosted by the AS directly, the means of communication
   between the AS and the service provided by this URI are out of scope
   for this specification.

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   The client instance MUST NOT modify the URI when launching it, in
   particular the client instance MUST NOT add any parameters to the
   URI.  The URI MUST be reachable from the end user's browser, though
   the URI MAY be opened on a separate device from the client instance
   itself.  The URI MUST be accessible from an HTTP GET request and MUST
   be protected by HTTPS or equivalent means.

4.1.2.  Interaction at the Static User Code URI

   When the end user is directed to enter a short code through the
   "user_code" (Section 3.3.3) mode, the client instance communicates
   the user code to the end user and directs the end user to enter that
   code at an associated URI.  The client instance MAY format the user
   code in such a way as to facilitate memorability and transfer of the
   code, so long as this formatting does not alter the value as accepted
   at the user code URI.  For example, a client instance receiving the
   user code "A1BC3DFF" could choose to display this to the user as
   "A1BC 3DFF", breaking up the long string into two shorter strings.
   In this example, the space in between the two parts would be removed
   upon its entry into the user code URI.

   This mode is designed to be used when the client instance is not able
   to communicate or facilitate launching an arbitrary URI.  The
   associated URI could be statically configured with the client
   instance or in the client software's documentation.  As a
   consequence, these URIs SHOULD be short.  The user code URI MUST be
   reachable from the end user's browser, though the URI is usually
   opened on a separate device from the client instance itself.  The URI
   MUST be accessible from an HTTP GET request and MUST be protected by
   HTTPS or equivalent means.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The value of the user code is used to
   identify the grant request being authorized.  If the user code cannot
   be associated with a currently active request, the AS MUST display an
   error to the RO and MUST NOT attempt to redirect the RO back to any
   client instance even if a redirect finish method is supplied
   (Section 2.5.2.1).  If the interaction component at the user code URI
   is not hosted by the AS directly, the means of communication between
   the AS and this URI, including communication of the user code itself,
   are out of scope for this specification.

   When the RO enters this code at the user code URI, the AS MUST
   uniquely identify the pending request that the code was associated
   with.  If the AS does not recognize the entered code, the interaction
   component MUST display an error to the user.  If the AS detects too
   many unrecognized code enter attempts, the interaction component

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   SHOULD display an error to the user indicating too many attempts and
   MAY take additional actions such as slowing down the input
   interactions.  The user should be warned as such an error state is
   approached, if possible.

4.1.3.  Interaction at a Dynamic User Code URI

   When the end user is directed to enter a short code through the
   "user_code_uri" (Section 3.3.4) mode, the client instance
   communicates the user code and associated URI to the end user and
   directs the end user to enter that code at the URI.  The client
   instance MAY format the user code in such a way as to facilitate
   memorability and transfer of the code, so long as this formatting
   does not alter the value as accepted at the user code URI.  For
   example, a client instance receiving the user code "A1BC3DFF" could
   choose to display this to the user as "A1BC 3DFF", breaking up the
   long string into two shorter strings.  In this example, the space in
   between the two parts would be removed upon its entry into the user
   code URI.

   This mode is used when the client instance is not able to facilitate
   launching a complex arbitrary URI but can communicate arbitrary
   values like URIs.  As a consequence, these URIs SHOULD be short to
   allow the URI to be typed by the end user.  The client instance MUST
   NOT modify the URI when communicating it to the end user; in
   particular the client instance MUST NOT add any parameters to the
   URI.  The user code URI MUST be reachable from the end user's
   browser, though the URI is usually be opened on a separate device
   from the client instance itself.  The URI MUST be accessible from an
   HTTP GET request and MUST be protected by HTTPS or equivalent means.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The value of the user code is used to
   identify the grant request being authorized.  If the user code cannot
   be associated with a currently active request, the AS MUST display an
   error to the RO and MUST NOT attempt to redirect the RO back to any
   client instance even if a redirect finish method is supplied
   (Section 2.5.2.1).  If the interaction component at the user code URI
   is not hosted by the AS directly, the means of communication between
   the AS and this URI, including communication of the user code itself,
   are out of scope for this specification.

   When the RO enters this code at the given URI, the AS MUST uniquely
   identify the pending request that the code was associated with.  If
   the AS does not recognize the entered code, the interaction component
   MUST display an error to the user.  If the AS detects too many
   unrecognized code enter attempts, the interaction component SHOULD

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   display an error to the user indicating too many attempts and MAY
   take additional actions such as slowing down the input interactions.
   The user should be warned as such an error state is approached, if
   possible.

4.1.4.  Interaction through an Application URI

   When the client instance is directed to launch an application through
   the "app" (Section 3.3.2) mode, the client launches the URI as
   appropriate to the system, such as through a deep link or custom URI
   scheme registered to a mobile application.  The means by which the AS
   and the launched application communicate with each other and perform
   any of the required actions are out of scope for this specification.

4.2.  Post-Interaction Completion

   If an interaction "finish" (Section 3.3.5) method is associated with
   the current request, the AS MUST follow the appropriate method upon
   completion of interaction in order to signal the client instance to
   continue, except for some limited error cases discussed below.  If a
   finish method is not available, the AS SHOULD instruct the RO to
   return to the client instance upon completion.

   The AS MUST create an interaction reference and associate that
   reference with the current interaction and the underlying pending
   request.  This interaction reference value MUST be sufficiently
   random so as not to be guessable by an attacker.  The interaction
   reference MUST be one-time-use to prevent interception and replay
   attacks.

   The AS MUST calculate a hash value based on the client instance and
   AS nonces and the interaction reference, as described in
   Section 4.2.3.  The client instance will use this value to validate
   the "finish" call.

   All interaction finish methods MUST define a way to convey the hash
   and interaction reference back to the client instance.  When an
   interaction finish method is used, the client instance MUST present
   the interaction reference back to the AS as part of its continuation
   request (Section 5.1).

   Note that in many error cases, such as when the RO has denied access,
   the "finish" method is still enacted by the AS.  This pattern allows
   the client instance to potentially recover from the error state by
   modifying its request or providing additional information directly to
   the AS in a continuation request.  The AS MUST NOT follow the
   "finish" method in the following circumstances:

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   *  The AS has determined that any URIs involved with the finish
      method are dangerous or blocked.

   *  The AS cannot determine which ongoing grant request is being
      referenced.

   *  The ongoing grant request has been cancelled or otherwise blocked.

4.2.1.  Completing Interaction with a Browser Redirect to the Callback
        URI

   When using the redirect interaction finish method defined in
   Section 2.5.2.1 and Section 3.3.5, the AS signals to the client
   instance that interaction is complete and the request can be
   continued by directing the RO (in their browser) back to the client
   instance's redirect URI.

   The AS secures this redirect by adding the hash and interaction
   reference as query parameters to the client instance's redirect URI.

   hash:  The interaction hash value as described in Section 4.2.3.
      REQUIRED.

   interact_ref:  The interaction reference generated for this
      interaction.  REQUIRED.

   The means of directing the RO to this URI are outside the scope of
   this specification, but common options include redirecting the RO
   from a web page and launching the system browser with the target URI.
   See Section 13.16 for considerations on which HTTP status code to use
   when redirecting a request that potentially contains credentials.

   NOTE: '\' line wrapping per RFC 8792

   https://client.example.net/return/123455\
     ?hash=x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY\
     &interact_ref=4IFWWIKYBC2PQ6U56NL1

   The client instance MUST be able to process a request on the URI.  If
   the URI is HTTP, the request MUST be an HTTP GET.

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   When receiving the request, the client instance MUST parse the query
   parameters to extract the hash and interaction reference values.  The
   client instance MUST calculate and validate the hash value as
   described in Section 4.2.3.  If the hash validates, the client
   instance sends a continuation request to the AS as described in
   Section 5.1 using the interaction reference value received here.  If
   the hash does not validate, the client instance MUST NOT send the
   interaction reference to the AS.

4.2.2.  Completing Interaction with a Direct HTTP Request Callback

   When using the push interaction finish method defined in
   Section 2.5.2.1 and Section 3.3.5, the AS signals to the client
   instance that interaction is complete and the request can be
   continued by sending an HTTP POST request to the client instance's
   callback URI.

   The entity message body is a JSON object consisting of the following
   two fields:

   hash (string):  The interaction hash value as described in
      Section 4.2.3.  REQUIRED.

   interact_ref (string)  The interaction reference generated for this
      interaction.  REQUIRED.

   POST /push/554321 HTTP/1.1
   Host: client.example.net
   Content-Type: application/json

   {
     "hash": "pjdHcrti02HLCwGU3qhUZ3wZXt8IjrV_BtE3oUyOuKNk",
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   Since the AS is making an outbound connection to a URI supplied by an
   outside party (the client instance), the AS MUST protect itself
   against SSRF attacks when making this call as discussed in
   Section 13.33.

   When receiving the request, the client instance MUST parse the JSON
   object and validate the hash value as described in Section 4.2.3.  If
   either fails, the client instance MUST return an unknown_interaction
   error (Section 3.6).  If the hash validates, the client instance
   sends a continuation request to the AS as described in Section 5.1
   using the interaction reference value received here.

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4.2.3.  Calculating the interaction hash

   The "hash" parameter in the request to the client instance's callback
   URI ties the front channel response to an ongoing request by using
   values known only to the parties involved.  This security mechanism
   allows the client instance to protect itself against several kinds of
   session fixation and injection attacks as discussed in Section 13.24
   and related sections.  The AS MUST always provide this hash, and the
   client instance MUST validate the hash when received.

   To calculate the "hash" value, the party doing the calculation
   creates a hash base string by concatenating the following values in
   the following order using a single newline (\n) character to separate
   them:

   *  the "nonce" value sent by the client instance in the interaction
      "finish" section of the initial request (Section 2.5.2)

   *  the AS's nonce value from the interaction finish response
      (Section 3.3.5)

   *  the "interact_ref" returned from the AS as part of the interaction
      finish method (Section 4.2)

   *  the grant endpoint URI the client instance used to make its
      initial request (Section 2)

   There is no padding or whitespace before or after any of the lines,
   and no trailing newline character.  The following example shows a
   constructed hash base string consisting of these four elements.

   VJLO6A4CATR0KRO
   MBDOFXG4Y5CVJCX821LH
   4IFWWIKYB2PQ6U56NL1
   https://server.example.com/tx

   The party then hashes the bytes of the ASCII encoding of this string
   with the appropriate algorithm based on the "hash_method" parameter
   under the "finish" key of the interaction finish request
   (Section 2.5.2).  The resulting byte array from the hash function is
   then encoded using URL-Safe Base64 with no padding [RFC4648].  The
   resulting string is the hash value.

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   If provided, the "hash_method" value MUST be one of the hash name
   strings defined in the IANA Named Information Hash Algorithm Registry
   (https://www.iana.org/assignments/named-information/named-
   information.xhtml#hash-alg).  If the "hash_method" value is not
   present in the client instance's request, the algorithm defaults to
   "sha-256".

   For example, the "sha-256" hash method consists of hashing the input
   string with the 256-bit SHA2 algorithm.  The following is the encoded
   "sha-256" hash of the above example hash base string.

   x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY

   For another example, the "sha3-512" hash method consists of hashing
   the input string with the 512-bit SHA3 algorithm.  The following is
   the encoded "sha3-512" hash of the above example hash base string.

   NOTE: '\' line wrapping per RFC 8792

   pyUkVJSmpqSJMaDYsk5G8WCvgY91l-agUPe1wgn-cc5rUtN69gPI2-S_s-Eswed8iB4\
     PJ_a5Hg6DNi7qGgKwSQ

5.  Continuing a Grant Request

   While it is possible for the AS to return an approved grant response
   (Section 3) with all the client instance's requested information
   (including access tokens (Section 3.2) and subject information
   (Section 3.4)) immediately, it's more common that the AS will place
   the grant request into the _pending_ state and require communication
   with the client instance several times over the lifetime of a grant
   request.  This is often part of facilitating interaction (Section 4),
   but it could also be used to allow the AS and client instance to
   continue negotiating the parameters of the original grant request
   (Section 2) through modification of the request.

   The ability to continue an already-started request allows the client
   instance to perform several important functions, including presenting
   additional information from interaction, modifying the initial
   request, and revoking a grant request in progress.

   To enable this ongoing negotiation, the AS provides a continuation
   API to the client software.  The AS returns a continue field in the
   response (Section 3.1) that contains information the client instance
   needs to access this API, including a URI to access as well as a
   special access token to use during the requests, called the
   _continuation access token_.

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   All requests to the continuation API are protected by a bound
   continuation access token.  The continuation access token is bound to
   the same key and method the client instance used to make the initial
   request (or its most recent rotation).  As a consequence, when the
   client instance makes any calls to the continuation URI, the client
   instance MUST present the continuation access token as described in
   Section 7.2 and present proof of the client instance's key (or its
   most recent rotation) by signing the request as described in
   Section 7.3.  The AS MUST validate the signature and ensure that it
   is bound to the appropriate key for the contination access token.

   Access tokens other than the continuation access tokens MUST NOT be
   usable for continuation requests.  Conversely, continuation access
   tokens MUST NOT be usable to make authorized requests to RS's, even
   if co-located within the AS.

   For example, here the client instance makes a POST request to a
   unique URI and signs the request with HTTP Message Signatures:

   POST /continue/KSKUOMUKM HTTP/1.1
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Host: server.example.com
   Content-Length: 0
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS MUST be able to tell from the client instance's request which
   specific ongoing request is being accessed, using a combination of
   the continuation URI and the continuation access token.  If the AS
   cannot determine a single active grant request to map the
   continuation request to, the AS MUST return an invalid_continuation
   error (Section 3.6).

   For example, here the client instance makes a POST request to a
   stable continuation endpoint URI with the interaction reference
   (Section 5.1), includes the access token, and signs with HTTP Message
   Signatures:

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   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   In this alternative example, the client instance had been provided a
   continuation URI unique to this ongoing grant request:

   POST /tx/rxgIIEVMBV-BQUO7kxbsp HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP eyJhbGciOiJub25lIiwidHlwIjoiYmFkIn0
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   In both cases, the AS determines which grant is being asked for based
   on the URI and continuation access token provided.

   If a wait parameter was included in the continuation response
   (Section 3.1), the client instance MUST NOT call the continuation URI
   prior to waiting the number of seconds indicated.  If no wait period
   is indicated, the client instance MUST NOT poll immediately and
   SHOULD wait at least 5 seconds.  If the client instance does not
   respect the given wait period, the AS MUST return the too_fast error
   (Section 3.6).

   The response from the AS is a JSON object of a grant response and MAY
   contain any of the fields described in Section 3, as described in
   more detail in the sections below.

   If the AS determines that the client instance can make further
   requests to the continuation API, the AS MUST include a new
   "continue" response (Section 3.1).  The new continue response MUST
   include a continuation access token as well, and this token SHOULD be
   a new access token, invalidating the previous access token.  If the
   AS does not return a new continue response, the client instance MUST

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   NOT make an additional continuation request.  If a client instance
   does so, the AS MUST return an invalid_continuation error
   (Section 3.6).

   For continuation functions that require the client instance to send a
   message body, the body MUST be a JSON object.

   For all requests to the grant continuation API, the AS MAY make use
   of long polling mechanisms such as discussed in [RFC6202].  That is
   to say, instead of returning the current status immediately, the long
   polling technique allows the AS additional time to process and
   fulfill the request before returning the HTTP response to the client
   instance.  For example, when the AS receives a continuation request
   but the grant request is in the _processing_ state, the AS could wait
   until the grant request has moved to the _pending_ or _approved_
   state before returning the response message.

5.1.  Continuing After a Completed Interaction

   When the AS responds to the client instance's finish method as in
   Section 4.2.1, this response includes an interaction reference.  The
   client instance MUST include that value as the field interact_ref in
   a POST request to the continuation URI.

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   Since the interaction reference is a one-time-use value as described
   in Section 4.2.1, if the client instance needs to make additional
   continuation calls after this request, the client instance MUST NOT
   include the interaction reference in subsequent calls.  If the AS
   detects a client instance submitting an interaction reference when
   the request is not in the _pending_ state, the AS MUST return a
   too_many_attempts error (Section 3.6) and SHOULD invalidate the
   ongoing request by moving it to the _finalized_ state.

   If the grant request is in the _approved_ state, the grant response
   (Section 3) MAY contain any newly-created access tokens (Section 3.2)
   or newly-released subject information (Section 3.4).  The response

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   MAY contain a new "continue" response (Section 3.1) as described
   above.  The response SHOULD NOT contain any interaction responses
   (Section 3.3).

   If the grant request is in the _pending_ state, the grant response
   (Section 3) MUST NOT contain access tokens or subject information,
   and MAY contain a new interaction responses (Section 3.3) to any
   interaction methods that have not been exhausted at the AS.

   For example, if the request is successful in causing the AS to issue
   access tokens and release opaque subject claims, the response could
   look like this:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

   With this example, the client instance can not make an additional
   continuation request because a continue field is not included.

   For another example, if the RO has denied the client instance's
   request, the AS responds with the following response:

   {
       "error": "user_denied",
       "continue": {
           "access_token": {
               "value": "33OMUKMKSKU80UPRY5NM"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       }
   }

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   In this example, the AS includes the continue field in the response.
   Therefore, the client instance can continue the grant negotiation
   process, perhaps modifying the request as discussed in Section 5.3.

5.2.  Continuing During Pending Interaction (Polling)

   When the client instance does not include a finish parameter, the
   client instance will often need to poll the AS until the RO has
   authorized the request.  To do so, the client instance makes a POST
   request to the continuation URI as in Section 5.1, but does not
   include a message body.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   If the grant request is in the _approved_ state, the grant response
   (Section 3) MAY contain any newly-created access tokens (Section 3.2)
   or newly-released subject claims (Section 3.4).  The response MAY
   contain a new "continue" response (Section 3.1) as described above.
   If a continue field is included, it SHOULD include a wait field to
   facilitate a reasonable polling rate by the client instance.  The
   response SHOULD NOT contain interaction responses (Section 3.3).

   If the grant request is in the _pending_ state, the grant response
   (Section 3) MUST NOT contain access tokens or subject information,
   and MAY contain a new interaction responses (Section 3.3) to any
   interaction methods that have not been exhausted at the AS.

   For example, if the request has not yet been authorized by the RO,
   the AS could respond by telling the client instance to make another
   continuation request in the future.  In this example, a new, unique
   access token has been issued for the call, which the client instance
   will use in its next continuation request.

   {
       "continue": {
           "access_token": {
               "value": "33OMUKMKSKU80UPRY5NM"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       }
   }

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   If the request is successful in causing the AS to issue access tokens
   and release subject information, the response could look like this
   example:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

   See Section 13.22 for considerations on polling for continuation
   without an interaction finish method.

   In error conditions, the AS responds to the client instance with the
   error code as discussed in Section 3.6.  For example, if the client
   instance has polled too many times before the RO has approved the
   request, the AS would respond with a message like this:

   {
       "error": "too_many_attempts"
   }

   Since this response does not include a continue section, the client
   instance cannot continue to poll the AS for additional updates and
   the grant request is _finalized_. If the client instance still needs
   access to the resource, it will need to start with a new grant
   request.

5.3.  Modifying an Existing Request

   The client instance might need to modify an ongoing request, whether
   or not tokens have already been issued or subject information has
   already been released.  In such cases, the client instance makes an
   HTTP PATCH request to the continuation URI and includes any fields it
   needs to modify.  Fields that aren't included in the request are
   considered unchanged from the original request.

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   A grant request associated with a modification request MUST be in the
   _approved_ or _pending_ state.  When the AS receives a valid
   modification request, the AS MUST place the grant request into the
   _processing_ state and re-evaluate the authorization in the new
   context created by the update request, since the extent and context
   of the request could have changed.

   The client instance MAY include the access_token and subject fields
   as described in Section 2.1 and Section 2.2.  Inclusion of these
   fields override any values in the initial request, which MAY trigger
   additional requirements and policies by the AS.  For example, if the
   client instance is asking for more access, the AS could require
   additional interaction with the RO to gather additional consent.  If
   the client instance is asking for more limited access, the AS could
   determine that sufficient authorization has been granted to the
   client instance and return the more limited access rights
   immediately.  If the grant request was previously in the _approved_
   state, the AS could decide to remember the larger scale of access
   rights associated with the grant request, allowing the client
   instance to make subsequent requests of different subsets of granted
   access.  The details of this processing are out of scope for this
   specification, but a one possible approach is as follows:

   1.  A client instance requests access to Foo, and is granted by the
       RO.  This results in an access token, AT1.

   2.  The client instance later modifies the grant request to include
       Foo and Bar together.  Since the client instance was previously
       granted Foo under this grant request, the RO is prompted to allow
       the client instance access to Foo and Bar together.  This results
       in a new access token, AT2 This access token has access to both
       Foo and Bar. The rights of the original access token AT1 are not
       modified.

   3.  The client instance makes another grant modification to ask only
       for Bar. Since the client instance was previously granted Foo and
       Bar together under this grant request, the RO is not prompted and
       the access to Bar is granted in a new access token, AT3.  This
       new access token does not allow access to Foo.

   4.  The original access token AT1 expires and the client seeks a new
       access token to replace it.  The client instance makes another
       grant modification to ask only for Foo. Since the client instance
       was previously granted Foo and Bar together under this grant
       request, the RO is not prompted and the access to Foo is granted
       in a new access token, AT4.  This new access token does not allow
       access to Bar.

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   All four access tokens are independent of each other and associated
   with the same underlying grant request.  Each of these access tokens
   could possibly also be rotated using token management, if available.
   For example, instead of asking for a new token to replace AT1, the
   client instance could ask for a refresh of AT1 using the rotation
   method of token management.  This would result in a refreshed AT1,
   potentially with a different token value and expiration from the
   original AT1 but with the same access rights of allowing only access
   to Foo.

   The client instance MAY include the interact field as described in
   Section 2.5.  Inclusion of this field indicates that the client
   instance is capable of driving interaction with the end user, and
   this field replaces any values from a previous request.  The AS MAY
   respond to any of the interaction responses as described in
   Section 3.3, just like it would to a new request.

   The client instance MAY include the user field as described in
   Section 2.4 to present new assertions or information about the end
   user.  The AS SHOULD check that this presented user information is
   consistent with any user information previously presented by the
   client instance or otherwise associated with this grant request.

   The client instance MUST NOT include the client section of the
   request, since the client instance is assumed not to have changed.
   Modification of client instance information, including rotation of
   keys associated with the client instance, is outside the scope of
   this specification.

   The client instance MUST NOT include post-interaction responses such
   as described in Section 5.1.

   Modification requests MUST NOT alter previously-issued access tokens.
   Instead, any access tokens issued from a continuation are considered
   new, separate access tokens.  The AS MAY revoke previously-issued
   access tokens after a modification has occurred.

   If the modified request can be granted immediately by the AS (the
   grant request is in the _approved_ state), the grant response
   (Section 3) MAY contain any newly-created access tokens (Section 3.2)
   or newly-released subject claims (Section 3.4).  The response MAY
   contain a new "continue" response (Section 3.1) as described above.
   If interaction can occur, the response SHOULD contain interaction
   responses (Section 3.3) as well.

   For example, a client instance initially requests a set of resources
   using references:

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

   Access is granted by the RO, and a token is issued by the AS.  In its
   final response, the AS includes a continue field, which includes a
   separate access token for accessing the continuation API:

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "access": [
               "read", "write"
           ]
       }
   }

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   This continue field allows the client instance to make an eventual
   continuation call.  Some time later, the client instance realizes
   that it no longer needs "write" access and therefore modifies its
   ongoing request, here asking for just "read" access instead of both
   "read" and "write" as before.

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read"
           ]
       }
       ...
   }

   The AS replaces the previous access from the first request, allowing
   the AS to determine if any previously-granted consent already
   applies.  In this case, the AS would determine that reducing the
   breadth of the requested access means that new access tokens can be
   issued to the client instance without additional interaction or
   consent.  The AS would likely revoke previously-issued access tokens
   that had the greater access rights associated with them, unless they
   had been issued with the durable flag.

   {
       "continue": {
           "access_token": {
               "value": "M33OMUK80UPRY5NMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "0EVKC7-2ZKwZM_6N760",
           "access": [
               "read"
           ]
       }
   }

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   For another example, the client instance initially requests read-only
   access but later needs to step up its access.  The initial request
   could look like this example.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

   Access is granted by the RO, and a token is issued by the AS.  In its
   final response, the AS includes a continue field:

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "access": [
               "read"
           ]
       }
   }

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   This allows the client instance to make an eventual continuation
   call.  The client instance later realizes that it now needs "write"
   access in addition to the "read" access.  Since this is an expansion
   of what it asked for previously, the client instance also includes a
   new interaction section in case the AS needs to interact with the RO
   again to gather additional authorization.  Note that the client
   instance's nonce and callback are different from the initial request.
   Since the original callback was already used in the initial exchange,
   and the callback is intended for one-time-use, a new one needs to be
   included in order to use the callback again.

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/654321",
               "nonce": "K82FX4T4LKLTI25DQFZC"
           }
       }
   }

   From here, the AS can determine that the client instance is asking
   for more than it was previously granted, but since the client
   instance has also provided a mechanism to interact with the RO, the
   AS can use that to gather the additional consent.  The protocol
   continues as it would with a new request.  Since the old access
   tokens are good for a subset of the rights requested here, the AS
   might decide to not revoke them.  However, any access tokens granted
   after this update process are new access tokens and do not modify the
   rights of existing access tokens.

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5.4.  Revoking a Grant Request

   If the client instance wishes to cancel an ongoing grant request and
   place it into the _finalized_ state, the client instance makes an
   HTTP DELETE request to the continuation URI.

   DELETE /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   If the request is successfully revoked, the AS responds with status
   code HTTP 204 (No Content).  The AS SHOULD revoke all associated
   access tokens, if possible.  The AS SHOULD disable all token rotation
   and other token management functions on such access tokens, if
   possible.  Once the grant request is in the _finalized_ state, it
   MUST NOT be moved to any other state.

6.  Token Management

   If an access token response includes the manage parameter as
   described in Section 3.2.1, the client instance MAY call this URI to
   manage the access token with the rotate and revoke actions defined in
   the following sections.  Other actions are undefined by this
   specification.

   The access token being managed acts as the access element for its own
   management API.  The client instance MUST present proof of an
   appropriate key along with the access token.

   If the token is sender-constrained (i.e., not a bearer token), it
   MUST be sent with the appropriate binding for the access token
   (Section 7.2) based on the key bound to the access token.

   If the token is a bearer token, the client instance MUST present
   proof of the client instance's key (Section 2.3) (or its most recent
   rotation) as described in Section 7.3.  Note that this is usually the
   same key used in the initial grant request.

   The AS MUST validate the proof and assure that it is associated with
   either the token itself or the client instance the token was issued
   to, as appropriate for the token's presentation type.

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6.1.  Rotating the Access Token

   If the client instance has an access token and that access token
   expires, the client instance might want to rotate the access token to
   a new value without expiration.  Rotating an access token consists of
   issuing a new access token in place of an existing access token, with
   the same rights and properties as the original token, apart from an
   updated token value and expiration time.

   To rotate an access token, the client instance makes an HTTP POST to
   the token management URI with no message body, sending the access
   token in the appropriate header and signing the request with the
   appropriate key.

   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The client instance can not request to alter the access rights
   associated with the access token during a rotation request.  To get
   an access token with different access rights for this grant request,
   the client instance has to call the continuation API's update
   (Section 5.3) functionality to get a new access token.  The client
   instance can also create a new grant request with the required access
   rights.

   The AS validates that the token presented is associated with the
   management URI, that the AS issued the token to the given client
   instance, and that the presented key is appropriate to the token.

   Note that in many cases, the access token will have expired for
   regular use at an RS.  To facilitate token rotation, the AS SHOULD
   honor the rotation request of the expired access token since it is
   likely that the client instance is attempting to refresh the expired
   token.  To support this, the AS MAY allow a longer lifetime for token
   management compared to its use at an RS.  An AS MUST NOT honor a
   rotation request for an access token that has been explicitly revoked
   or otherwise disabled.

   If the token is validated and the key is appropriate for the request,
   the AS MUST invalidate the current access token value associated with
   this URI, if possible.  Note that stateless access tokens can make
   proactive revocation difficult within a system, see Section 13.31.

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   For successful rotations, the AS responds with an HTTP 200 with a
   JSON body consisting of the rotated access token in the access_token
   field described in Section 3.2.1.  The value of the access token MUST
   NOT be the same as the current value of the access token used to
   access the management API.  The response MUST include an access token
   management URI, and the value of this URI MAY be different from the
   URI used by the client instance to make the rotation call.  The
   client instance MUST use this new URI to manage the rotated access
   token.

   The access rights in the access array for the rotated access token
   MUST be included in the response and MUST be the same as the token
   before rotation.

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "FP6A8H6HY37MH13CK76LBZ6Y1UADG6VEUPEER5H2",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "expires_in": 3600,
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "read", "dolphin-metadata"
           ]
       }
   }

   If the AS is unable or unwilling to rotate the value of the access
   token, the AS responds with an invalid_rotation error (Section 3.6).
   Upon receiving such an error, the client instance SHOULD consider the
   access token to not have changed its state.

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6.1.1.  Binding a New Key to the Rotated Access Token

   If the client instance wishes to bind a new presentation key to an
   access token, the client instance MUST present both the new key and
   the proof of previous key material in the access token rotation
   request.  The client instance makes an HTTP POST as a JSON object
   with the following field:

   key:  The new key value or reference in the format described in
      Section 7.1.  Note that keys passed by value are always public
      keys.  REQUIRED when doing key rotation.

   The proof method and parameters for the new key MUST be the same as
   those established for the previous key.

   The client instance MUST prove possession of both the currently-bound
   key and the newly-requested key simultaneously in the rotation
   request.  Specifically, the signature from the previous key MUST
   cover the value or reference of the new key, and the signature of the
   new key MUST cover the signature value of the old key.  The means of
   doing so varies depending on the proofing method in use.  For
   example, the HTTP Message Signatures proofing method uses multiple
   signatures in the request as described in Section 7.3.1.1, as shown
   in this example.

   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=..., sig2=("signature";key=sig1)...
   Signature: sig1=..., sig2=...
   Content-Digest: sha-256=...

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

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   Failure to present the appropriate proof of either the new key or the
   previous key for the access token, as defined by the proof method,
   MUST result in an invalid_rotation error code from the AS
   (Section 3.6).

   An attempt to change the proof method or parameters, including an
   attempt to rotate the key of a bearer token (which has no key), MUST
   result in an invalid_rotation error code returned from the AS
   (Section 3.6).

   If the AS does not allow rotation of the access token's key for any
   reason, including but not limited to lack of permission for this
   client instance or lack of capability by the AS, the AS MUST return a
   key_rotation_not_supported error code (Section 3.6).

6.2.  Revoking the Access Token

   If the client instance wishes to revoke the access token proactively,
   such as when a user indicates to the client instance that they no
   longer wish for it to have access or the client instance application
   detects that it is being uninstalled, the client instance can use the
   token management URI to indicate to the AS that the AS should
   invalidate the access token for all purposes.

   The client instance makes an HTTP DELETE request to the token
   management URI, presenting the access token and signing the request
   with the appropriate key.

   DELETE /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=...
   Signature: sig1=...

   If the key presented is associated with the token (or the client
   instance, in the case of a bearer token), the AS MUST invalidate the
   access token, if possible, and return an HTTP 204 response code.

   204 No Content

   Though the AS MAY revoke an access token at any time for any reason,
   the token management function is specifically for the client
   instance's use.  If the access token has already expired or has been
   revoked through other means, the AS SHOULD honor the revocation
   request to the token management URI as valid, since the end result is
   still the token not being usable.

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7.  Securing Requests from the Client Instance

   In GNAP, the client instance secures its requests to the AS and RS by
   presenting an access token, presenting proof of a key that it
   possesses (aka, a "key proof"), or both an access token and key proof
   together.

   *  When an access token is used with a key proof, this is a bound
      token request.  This type of request is used for calls to the RS
      as well as the AS during grant negotiation.

   *  When a key proof is used with no access token, this is a non-
      authorized signed request.  This type of request is used for calls
      to the AS to initiate a grant negotiation.

   *  When an access token is used with no key proof, this is a bearer
      token request.  This type of request is used only for calls to the
      RS, and only with access tokens that are not bound to any key as
      described in Section 3.2.1.

   *  When neither an access token nor key proof are used, this is an
      unsecured request.  This type of request is used optionally for
      calls to the RS as part of an RS-first discovery process as
      described in Section 9.1.

7.1.  Key Formats

   Several different places in GNAP require the presentation of key
   material by value or by reference.  Key material sent by value is
   sent using a JSON object with several fields described in this
   section.

   All keys are associated with a specific key proofing method.  The
   proofing method associated with the key is indicated using the proof
   field of the key object.

   proof (string or object):  The form of proof that the client instance
      will use when presenting the key.  The valid values of this field
      and the processing requirements for each are detailed in
      Section 7.3.  REQUIRED.

   A key presented by value MUST be a public key and MUST be presented
   in one and only one supported format, as discussed in Section 13.34.
   Note that while most formats present the full value of the public
   key, some formats present a value cryptographically derived from the
   public key.  See additional discussion of the presentation of public
   keys in Section 13.5.

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   jwk (object):  The public key and its properties represented as a
      JSON Web Key [RFC7517].  A JWK MUST contain the alg (Algorithm)
      and kid (Key ID) parameters.  The alg parameter MUST NOT be
      "none".  The x5c (X.509 Certificate Chain) parameter MAY be used
      to provide the X.509 representation of the provided public key.
      OPTIONAL.

   cert (string):  PEM serialized value of the certificate used to sign
      the request, with optional internal whitespace per [RFC7468].  The
      PEM header and footer are optionally removed.  OPTIONAL.

   cert#S256 (string):  The certificate thumbprint calculated as per
      OAuth-MTLS [RFC8705] in base64 URL encoding.  Note that this
      format does not include the full public key.  OPTIONAL.

   Additional key formats are defined in the Key Formats Registry
   (Section 11.15).

   This non-normative example shows a single key presented in two
   different formats.  This example key is intended to be used with the
   HTTP Message Signatures (Section 7.3.1) proofing mechanism, as
   indicated by the httpsig value of the proof field.

   As a JSON Web Key:

   "key": {
       "proof": "httpsig",
       "jwk": {
           "kty": "RSA",
           "e": "AQAB",
           "kid": "xyz-1",
           "alg": "RS256",
           "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
       }
   }

   As a certificate in PEM format:

   "key": {
       "proof": "httpsig",
       "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
   }

   When the key is presented in GNAP, proof of this key material MUST be
   used to bind the request, the nature of which varies with the
   location in the protocol the key is used.  For a key used as part of
   a client instance's initial request in Section 2.3, the key value
   represents the client instance's public key, and proof of that key

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   MUST be presented in that request.  For a key used as part of an
   access token response in Section 3.2.1, the proof of that key MUST be
   used when the client instance later presents the access token to the
   RS.

7.1.1.  Key References

   Keys in GNAP can also be passed by reference such that the party
   receiving the reference will be able to determine the appropriate
   keying material for use in that part of the protocol.  Key references
   are a single opaque string.

       "key": "S-P4XJQ_RYJCRTSU1.63N3E"

   Keys referenced in this manner MAY be shared symmetric keys.  See the
   additional considerations for symmetric keys in Section 13.5.  The
   key reference MUST NOT contain any unencrypted private or shared
   symmetric key information.

   Keys referenced in this manner MUST be bound to a single proofing
   mechanism.

   The means of dereferencing this reference to a key value and proofing
   mechanism are out of scope for this specification.  Commonly, key
   references are created by the AS and are not necessarily needed to be
   understood by the client.  These types of key references are an
   internal reference to the AS, such as an identifier of a record in a
   database.  In other applications, it can be useful to use key
   references that are resolvable by both clients and AS, which could be
   accomplished by a client publishing a public key at a URI, for
   example.  For interoperability, this method could later be described
   as an extension, but doing so is out of scope for this specification.

7.1.2.  Key Protection

   The security of GNAP relies on the cryptographic security of the keys
   themselves.  When symmetric keys are used in GNAP, a key management
   system or secure key derivation mechanism MUST be used to supply the
   keys.  Symmetric keys MUST NOT be a human memorable password or a
   value derived from one.  Symmetric keys MUST NOT be passed by value
   from the client instance to the AS.

   Additional security considerations apply when rotating keys
   (Section 13.21).

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7.2.  Presenting Access Tokens

   Access tokens are issued to client instances in GNAP to allow the
   client instance to make an authorized call to an API.  The method the
   client instance uses to send an access token depends on whether the
   token is bound to a key, and if so which proofing method is
   associated with the key.  This information is conveyed by the key
   parameter and the bearer flag in the access token response structure
   (Section 3.2.1).

   If the flags field does not contain the bearer flag and the key is
   absent, the access token MUST be sent using the same key and proofing
   mechanism that the client instance used in its initial request (or
   its most recent rotation).

   If the flags field does not contain the bearer flag and the key value
   is an object as described in Section 7.1, the access token MUST be
   sent using the key and proofing mechanism defined by the value of the
   proof field within the key object.

   The access token MUST be sent using the HTTP "Authorization" request
   header field and the "GNAP" authorization scheme along with a key
   proof as described in Section 7.3 for the key bound to the access
   token.  For example, an access token bound using HTTP Message
   Signatures would be sent as follows:

   NOTE: '\' line wrapping per RFC 8792

   GET /stuff HTTP/1.1
   Host: resource.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=("@method" "@target-uri" "authorization")\
     ;created=1618884473;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"
   Signature: sig1=:FQ+EjWqc38uLFByKa5y+c4WyYYwCTGUhidWKfr5L1Cha8FiPEw\
     DxG7nWttpBLS/B6VLfkZJogPbclySs9MDIsAIJwHnzlcJjwXWR2lfvm2z3X7EkJHm\
     Zp4SmyKOS34luAiKR1xwf32NYFolHmZf/SbHZJuWvQuS4U33C+BbsXz8MflFH1Dht\
     H/C1E5i244gSbdLCPxzABc/Q0NHVSLo1qaouYIvnxXB8OT3K7mwWjsLh1GC5vFThb\
     3XQ363r6f0OPRa4qWHhubR/d/J/lNOjbBdjq9AJ69oqNJ+A2XT+ZCrVasEJE0OBvD\
     auQoiywhb8BMB7+PEINsPk5/8UvaNxbw==:

   If the flags field contains the bearer flag, the access token is a
   bearer token that MUST be sent using the Authorization Request Header
   Field method defined in [RFC6750].

   Authorization: Bearer OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0

   The Form-Encoded Body Parameter and URI Query Parameter methods of
   [RFC6750] MUST NOT be used for GNAP access tokens.

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7.3.  Proving Possession of a Key with a Request

   Any keys presented by the client instance to the AS or RS MUST be
   validated as part of the request in which they are presented.  The
   type of binding used is indicated by the proof parameter of the key
   object in Section 7.1.  Key proof methods are specified either by a
   string, which consists of the key proof method name on its own, or by
   a JSON object with the required field method:

   method:  The name of the key proofing method to be used.  REQUIRED.

   Individual methods defined as objects MAY define additional
   parameters as members in this object.

   Values for the method defined by this specification are as follows:

   "httpsig" (string or object):  HTTP Signing signature headers.  See
      Section 7.3.1.

   "mtls" (string):  Mutual TLS certificate verification.  See
      Section 7.3.2.

   "jwsd" (string):  A detached JWS signature header.  See
      Section 7.3.3.

   "jws" (string):  Attached JWS payload.  See Section 7.3.4.

   Additional proofing methods are defined by the Key Proofing Methods
   Registry (Section 11.14).

   Proof methods MAY be defined as both an object and a string.  For
   example, the httpsig method can be specified as an object with its
   parameters explicitly declared, such as:

   {
       "proof": {
           "method": "httpsig",
           "alg": "ecdsa-p384-sha384",
           "content-digest-alg": "sha-256"
       }
   }

   The httpsig method also defines defines default behavior when it is
   passed as a string form, using the signature algorithm specified by
   the associated key material and the content digest is calculated
   using sha-256.  This configuration can be selected using the
   following shortened form:

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   {
       "proof": "httpsig"
   }

   All key binding methods used by this specification MUST cover all
   relevant portions of the request, including anything that would
   change the nature of the request, to allow for secure validation of
   the request.  Relevant aspects include the URI being called, the HTTP
   method being used, any relevant HTTP headers and values, and the HTTP
   message body itself.  The verifier of the signed message MUST
   validate all components of the signed message to ensure that nothing
   has been tampered with or substituted in a way that would change the
   nature of the request.  Key binding method definitions SHOULD
   enumerate how these requirements are fulfilled.

   When a key proofing mechanism is bound to an access token, the key
   being presented MUST be the key associated with the access token and
   the access token MUST be covered by the signature method of the
   proofing mechanism.

   The key binding methods in this section MAY be used by other
   components making calls as part of GNAP, such as the extensions
   allowing the RS to make calls to the AS defined in
   [I-D.ietf-gnap-resource-servers].  To facilitate this extended use,
   the sections below are defined in generic terms of the "signer" and
   "verifier" of the HTTP message.  In the core functions of GNAP
   specified in this document, the "signer" is the client instance and
   the "verifier" is the AS (for grant requests) or RS (for resource
   requests), as appropriate.

   When used for delegation in GNAP, these key binding mechanisms allow
   the AS to ensure that the keys presented by the client instance in
   the initial request are in control of the party calling any follow-up
   or continuation requests.  To facilitate this requirement, the
   continuation response (Section 3.1) includes an access token bound to
   the client instance's key (Section 2.3), and that key (or its most
   recent rotation) MUST be proved in all continuation requests
   (Section 5).  Token management requests (Section 6) are similarly
   bound to either the access token's own key or, in the case of bearer
   tokens, the client instance's key.

   In the following sections, unless otherwise noted, the RS256 JOSE
   Signature Algorithm is applied using the following RSA key (presented
   here in JWK format):

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   {
       "kid": "gnap-rsa",
       "p": "xS4-YbQ0SgrsmcA7xDzZKuVNxJe3pCYwdAe6efSy4hdDgF9-vhC5gjaRk\
           i1wWuERSMW4Tv44l5HNrL-Bbj_nCJxr_HAOaesDiPn2PnywwEfg3Nv95Nn-\
           eilhqXRaW-tJKEMjDHu_fmJBeemHNZI412gBnXdGzDVo22dvYoxd6GM",
       "kty": "RSA",
       "q": "rVdcT_uy-CD0GKVLGpEGRR7k4JO6Tktc8MEHkC6NIFXihk_6vAIOCzCD6\
           LMovMinOYttpRndKoGTNdJfWlDFDScAs8C5n2y1STCQPRximBY-bw39-aZq\
           JXMxOLyPjzuVgiTOCBIvLD6-8-mvFjXZk_eefD0at6mQ5qV3U1jZt88",
       "d": "FHlhdTF0ozTliDxMBffT6aJVKZKmbbFJOVNten9c3lXKB3ux3NAb_D2dB\
           7inp9EV23oWrDspFtvCvD9dZrXgRKMHofkEpo_SSvBZfgtH-OTkbY_TqtPF\
           FLPKAw0JX5cFPnn4Q2xE4n-dQ7tpRCKl59vZLHBrHShr90zqzFp0AKXU5fj\
           b1gC9LPwsFA2Fd7KXmI1drQQEVq9R-o18Pnn4BGQNQNjO_VkcJTiBmEIVT_\
           KJRPdpVJAmbgnYWafL_hAfeb_dK8p85yurEVF8nCK5oO3EPrqB7IL4UqaEn\
           5Sl3u0j8x5or-xrrAoNz-gdOv7ONfZY6NFoa-3f8q9wBAHUuQ",
       "e": "AQAB",
       "qi": "ogpNEkDKg22Rj9cDV_-PJBZaXMk66Fp557RT1tafIuqJRHEufSOYnsto\
           bWPJ0gHxv1gVJw3gm-zYvV-wTMNgr2wVsBSezSJjPSjxWZtmT2z68W1DuvK\
           kZy15vz7Jd85hmDlriGcXNCoFEUsGLWkpHH9RwPIzguUHWmTt8y0oXyI",
       "dp": "dvCKGI2G7RLh3WyjoJ_Dr6hZ3LhXweB3YcY3qdD9BnxZ71mrLiMQg4c_\
           EBnwqCETN_5sStn2cRc2JXnvLP3G8t7IFKHTT_i_TSTacJ7uT04MSa053Y3\
           RfwbvLjRNPR0UKAE3ZxROUoIaVNuU_6-QMf8-2ilUv2GIOrCN87gP_Vk",
       "alg": "RS256",
       "dq": "iMZmELaKgT9_W_MRT-UfDWtTLeFjIGRW8aFeVmZk9R7Pnyt8rNzyN-IQ\
           M40ql8u8J6vc2GmQGfokLlPQ6XLSCY68_xkTXrhoU1f-eDntkhP7L6XawSK\
           Onv5F2H7wyBQ75HUmHTg8AK2B_vRlMyFKjXbVlzKf4kvqChSGEz4IjQ",
       "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8BfYdHsFzAt\
           YKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZGYX\
           jHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZx\
           e0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0\
           bunS0K3bA_3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kO\
           zywzwPTuq-cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
   }

   Key proofing methods SHOULD define a mechanism to allow the rotation
   of keys discussed in Section 6.1.1.  Key rotation mechanisms MUST
   define a way for presenting proof of two keys simultaneously with the
   following attributes:

   *  The value of or reference to the new key material MUST be signed
      by the existing key.  Generally speaking, this amounts to using
      the existing key to sign the body of the message.

   *  The signature of the old key MUST be signed by the new key.
      Generally speaking, this means including the signature value of
      the old key under the coverage of the new key.

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7.3.1.  HTTP Message Signatures

   This method is indicated by the method value httpsig and can be
   declared in either object form or string form.

   When the proof method is specified in object form, the following
   parameters are defined:

   alg:  The HTTP signature algorithm, from the HTTP Signature Algorithm
      registry.  REQUIRED.

   content-digest-alg:  The algorithm used for the Content-Digest field,
      used to protect the body when present in the message.  REQUIRED.

   This example uses the ECDSA signing algorithm over the P384 curve and
   the SHA-512 hashing algorithm for the content digest.

   {
       "proof": {
           "method": "httpsig",
           "alg": "ecdsa-p384-sha384",
           "content-digest-alg": "sha-512"
       }
   }

   When the proof method is specified in string form, the signing
   algorithm MUST be derived from the key material (such as using the
   JWS algorithm in a JWK formatted key), and the content digest
   algorithm MUST be sha-256.

   {
       "proof": "httpsig"
   }

   When using this method, the signer creates an HTTP Message Signature
   as described in [I-D.ietf-httpbis-message-signatures].  The covered
   components of the signature MUST include the following:

   "@method":  The method used in the HTTP request.

   "@target-uri":  The full request URI of the HTTP request.

   When the message contains a request body, the covered components MUST
   also include the following:

   "content-digest":  The Content-Digest header as defined in

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      [I-D.ietf-httpbis-digest-headers].  When the request message has a
      body, the signer MUST calculate this field value and include the
      field in the request.  The verifier MUST validate this field
      value.  REQUIRED when the message request contains a message body.

   When the request is bound to an access token, the covered components
   MUST also include the following:

   "authorization":  The Authorization header used to present the access
      token as discussed in Section 7.2.

   Other message components MAY also be included.

   The signer MUST include the tag signature parameter with the value
   gnap, and the verifier MUST verify that the parameter exists with
   this value.  The signer MUST include the created signature parameter
   with a timestamp of when the signature was created, and the verifier
   MUST ensure that the creation timestamp is sufficiently close to the
   current time given expected network delay and clock skew.  The signer
   SHOULD include the nonce parameter with a unique and unguessable
   value.  When included, the verifier MUST determine that the nonce
   value is unique within a reasonably short time period such as several
   minutes.

   If the signer's key presented is a JWK, the keyid parameter of the
   signature MUST be set to the kid value of the JWK, the signing
   algorithm used MUST be the JWS algorithm denoted by the key's alg
   field of the JWK.

   The explicit alg signature parameter MUST NOT be included in the
   signature, since the algorithm will be derived either from the key
   material or from the proof value.

   In this example, the message body is the following JSON object:

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   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "PS512",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   This body is hashed for the Content-Digest header using sha-256 into
   the following encoded value:

   sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAGg=:

   The HTTP message signature input string is calculated to be the
   following:

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   "@method": POST
   "@target-uri": https://server.example.com/gnap
   "content-digest": \
     sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAGg=:
   "content-length": 988
   "content-type": application/json
   "@signature-params": ("@method" "@target-uri" "content-digest" \
     "content-length" "content-type");created=1618884473\
     ;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"

   This leads to the following full HTTP message request:

   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 988
   Content-Digest: sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAG\
     g=:
   Signature-Input: sig1=("@method" "@target-uri" "content-digest" \
     "content-length" "content-type");created=1618884473\
     ;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"
   Signature: sig1=:c2uwTa6ok3iHZsaRKl1ediKlgd5cCAYztbym68XgX8gSOgK0Bt\
     +zLJ19oGjSAHDjJxX2gXP2iR6lh9bLMTfPzbFVn4Eh+5UlceP+0Z5mES7v0R1+eHe\
     OqBl0YlYKaSQ11YT7n+cwPnCSdv/6+62m5zwXEEftnBeA1ECorfTuPtau/yrTYEvD\
     9A/JqR2h9VzAE17kSlSSsDHYA6ohsFqcRJavX29duPZDfYgkZa76u7hJ23yVxoUpu\
     2J+7VUdedN/72N3u3/z2dC8vQXbzCPTOiLru12lb6vnBZoDbUGsRR/zHPauxhj9T+\
     218o5+tgwYXw17othJSxIIOZ9PkIgz4g==:

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {

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           "proof": "httpsig",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "PS512",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   The verifier MUST ensure that the signature covers all required
   message components.  If the HTTP Message includes a message body, the
   verifier MUST calculate and verify the value of the Content-Digest
   header.  The verifier MUST validate the signature against the
   expected key of the signer.

   A received message MAY include multiple signatures, each with its own
   label.  The verifier MUST examine all included signatures until it
   finds (at least) one that's acceptable according to its policy and
   meets the requirements in this section.

7.3.1.1.  Key Rotation using HTTP Message Signatures

   When rotating a key using HTTP Message Signatures, the message, which
   includes the new public key value or reference, is first signed with
   the old key.  The message is then signed again with the new key by
   including the signature from the old key under the signature of the
   new key.

   For example, the following request to the token management endpoint
   for rotating a token value contains both the new key in the request.
   The message is first signed using the old key and the resulting
   signature is placed in "sig1":

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   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=("authorization" "@method")\
       ;keyid="xyz-1";created=161888447;tag="gnap"
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

   The signer then creates a new signature using the new key using the
   signature value as its input to the signature base.  Since the
   existing signature covers the required parts of the message, they do
   not need to be repeated.

   "signature";key="sig1"
   "@signature-input": ("signature";key="sig1");keyid="xyz-2"\
     ;tag="gnap";created=161888447

   This signature is then added to the message:

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   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=("authorization" "@method")\
       ;keyid="xyz-1";created=161888447;tag="gnap", \
       sig2=("signature";key="sig1");keyid="xyz-2";tag="gnap";\
       ;created=161888447
   Signature: sig1=..., sig2=...
   Content-Digest: sha-256=...

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

   The verifier MUST validate both signatures before processing the
   request for key rotation.

7.3.2.  Mutual TLS

   This method is indicated by the method value mtls in string form.

   {
       "proof": "mtls"
   }

   The signer presents its TLS client certificate during TLS negotiation
   with the verifier.

   In this example, the certificate is communicated to the application
   through the Client-Cert header from a TLS reverse proxy as per
   [I-D.ietf-httpbis-client-cert-field], leading to the following full
   HTTP request message:

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/jose
   Content-Length: 1567
   Client-Cert: \
     :MIIC6jCCAdKgAwIBAgIGAXjw74xPMA0GCSqGSIb3DQEBCwUAMDYxNDAyBgNVBAMM\

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     K05JWU15QmpzRGp5QkM5UDUzN0Q2SVR6a3BEOE50UmppOXlhcEV6QzY2bVEwHhcN\
     MjEwNDIwMjAxODU0WhcNMjIwMjE0MjAxODU0WjA2MTQwMgYDVQQDDCtOSVlNeUJq\
     c0RqeUJDOVA1MzdENklUemtwRDhOdFJqaTl5YXBFekM2Nm1RMIIBIjANBgkqhkiG\
     9w0BAQEFAAOCAQ8AMIIBCgKCAQEAhYOJ+XOKISdMMShn/G4W9m20mT0VWtQBsmBB\
     kI2cmRt4Ai8BfYdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8I\
     kZ8NMwSrcUIBZGYXjHpwjzvfGvXH/5KJlnR3/uRUp4Z4Ujk2bCaKegDn11V2vxE4\
     1hqaPUnhRZxe0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo+\
     uv4BC0bunS0K3bA/3UgVp7zBlQFoFnLTO2uWp/muLEWGl67gBq9MO3brKXfGhi3k\
     OzywzwPTuq+cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQIDAQABMA0GCSqG\
     SIb3DQEBCwUAA4IBAQBnYFK0eYHy+hVf2D58usj39lhL5znb/q9G35GBd/XsWfCE\
     wHuLOSZSUmG71bZtrOcx0ptle9bp2kKl4HlSTTfbtpuG5onSa3swRNhtKtUy5NH9\
     W/FLViKWfoPS3kwoEpC1XqKY6l7evoTCtS+kTQRSrCe4vbNprCAZRxz6z1nEeCgu\
     NMk38yTRvx8ihZpVOuU+Ih+dOtVe/ex5IAPYxlQsvtfhsUZqc7IyCcy72WHnRHlU\
     fn3pJm0S5270+Yls3Iv6h3oBAP19i906UjiUTNH3g0xMW+V4uLxgyckt4wD4Mlyv\
     jnaQ7Z3sR6EsXMocAbXHIAJhwKdtU/fLgdwL5vtx:

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "mtls",
           "cert": "MIIC6jCCAdKgAwIBAgIGAXjw74xPMA0GCSqGSIb3DQEBCwUAMD\
     YxNDAyBgNVBAMMK05JWU15QmpzRGp5QkM5UDUzN0Q2SVR6a3BEOE50UmppOXlhcEV\
     6QzY2bVEwHhcNMjEwNDIwMjAxODU0WhcNMjIwMjE0MjAxODU0WjA2MTQwMgYDVQQD\
     DCtOSVlNeUJqc0RqeUJDOVA1MzdENklUemtwRDhOdFJqaTl5YXBFekM2Nm1RMIIBI\
     jANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAhYOJ+XOKISdMMShn/G4W9m20mT\
     0VWtQBsmBBkI2cmRt4Ai8BfYdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8\
     KowlyVy8IkZ8NMwSrcUIBZGYXjHpwjzvfGvXH/5KJlnR3/uRUp4Z4Ujk2bCaKegDn\
     11V2vxE41hqaPUnhRZxe0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDad\
     z8BkPo+uv4BC0bunS0K3bA/3UgVp7zBlQFoFnLTO2uWp/muLEWGl67gBq9MO3brKX\
     fGhi3kOzywzwPTuq+cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQIDAQABMA0\
     GCSqGSIb3DQEBCwUAA4IBAQBnYFK0eYHy+hVf2D58usj39lhL5znb/q9G35GBd/Xs\
     WfCEwHuLOSZSUmG71bZtrOcx0ptle9bp2kKl4HlSTTfbtpuG5onSa3swRNhtKtUy5\
     NH9W/FLViKWfoPS3kwoEpC1XqKY6l7evoTCtS+kTQRSrCe4vbNprCAZRxz6z1nEeC\
     guNMk38yTRvx8ihZpVOuU+Ih+dOtVe/ex5IAPYxlQsvtfhsUZqc7IyCcy72WHnRHl\

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     Ufn3pJm0S5270+Yls3Iv6h3oBAP19i906UjiUTNH3g0xMW+V4uLxgyckt4wD4Mlyv\
     jnaQ7Z3sR6EsXMocAbXHIAJhwKdtU/fLgdwL5vtx"
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       },
       "subject": {
           "formats": ["iss_sub", "opaque"]
       }
   }

   The verifier compares the TLS client certificate presented during
   mutual TLS negotiation to the expected key of the signer.  Since the
   TLS connection covers the entire message, there are no additional
   requirements to check.

   Note that in many instances, the verifier will not do a full
   certificate chain validation of the presented TLS client certificate,
   as the means of trust for this certificate could be in something
   other than a PKI system, such as a static registration or trust-on-
   first-use.  See Section 13.17 and Section 13.18 for some additional
   considerations for this key proofing method.

7.3.2.1.  Key Rotation using MTLS

   Since it is not possible to present two client authenticated
   certificates to a mutual TLS connection simultaneously, dynamic key
   rotation for this proofing method is not defined.  Instead, key
   rotation for MTLS-based client instances is expected to be managed
   through deployment practices, as discussed in Section 13.18.

7.3.3.  Detached JWS

   This method is indicated by the method value jwsd in string form.

   {
       "proof": "jwsd"
   }

   The signer creates a JWS [RFC7515] object as follows:

   To protect the request, the JOSE header of the signature contains the
   following claims:

   kid (string):  The key identifier.  REQUIRED if the key is presented
      in JWK format, this MUST be the value of the kid field of the key.

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   alg (string):  The algorithm used to sign the request.  MUST be
      appropriate to the key presented.  If the key is presented as a
      JWK, this MUST be equal to the alg parameter of the key.  MUST NOT
      be none.  REQUIRED.

   typ (string):  The type header, value ”gnap-binding+jwsd”. REQUIRED.

   htm (string):  The HTTP Method used to make this request, as a case-
      sensitive ASCII string.  Note that most public HTTP methods are in
      uppercase ASCII by convention.  REQUIRED.

   uri (string):  The HTTP URI used for this request.  This value MUST
      be an absolute URI, including all path and query components and no
      fragment component.  REQUIRED.

   created (integer):  A timestamp of when the signature was created, in
      integer seconds since UNIX Epoch.  REQUIRED.

   When the request is bound to an access token, the JOSE header MUST
   also include the following:

   ath (string):  The hash of the access token.  The value MUST be the
      result of Base64url encoding (with no padding) the SHA-256 digest
      of the ASCII encoding of the associated access token's value.
      REQUIRED.

   If the HTTP request has a message body, such as an HTTP POST or PUT
   method, the payload of the JWS object is the Base64url encoding
   (without padding) of the SHA256 digest of the bytes of the body.  If
   the request being made does not have a message body, such as an HTTP
   GET, OPTIONS, or DELETE method, the JWS signature is calculated over
   an empty payload.

   The signer presents the signed object in compact form [RFC7515] in
   the Detached-JWS HTTP Header field.

   In this example, the JOSE Header contains the following parameters:

   {
       "alg": "RS256",
       "kid": "gnap-rsa",
       "uri": "https://server.example.com/gnap",
       "htm": "POST",
       "typ": "gnap-binding+jwsd",
       "created": 1618884475
   }

   The request body is the following JSON object:

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   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jwsd",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   This is hashed to the following Base64 encoded value:

   PGiVuOZUcN1tRtUS6tx2b4cBgw9mPgXG3IPB3wY7ctc

   This leads to the following full HTTP request message:

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   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 983
   Detached-JWS: eyJhbGciOiJSUzI1NiIsImNyZWF0ZWQiOjE2MTg4ODQ0NzUsImh0b\
     SI6IlBPU1QiLCJraWQiOiJnbmFwLXJzYSIsInR5cCI6ImduYXAtYmluZGluZytqd3\
     NkIiwidXJpIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20vZ25hcCJ9.PGiVuO\
     ZUcN1tRtUS6tx2b4cBgw9mPgXG3IPB3wY7ctc.fUq-SV-A1iFN2MwCRW_yolVtT2_\
     TZA2h5YeXUoi5F2Q2iToC0Tc4drYFOSHIX68knd68RUA7yHqCVP-ZQEd6aL32H69e\
     9zuMiw6O_s4TBKB3vDOvwrhYtDH6fX2hP70cQoO-47OwbqP-ifkrvI3hVgMX9TfjV\
     eKNwnhoNnw3vbu7SNKeqJEbbwZfpESaGepS52xNBlDNMYBQQXxM9OqKJaXffzLFEl\
     -Xe0UnfolVtBraz3aPrPy1C6a4uT7wLda3PaTOVtgysxzii3oJWpuz0WP5kRujzDF\
     wX_EOzW0jsjCSkL-PXaKSpZgEjNjKDMg9irSxUISt1C1T6q3SzRgfuQ

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jwsd",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {

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           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   When the verifier receives the Detached-JWS header, it MUST parse and
   validate the JWS object.  The signature MUST be validated against the
   expected key of the signer.  If the HTTP message request contains a
   body, the verifier MUST calculate the hash of body just as the signer
   does, with no normalization or transformation of the request.  All
   required fields MUST be present and their values MUST be valid.  All
   fields MUST match the corresponding portions of the HTTP message.
   For example, the htm field of the JWS header has to be the same as
   the HTTP verb used in the request.

   Note that this proof method depends on a specific cryptographic
   algorithm, SHA-256, in two ways: the ath hash algorithm is hardcoded,
   and computing the payload of the detached/attached signature also
   uses a hardcoded hash.  A future version of this document may address
   crypto-agility for both these uses by replacing ath with a new header
   that upgrades the algorithm, and possibly defining a new JWS header
   that indicates the HTTP content's hash method.

7.3.3.1.  Key Rotation using Detached JWS

   When rotating a key using Detached JWS, the message, which includes
   the new public key value or reference, is first signed with the old
   key as described above using a JWS object with typ header value
   ”gnap-binding-rotation+jwsd”. The value of the JWS object is then
   taken as the payload of a new JWS object, to be signed by the new key
   using the parameters above.

   The value of the new JWS object is sent in the Detached-JWS header.

7.3.4.  Attached JWS

   This method is indicated by the method value jws in string form.

   {
       "proof": "jws"
   }

   The signer creates a JWS [RFC7515] object as follows:

   To protect the request, the JWS header contains the following claims.

   kid (string):  The key identifier.  REQUIRED if the key is presented

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      in JWK format, this MUST be the value of the kid field of the key.

   alg (string):  The algorithm used to sign the request.  MUST be
      appropriate to the key presented.  If the key is presented as a
      JWK, this MUST be equal to the alg parameter of the key.  MUST NOT
      be none.  REQUIRED.

   typ (string):  The type header, value ”gnap-binding+jwsd”. REQUIRED.

   htm (string):  The HTTP Method used to make this request, as a case-
      sensitive ASCII string.  (Note that most public HTTP methods are
      in uppercase.)  REQUIRED.

   uri (string):  The HTTP URI used for this request, including all path
      and query components and no fragment component.  REQUIRED.

   created (integer):  A timestamp of when the signature was created, in
      integer seconds since UNIX Epoch.  REQUIRED.

   When the request is bound to an access token, the JOSE header MUST
   also include the following:

   ath (string):  The hash of the access token.  The value MUST be the
      result of Base64url encoding (with no padding) the SHA-256 digest
      of the ASCII encoding of the associated access token's value.
      REQUIRED.

   If the HTTP request has a message body, such as an HTTP POST or PUT
   method, the payload of the JWS object is the JSON serialized body of
   the request, and the object is signed according to JWS and serialized
   into compact form [RFC7515].  The signer presents the JWS as the body
   of the request along with a content type of application/jose.  The
   verifier MUST extract the payload of the JWS and treat it as the
   request body for further processing.

   If the request being made does not have a message body, such as an
   HTTP GET, OPTIONS, or DELETE method, the JWS signature is calculated
   over an empty payload and passed in the Detached-JWS header as
   described in Section 7.3.3.

   In this example, the JOSE header contains the following parameters:

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   {
       "alg": "RS256",
       "kid": "gnap-rsa",
       "uri": "https://server.example.com/gnap",
       "htm": "POST",
       "typ": "gnap-binding+jwsd",
       "created": 1618884475
   }

   The request body, used as the JWS Payload, is the following JSON
   object:

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   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jws",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       },
       "subject": {
           "formats": ["iss_sub", "opaque"]
       }
   }

   This leads to the following full HTTP request message:

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   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/jose
   Content-Length: 1047

   eyJhbGciOiJSUzI1NiIsImNyZWF0ZWQiOjE2MTg4ODQ0NzUsImh0bSI6IlBPU1QiLCJ\
   raWQiOiJnbmFwLXJzYSIsInR5cCI6ImduYXAtYmluZGluZytqd3NkIiwidXJpIjoiaH\
   R0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20vZ25hcCJ9.CnsKICAgICJhY2Nlc3NfdG9r\
   ZW4iOiB7CiAgICAgICAgImFjY2VzcyI6IFsKICAgICAgICAgICAgImRvbHBoaW4tbWV\
   0YWRhdGEiCiAgICAgICAgXQogICAgfSwKICAgICJpbnRlcmFjdCI6IHsKICAgICAgIC\
   Aic3RhcnQiOiBbInJlZGlyZWN0Il0sCiAgICAgICAgImZpbmlzaCI6IHsKICAgICAgI\
   CAgICAgIm1ldGhvZCI6ICJyZWRpcmVjdCIsCiAgICAgICAgICAgICJ1cmkiOiAiaHR0\
   cHM6Ly9jbGllbnQuZm9vL2NhbGxiYWNrIiwKICAgICAgICAgICAgIm5vbmNlIjogIlZ\
   KTE82QTRDQVlMQlhIVFIwS1JPIgogICAgICAgIH0KICAgIH0sCiAgICAiY2xpZW50Ij\
   ogewogICAgICAicHJvb2YiOiAiandzIiwKICAgICAgImtleSI6IHsKICAgICAgICAia\
   ndrIjogewogICAgICAgICAgICAia2lkIjogImduYXAtcnNhIiwKICAgICAgICAgICAg\
   Imt0eSI6ICJSU0EiLAogICAgICAgICAgICAiZSI6ICJBUUFCIiwKICAgICAgICAgICA\
   gImFsZyI6ICJSUzI1NiIsCiAgICAgICAgICAgICJuIjogImhZT0otWE9LSVNkTU1TaG\
   5fRzRXOW0yMG1UMFZXdFFCc21CQmtJMmNtUnQ0QWk4QmZZZEhzRnpBdFlLT2pwQlIxU\
   nBLcEptVkt4SUdOeTBnNlozYWQyWFlzaDhLb3dseVZ5OElrWjhOTXdTcmNVSUJaR1lY\
   akhwd2p6dmZHdlhIXzVLSmxuUjNfdVJVcDRaNFVqazJiQ2FLZWdEbjExVjJ2eEU0MWh\
   xYVBVbmhSWnhlMGpSRVRkZHpzRTNtdTFTSzhkVENST2p3VWwxNG1VTm84aVRyVG00bj\
   BxRGFkejhCa1BvLXV2NEJDMGJ1blMwSzNiQV8zVWdWcDd6QmxRRm9GbkxUTzJ1V3Bfb\
   XVMRVdHbDY3Z0JxOU1PM2JyS1hmR2hpM2tPenl3endQVHVxLWNWUUR5RU43YUwwU3hD\
   YjNIYzRJZHFEYU1nOHFIVXlPYnBQaXREUSIKICAgICAgICB9CiAgICAgIH0KICAgICA\
   gImRpc3BsYXkiOiB7CiAgICAgICAgIm5hbWUiOiAiTXkgQ2xpZW50IERpc3BsYXkgTm\
   FtZSIsCiAgICAgICAgInVyaSI6ICJodHRwczovL2NsaWVudC5mb28vIgogICAgICB9L\
   AogICAgfSwKICAgICJzdWJqZWN0IjogewogICAgICAgICJmb3JtYXRzIjogWyJpc3Nf\
   c3ViIiwgIm9wYXF1ZSJdCiAgICB9Cn0K.MwNoVMQp5hVxI0mCs9LlOUdFtkDXaA1_eT\
   vOXq7DOGrtDKH7q4vP2xUq3fH2jRAZqnobo0WdPP3eM3NH5QUjW8pa6_QpwdIWkK7r-\
   u_52puE0lPBp7J4U2w4l9gIbg8iknsmWmXeY5F6wiGT8ptfuEYGgmloAJd9LIeNvD3U\
   LW2h2dz1Pn2eDnbyvgB0Ugae0BoZB4f69fKWj8Z9wvTIjk1LZJN1PcL7_zT8Lrlic9a\
   PyzT7Q9ovkd1s-4whE7TrnGUzFc5mgWUn_gsOpsP5mIIljoEEv-FqOW2RyNYulOZl0Q\
   8EnnDHV_vPzrHlUarbGg4YffgtwkQhdK72-JOxYQ

   When the verifier receives an attached JWS request, it MUST parse and
   validate the JWS object.  The signature MUST be validated against the
   expected key of the signer.  All required fields MUST be present and
   their values MUST be valid.  All fields MUST match the corresponding
   portions of the HTTP message.  For example, the htm field of the JWS
   header has to be the same as the HTTP verb used in the request.

   Note that this proof method depends on a specific cryptographic
   algorithm, SHA-256, in two ways: the ath hash algorithm is hardcoded,
   and computing the payload of the detached/attached signature also
   uses a hardcoded hash.  A future version of this document may address

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   crypto-agility for both these uses by replacing ath with a new header
   that upgrades the algorithm, and possibly defining a new header that
   indicates the HTTP content's hash method.

7.3.4.1.  Key Rotation using Attached JWS

   When rotating a key using Attached JWS, the message, which includes
   the new public key value or reference, is first signed with the old
   key using a JWS object with typ header value ”gnap-binding-
   rotation+jwsd”. The value of the JWS object is then taken as the
   payload of a new JWS object, to be signed by the new key.

8.  Resource Access Rights

   GNAP provides a rich structure for describing the protected resources
   hosted by RSs and accessed by client software.  This structure is
   used when the client instance requests an access token (Section 2.1)
   and when an access token is returned (Section 3.2).

   The root of this structure is a JSON array.  The elements of the JSON
   array represent rights of access that are associated with the the
   access token.  Individual rights of access can be defined by the RS
   as either an object or a string.  The resulting access is the union
   of all elements within the array.

   The access associated with the access token is described using
   objects that each contain multiple dimensions of access.  Each object
   contains a REQUIRED type property that determines the type of API
   that the token is used for.

   type (string):  The type of resource request as a string.  This field
      MAY define which other fields are allowed in the request object.
      REQUIRED.

   The value of the type field is under the control of the AS.  This
   field MUST be compared using an exact byte match of the string value
   against known types by the AS.  The AS MUST ensure that there is no
   collision between different authorization data types that it
   supports.  The AS MUST NOT do any collation or normalization of data
   types during comparison.  It is RECOMMENDED that designers of
   general-purpose APIs use a URI for this field to avoid collisions
   between multiple API types protected by a single AS.

   While it is expected that many APIs will have their own properties, a
   set of common properties are defined here.  Specific API
   implementations SHOULD re-use these fields with the same semantics
   and syntax.  The available values for these properties are determined
   by the API being protected at the RS.  This specification does not

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   require the use of any of these common fields by an API definition,
   but instead provides them as reusable generic components for API
   designers to make use of.  The allowable values of all fields are
   determined by the API being protected, as defined by a particular
   type value.

   actions (array of strings):  The types of actions the client instance
      will take at the RS as an array of strings.  For example, a client
      instance asking for a combination of "read" and "write" access.

   locations (array of strings):  The location of the RS as an array of
      strings.  These strings are typically URIs identifying the
      location of the RS.

   datatypes (array of strings):  The kinds of data available to the
      client instance at the RS's API as an array of strings.  For
      example, a client instance asking for access to raw "image" data
      and "metadata" at a photograph API.

   identifier (string):  A string identifier indicating a specific
      resource at the RS.  For example, a patient identifier for a
      medical API or a bank account number for a financial API.

   privileges (array of strings):  The types or levels of privilege
      being requested at the resource.  For example, a client instance
      asking for administrative level access, or access when the
      resource owner is no longer online.

   The following non-normative example is describing three kinds of
   access (read, write, delete) to each of two different locations and
   two different data types (metadata, images) for a single access token
   using the fictitious photo-api type definition.

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   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write",
               "delete"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ]
       }
   ]

   The access requested for each object in the array is the cross-
   product of all fields of the object.  That is to say, the object
   represents a request for all actions listed to be used at all
   locations listed for all possible datatypes listed within the object.
   Assuming the request above was granted, the client instance could
   assume that it would be able to do a read action against the images
   on the first server as well as a delete action on the metadata of the
   second server, or any other combination of these fields, using the
   same access token.

   To request a different combination of access, such as requesting one
   of the possible actions against one of the possible locations and a
   different choice of possible actions against a different one of the
   possible locations, the client instance can include multiple separate
   objects in the resources array.  The total access rights for the
   resulting access token is the union of all objects.  The following
   non-normative example uses the same fictitious photo-api type
   definition to request a single access token with more specifically
   targeted access rights by using two discrete objects within the
   request.

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   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read"
           ],
           "locations": [
               "https://server.example.net/"
           ],
           "datatypes": [
               "images"
           ]
       },
       {
           "type": "photo-api",
           "actions": [
               "write",
               "delete"
           ],
           "locations": [
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata"
           ]
       }
   ]

   The access requested here is for read access to images on one server
   while simultaneously requesting write and delete access for metadata
   on a different server, but importantly without requesting write or
   delete access to images on the first server.

   It is anticipated that API designers will use a combination of common
   fields defined in this specification as well as fields specific to
   the API itself.  The following non-normative example shows the use of
   both common and API-specific fields as part of two different
   fictitious API type values.  The first access request includes the
   actions, locations, and datatypes fields specified here as well as
   the API-specific geolocation field.  The second access request
   includes the actions and identifier fields specified here as well as
   the API-specific currency field.

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   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ],
           "geolocation": [
               { lat: -32.364, lng: 153.207 },
               { lat: -35.364, lng: 158.207 }
           ]
       },
       {
           "type": "financial-transaction",
           "actions": [
               "withdraw"
           ],
           "identifier": "account-14-32-32-3",
           "currency": "USD"
       }
   ]

   If this request is approved, the resulting access token's access
   rights will be the union of the requested types of access for each of
   the two APIs, just as above.

8.1.  Requesting Resources By Reference

   Instead of sending an object describing the requested resource
   (Section 8), access rights MAY be communicated as a string known to
   the AS representing the access being requested.  Just like access
   rights communicated as an object, access rights communicated as
   reference strings indicate a specific access at a protected resource.
   In the following non-normative example, three distinct resource
   access rights are being requested.

   "access": [
       "read", "dolphin-metadata", "some other thing"
   ]

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   This value is opaque to the client instance and MAY be any valid JSON
   string, and therefore could include spaces, unicode characters, and
   properly escaped string sequences.  However, in some situations the
   value is intended to be seen and understood by the client software's
   developer.  In such cases, the API designer choosing any such human-
   readable strings SHOULD take steps to ensure the string values are
   not easily confused by a developer, such as by limiting the strings
   to easily disambiguated characters.

   This functionality is similar in practice to OAuth 2.0's scope
   parameter [RFC6749], where a single string represents the set of
   access rights requested by the client instance.  As such, the
   reference string could contain any valid OAuth 2.0 scope value as in
   Appendix C.5.  Note that the reference string here is not bound to
   the same character restrictions as in OAuth 2.0's scope definition.

   A single access array MAY include both object-type and string-type
   resource items.  In this non-normative example, the client instance
   is requesting access to a photo-api and financial-transaction API
   type as well as the reference values of read, dolphin-metadata, and
   some other thing.

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   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write",
               "delete"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ]
       },
       "read",
       "dolphin-metadata",
       {
           "type": "financial-transaction",
           "actions": [
               "withdraw"
           ],
           "identifier": "account-14-32-32-3",
           "currency": "USD"
       },
       "some other thing"
   ]

   The requested access is the union of all elements of the array,
   including both objects and reference strings.

   In order to facilitate the use of both object and reference strings
   to access the same kind of APIs, the API designer can define a clear
   mapping between these forms.  One possible approach for choosing
   reference string values is to use the same value as the type
   parameter from the fully-specified object, with the API defining a
   set of default behaviors in this case.  For example, an API
   definition could declare the following string:

   "access": [
       "photo-api"
   ]

   As being equivalent to the following fully-defined object:

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   "access": [
       {
           "type": "photo-api",
           "actions": [ "read", "write", "delete" ],
           "datatypes": [ "metadata", "image" ]
       }
   ]

   The exact mechanisms for relating reference strings is up to the API
   designer.  These are enforced by the AS, and the details are out of
   scope for this specification.

9.  Discovery

   By design, GNAP minimizes the need for any pre-flight discovery.  To
   begin a request, the client instance only needs to know the grant
   endpoint of the AS (a single URI) and which keys it will use to sign
   the request.  Everything else can be negotiated dynamically in the
   course of the protocol.

   However, the AS can have limits on its allowed functionality.  If the
   client instance wants to optimize its calls to the AS before making a
   request, it MAY send an HTTP OPTIONS request to the grant request
   endpoint to retrieve the server's discovery information.  The AS MUST
   respond with a JSON document with Content-Type application/json
   containing a single object with the following fields:

   grant_request_endpoint (string):  The location of the AS's grant
      request endpoint.  The location MUST be an absolute URL [RFC3986]
      with a scheme component (which MUST be "https"), a host component,
      and optionally, port, path and query components and no fragment
      components.  This URL MUST match the URL the client instance used
      to make the discovery request.  REQUIRED.

   interaction_start_modes_supported (array of strings):  A list of the
      AS's interaction start methods.  The values of this list
      correspond to the possible values for the interaction start
      section (Section 2.5.1) of the request and MUST be values from the
      Interaction Start Modes Registry (Section 11.7).  OPTIONAL.

   interaction_finish_methods_supported (array of strings):  A list of
      the AS's interaction finish methods.  The values of this list
      correspond to the possible values for the method element of the
      interaction finish section (Section 2.5.2) of the request and MUST
      be values from the Interaction Finish Methods Registry
      (Section 11.8).  OPTIONAL.

   key_proofs_supported (array of strings):  A list of the AS's

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      supported key proofing mechanisms.  The values of this list
      correspond to possible values of the proof field of the key
      section (Section 7.1) of the request and MUST be values from the
      Key Proofing Methods Registry (Section 11.14).  OPTIONAL.

   sub_id_formats_supported (array of strings):  A list of the AS's
      supported subject identifier formats.  The values of this list
      correspond to possible values of the subject identifier section
      (Section 2.2) of the request and MUST be values from the Subject
      Identifier Formats Registry established by
      [I-D.ietf-secevent-subject-identifiers].  OPTIONAL.

   assertion_formats_supported (array of strings):  A list of the AS's
      supported assertion formats.  The values of this list correspond
      to possible values of the subject assertion section (Section 2.2)
      of the request and MUST be values from the Assertion Formats
      Registry (Section 11.4).  OPTIONAL.

   key_rotation_supported (boolean):  The boolean "true" indicates that
      rotation of access token bound keys by the client (Section 6.1.1)
      is supported by the AS.  The absence of this field or a boolean
      "false" value indicates that this feature is not supported.

   The information returned from this method is for optimization
   purposes only.  The AS MAY deny any request, or any portion of a
   request, even if it lists a capability as supported.  For example, a
   given client instance can be registered with the mtls key proofing
   mechanism, but the AS also returns other proofing methods from the
   discovery document, then the AS will still deny a request from that
   client instance using a different proofing mechanism.  Similarly, an
   AS with key_rotation_supported set to "true" can still deny any
   request for rotating any access token's key for a variety of reasons.

   Additional fields can be defined the Authorization Server Discovery
   Fields Registry (Section 11.16).

9.1.  RS-first Method of AS Discovery

   If the client instance calls an RS without an access token, or with
   an invalid access token, the RS SHOULD be explicit about the fact
   that GNAP needs to be used to access the resource, by responding with
   the WWW-Authenticate header field and a GNAP challenge.

   In some situations, the client instance might want to know with which
   specific AS it needs to negotiate for access to that RS.  The RS MAY
   additionally return the address of the GNAP endpoint in the as_uri
   parameter, a referrer parameter to indicate which RS initiated the
   discovery process, and an opaque access reference.  The client

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   instance SHOULD then use both the referrer and access parameters in
   its access token request.  The referrer parameter MUST be the URI of
   the RS, and the client instance MUST check its value to protect
   itself.  The opaque access reference MUST be sufficient for at least
   the action the client instance was attempting to take at the RS and
   MAY be more powerful.

   The means for the RS to determine the value for the access reference
   are out of scope of this specification, but some dynamic methods are
   discussed in [I-D.ietf-gnap-resource-servers].

   When receiving the following response from the RS:

   NOTE: '\' line wrapping per RFC 8792

   WWW-Authenticate: \
     GNAP as_uri=https://as.example/tx\
     ;access=FWWIKYBQ6U56NL1\
     ;referrer=https://rs.example

   The client instance then makes a request to the as_uri as described
   in Section 2, with the value of referrer passed as an HTTP Referer
   header field and the access reference passed unchanged into the
   access array in the access_token portion of the request.  The client
   instance MAY request additional resources and other information.

   In this non-normative example, the client instance is requesting a
   single access token using the opaque access reference FWWIKYBQ6U56NL1
   received from the RS in addition to the dolphin-metadata that the
   client instance has been configured with out of band.

   POST /tx HTTP/1.1
   Host: as.example
   Referer: https://rs.example/resource
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "FWWIKYBQ6U56NL1",
               "dolphin-metadata"
           ]
       },
       "client": "KHRS6X63AJ7C7C4AZ9AO"
   }

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   The client instance includes the Referer header field as a way for
   the AS to know that the process is initiated through a discovery
   process at the RS.

   If issued, the resulting access token would contain sufficient access
   to be used at both referenced resources.

   Security considerations, especially related to the potential of a
   compromised RS (Section 13.36) redirecting the requests of an
   otherwise properly authenticated client, need to be carefully
   considered when allowing such a discovery process.  This risk can be
   mitigated by an alternative pre-registration process so that the
   client knows which AS protects which RS.  There are also privacy
   considerations related to revealing which AS is protecting a given
   resource, discussed in Section 14.4.1.

9.2.  Dynamic grant endpoint discovery

   Additional methods of discovering the appropriate grant endpoint for
   a given application are outside the scope of this specification.
   This limitation is intentional, as many applications rely on static
   configuration between the client instance and AS, as is common in
   OAuth 2.0.  However, the dynamic nature of GNAP makes it a prime
   candidate for other extensions defining methods for discovery of the
   appropriate AS grant endpoint at runtime.  Advanced use cases could
   define contextual methods for contextually providing this endpoint to
   the client instance securely.  Furthermore, GNAP's design
   intentionally requires the client instance to only know the grant
   endpoint and not additional parameters, since other functions and
   values can be disclosed and negotiated during the grant process.

10.  Acknowledgements

   The editors would like to thank the feedback of the following
   individuals for their reviews, implementations, and contributions:
   Åke Axeland, Aaron Parecki, Adam Omar Oueidat, Andrii Deinega,
   Annabelle Backman, Dick Hardt, Dmitri Zagidulin, Dmitry Barinov,
   Fabien Imbault, Florian Helmschmidt, Francis Pouatcha, George
   Fletcher, Haardik Haardik, Hamid Massaoud, Jacky Yuan, Joseph Heenan,
   Justin Richer, Kathleen Moriarty, Leif Johansson, Mike Jones, Mike
   Varley, Nat Sakimura, Takahiko Kawasaki, Takahiro Tsuchiya, Yaron
   Sheffer.

   The editors would also like to thank the GNAP working group design
   team of Kathleen Moriarty, Fabien Imbault, Dick Hardt, Mike Jones,
   and Justin Richer, who incorporated elements from the XAuth and XYZ
   proposals to create the first version of this document.

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   In addition, the editors would like to thank Aaron Parecki and Mike
   Jones for insights into how to integrate identity and authentication
   systems into the core protocol, and Justin Richer and Dick Hardt for
   the use cases, diagrams, and insights provided in the XYZ and XAuth
   proposals that have been incorporated here.  The editors would like
   to especially thank Mike Varley and the team at SecureKey for
   feedback and development of early versions of the XYZ protocol that
   fed into this standards work.

   Finally, the editors want to acknowledge the immense contributions of
   Aaron Parecki to the content of this document.  We thank him for his
   insight, input, and hard work, without which GNAP would not have
   grown to what it is.

11.  IANA Considerations

   IANA is requested to create 16 registries for the Grant Negotiation
   and Authorization Protocol and to populate those registries with
   initial values as described in this section.

   All use of value typing is based on [RFC8259] data types and MUST be
   one of the following: number, object, string, boolean, or array.
   When the type is array, the contents of the array MUST be specified,
   as in "array of objects" when one subtype is allowed or "array of
   strings/objects" when multiple simultaneous subtypes are allowed.
   When the type is object, the structure of the object MUST be
   specified in the definition.  If a parameter is available in
   different types, each type SHOULD be registered separately.

   General guidance for extension parameters is found in Appendix E.

11.1.  Grant Request Parameters

   This document defines a GNAP grant request, for which IANA is asked
   to create and maintain a new registry titled "Grant Request
   Parameters".  Initial values for this registry are given in
   Section 11.1.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

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   The Designated Expert (DE) is expected to ensure that all
   registrations follow the template presented in Section 11.1.1.  The
   DE is expected to ensure that the request parameter's definition is
   sufficiently orthogonal to existing functionality provided by
   existing parameters.  The DE is expected to ensure that registrations
   for the same name with different types are sufficiently close in
   functionality so as not to cause confusion for developers.  The DE is
   expected to ensure that the request parameter's definition specifies
   the expected behavior of the AS in response to the request parameter
   for each potential state of the grant request.

11.1.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.1.2.  Initial Contents

      +==============+==================+===========================+
      | Name         | Type             | Specification document(s) |
      +==============+==================+===========================+
      | access_token | object           | Section 2.1.1 of RFC nnnn |
      +--------------+------------------+---------------------------+
      | access_token | array of objects | Section 2.1.2 of RFC nnnn |
      +--------------+------------------+---------------------------+
      | subject      | object           | Section 2.2 of RFC nnnn   |
      +--------------+------------------+---------------------------+
      | client       | object           | Section 2.3 of RFC nnnn   |
      +--------------+------------------+---------------------------+
      | client       | string           | Section 2.3.1 of RFC nnnn |
      +--------------+------------------+---------------------------+
      | user         | object           | Section 2.4 of RFC nnnn   |
      +--------------+------------------+---------------------------+
      | user         | string           | Section 2.4.1 of RFC nnnn |
      +--------------+------------------+---------------------------+
      | interact     | object           | Section 2.5 of RFC nnnn   |
      +--------------+------------------+---------------------------+
      | interact_ref | string           | Section 5.1 of RFC nnnn   |
      +--------------+------------------+---------------------------+

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                                  Table 1

11.2.  Access Token Flags

   This document defines a GNAP access token flags, for which IANA is
   asked to create and maintain a new registry titled "Access Token
   Flags".  Initial values for this registry are given in
   Section 11.2.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.2.1.  The DE is expected to ensure
   that the flag specifies whether it applies to requests for tokens to
   the AS, responses with tokens from the AS, or both.

11.2.1.  Registration Template

   Name:
      An identifier for the parameter.

   Allowed Use:
      Where the flag is allowed to occur.  Possible values are
      "Request", "Response", and "Request, Response".

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.2.2.  Initial Contents

        +=========+===================+===========================+
        | Name    | Allowed Use       | Specification document(s) |
        +=========+===================+===========================+
        | bearer  | Request, Response | Section 2.1.1 and         |
        |         |                   | Section 3.2.1 of RFC nnnn |
        +---------+-------------------+---------------------------+
        | durable | Response          | Section 3.2.1 of RFC nnnn |
        +---------+-------------------+---------------------------+

                                  Table 2

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11.3.  Subject Information Request Fields

   This document defines a means to request subject information from the
   AS to the client instance, for which IANA is asked to create and
   maintain a new registry titled "Subject Information Request Fields".
   Initial values for this registry are given in Section 11.3.2.  Future
   assignments and modifications to existing assignment are to be made
   through the Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.3.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.

11.3.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.3.2.  Initial Contents

   +===================+==================+===========================+
   | Name              | Type             | Specification document(s) |
   +===================+==================+===========================+
   | sub_id_formats    | array of strings | Section 2.2 of RFC nnnn   |
   +-------------------+------------------+---------------------------+
   | assertion_formats | array of strings | Section 2.2 of RFC nnnn   |
   +-------------------+------------------+---------------------------+
   | sub_ids           | array of objects | Section 2.2 of RFC nnnn   |
   +-------------------+------------------+---------------------------+

                                 Table 3

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11.4.  Assertion Formats

   This document defines a means to pass identity assertions between the
   AS and client instance, for which IANA is asked to create and
   maintain a new registry titled "Assertion Formats".  Initial values
   for this registry are given in Section 11.4.2.  Future assignments
   and modifications to existing assignment are to be made through the
   Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.4.1.  The DE is expected to ensure
   that the definition specifies the serialization format of the
   assertion value as used within GNAP.

11.4.1.  Registration Template

   Name:
      An identifier for the assertion format.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.4.2.  Initial Contents

          +==========+=========================================+
          | Name     | Specification document(s)               |
          +==========+=========================================+
          | id_token | Section 2.2 and Section 3.4 of RFC nnnn |
          +----------+-----------------------------------------+
          | saml2    | Section 2.2 and Section 3.4 of RFC nnnn |
          +----------+-----------------------------------------+

                                 Table 4

11.5.  Client Instance Fields

   This document defines a means to send information about the client
   instance, for which IANA is asked to create and maintain a new
   registry titled "Client Instance Fields".  Initial values for this
   registry are given in Section 11.5.2.  Future assignments and
   modifications to existing assignment are to be made through the
   Expert Review registration policy [RFC8126].

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   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.5.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.

11.5.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.5.2.  Initial Contents

             +==========+========+===========================+
             | Name     | Type   | Specification document(s) |
             +==========+========+===========================+
             | key      | object | Section 7.1 of RFC nnnn   |
             +----------+--------+---------------------------+
             | key      | string | Section 7.1.1 of RFC nnnn |
             +----------+--------+---------------------------+
             | class_id | string | Section 2.3 of RFC nnnn   |
             +----------+--------+---------------------------+
             | display  | object | Section 2.3.2 of RFC nnnn |
             +----------+--------+---------------------------+

                                  Table 5

11.6.  Client Instance Display Fields

   This document defines a means to send end-user facing displayable
   information about the client instance, for which IANA is asked to
   create and maintain a new registry titled "Client Instance Display
   Fields".  Initial values for this registry are given in
   Section 11.6.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

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   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.6.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.

11.6.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.6.2.  Initial Contents

             +==========+========+===========================+
             | Name     | Type   | Specification document(s) |
             +==========+========+===========================+
             | name     | string | Section 2.3.2 of RFC nnnn |
             +----------+--------+---------------------------+
             | uri      | string | Section 2.3.2 of RFC nnnn |
             +----------+--------+---------------------------+
             | logo_uri | string | Section 2.3.2 of RFC nnnn |
             +----------+--------+---------------------------+

                                  Table 6

11.7.  Interaction Start Modes

   This document defines a means for the client instance to begin
   interaction between the end-user and the AS, for which IANA is asked
   to create and maintain a new registry titled "Interaction Start
   Modes".  Initial values for this registry are given in
   Section 11.7.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.7.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for

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   developers.  The DE is expected to ensure that any registration using
   an "object" type declares all additional parameters, their
   optionality, and purpose.  The DE is expected to ensure that all
   start modes clearly define what actions the client is expected to
   take to begin interaction, what the expected user experience is, and
   any security considerations for this communication from either party.
   The DE is expected to ensure that all start modes document
   incompatibilities with other start modes or finish methods, if
   applicable.

11.7.1.  Registration Template

   Mode:
      An identifier for the interaction start mode.

   Type:
      The JSON type for the value, either "string" or "object", as
      described in Section 2.5.1.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.7.2.  Initial Contents

         +===============+========+=============================+
         | Mode          | Type   | Specification document(s)   |
         +===============+========+=============================+
         | redirect      | string | Section 2.5.1.1 of RFC nnnn |
         +---------------+--------+-----------------------------+
         | app           | string | Section 2.5.1.2 of RFC nnnn |
         +---------------+--------+-----------------------------+
         | user_code     | string | Section 2.5.1.3 of RFC nnnn |
         +---------------+--------+-----------------------------+
         | user_code_uri | string | Section 2.5.1.4 of RFC nnnn |
         +---------------+--------+-----------------------------+

                                 Table 7

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11.8.  Interaction Finish Methods

   This document defines a means for the client instance to be notified
   of the end of interaction between the end-user and the AS, for which
   IANA is asked to create and maintain a new registry titled
   "Interaction Finish Methods".  Initial values for this registry are
   given in Section 11.8.2.  Future assignments and modifications to
   existing assignment are to be made through the Expert Review
   registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.8.1.  The DE is expected to ensure
   that all finish methods clearly define what actions the AS is
   expected to take, what listening methods the client instance needs to
   enable, and any security considerations for this communication from
   either party.  The DE is expected to ensure that all finish methods
   document incompatibilities with any start modes, if applicable.

11.8.1.  Registration Template

   Method:
      An identifier for the interaction finish method.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.8.2.  Initial Contents

                +==========+=============================+
                | Mode     | Specification document(s)   |
                +==========+=============================+
                | redirect | Section 2.5.2.1 of RFC nnnn |
                +----------+-----------------------------+
                | push     | Section 2.5.2.2 of RFC nnnn |
                +----------+-----------------------------+

                                 Table 8

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11.9.  Interaction Hints

   This document defines a set of hints that a client instance can
   provide to the AS to facilitate interaction with the end user, for
   which IANA is asked to create and maintain a new registry titled
   "Interaction Hints".  Initial values for this registry are given in
   Section 11.9.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.9.1.  The DE is expected to ensure
   that all interaction hints clearly document the expected behaviors of
   the AS in response to the hint, and that an AS not processing the
   hint does not impede the operation of the AS or client instance.

11.9.1.  Registration Template

   Name:
      An identifier for the parameter.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.9.2.  Initial Contents

                +============+===========================+
                | Mode       | Specification document(s) |
                +============+===========================+
                | ui_locales | Section 2.5.3 of RFC nnnn |
                +------------+---------------------------+

                                 Table 9

11.10.  Grant Response Parameters

   This document defines a GNAP grant response, for which IANA is asked
   to create and maintain a new registry titled "Grant Response
   Parameters".  Initial values for this registry are given in
   Section 11.10.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

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   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.10.1.  The DE is expected to ensure
   that the response parameter's definition is sufficiently orthogonal
   to existing functionality provided by existing parameters.  The DE is
   expected to ensure that registrations for the same name with
   different types are sufficiently close in functionality so as not to
   cause confusion for developers.  The DE is expected to ensure that
   the response parameter's definition specifies grant states for which
   the client instance can expect this parameter to appear in a response
   message.

11.10.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.10.2.  Initial Contents

      +=============+==================+===========================+
      | Name        | Type             | Specification document(s) |
      +=============+==================+===========================+
      | continue    | object           | Section 3.1 of RFC nnnn   |
      +-------------+------------------+---------------------------+
      | acces_token | object           | Section 3.2.1 of RFC nnnn |
      +-------------+------------------+---------------------------+
      | acces_token | array of objects | Section 3.2.2 of RFC nnnn |
      +-------------+------------------+---------------------------+
      | interact    | object           | Section 3.3 of RFC nnnn   |
      +-------------+------------------+---------------------------+
      | subject     | object           | Section 3.4 of RFC nnnn   |
      +-------------+------------------+---------------------------+
      | instance_id | string           | Section 3.5 of RFC nnnn   |
      +-------------+------------------+---------------------------+
      | error       | object           | Section 3.6 of RFC nnnn   |
      +-------------+------------------+---------------------------+

                                 Table 10

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11.11.  Interaction Mode Responses

   This document defines a means for the AS to provide to the client
   instance information that is required to complete a particular
   interaction mode, for which IANA is asked to create and maintain a
   new registry titled "Interaction Mode Responses".  Initial values for
   this registry are given in Section 11.11.2.  Future assignments and
   modifications to existing assignment are to be made through the
   Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.11.1.  If the name of the
   registration matches the name of an interaction start mode, the DE is
   expected to ensure that the response parameter is unambiguously
   associated with the interaction start mode of the same name.

11.11.1.  Registration Template

   Name:
      An identifier for the parameter.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.11.2.  Initial Contents

               +===============+===========================+
               | Name          | Specification document(s) |
               +===============+===========================+
               | redirect      | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+
               | app           | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+
               | user_code     | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+
               | user_code_uri | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+
               | finish        | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+
               | expires_in    | Section 3.3 of RFC nnnn   |
               +---------------+---------------------------+

                                  Table 11

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11.12.  Subject Information Response Fields

   This document defines a means to return subject information from the
   AS to the client instance, for which IANA is asked to create and
   maintain a new registry titled "Subject Information Response Fields".
   Initial values for this registry are given in Section 11.12.2.
   Future assignments and modifications to existing assignment are to be
   made through the Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.12.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.

11.12.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.12.2.  Initial Contents

       +============+==================+===========================+
       | Name       | Type             | Specification document(s) |
       +============+==================+===========================+
       | sub_ids    | array of objects | Section 3.4 of RFC nnnn   |
       +------------+------------------+---------------------------+
       | assertions | array of objects | Section 3.4 of RFC nnnn   |
       +------------+------------------+---------------------------+
       | updated_at | string           | Section 3.4 of RFC nnnn   |
       +------------+------------------+---------------------------+

                                  Table 12

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11.13.  Error Codes

   This document defines a set of errors that the AS can return to the
   client instance, for which IANA is asked to create and maintain a new
   registry titled "Error Codes".  Initial values for this registry are
   given in Section 11.13.2.  Future assignments and modifications to
   existing assignment are to be made through the Expert Review
   registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.13.1.  The DE is expected to ensure
   that the error response is sufficiently unique from other errors to
   provide actionable information to the client instance.  The DE is
   expected to ensure that the definition of the error response
   specifies all conditions in which the error response is returned, and
   what the client instance's expected action is.

11.13.1.  Registration Template

   Error:
      A unique string code for the error.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

11.13.2.  Initial Contents

        +============================+===========================+
        | Error                      | Specification document(s) |
        +============================+===========================+
        | invalid_request            | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | invalid_client             | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | invalid_interaction        | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | invalid_flag               | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | invalid_rotation           | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | key_rotation_not_supported | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | invalid_continuation       | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | user_denied                | Section 3.6 of RFC nnnn   |

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        +----------------------------+---------------------------+
        | request_denied             | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | unknown_interaction        | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | too_fast                   | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+
        | too_many_attempts          | Section 3.6 of RFC nnnn   |
        +----------------------------+---------------------------+

                                 Table 13

11.14.  Key Proofing Methods

   This document defines methods that the client instance can use to
   prove possession of a key, for which IANA is asked to create and
   maintain a new registry titled "Key Proofing Methods".  Initial
   values for this registry are given in Section 11.14.2.  Future
   assignments and modifications to existing assignment are to be made
   through the Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.14.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.  The DE is expected to ensure that the proofing method
   provides sufficient coverage of and binding to the protocol messages
   to which it is applied.

11.14.1.  Registration Template

   Method:
      A unique string code for the key proofing method.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

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11.14.2.  Initial Contents

             +=========+========+===========================+
             | Method  | Type   | Specification document(s) |
             +=========+========+===========================+
             | httpsig | string | Section 7.3.1 of RFC nnnn |
             +---------+--------+---------------------------+
             | httpsig | object | Section 7.3.1 of RFC nnnn |
             +---------+--------+---------------------------+
             | mtls    | string | Section 7.3.2 of RFC nnnn |
             +---------+--------+---------------------------+
             | jwsd    | string | Section 7.3.3 of RFC nnnn |
             +---------+--------+---------------------------+
             | jws     | string | Section 7.3.4 of RFC nnnn |
             +---------+--------+---------------------------+

                                 Table 14

11.15.  Key Formats

   This document defines formats for a public key value, for which IANA
   is asked to create and maintain a new registry titled "Key Formats".
   Initial values for this registry are given in Section 11.15.2.
   Future assignments and modifications to existing assignment are to be
   made through the Expert Review registration policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.15.1.  The DE is expected to ensure
   the key format specifies the structure and serialization of the key
   material.

11.15.1.  Registration Template

   Format:
      A unique string code for the key format.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

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11.15.2.  Initial Contents

                 +===========+===========================+
                 | Format    | Specification document(s) |
                 +===========+===========================+
                 | jwk       | Section 7.1 of RFC nnnn   |
                 +-----------+---------------------------+
                 | cert      | Section 7.1 of RFC nnnn   |
                 +-----------+---------------------------+
                 | cert#S256 | Section 7.1 of RFC nnnn   |
                 +-----------+---------------------------+

                                  Table 15

11.16.  Authorization Server Discovery Fields

   This document defines a discovery document for an AS, for which IANA
   is asked to create and maintain a new registry titled "Authorization
   Server Discovery Fields".  Initial values for this registry are given
   in Section 11.16.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126].

   The DE is expected to ensure that all registrations follow the
   template presented in Section 11.16.1.  The DE is expected to ensure
   that registrations for the same name with different types are
   sufficiently close in functionality so as not to cause confusion for
   developers.  The DE is expected to ensure that the values in the
   discovery document are sufficient to provide optimization and hints
   to the client instance, but that knowledge of the discovered value is
   not required for starting a transaction with the AS.

11.16.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Specification document(s):
      Reference to the document(s) that specify the value, preferably
      including a URI that can be used to retrieve a copy of the
      document(s).  An indication of the relevant sections may also be
      included but is not required.

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11.16.2.  Initial Contents

    +======================================+==========+===============+
    | Name                                 | Type     | Specification |
    |                                      |          | document(s)   |
    +======================================+==========+===============+
    | grant_request_endpoint               | string   | Section 9 of  |
    |                                      |          | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | interaction_start_modes_supported    | array of | Section 9 of  |
    |                                      | strings  | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | interaction_finish_methods_supported | array of | Section 9 of  |
    |                                      | strings  | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | key_proofs_supported                 | array of | Section 9 of  |
    |                                      | strings  | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | sub_id_formats_supported             | array of | Section 9 of  |
    |                                      | strings  | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | assertion_formats_supported          | array of | Section 9 of  |
    |                                      | strings  | RFC nnnn      |
    +--------------------------------------+----------+---------------+
    | key_rotation_supported               | boolean  | Section 9 of  |
    |                                      |          | RFC nnnn      |
    +--------------------------------------+----------+---------------+

                                  Table 16

12.  Implementation Status

      Note: To be removed by RFC editor before publication.

   *GNAP Authorization Service in Rust* implementation by David Skyberg.
   https://github.com/dskyberg/gnap (https://github.com/dskyberg/gnap)
   Prototype implementation of AS and client in Rust.  MIT license.

   *GNAP JS Client* from Interop Alliance, implementation by Dmitri
   Zagidulin. https://github.com/interop-alliance/gnap-client-js
   (https://github.com/interop-alliance/gnap-client-js) Prototype
   implementation of client in JavaScript.  MIT License.

   *Rafiki* from Interledger Foundation. https://github.com/interledger/
   rafiki (https://github.com/interledger/rafiki) Production
   implementation of AS in JavaScript.  Apache 2.0 license.

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   *Sample GNAP Client in PHP* implementation by Aaron Parecki.
   https://github.com/aaronpk/gnap-client-php
   (https://github.com/aaronpk/gnap-client-php) Prototype implementation
   of web application client and CLI client in PHP, with common support
   library.  CC0 license.

   *SUNET Auth Server* from SUNET. https://github.com/SUNET/sunet-auth-
   server (https://github.com/SUNET/sunet-auth-server) Production
   implementation of AS in Python.  BSD license.

   *Verified.ME* from SecureKey. https://verified.me/
   (https://verified.me/) Production implementation of AS, client and
   RS.  Proprietary license.

   *XYZ* from Bespoke Engineering, implementation by Justin Richer.
   https://github.com/bspk/oauth.xyz-java (https://github.com/bspk/
   oauth.xyz-java) Advanced prototype implementation of AS, client, and
   RS in Java, with common support library.  Prototype implementation of
   SPA client in JavaScript.  Apache 2.0 license.

13.  Security Considerations

   In addition to the normative requirements in this document,
   implementors are strongly encouraged to consider these additional
   security considerations in implementations and deployments of GNAP.

13.1.  TLS Protection in Transit

   All requests in GNAP have to be made over TLS or equivalent as
   outlined in [BCP195] to protect the contents of the request and
   response from manipulation and interception by an attacker.  This
   includes all requests from a client instance to the AS, all requests
   from the client instance to an RS, any requests back to a client
   instance such as the push-based interaction finish method, and any
   back-end communications such as from an RS to an AS as described in
   [I-D.ietf-gnap-resource-servers].  Additionally, all requests between
   a browser and other components, such as during redirect-based
   interaction, need to be made over TLS or use equivalent protection.

   Even though requests from the client instance to the AS are signed,
   the signature method alone does not protect the request from
   interception by an attacker.  TLS protects the response as well as
   the request, preventing an attacker from intercepting requested
   information as it is returned.  This is particularly important in the
   core protocol for security artifacts such as nonces and for personal
   information such as subject information.

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   The use of key-bound access tokens does not negate the requirement
   for protecting calls to the RS with TLS.  While the keys and
   signatures associated a bound access token will prevent an attacker
   from using a stolen token, without TLS an attacker would be able to
   watch the data being sent to the RS and returned from the RS during
   legitimate use of the client instance under attack.  Additionally,
   without TLS an attacker would be able to profile the calls made
   between the client instance and RS, possibly gaining information
   about the functioning of the API between the client software and RS
   software that would be otherwise unknown to the attacker.

   TLS or equivalent protection also needs to be used between the
   browser and any other components.  This applies during initial
   redirects to an AS's components during interaction, during any
   interaction with the resource owner, and during any redirect back to
   the client instance.  Without TLS protection on these portions of the
   process, an attacker could wait for a valid request to start and then
   take over the resource owner's interaction session.

13.2.  Signing Requests from the Client Software

   Even though all requests in GNAP need to be transmitted over TLS or
   its equivalent, the use of TLS alone is not sufficient to protect all
   parts of a multi-party and multi-stage protocol like GNAP, and TLS is
   not targeted at tying multiple requests to each other over time.  To
   account for this, GNAP makes use of message-level protection and key
   presentation mechanisms that strongly associate a request with a key
   held by the client instance (see Section 7).

   During the initial request from a client instance to the AS, the
   client instance has to identify and prove possession of a
   cryptographic key.  If the key is known to the AS, such as if it is
   previously registered or dereferenceable to a trusted source, the AS
   can associate a set of policies to the client instance identified by
   the key.  Without the requirement that the client instance prove that
   it holds that key, the AS could not trust that the connection came
   from any particular client and could not apply any associated
   policies.

   Even more importantly, the client instance proving possession of a
   key on the first request allows the AS to associate future requests
   with each other by binding all future requests in that transaction to
   the same key.  The access token used for grant continuation is bound
   to the same key and proofing mechanism used by the client instance in
   its initial request, which means that the client instance needs to
   prove possession of that same key in future requests allowing the AS
   to be sure that the same client instance is executing the follow-ups
   for a given ongoing grant request.  Therefore, the AS has to ensure

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   that all subsequent requests for a grant are associated with the same
   key that started the grant, or the most recent rotation of that key.
   This need holds true even if the initial key is previously unknown to
   the AS, such as would be the case when a client instance creates an
   ephemeral key for its request.  Without this ongoing association, an
   attacker would be able to impersonate a client instance in the midst
   of a grant request, potentially stealing access tokens and subject
   information with impunity.

   Additionally, all access tokens in GNAP default to be associated with
   the key that was presented during the grant request that created the
   access token.  This association allows an RS to know that the
   presenter of the access token is the same party that the token was
   issued to, as identified by their keys.  While non-bound bearer
   tokens are an option in GNAP, these types of tokens have their own
   tradeoffs discussed in Section 13.7.

   TLS functions at the transport layer, ensuring that only the parties
   on either end of that connection can read the information passed
   along that connection.  Each time a new connection is made, such as
   for a new HTTP request, a new trust is re-established that is mostly
   unrelated to previous connections.  While modern TLS does make use of
   session resumption, this still needs to be augmented with
   authentication methods to determine the identity of parties on the
   connections.  In other words, it is not possible with TLS alone to
   know that the same party is making a set of calls over time, since
   each time a new TLS connection is established, both the client and
   the server (or the server only when using Section 7.3.2) have to
   validate the other party's identity.  Such a verification can be
   achieved via methods described in [I-D.ietf-uta-rfc6125bis], but
   these are not enough to establish the identity of the client instance
   in many cases.

   To counter this, GNAP defines a set of key binding methods in
   Section 7.3 that allow authentication and proof of possession by the
   caller, which is usually the client instance.  These methods are
   intended to be used in addition to TLS on all connections.

13.3.  Protection of Client Instance Key Material

   Client instances are identified by their unique keys, and anyone with
   access to a client instance's key material will be able to
   impersonate that client instance to all parties.  This is true for
   both calls to the AS as well as calls to an RS using an access token
   bound to the client instance's unique key.  As a consequence, it is
   of utmost importance for a client instance to protect its private key
   material.

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   Different types of client software have different methods for
   creating, managing, and registering keys.  GNAP explicitly allows for
   ephemeral clients (such as SPAs) and single-user clients (such as
   mobile applications) to create and present their own keys during the
   initial grant request without any explicit pre-registration step.
   The client software can securely generate a keypair on-device and
   present the public key, along with proof of holding the associated
   private key, to the AS as part of the initial request.  To facilitate
   trust in these ephemeral keys, GNAP further allows for an extensible
   set of client information to be passed with the request.  This
   information can include device posture and third-party attestations
   of the client software's provenance and authenticity, depending on
   the needs and capabilities of the client software and its deployment.

   From GNAP's perspective, each distinct key is a different client
   instance.  However, multiple client instances can be grouped together
   by an AS policy and treated similarly to each other.  For instance,
   if an AS knows of several different keys for different servers within
   a cluster, the AS can decide that authorization of one of these
   servers applies to all other servers within the cluster.  An AS that
   chooses to do this needs to be careful with how it groups different
   client keys together in its policy, since the breach of one instance
   would have direct effects on the others in the cluster.

   Additionally, if an end user controls multiple instances of a single
   type of client software, such as having an application installed on
   multiple devices, each of these instances is expected to have a
   separate key and be issued separate access tokens.  However, if the
   AS is able to group these separate instances together as described
   above, it can streamline the authorization process for new instances
   of the same client software.  For example, if two client instances
   can present proof of a valid installation of a piece of client
   software, the AS would be able to associate the approval of the first
   instance of this software to all related instances.  The AS could
   then choose to bypass an explicit prompt of the resource owner for
   approval during authorization, since such approval has already been
   given.  An AS doing such a process would need to take assurance
   measures that the different instances are in fact correlated and
   authentic, as well as ensuring the expected resource owner is in
   control of the client instance.

   Finally, if multiple instances of client software each have the same
   key, then from GNAP's perspective, these are functionally the same
   client instance as GNAP has no reasonable way to differentiate
   between them.  This situation could happen if multiple instances
   within a cluster can securely share secret information among
   themselves.  Even though there are multiple copies of the software,
   the shared key makes these copies all present as a single instance.

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   It is considered bad practice to share keys between copies of
   software unless they are very tightly integrated with each other and
   can be closely managed.  It is particularly bad practice to allow an
   end user to copy keys between client instances and to willingly use
   the same key in multiple instances.

13.4.  Protection of Authorization Server

   The AS performs critical functions in GNAP, including authenticating
   client software, managing interactions with end users to gather
   consent and provide notice, and issuing access tokens for client
   instances to present to resource servers.  As such, protecting the AS
   is central to any GNAP deployment.

   If an attacker is able to gain control over an AS, they would be able
   to create fraudulent tokens and manipulate registration information
   to allow for malicious clients.  These tokens and clients would be
   trusted by other components in the ecosystem under the protection of
   the AS.

   If the AS is using signed access tokens, an attacker in control of
   the AS's signing keys would be able to manufacture fraudulent tokens
   for use at RS's under the protection of the AS.

   If an attacker is able to impersonate an AS, they would be able to
   trick legitimate client instances into making signed requests for
   information which could potentially be proxied to a real AS.  To
   combat this, all communications to the AS need to be made over TLS or
   its equivalent, and the software making the connection has to
   validate the certificate chain of the host it is connecting to.

   Consequently, protecting, monitoring, and auditing the AS is
   paramount to preserving the security of a GNAP-protected ecosystem.
   The AS presents attackers with a valuable target for attack.
   Fortunately, the core focus and function of the AS is to provide
   security for the ecosystem, unlike the RS whose focus is to provide
   an API or the client software whose focus is to access the API.

13.5.  Symmetric and Asymmetric Client Instance Keys

   Many of the cryptographic methods used by GNAP for key-proofing can
   support both asymmetric and symmetric cryptography, and can be
   extended to use a wide variety of mechanisms.  Implementers will find
   useful the available guidelines on cryptographic key management
   provided in [RFC4107].  While symmetric cryptographic systems have
   some benefits in speed and simplicity, they have a distinct drawback
   that both parties need access to the same key in order to do both
   signing and verification of the message.  This means that when the

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   client instance calls the AS to request a token, the AS needs to know
   the exact value of the client instance's key (or be able to derive
   it) in order to validate the key proof signature.  With asymmetric
   keys, the client needs only to send its public key to the AS to allow
   for verification that the client holds the associated private key,
   regardless of whether that key was pre-registered or not with the AS.

   Symmetric keys also have the expected advantage of providing better
   protection against quantum threats in the future.  Also, these types
   of keys (and their secure derivations) are widely supported among
   many cloud-based key management systems.

   When used to bind to an access token, a key value must be known by
   the RS in order to validate the proof signature on the request.
   Common methods for communicating these proofing keys include putting
   information in a structured access token and allowing the RS to look
   up the associated key material against the value of the access token.
   With symmetric cryptography, both of these methods would expose the
   signing key to the RS, and in the case of an structured access token,
   potentially to any party that can see the access token itself unless
   the token's payload has been encrypted.  Any of these parties would
   then be able to make calls using the access token by creating a valid
   signature using the shared key.  With asymmetric cryptography, the RS
   needs to know only the public key associated with the token in order
   to validate the request, and therefore the RS cannot create any new
   signed calls.

   While both signing approaches are allowed, GNAP treats these two
   classes of keys somewhat differently.  Only the public portion of
   asymmetric keys are allowed to be sent by value in requests to the AS
   when establishing a connection.  Since sending a symmetric key (or
   the private portion of an asymmetric key) would expose the signing
   material to any parties on the request path, including any attackers,
   sending these kinds of keys by value is prohibited.  Symmetric keys
   can still be used by client instances, but only if the client
   instance can send a reference to the key and not its value.  This
   approach allows the AS to use pre-registered symmetric keys as well
   as key derivation schemes to take advantage of symmetric cryptography
   but without requiring key distribution at runtime, which would expose
   the keys in transit.

   Both the AS and client software can use systems such as hardware
   security modules to strengthen their key security storage and
   generation for both asymmetric and symmetric keys (see also
   Section 7.1.2).

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13.6.  Generation of Access Tokens

   The content of access tokens need to be such that only the generating
   AS would be able to create them, and the contents cannot be
   manipulated by an attacker to gain different or additional access
   rights.

   One method for accomplishing this is to use a cryptographically
   random value for the access token, generated by the AS using a secure
   randomization function with sufficiently high entropy.  The odds of
   an attacker guessing the output of the randomization function to
   collide with a valid access token are exceedingly small, and even
   then the attacker would not have any control over what the access
   token would represent since that information would be held close by
   the AS.

   Another method for accomplishing this is to use a structured token
   that is cryptographically signed.  In this case, the payload of the
   access token declares to the RS what the token is good for, but the
   signature applied by the AS during token generation covers this
   payload.  Only the AS can create such a signature and therefore only
   the AS can create such a signed token.  The odds of an attacker being
   able to guess a signature value with a useful payload are exceedingly
   small.  This technique only works if all targeted RS's check the
   signature of the access token.  Any RS that does not validate the
   signature of all presented tokens would be susceptible to injection
   of a modified or falsified token.  Furthermore, an AS has to
   carefully protect the keys used to sign access tokens, since anyone
   with access to these signing keys would be able to create seemingly-
   valid access tokens using them.

13.7.  Bearer Access Tokens

   Bearer access tokens can be used by any party that has access to the
   token itself, without any additional information.  As a natural
   consequence, any RS that a bearer token is presented to has the
   technical capability of presenting that bearer token to another RS,
   as long as the token is valid.  It also means that any party that is
   able capture of the token value in storage or in transit is able to
   use the access token.  While bearer tokens are inherently simpler,
   this simplicity has been misapplied and abused in making needlessly
   insecure systems.  The downsides of bearer tokens have become more
   pertinent lately as stronger authentication systems have caused some
   attacks to shift to target tokens and APIs.

   In GNAP, key-bound access tokens are the default due to their higher
   security properties.  While bearer tokens can be used in GNAP, their
   use should be limited to cases where the simplicity benefits outweigh

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   the significant security downsides.  One common deployment pattern is
   to use a gateway that takes in key-bound tokens on the outside, and
   verifies the signatures on the incoming requests, but translates the
   requests to a bearer token for use by trusted internal systems.  The
   bearer tokens are never issued or available outside of the internal
   systems, greatly limiting the exposure of the less secure tokens but
   allowing the internal deployment to benefit from the advantages of
   bearer tokens.

13.8.  Key-Bound Access Tokens

   Key-bound access tokens, as the name suggests, are bound to a
   specific key and must be presented along with proof of that key
   during use.  The key itself is not presented at the same time as the
   token, so even if a token value is captured, it cannot be used to
   make a new request.  This is particularly true for an RS, which will
   see the token value but will not see the keys used to make the
   request (assuming asymmetric cryptography is in use, see
   Section 13.5).

   Key-bound access tokens provide this additional layer of protection
   only when the RS checks the signature of the message presented with
   the token.  Acceptance of an invalid presentation signature, or
   failure to check the signature entirely, would allow an attacker to
   make calls with a captured access token without having access to the
   related signing key material.

   In addition to validating the signature of the presentation message
   itself, the RS also needs to ensure that the signing key used is
   appropriate for the presented token.  If an RS does not ensure that
   the right keys were used to sign a message with a specific token, an
   attacker would be able to capture an access token and sign the
   request with their own keys, thereby negating the benefits of using
   key-bound access tokens.

   The RS also needs to ensure that sufficient portions of the message
   are covered by the signature.  Any items outside the signature could
   still affect the API's processing decisions, but these items would
   not be strongly bound to the token presentation.  As such, an
   attacker could capture a valid request, then manipulate portions of
   the request outside of the signature envelope in order to cause
   unwanted actions at the protected API.

   Some key-bound tokens are susceptible to replay attacks, depending on
   the details of the signing method used.  Key proofing mechanisms used
   with access tokens therefore need to use replay protection mechanisms
   covered under the signature such as a per-message nonce, a reasonably
   short time validity window, or other uniqueness constraints.  The

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   details of using these will vary depending on the key proofing
   mechanism in use, but for example, HTTP Message Signatures has both a
   created and nonce signature parameter as well as the ability to cover
   significant portions of the HTTP message.  All of these can be used
   to limit the attack surface.

13.9.  Exposure of End-user Credentials to Client Instance

   As a delegation protocol, one of the main goals of GNAP is to prevent
   the client software from being exposed to any credentials or
   information about the end user or resource owner as a requirement of
   the delegation process.  By using the variety of interaction
   mechanisms, the resource owner can interact with the AS without ever
   authenticating to the client software, and without the client
   software having to impersonate the resource owner through replay of
   their credentials.

   Consequently, no interaction methods defined in the GNAP core require
   the end user to enter their credentials, but it is technologically
   possible for an extension to be defined to carry such values.  Such
   an extension would be dangerous as it would allow rogue client
   software to directly collect, store, and replay the end user's
   credentials outside of any legitimate use within a GNAP request.

   The concerns of such an extension could be mitigated through use of a
   challenge and response unlocked by the end user's credentials.  For
   example, the AS presents a challenge as part of an interaction start
   method, and the client instance signs that challenge using a key
   derived from a password presented by the end user.  It would be
   possible for the client software to collect this password in a secure
   software enclave without exposing the password to the rest of the
   client software or putting it across the wire to the AS.  The AS can
   validate this challenge response against a known password for the
   identified end user.  While an approach such as this does not remove
   all of the concerns surrounding such a password-based scheme, it is
   at least possible to implement in a more secure fashion than simply
   collecting and replaying the password.  Even so, such schemes should
   only ever be used by trusted clients due to the ease of abusing them.

13.10.  Mixing Up Authorization Servers

   If a client instance is able to work with multiple AS's
   simultaneously, it is possible for an attacker to add a compromised
   AS to the client instance's configuration and cause the client
   software to start a request at the compromised AS.  This AS could
   then proxy the client's request to a valid AS in order to attempt to
   get the resource owner to approve access for the legitimate client
   instance.

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   A client instance needs to always be aware of which AS it is talking
   to throughout a grant process, and ensure that any callback for one
   AS does not get conflated with the callback to different AS.  The
   interaction finish hash calculation in Section 4.2.3 allows a client
   instance to protect against this kind of substitution, but only if
   the client instance validates the hash.  If the client instance does
   not use an interaction finish method or does not check the
   interaction finish hash value, the compromised AS can be granted a
   valid access token on behalf of the resource owner.  See
   [AXELAND2021] for details of one such attack, which has been since
   addressed in this document by including the grant endpoint in the
   interaction hash calculation.  Note that the client instance still
   needs to validate the hash for the attack to be prevented.

13.11.  Processing of Client-Presented User Information

   GNAP allows the client instance to present assertions and identifiers
   of the current user to the AS as part of the initial request.  This
   information should only ever be taken by the AS as a hint, since the
   AS has no way to tell if the represented person is present at the
   client software, without using an interaction mechanism.  This
   information does not guarantee the given user is there, but it does
   constitute a statement by the client software that the AS can take
   into account.

   For example, if a specific user is claimed to be present prior to
   interaction, but a different user is shown to be present during
   interaction, the AS can either determine this to be an error or
   signal to the client instance through returned subject information
   that the current user has changed from what the client instance
   thought.  This user information can also be used by the AS to
   streamline the interaction process when the user is present.  For
   example, instead of having the user type in their account identifier
   during interaction at a redirected URI, the AS can immediately
   challenge the user for their account credentials.  Alternatively, if
   an existing session is detected, the AS can determine that it matches
   the identifier provided by the client and subsequently skip an
   explicit authentication event by the resource owner.

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   In cases where the AS trusts the client software more completely, due
   to policy or by previous approval of a given client instance, the AS
   can take this user information as a statement that the user is
   present and could issue access tokens and release subject information
   without interaction.  The AS should only take such action in very
   limited circumstances, as a client instance could assert whatever it
   likes for the user's identifiers in its request.  The AS can limit
   the possibility of this by issuing randomized opaque identifiers to
   client instances to represent different end user accounts after an
   initial login.

   When a client instance presents an assertion to the AS, the AS needs
   to evaluate that assertion.  Since the AS is unlikely to be the
   intended audience of an assertion held by the client software, the AS
   will need to evaluate the assertion in a different context.  Even in
   this case, the AS can still evaluate that the assertion was generated
   by a trusted party, was appropriately signed, and is within any time
   validity windows stated by the assertion.  If the client instance's
   audience identifier is known to the AS and can be associated with the
   client instance's presented key, the AS can also evaluate that the
   appropriate client instance is presenting the claimed assertion.  All
   of this will prevent an attacker from presenting a manufactured
   assertion, or one captured from an untrusted system.  However,
   without validating the audience of the assertion, a captured
   assertion could be presented by the client instance to impersonate a
   given end user.  In such cases, the assertion offers little more
   protection than a simple identifier would.

   A special case exists where the AS is the generator of the assertion
   being presented by the client instance.  In these cases, the AS can
   validate that it did issue the assertion and it is associated with
   the client instance presenting the assertion.

13.12.  Client Instance Pre-registration

   Each client instance is identified by its own unique key, and for
   some kinds of client software such as a web server or backend system,
   this identification can be facilitated by registering a single key
   for a piece of client software ahead of time.  This registration can
   be associated with a set of display attributes to be used during the
   authorization process, identifying the client software to the user.
   In these cases, it can be assumed that only one instance of client
   software will exist, likely to serve many different users.

   A client's registration record needs to include its identifying key.
   Furthermore, it is the case that any clients using symmetric
   cryptography for key proofing mechanisms need to have their keys pre-
   registered.  The registration should also include any information

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   that would aid in the authorization process, such as a display name
   and logo.  The registration record can also limit a given client to
   ask for certain kinds of information and access, or be limited to
   specific interaction mechanisms at runtime.

   It also is sensible to pre-register client instances when the
   software is acting autonomously, without the need for a runtime
   approval by a resource owner or any interaction with an end user.  In
   these cases, an AS needs to rest on the trust decisions that have
   been determined prior to runtime in determining what rights and
   tokens to grant to a given client instance.

   However, it does not make sense to pre-register many types of
   clients.  Single-page applications (SPAs) and mobile/desktop
   applications in particular present problems with pre-registration.
   For SPAs, the instances are ephemeral in nature and long-term
   registration of a single instance leads to significant storage and
   management overhead at the AS.  For mobile applications, each
   installation of the client software is a separate instance, and
   sharing a key among all instances would be detrimental to security as
   the compromise of any single installation would compromise all copies
   for all users.

   An AS can treat these classes of client software differently from
   each other, perhaps by allowing access to certain high-value APIs
   only to pre-registered known clients, or by requiring an active end
   user delegation of authority to any client software not pre-
   registered.

   An AS can also provide warnings and caveats to resource owners during
   the authorization process, allowing the user to make an informed
   decision regarding the software they are authorizing.  For example,
   if the AS has done vetting of the client software and this specific
   instance, it can present a different authorization screen compared to
   a client instance that is presenting all of its information at
   runtime.

   Finally, an AS can use platform attestations and other signals from
   the client instance at runtime to determine whether the software
   making the request is legitimate or not.  The details of such
   attestations are outside the scope of the core protocol, but the
   client portion of a grant request provides a natural extension point
   to such information through the Client Instance Fields registry
   (Section 11.5).

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13.13.  Client Instance Impersonation

   If client instances are allowed to set their own user-facing display
   information, such as a display name and website URL, a malicious
   client instance could impersonate legitimate client software for the
   purposes of tricking users into authorizing the malicious client.

   Requiring clients to pre-register does not fully mitigate this
   problem since many pre-registration systems have self-service portals
   for management of client registration, allowing authenticated
   developers to enter self-asserted information into the management
   portal.

   An AS can mitigate this by actively filtering all self-asserted
   values presented by client software, both dynamically as part of GNAP
   and through a registration portal, to limit the kinds of
   impersonation that would be done.

   An AS can also warn the resource owner about the provenance of the
   information it is displaying, allowing the resource owner to make a
   more informed delegation decision.  For example, an AS can visually
   differentiate between a client instance that can be traced back to a
   specific developer's registration and an instance that has self-
   asserted its own display information.

13.14.  Interception of Information in the Browser

   Most information passed through the web-browser is susceptible to
   interception and possible manipulation by elements within the browser
   such as scripts loaded within pages.  Information in the URI is
   exposed through browser and server logs, and can also leak to other
   parties through HTTP Referer headers.

   GNAP's design limits the information passed directly through the
   browser, allowing for opaque URIs in most circumstances.  For the
   redirect-based interaction finish mechanism, named query parameters
   are used to carry unguessable opaque values.  For these, GNAP
   requires creation and validation of a cryptographic hash to protect
   the query parameters added to the URI and associate them with an
   ongoing grant process and values not passed in the URI.  The client
   instance has to properly validate this hash to prevent an attacker
   from injecting an interaction reference intended for a different AS
   or client instance.

   Several interaction start mechanisms use URIs created by the AS and
   passed to the client instance.  While these URIs are opaque to the
   client instance, it's possible for the AS to include parameters,
   paths, and other pieces of information that could leak security data

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   or be manipulated by a party in the middle of the transaction.  An AS
   implementation can avoid this problem by creating URIs using
   unguessable values that are randomized for each new grant request.

13.15.  Callback URI Manipulation

   The callback URI used in interaction finish mechanisms is defined by
   the client instance.  This URI is opaque to the AS, but can contain
   information relevant to the client instance's operations.  In
   particular, the client instance can include state information to
   allow the callback request to be associated with an ongoing grant
   request.

   Since this URI is exposed to the end user's browser, it is
   susceptible to both logging and manipulation in transit before the
   request is made to the client software.  As such, a client instance
   should never put security-critical or private information into the
   callback URI in a cleartext form.  For example, if the client
   software includes a post-redirect target URI in its callback URI to
   the AS, this target URI could be manipulated by an attacker, creating
   an open redirector at the client.  Instead, a client instance can use
   an unguessable identifier in the URI that can then be used by the
   client software to look up the details of the pending request.  Since
   this approach requires some form of statefulness by the client
   software during the redirection process, clients that are not capable
   of holding state through a redirect should not use redirect-based
   interaction mechanisms.

13.16.  Redirection Status Codes

   As already described in [I-D.ietf-oauth-security-topics], a server
   should never use the HTTP 307 status code to redirect a request that
   potentially contains user credentials.  If an HTTP redirect is used
   for such a request, the HTTP status code 303 "See Other" should be
   used instead.

   The status code 307, as defined in the HTTP standard [RFC7231],
   requires the user agent to preserve the method and body of a request,
   thus submitting the body of the POST request to the redirect target.
   In the HTTP standard [RFC7231], only the status code 303
   unambiguously enforces rewriting the HTTP POST request to an HTTP GET
   request, which eliminates the POST body from the redirected request.
   For all other status codes, including status code 302, user agents
   are allowed not to rewrite a POST request into a GET request and thus
   to resubmit the body.

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   The use of status code 307 results in a vulnerability when using the
   redirect interaction finish method (Section 3.3.5).  With this
   method, the AS potentially prompts the RO to enter their credentials
   in a form that is then submitted back to the AS (using an HTTP POST
   request).  The AS checks the credentials and, if successful, may
   directly redirect the RO to the client instance's redirect URI.  Due
   to the use of status code 307, the RO's user agent now transmits the
   RO's credentials to the client instance.  A malicious client instance
   can then use the obtained credentials to impersonate the RO at the
   AS.

   Redirection away from the initial URI in an interaction session could
   also leak information found in that initial URI through the HTTP
   Referer header field, which would be sent by the user agent to the
   redirect target.  To avoid such leakage, a server can first redirect
   to an internal interstitial page without any identifying or sensitive
   information on the URI before processing the request.  When the user
   agent is ultimately redirected from this page, no part of the
   original interaction URI will be found in the Referer header.

13.17.  MTLS Message Integrity

   The MTLS key proofing mechanism (Section 7.3.2) provides a means for
   a client instance to present a key using a certificate at the TLS
   layer.  Since TLS protects the entire HTTP message in transit,
   verification of the TLS client certificate presented with the message
   provides a sufficient binding between the two.  However, since TLS is
   functioning at a separate layer from HTTP, there is no direct
   connection between the TLS key presentation and the message itself,
   other than the fact that the message was presented over the TLS
   channel.  That is to say, any HTTP message can be presented over the
   TLS channel in question with the same level of trust.  The verifier
   is responsible for ensuring the key in the TLS client certificate is
   the one expected for a particular request.  For example, if the
   request is a grant request (Section 2), the AS needs to compare the
   TLS client certificate presented at the TLS layer to the key
   identified in the request body itself (either by value or through a
   referenced identifier).

   Furthermore, the prevalence of the TLS-terminating reverse proxy
   (TTRP) pattern in deployments adds a wrinkle to the situation.  In
   this common pattern, the TTRP validates the TLS connection and then
   forwards the HTTP message contents onward to an internal system for
   processing.  The system processing the HTTP message no longer has
   access to the original TLS connection's information and context.  To
   compensate for this, the TTRP could inject the TLS client certificate
   into the forwarded request as a header parameter using
   [I-D.ietf-httpbis-client-cert-field], giving the downstream system

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   access to the certificate information.  The TTRP has to be trusted to
   provide accurate certificate information, and the connection between
   the TTRP and the downstream system also has to be protected.  The
   TTRP could provide some additional assurance, for example, by adding
   its own signature to the Client-Cert header field using
   [I-D.ietf-httpbis-message-signatures].  This signature would be
   effectively ignored by GNAP (since it would not use GNAP's tag
   parameter value) but would be understood by the downstream service as
   part of its deployment.

   Additional considerations for different types of deployment patterns
   and key distribution mechanisms for MTLS are found in Section 13.18.

13.18.  MTLS Deployment Patterns

   GNAP does not specify how a client instance's keys could be made
   known to the AS ahead of time.  Public Key Infrastructure (PKI) can
   be used to manage the keys used by client instances when calling the
   AS, allowing the AS to trust a root key from a trusted authority.
   This method is particularly relevant to the MTLS key proofing method,
   where the client instance presents its certificate to the AS as part
   of the TLS connection.  An AS using PKI to validate the MTLS
   connection would need to ensure that the presented certificate was
   issued by a trusted certificate authority before allowing the
   connection to continue.  PKI-based certificates would allow a key to
   be revoked and rotated through management at the certificate
   authority without requiring additional registration or management at
   the AS.  PKI has historically been difficult to deploy, especially at
   scale, but it remains an appropriate solution for systems where the
   required overhead is not an impediment.

   MTLS in GNAP need not use a PKI backing, as self-signed certificates
   and certificates from untrusted authorities can still be presented as
   part of a TLS connection.  In this case, the verifier would validate
   the connection but accept whatever certificate was presented by the
   client software.  This specific certificate would then be bound to
   all future connections from that client software by being bound to
   the resulting access tokens, in a trust-on-first-use pattern.  See
   Section 13.17 for more considerations on MTLS as a key proofing
   mechanism.

13.19.  Interception of Responses from the AS

   Responses from the AS contain information vital to both the security
   and privacy operations of GNAP.  This information includes nonces
   used in cryptographic calculations, subject identifiers, assertions,
   public keys, and information about what client software is requesting
   and was granted.

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   In addition, if bearer tokens are used or keys are issued alongside a
   bound access token, the response from the AS contains all information
   necessary for use of the contained access token.  Any party that is
   capable of viewing such a response, such as an intermediary proxy,
   would be able to exfiltrate and use this token.  If the access token
   is instead bound to the client instance's presented key,
   intermediaries no longer have sufficient information to use the
   token.  They can still, however, gain information about the end user
   as well as the actions of the client software.

13.20.  Key Distribution

   GNAP does not define ways for the client instances keys to be
   provided to the client instances, particularly in light of how those
   keys are made known to the AS.  These keys could be generated
   dynamically on the client software or pre-registered at the AS in a
   static developer portal.  The keys for client instances could also be
   distributed as part of the deployment process of instances of the
   client software.  For example, an application installation framework
   could generate a keypair for each copy of client software, then both
   install it into the client software upon installation and registering
   that instance with the AS.

   Alternatively, it's possible for the AS to generate keys to be used
   with access tokens that are separate from the keys used by the client
   instance to request tokens.  In this method, the AS would generate
   the asymmetric keypair or symmetric key and return the public key or
   key reference, to the client instance alongside the access token
   itself.  The means for the AS to return generated key values to the
   client instance are out of scope, since GNAP does not allow the
   transmission of private or shared key information within the protocol
   itself.

   Additionally, if the token is bound to a key other than the client
   instance's presented key, this opens a possible attack surface for an
   attacker's AS to request an access token then substitute their own
   key material in the response to the client instance.  The attacker's
   AS would need to be able to use the same key as the client instance,
   but this setup would allow an attacker's AS to make use of a
   compromised key within a system.  This attack can be prevented by
   only binding access tokens to the client instance's presented keys,
   and by having client instances have a strong association between
   which keys they expect to use and the AS they expect to use them on.
   This attack is also only able to be propagated on client instances
   that talk to more than one AS at runtime, which can be limited by the
   client software.

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13.21.  Key Rotation Policy

   When keys are rotated, there could be a delay in the propagation of
   that rotation to various components in the AS's ecosystem.  The AS
   can define its own policy regarding the timeout of the previously-
   bound key, either making it immediately obsolete or allowing for a
   limited grace period during which both the previously-bound key and
   the current key can be used for signing requests.  Such a grace
   period can be useful when there are multiple running copies of the
   client that are coordinated with each other.  For example, the client
   software could be deployed as a cloud service with multiple
   orchestrated nodes.  Each of these copies is deployed using the same
   key and therefore all the nodes represent the same client instance to
   the AS.  In such cases, it can be difficult, or even impossible, to
   update the keys on all these copies in the same instant.

   The need for accommodating such known delays in the system needs to
   be balanced with the risk of allowing an old key to still be used.
   Narrowly restricting the exposure opportunities for exploit at the AS
   in terms of time, place, and method makes exploit significantly more
   difficult, especially if the exception happens only once.  For
   example, the AS can reject requests from the previously-bound key (or
   any previous one before it) to cause rotation to a new key, or at
   least ensure that the rotation happens in an idempotent way to the
   same new key.

   See also the related considerations for token values in
   Section 13.32.

13.22.  Interaction Finish Modes and Polling

   During the interaction process, the client instance usually hands
   control of the user experience over to another component, be it the
   system browser, another application, or some action the resource
   owner is instructed to take on another device.  By using an
   interaction finish method, the client instance can be securely
   notified by the AS when the interaction is completed and the next
   phase of the protocol should occur.  This process includes
   information that the client instance can use to validate the finish
   call from the AS and prevent some injection, session hijacking, and
   phishing attacks.

   Some types of client deployment are unable to receive an interaction
   finish message.  Without an interaction finish method to notify it,
   the client instance will need to poll the grant continuation API
   while waiting for the resource owner to approve or deny the request.
   An attacker could take advantage of this situation by capturing the
   interaction start parameters and phishing a legitimate user into

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   authorizing the attacker's waiting client instance, which would in
   turn have no way of associating the completed interaction from the
   targeted user with the start of the request from the attacker.

   However, it is important to note that this pattern is practically
   indistinguishable from some legitimate use cases.  For example, a
   smart device emits a code for the resource owner to enter on a
   separate device.  The smart device has to poll because the expected
   behavior is that the interaction will take place on the separate
   device, without a way to return information to the original device's
   context.

   As such, developers need to weigh the risks of forgoing an
   interaction finish method against the deployment capabilities of the
   client software and its environment.  Due to the increased security,
   an interaction finish method should be employed whenever possible.

13.23.  Session Management for Interaction Finish Methods

   When using an interaction finish method such as redirect or push, the
   client instance receives an unsolicited inbound request from an
   unknown party (in most cases over HTTP).  The client instance needs
   to be able to successfully associate this incoming request with a
   specific pending grant request being managed by the client instance.
   If the client instance is not careful and precise about this, an
   attacker could associate their own session at the client instance
   with a stolen interaction response.  The means of preventing this
   varies by the type of client software and interaction methods in use.
   Some common patterns are enumerated here.

   If the end user interacts with the client instance through a web
   browser and the redirect interaction finish method is used, the
   client instance can ensure that the incoming HTTP request from the
   finish method is presented in the same browser session that the grant
   request was started in.  This technique is particularly useful when
   the redirect interaction start mode is used as well, since in many
   cases the end user will follow the redirection with the same browser
   that they are using to interact with the client instance.  The client
   instance can then store the relevant pending grant information in the
   session, either in the browser storage directly (such as with a
   single-page application) or in an associated session store on a back-
   end server.  In both cases, when the incoming request reaches the
   client instance, the session information can be used to ensure that
   the same party that started the request is present as the request
   finishes.

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   Ensuring that the same party that started a request is present when
   that request finishes can prevent phishing attacks, where an attacker
   starts a request at an honest client instance and tricks an honest RO
   into authorizing it.  For example, if an honest end user (that also
   acts as the RO) wants to start a request through a client instance
   controlled by the attacker, the attacker can start a request at an
   honest client instance and then redirect the honest end user to the
   interaction URI from the attackers session with the honest client
   instance.  If the honest end user then fails to realize that they are
   not authorizing the attacker-controlled client instance (with which
   it started its request) but instead the honest client instance when
   interacting with the AS, the attacker's session with the honest
   client instance would be authorized.  This would give the attacker
   access to the honest end user's resources that the honest client
   instance is authorized to access.  However, if after the interaction
   the AS redirects the honest end user back to the client instance
   whose grant request the end user just authorized, the honest end user
   is redirected to the honest client instance.  The honest client
   instance can then detect that the end user is not the party that
   started the request, since the request at the honest client instance
   was started by the attacker.  This detection can prevent the attack.
   This is related to the discussion in Section 13.13, because again the
   attack can be prevented by the AS informing the user as much as
   possible about the client instance that is to be authorized.

   If the end user does not interact with the client instance through a
   web browser or the interaction start method does not use the same
   browser or device that the end user is interacting through (such as
   the launch of a second device through a scannable code or
   presentation of a user code) the client instance will not be able to
   strongly associate an incoming HTTP request with an established
   session with the end user.  This is also true when the push
   interaction finish method is used, since the HTTP request comes
   directly from the interaction component of the AS.  In these
   circumstances, the client instance can at least ensure that the
   incoming HTTP request can be uniquely associated with an ongoing
   grant request by making the interaction finish callback URI unique
   for the grant when making the interaction request (Section 2.5.2).
   Mobile applications and other client instances that generally serve
   only a single end user at a time can use this unique incoming URL to
   differentiate between a legitimate incoming request and an attacker's
   stolen request.

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13.24.  Calculating Interaction Hash

   The calculation of the interaction hash value provides defence in
   depth, allowing a client instance to protect itself from spurious
   injection of interaction references when using an interaction finish
   method.  The AS is protected during this attack through the
   continuation access token being bound to the expected interaction
   reference, but without hash calculation, the attacker could cause the
   client to make an HTTP request on command.  With both of these in
   place, an attacker attempting to substitute the interaction reference
   is stopped in several places.

    .----.        .------.       +--------+      +--------+
   | User |      |Attacker|      | Client |      |   AS   |
   |      |      |        |      |Instance|      |        |
   |      |      |        |      |        |      |        |
   |      |      |        +=(1)=>|        |      |        |
   |      |      |        |      |        +-(2)->|        |
   |      |      |        |      |        |<-(3)-+        |
   |      |      |        |<=(4)=+        |      |        |
   |      |      |        |      |        |      |        |
   |      |      |        +==(5)================>|        |
   |      |      |        |      |        |      |        |
   |      |      |        |<================(6)==+        |
   |      |      |        |      |        |      |        |
   |      +==(A)================>|        |      |        |
   |      |      |        |      |        +-(B)->|        |
   |      |      |        |      |        |<-(C)-+        |
   |      |<=================(D)=+        |      |        |
   |      |      |        |      |        |      |        |
   |      +==(E)================================>|        |
   |      |      |        |      |        |      |        |
   |      |<=(7)=+        |      |        |      |        |
   |      |      |        |      |        |      |        |
   |      +==(F)================>|        |      |        |
   |      |      |        |      |        +-(G)->|        |
   |      |      |        |      |        |      |        |
    `----`        `------`       +--------+      +--------+

   *  Prerequesits: The client instance can allow multiple end users to
      access the same AS.  The attacker is attempting to associate their
      rights with the target user's session.

   *  (1) The attacker starts a session at the client instance.

   *  (2) The client instance creates a grant request with nonce CN1.

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   *  (3) The AS responds to the grant request with a need to interact,
      nonce SN1, and a continuation token, CT1.

   *  (4) The client instructs the attacker to interact at the AS.

   *  (5) The attacker interacts at the AS.

   *  (6) The AS completes the interact finish with interact ref IR1 and
      interact hash IH1 calculated from (CN1 + SN1 + IR1 + AS).  The
      attacker prevents IR1 from returning to the client instance.

   *  (A) The target user starts a session at the client instance.

   *  (B) The client instance creates a grant request with nonce CN2.

   *  (C) The AS responds to the grant request with a need to interact,
      nonce SN2, and a continuation token, CT2.

   *  (D) The client instance instructs the user to interact at the AS.

   *  (E) The target user interacts at the AS.

   *  (7) Before the target user can complete their interaction, the
      attacker delivers their own interact ref IR1 into the user's
      session.  The attacker cannot calculate the appropriate hash
      because the attacker does not have access to CN2 and SN2.

   *  (F) The target user triggers the interaction finish in their own
      session with the attacker's IR1.

   *  (G) If the client instance is checking the interaction hash, the
      attack stops here because the hash calculation of (CN2 + SN2 + IR1
      + AS) will fail.  If the client instance does not check the
      interaction hash, the AS will reject the interaction request
      because it is presented against CT2 and not CT1 as expected.

13.25.  Storage of Information During Interaction and Continuation

   When starting an interactive grant request, a client application has
   a number of protocol elements that it needs to manage, including
   nonces, references, keys, access tokens, and other elements.  During
   the interaction process, the client instance usually hands control of
   the user experience over to another component, be it the system
   browser, another application, or some action the resource owner is
   instructed to take on another device.  In order for the client
   instance to make its continuation call, it will need to recall all of
   these protocol elements at a future time.  Usually this means the
   client instance will need to store these protocol elements in some

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   retrievable fashion.

   If the security protocol elements are stored on the end user's
   device, such as in browser storage or in local application data
   stores, capture and exfiltration of this information could allow an
   attacker to continue a pending transaction instead of the client
   instance.  Client software can make use of secure storage mechanisms,
   including hardware-based key and data storage, to prevent such
   exfiltration.

   Note that in GNAP, the client instance has to choose its interaction
   finish URI prior to making the first call to the AS.  As such, the
   interaction finish URI will often have a unique identifier for the
   ongoing request, allowing the client instance to access the correct
   portion of its storage.  Since this URI is passed to other parties
   and often used through a browser, this URI should not contain any
   security-sensitive information that would be valuable to an attacker,
   such as any token identifier, nonce, or user information.  Instead, a
   cryptographically random value is suggested, and that value should be
   used to index into a secure session or storage mechanism.

13.26.  Denial of Service (DoS) through Grant Continuation

   When a client instance starts off an interactive process, it will
   eventually need to continue the grant request in a subsequent message
   to the AS.  It's possible for a naive client implementation to
   continuously send continuation requests to the AS while waiting for
   approval, especially if no interaction finish method is used.  Such
   constant requests could overwhelm the AS's ability to respond to both
   these and other requests.

   To mitigate this for well-behaved client software, the continuation
   response contains a wait parameter that is intended to tell the
   client instance how long it should wait until making its next
   request.  This value can be used to back off client software that is
   checking too quickly by returning increasing wait times for a single
   client instance.

   If client software ignores the wait value and makes its continuation
   calls too quickly, or if the client software assumes the absence of
   the wait values means it should poll immediately, the AS can choose
   to return errors to the offending client instance, including possibly
   canceling the ongoing grant request.  With well-meaning client
   software these errors can indicate a need to change the client
   software's programmed behavior.

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13.27.  Exhaustion of Random Value Space

   Several parts of the GNAP process make use of unguessable randomized
   values, such as nonces, tokens, user codes, and randomized URIs.
   Since these values are intended to be unique, a sufficiently powerful
   attacker could make a large number of requests to trigger generation
   of randomized values in an attempt to exhaust the random number
   generation space.  While this attack is particularly applicable to
   the AS, client software could likewise be targeted by an attacker
   triggering new grant requests against an AS.

   To mitigate this, software can ensure that its random values are
   chosen from a significantly large pool that exhaustion of that pool
   is prohibitive for an attacker.  Additionally, the random values can
   be time-boxed in such a way as their validity windows are reasonably
   short.  Since many of the random values used within GNAP are used
   within limited portions of the protocol, it is reasonable for a
   particular random value to be valid for only a small amount of time.
   For example, the nonces used for interaction finish hash calculation
   need only to be valid while the client instance is waiting for the
   finish callback and can be functionally expired when the interaction
   has completed.  Similarly, artifacts like access tokens and the
   interaction reference can be limited to have lifetimes tied to their
   functional utility.  Finally, each different category of artifact
   (nonce, token, reference, identifier, etc.) can be generated from a
   separate random pool of values instead of a single global value
   space.

13.28.  Front-channel URIs

   Some interaction methods in GNAP make use of URIs accessed through
   the end user's browser, known collectively as front-channel
   communication.  These URIs are most notably present in the redirect
   interaction start method and the redirect interaction finish mode.
   Since these URIs are intended to be given to the end user, the end
   user and their browser will be subjected to anything hosted at that
   URI including viruses, malware, and phishing scams.  This kind of
   risk is inherent to all redirection-based protocols, including GNAP
   when used in this way.

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   When talking to a new or unknown AS, a client instance might want to
   check the URI from the interaction start against a blocklist and warn
   the end user before redirecting them.  Many client instances will
   provide an interstitial message prior to redirection in order to
   prepare the user for control of the user experience being handed to
   the domain of the AS, and such a method could be used to warn the
   user of potential threats.  For instance, a rogue AS impersonating a
   well-known service provider.  Client software can also prevent this
   by managing an allowlist of known and trusted AS's.

   Alternatively, an attacker could start a GNAP request with a known
   and trusted AS but include their own attack site URI as the callback
   for the redirect finish method.  The attacker would then send the
   interaction start URI to the victim and get them to click on it.
   Since the URI is at the known AS, the victim is inclined to do so.
   The victim will then be prompted to approve the attacker's
   application, and in most circumstances the victim will then be
   redirected to the attacker's site whether or not the user approved
   the request.  The AS could mitigate this partially by using a
   blocklist and allowlist of interaction finish URIs during the client
   instance's initial request, but this approach can be especially
   difficult if the URI has any dynamic portion chosen by the client
   software.  The AS can couple these checks with policies associated
   with the client instance that has been authenticated in the request.
   If the AS has any doubt about the interaction finish URI, the AS can
   provide an interstitial warning to the end user before processing the
   redirect.

   Ultimately, all protocols that use redirect-based communication
   through the user's browser are susceptible to having an attacker try
   to co-opt one or more of those URIs in order to harm the user.  It is
   the responsibility of the AS and the client software to provide
   appropriate warnings, education, and mitigation to protect end users.

13.29.  Processing Assertions

   Identity assertions can be used in GNAP to convey subject
   information, both from the AS to the client instance in a response
   (Section 3.4) and from the client instance to the AS in a request
   (Section 2.2).  In both of these circumstances, when an assertion is
   passed in GNAP, the receiver of the assertion needs to parse and
   process the assertion.  As assertions are complex artifacts with
   their own syntax and security, special care needs to be taken to
   prevent the assertion values from being used as an attack vector.

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   All assertion processing needs to account for the security aspects of
   the assertion format in use.  In particular, the processor needs to
   parse the assertion from a JSON string object, and apply the
   appropriate cryptographic processes to ensure the integrity of the
   assertion.

   For example, when SAML 2 assertions are used, the receiver has to
   parse an XML document.  There are many well-known security
   vulnerabilities in XML parsers, and the XML standard itself can be
   attacked through the use of processing instructions and entity
   expansions to cause problems with the processor.  Therefore, any
   system capable of processing SAML 2 assertions also needs to have a
   secure and correct XML parser.  In addition to this, the SAML 2
   specification uses XML Signatures, which have their own
   implementation problems that need to be accounted for.  Similar
   requirements exist for OpenID Connect's ID token, which is based on
   the JSON Web Token (JWT) format and the related JSON Object Signing
   And Encryption (JOSE) cryptography suite.

13.30.  Stolen Token Replay

   If a client instance can request tokens at multiple AS's, and the
   client instance uses the same keys to make its requests across those
   different AS's, then it is possible for an attacker to replay a
   stolen token issued by an honest AS from a compromised AS, thereby
   binding the stolen token to the client instance's key in a different
   context.  The attacker can manipulate the client instance into using
   the stolen token at an RS, particularly at an RS that is expecting a
   token from the honest AS.  Since the honest AS issued the token and
   the client instance presents the token with its expected bound key,
   the attack succeeds.

   This attack has several preconditions.  In this attack, the attacker
   does not need access to the client instance's key and cannot use the
   stolen token directly at the RS, but the attacker is able to get the
   access token value in some fashion.  The client instance also needs
   to be configured to talk to multiple AS's, including the attacker's
   controlled AS.  Finally, the client instance needs to be able to be
   manipulated by the attacker to call the RS while using a token issued
   from the stolen AS.  The RS does not need to be compromised or made
   to trust the attacker's AS.

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   To protect against this attack, the client instance can use a
   different key for each AS that it talks to.  Since the replayed token
   will be bound to the key used at the honest AS, the uncompromised RS
   will reject the call since the client instance will be using the key
   used at the attacker's AS instead with the same token.  When the MTLS
   key proofing method is used, a client instance can use self-signed
   certificates to use a different key for each AS that it talks to, as
   discussed in Section 13.18.

   Additionally, the client instance can keep a strong association
   between the RS and a specific AS that it trusts to issue tokens for
   that RS.  This strong binding also helps against some forms of AS
   mix-up attacks (Section 13.10).  Managing this binding is outside the
   scope of GNAP core, but it can be managed either as a configuration
   element for the client instance or dynamically through discovering
   the AS from the RS (Section 9.1).

   The details of this attack are available in [HELMSCHMIDT2022] with
   additional discussion and considerations.

13.31.  Self-contained Stateless Access Tokens

   The contents and format of the access token are at the discretion of
   the AS, and are opaque to the client instance within GNAP.  As
   discussed in the companion document,
   [I-D.ietf-gnap-resource-servers], the AS and RS can make use of
   stateless access tokens with an internal structure and format.  These
   access tokens allow an RS to validate the token without having to
   make any external calls at runtime, allowing for benefits in some
   deployments, the discussion of which are outside the scope of this
   specification.

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   However, the use of such self-contained access tokens has an effect
   on the ability of the AS to provide certain functionality defined
   within this specification.  Specifically, since the access token is
   self-contained, it is difficult or impossible for an AS to signal to
   all RS's within an ecosystem when a specific access token has been
   revoked.  Therefore, an AS in such an ecosystem should probably not
   offer token revocation functionality to client instances, since the
   client instance's calls to such an endpoint is effectively
   meaningless.  However, a client instance calling the token revocation
   function will also throw out its copy of the token, so such a placebo
   endpoint might not be completely meaningless.  Token rotation
   similarly difficult because the AS has to revoke the old access token
   after a rotation call has been made.  If the access tokens are
   completely self-contained and non-revocable, this means that there
   will be a period of time during which both the old and new access
   tokens are valid and usable, which is an increased security risk for
   the environment.

   These problems can be mitigated by keeping the validity time windows
   of self-contained access tokens reasonably short, limiting the time
   after a revocation event that a revoked token could be used.
   Additionally, the AS could proactively signal to RS's under its
   control identifiers for revoked tokens that have yet to expire.  This
   type of information push would be expected to be relatively small and
   infrequent, and its implementation is outside the scope of this
   specification.

13.32.  Network Problems and Token and Grant Management

   If a client instance makes a call to rotate an access token but the
   network connection is dropped before the client instance receives the
   response with the new access token, the system as a whole can end up
   in an inconsistent state, where the AS has already rotated the old
   access token and invalidated it, but the client instance only has
   access to the invalidated access token and not the newly rotated
   token value.  If the client instance retries the rotation request, it
   would fail because the client is no longer presenting a valid and
   current access token.  A similar situation can occur during grant
   continuation, where the same client instance calls to continue or
   update a grant request without successfully receiving the results of
   the update.

   To combat this, both grant Management (Section 5) and token
   management (Section 6) can be designed to be idempotent, where
   subsequent calls to the same function with the same credentials are
   meant to produce the same results.  For example, multiple calls to
   rotate the same access token need to result in the same rotated token
   value, within a reasonable time window.

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   In practice, an AS can hold on to an old token value for such limited
   purposes.  For example, to support rotating access tokens over
   unreliable networks, the AS receives the initial request to rotate an
   access token and creates a new token value and returns it.  The AS
   also marks the old token value as having been used to create the
   newly-rotated token value.  If the AS sees the old token value within
   a small enough time window, such as a few seconds since the first
   rotation attempt, the AS can return the same rotated access token
   value.  Furthermore, once the system has seen the newly-rotated token
   in use, the original token can be discarded because the client
   instance has proved that it did receive the token.  The result of
   this is a system that is eventually self-consistent without placing
   an undue complexity burden on the client instance to manage
   problematic networks.

13.33.  Server-side Request Forgery (SSRF)

   There are several places within GNAP where a URI can be given to a
   party causing it to fetch that URI during normal operation of the
   protocol.  If an attacker is able to control the value of one of
   these URIs within the protocol, the attacker could cause the target
   system to execute a request on a URI that is within reach of the
   target system but normally unavailable to the attacker.  For example,
   an attacker sending a URL of http://localhost/admin to cause the
   server to access an internal function on itself, or
   https://192.168.0.14/ to call a service behind a firewall.  Even if
   the attacker does not gain access to the results of the call, the
   side effects of such requests coming from a trusted host can be
   problematic to the security and sanctity of such otherwise unexposed
   endpoints.  This can be particularly problematic if such a URI is
   used to call non-HTTP endpoints, such as remote code execution
   services local to the AS.

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   In GNAP, the most vulnerable place in the core protocol is the
   push-based post-interaction finish method (Section 4.2.2), as the
   client instance is less trusted than the AS and can use this method
   to make the AS call an arbitrary URI.  While it is not required by
   the protocol, the AS can fetch other client-instance provided URIs
   such as the logo image or home page, for verification or privacy-
   preserving purposes before displaying them to the resource owner as
   part of a consent screen.  Even if the AS does not fetch these URIs,
   their use in GNAP's normal operation could cause an attack against
   the end user's browser as it fetches these same attack URIs.
   Furthermore, extensions to GNAP that allow or require URI fetch could
   also be similarly susceptible, such as a system for having the AS
   fetch a client instance's keys from a presented URI instead of the
   client instance presenting the key by value.  Such extensions are
   outside the scope of this specification, but any system deploying
   such an extension would need to be aware of this issue.

   To help mitigate this problem, similar approaches to protecting
   parties against malicious redirects (Section 13.28) can be used.  For
   example, all URIs that can result in a direct request being made by a
   party in the protocol can be filtered through an allowlist or
   blocklist.  For example, an AS that supports the push based
   interaction finish can compare the callback URI in the interaction
   request to a known URI for a pre-registered client instance, or it
   can ensure that the URI is not on a blocklist of sensitive URLs such
   as internal network addresses.  However, note that because these
   types of calls happen outside of the view of human interaction, it is
   not usually feasible to provide notification and warning to someone
   before the request needs to be executed, as is the case with
   redirection URLs.  As such, SSRF is somewhat more difficult to manage
   at runtime, and systems should generally refuse to fetch a URI if
   unsure.

13.34.  Multiple Key Formats

   All keys presented by value are allowed to be in only a single
   format.  While it would seem beneficial to allow keys to be sent in
   multiple formats, in case the receiver doesn't understand one or more
   of the formats used, there would be security issues with such a
   feature.  If multiple keys formats were allowed, receivers of these
   key definitions would need to be able to make sure that it's the same
   key represented in each field and not simply use one of the key
   formats without checking for equivalence.  If equivalence were not
   carefully checked, it is possible for an attacker to insert their own
   key into one of the formats without needing to have control over the
   other formats.  This could potentially lead to a situation where one
   key is used by part of the system (such as identifying the client
   instance) and a different key in a different format in the same

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   message is used for other things (such as calculating signature
   validity).  However, in such cases, it is impossible for the receiver
   to ensure that all formats contain the same key information since it
   is assumed that the receiver cannot understand all of the formats.

   To combat this, all keys presented by value have to be in exactly one
   supported format known by the receiver as discussed in Section 7.1.
   In most cases, a client instance is going to be configured with its
   keys in a single format, and it will simply present that format as-is
   to the AS in its request.  A client instance capable of multiple
   formats can use AS discovery (Section 9) to determine which formats
   are supported, if desired.  An AS should be generous in supporting
   many different key formats to allow different types of client
   software and client instance deployments.  An AS implementation
   should try to support multiple formats to allow a variety of client
   software to connect.

13.35.  Asynchronous Interactions

   GNAP allows the RO to be contacted by the AS asynchronously, outside
   the regular flow of the protocol.  This allows for some advanced use
   cases, such as cross-user authentication or information release, but
   such advanced use cases have some distinct issues that implementors
   need to be fully aware of before using these features.

   First, in many applications, the return of a subject information to
   the client instance could indicate to the client instance that the
   end-user is the party represented by that information, functionally
   allowing the end-user to authenticate to the client application.
   While the details of a fully functional authentication protocol are
   outside the scope of GNAP, it is a common exercise for a client
   instance to be requesting information about the end user.  This is
   facilitated by the several interaction methods (Section 4.1) defined
   in GNAP that allow the end user to begin interaction directly with
   the AS.  However, when the subject of the information is
   intentionally not the end-user, the client application will need some
   way to differentiate between requests for authentication of the end
   user and requests for information about a different user.  Confusing
   these states could lead to an attacker having their account
   associated with a privileged user.  Client instances can mitigate
   this by having distinct code paths for primary end user
   authentication and requesting subject information about secondary
   users, such as in a call center.  In such use cases, the client
   software used by the resource owner (the caller) and the end-user
   (the agent) are generally distinct, allowing the AS to differentiate
   between the agent's corporate device making the request and the
   caller's personal device approving the request.

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   Second, RO's interacting asynchronously do not usually have the same
   context as an end user in an application attempting to perform the
   task needing authorization.  As such, the asynchronous requests for
   authorization coming to the RO from the AS might have very little to
   do with what the RO is doing at the time.  This situation can
   consequently lead to authorization fatigue on the part of the RO,
   where any incoming authorization request is quickly approved and
   dispatched without the RO making a proper verification of the
   request.  An attacker can exploit this fatigue and get the RO to
   authorize the attacker's system for access.  To mitigate this, AS
   systems deploying asynchronous authorization should only prompt the
   RO when the RO is expecting such a request, and significant user
   experience engineering efforts need to be employed to ensure the RO
   can clearly make the appropriate security decision.  Furthermore,
   audit capability, and the ability to undo access decisions that may
   be ongoing, is particularly important in the asynchronous case.

13.36.  Compromised RS

   An attacker may aim to gain access to confidential or sensitive
   resources.  The measures for hardening and monitoring resource server
   systems (beyond protection with access tokens) is out of the scope of
   this document, but the use of GNAP to protect a system does not
   absolve the resource server of following best practices.  GNAP
   generally considers a breach can occur, and therefore advises to
   prefer key-bound tokens whenever possible, which at least limits the
   impact of access token leakage by a compromised or malicious RS.

14.  Privacy Considerations

   The privacy considerations in this section are modeled after the list
   of privacy threats in [RFC6973], "Privacy Considerations for Internet
   Protocols", and either explain how these threats are mitigated or
   advise how the threats relate to GNAP.

14.1.  Surveillance

   Surveillance is the observation or monitoring of an individual's
   communications or activities.  Surveillance can be conducted by
   observers or eavesdroppers at any point along the communications
   path.

   GNAP assumes the TLS protection used throughout the spec is intact.
   Without the protection of TLS, there are many points throughout the
   use of GNAP that would lead to possible surveillance.  Even with the
   proper use of TLS, surveillance could occur by several parties
   outside of the TLS-protected channels, as discussed in the sections
   below.

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14.1.1.  Surveillance by the Client

   The purpose of GNAP is to authorize clients to be able to access
   information on behalf of a user.  So while it is expected that the
   client may be aware of the user's identity as well as data being
   fetched for that user, in some cases the extent of the client may be
   beyond what the user is aware of.  For example, a client may be
   implemented as multiple distinct pieces of software, such as a
   logging service or a mobile app that reports usage data to an
   external backend service.  Each of these pieces could gain
   information about the user without the user being aware of this
   action.

14.1.2.  Surveillance by the Authorization Server

   The role of the authorization server is to manage the authorization
   of client instances to protect access to the user's data.  In this
   role, the authorization server is by definition aware of each
   authorization of a client instance by a user.  When the authorization
   server shares user information with the client instance, it needs to
   make sure that it has the permission from that user to do so.

   Additionally, as part of the authorization grant process, the
   authorization server may be aware of which resource servers the
   client intends to use an access token at.  However, it is possible to
   design a system using GNAP in which this knowledge is not made
   available to the authorization server, such as by avoiding the use of
   the locations object in the authorization request.

   If the authorization server's implementation of access tokens is such
   that it requires a resource server call back to the authorization
   server to validate them, then the authorization server will be aware
   of which resource servers are actively in use and by which users and
   which clients.  To avoid this possibility, the authorization server
   would need to structure access tokens in such a way that they can be
   validated by the resource server without notifying the authorization
   server that the token is being validated.

14.2.  Stored Data

   Several parties in the GNAP process are expected to persist data at
   least temporarily, if not semi-permanently, for the normal
   functioning of the system.  If compromised, this could lead to
   exposure of sensitive information.  This section documents the
   potentially sensitive information each party in GNAP is expected to
   store for normal operation.  Naturally it is possible that any party
   is storing information for longer than technically necessary of the
   protocol mechanics (such as audit logs, etc).

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   The authorization server is expected to store subject identifiers for
   users indefinitely, in order to be able to include them in the
   responses to clients.  The authorization server is also expected to
   store client key identifiers associated with display information
   about the client such as its name and logo.

   The client is expected to store its client instance key indefinitely,
   in order to authenticate to the authorization server for the normal
   functioning of the GNAP flows.  Additionally, the client will be
   temporarily storing artifacts issued by the authorization server
   during a flow, and these artifacts ought to be discarded by the
   client when the transaction is complete.

   The resource server is not required to store any state for its normal
   operation, as far as its part in implementing GNAP.  Depending on the
   implementation of access tokens, the resource server may need to
   cache public keys from the authorization server in order to validate
   access tokens.

14.3.  Intrusion

   Intrusion refers to the ability of various parties to send
   unsolicited messages or cause denial of service for unrelated
   parties.

   If the resource owner is different from the end user, there is an
   opportunity for the end user to cause unsolicited messages to be sent
   to the resource owner if the system prompts the resource owner for
   consent when an end user attempts to access their data.

   The format and contents of subject identifiers are intentionally not
   defined by GNAP.  If the authorization server uses values for subject
   identifiers that are also identifiers for communication channels,
   (e.g. an email address or phone number), this opens up the
   possibility for a client to learn this information when it was not
   otherwise authorized to access this kind of data about the user.

14.4.  Correlation

   The threat of correlation is the combination of various pieces of
   information related to an individual in a way that defies their
   expectations of what others know about them.

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14.4.1.  Correlation by Clients

   The biggest risk of correlation in GNAP is when an authorization
   server returns stable consistent user identifiers to multiple
   different applications.  In this case, applications created by
   different parties would be able to correlate these user identifiers
   out of band in order to know which users they have in common.

   The most common example of this in practice is tracking for
   advertising purposes, such that a client shares their list of user
   IDs with an ad platform that is then able to retarget ads to
   applications created by other parties.  In contrast, a positive
   example of correlation is a corporate acquisition where two
   previously unrelated clients now do need to be able to identify the
   same user between the two clients, such as when software systems are
   intentionally connected by the end user.

   Another means of correlation comes from the use of RS-first discovery
   (Section 9.1).  A client instance knowing nothing other than an RS's
   URL could make an unauthenticated call to the RS and learn which AS
   protects the resources there.  If the client instance knows something
   about the AS, such as it being a single-user AS or belonging to a
   specific organization, the client instance could, through
   association, learn things about the resource without ever gaining
   access to the resource itself.

14.4.2.  Correlation by Resource Servers

   Unrelated resource servers also have an opportunity to correlate
   users if the authorization server includes stable user identifiers in
   access tokens or in access token introspection responses.

   In some cases a resource server may not actually need to be able to
   identify users, (such as a resource server providing access to a
   company cafeteria menu which only needs to validate whether the user
   is a current employee), so authorization servers should be thoughtful
   of when user identifiers are actually necessary to communicate to
   resource servers for the functioning of the system.

   However, note that the lack of inclusion of a user identifier in an
   access token may be a risk if there is a concern that two users may
   voluntarily share access tokens between them in order to access
   protected resources.  For example, if a website wants to limit access
   to only people over 18, and such does not need to know any user
   identifiers, an access token may be issued by an AS contains only the
   claim "over 18".  If the user is aware that this access token doesn't
   reference them individually, they may be willing to share the access
   token with a user who is under 18 in order to let them get access to

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   the website.  (Note that the binding of an access token to a non-
   extractable client instance key also prevents the access token from
   being voluntarily shared.)

14.4.3.  Correlation by Authorization Servers

   Clients are expected to be identified by their client instance key.
   If a particular client instance key is used at more than one
   authorization server, this could open up the possibility for multiple
   unrelated authorization servers to correlate client instances.  This
   is especially a problem in the common case where a client instance is
   used by a single individual, as it would allow the authorization
   servers to correlate that individual between them.  If this is a
   concern of a client, the client should use distinct keys with each
   authorization server.

14.5.  Disclosure in Shared References

   Throughout many parts of GNAP, the parties pass shared references
   between each other, sometimes in place of the values themselves.  For
   example the interact_ref value used throughout the flow.  These
   references are intended to be random strings and should not contain
   any private or sensitive data that would potentially leak information
   between parties.

15.  References

15.1.  Normative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", May 2015,
              <https://www.rfc-editor.org/info/bcp195>.

   [I-D.ietf-gnap-resource-servers]
              Richer, J., Parecki, A., and F. Imbault, "Grant
              Negotiation and Authorization Protocol Resource Server
              Connections", Work in Progress, Internet-Draft, draft-
              ietf-gnap-resource-servers-02, 11 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-gnap-
              resource-servers-02>.

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   [I-D.ietf-httpbis-digest-headers]
              Polli, R. and L. Pardue, "Digest Fields", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-digest-
              headers-10, 19 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              digest-headers-10>.

   [I-D.ietf-httpbis-message-signatures]
              Backman, A., Richer, J., and M. Sporny, "HTTP Message
              Signatures", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-message-signatures-16, 6 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              message-signatures-16>.

   [I-D.ietf-oauth-rar]
              Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", Work in Progress, Internet-
              Draft, draft-ietf-oauth-rar-23, 30 January 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              rar-23>.

   [I-D.ietf-secevent-subject-identifiers]
              Backman, A., Scurtescu, M., and P. Jain, "Subject
              Identifiers for Security Event Tokens", Work in Progress,
              Internet-Draft, draft-ietf-secevent-subject-identifiers-
              16, 14 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              secevent-subject-identifiers-16>.

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
              DOI 10.17487/RFC2397, August 1998,
              <https://www.rfc-editor.org/rfc/rfc2397>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/rfc/rfc3339>.

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/rfc/rfc4107>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/rfc/rfc5646>.

   [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
              "Known Issues and Best Practices for the Use of Long
              Polling and Streaming in Bidirectional HTTP", RFC 6202,
              DOI 10.17487/RFC6202, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6202>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6750>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7231>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7234>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/rfc/rfc7468>.

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   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/rfc/rfc7515>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7517>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8705>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8792>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [SAML2]    Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", March 2005,
              <https://docs.oasis-open.org/security/saml/v2.0/saml-core-
              2.0-os.pdf>.

   [SP80063C] Grassi, P., Nadeau, E., Richer, J., Squire, S., Fenton,
              J., Lefkovitz, N., Danker, J., Choong, Y., Greene, K., and
              M. Theofanos, "Digital Identity Guidelines: Federation and
              Assertions", June 2017,
              <https://doi.org/10.6028/NIST.SP.800-63c>.

15.2.  Informative References

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   [AXELAND2021]
              Axeland, Å. and O. Oueidat, "Security Analysis of Attack
              Surfaces on the Grant Negotiation and Authorization
              Protocol", 2021,
              <https://odr.chalmers.se/handle/20.500.12380/304105>.

   [HELMSCHMIDT2022]
              Helmschmidt, F., "Security Analysis of the Grant
              Negotiation and Authorization Protocol", 2022,
              <http://dx.doi.org/10.18419/opus-12203>.

   [I-D.ietf-httpbis-client-cert-field]
              Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-client-cert-field-04, 2 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              client-cert-field-04>.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-21, 27 September 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-21>.

   [I-D.ietf-uta-rfc6125bis]
              Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              Work in Progress, Internet-Draft, draft-ietf-uta-
              rfc6125bis-10, 25 January 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              rfc6125bis-10>.

   [promise-theory]
              Burgess, M. and J. Bergstra, "Promise theory", January
              2014, <http://markburgess.org/promises.html>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/rfc/rfc6973>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

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Appendix A.  Document History

      Note: To be removed by RFC editor before publication.

   *  -13

      -  Editoral changes from chair review.

      -  Clarify that user codes are ungessable.

      -  Fix user code examples.

      -  Clarify expectations for extensions to interaction start and
         finish methods.

      -  Fix references.

      -  Add IANA designated expert instructions.

      -  Clarify new vs. updated access tokens, and call out no need for
         refresh tokens in OAuth 2 comparison section.

      -  Add instructions on assertion processing.

      -  Explicitly list user reference lifetime management.

   *  -12

      -  Make default hash algorithm SHA256 instead of SHA3-512.

      -  Remove previous_key from key rotation.

      -  Defined requirements for key rotation methods.

      -  Add specificity to context of subject identifier being the AS.

      -  Editorial updates and protocol clarification.

   *  -11

      -  Error as object or string, more complete set of error codes

      -  Added key rotation in token management.

      -  Restrict keys to a single format per message.

      -  Discussed security issues of multiple key formats.

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      -  Make token character set more strict.

      -  Add note on long-polling in continuation requests.

      -  Removed "Models" section.

      -  Rewrote guidance and requirements for extensions.

      -  Require all URIs to be absolute throughout protocol.

      -  Make response from RS a "SHOULD" instead of a "MAY".

      -  Added a way for the client instance to ask for a specific
         user's information, separate from the end-user.

      -  Added security considerations for asynchronous authorization.

      -  Added security considerations for compromised RS.

      -  Added interoperability profiles.

      -  Added implementation status section.

   *  -10

      -  Added note on relating access rights sent as strings to rights
         sent as objects.

      -  Expand proofing methods to allow definition by object, with
         single string as optimization for common cases.

      -  Removed "split_token" functionality.

      -  Collapse "user_code" into a string instead of an object.

      -  References hash algorithm identifiers from the existing IANA
         registry

      -  Allow interaction responses to time out.

      -  Added explicit protocol state discussion.

      -  Added RO policy use case.

   *  -09

      -  Added security considerations on redirection status codes.

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      -  Added security considerations on cuckoo token attack.

      -  Made token management URL required on token rotation.

      -  Added considerations on token rotation and self-contained
         tokens.

      -  Added security considerations for SSRF.

      -  Moved normative requirements about end user presence to
         security considerations.

      -  Clarified default wait times for continuation requests
         (including polling).

      -  Clarified URI vs. URL.

      -  Added "user_code_uri" mode, removed "uri" from "user_code"
         mode.

      -  Consistently formatted all parameter lists.

      -  Updated examples for HTTP Signatures.

   *  -08

      -  Update definition for "Client" to account for the case of no
         end user.

      -  Change definition for "Subject".

      -  Expanded security and privacy considerations for more
         situations.

      -  Added cross-links from security and privacy considerations.

      -  Editorial updates.

   *  -07

      -  Replace user handle by opaque identifier

      -  Added trust relationships

      -  Added privacy considerations section

      -  Added security considerations.

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   *  -06

      -  Removed "capabilities" and "existing_grant" protocol fields.

      -  Removed separate "instance_id" field.

      -  Split "interaction_methods_supported" into
         "interaction_start_modes_supported" and
         "interaction_finish_methods_supported".

      -  Added AS endpoint to hash calculation to fix mix-up attack.

      -  Added "privileges" field to resource access request object.

      -  Moved client-facing RS response back from GNAP-RS document.

      -  Removed oauthpop key binding.

      -  Removed dpop key binding.

      -  Added example DID identifier.

      -  Changed token response booleans to flag structure to match
         request.

      -  Updated signature examples to use HTTP Message Signatures.

   *  -05

      -  Changed "interaction_methods" to
         "interaction_methods_supported".

      -  Changed "key_proofs" to "key_proofs_supported".

      -  Changed "assertions" to "assertions_supported".

      -  Updated discovery and field names for subject formats.

      -  Add an appendix to provide protocol rationale, compared to
         OAuth2.

      -  Updated subject information definition.

      -  Refactored the RS-centric components into a new document.

      -  Updated cryptographic proof of possession methods to match
         current reference syntax.

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      -  Updated proofing language to use "signer" and "verifier"
         generically.

      -  Updated cryptographic proof of possession examples.

      -  Editorial cleanup and fixes.

      -  Diagram cleanup and fixes.

   *  -04

      -  Updated terminology.

      -  Refactored key presentation and binding.

      -  Refactored "interact" request to group start and end modes.

      -  Changed access token request and response syntax.

      -  Changed DPoP digest field to 'htd' to match proposed FAPI
         profile.

      -  Include the access token hash in the DPoP message.

      -  Removed closed issue links.

      -  Removed function to read state of grant request by client.

      -  Closed issues related to reading and updating access tokens.

   *  -03

      -  Changed "resource client" terminology to separate "client
         instance" and "client software".

      -  Removed OpenID Connect "claims" parameter.

      -  Dropped "short URI" redirect.

      -  Access token is mandatory for continuation.

      -  Removed closed issue links.

      -  Editorial fixes.

   *  -02

      -  Moved all "editor's note" items to GitHub Issues.

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      -  Added JSON types to fields.

      -  Changed "GNAP Protocol" to "GNAP".

      -  Editorial fixes.

   *  -01

      -  "updated_at" subject info timestamp now in ISO 8601 string
         format.

      -  Editorial fixes.

      -  Added Aaron and Fabien as document authors.

   *  -00

      -  Initial working group draft.

Appendix B.  Compared to OAuth 2.0

   GNAP's protocol design differs from OAuth 2.0's in several
   fundamental ways:

   1.  *Consent and authorization flexibility:*

       OAuth 2.0 generally assumes the user has access to a web browser.
       The type of interaction available is fixed by the grant type, and
       the most common interactive grant types start in the browser.
       OAuth 2.0 assumes that the user using the client software is the
       same user that will interact with the AS to approve access.

       GNAP allows various patterns to manage authorizations and
       consents required to fulfill this requested delegation, including
       information sent by the client instance, information supplied by
       external parties, and information gathered through the
       interaction process.  GNAP allows a client instance to list
       different ways that it can start and finish an interaction, and
       these can be mixed together as needed for different use cases.
       GNAP interactions can use a browser, but don’t have to.  Methods
       can use inter-application messaging protocols, out-of-band data
       transfer, or anything else.  GNAP allows extensions to define new
       ways to start and finish an interaction, as new methods and
       platforms are expected to become available over time.  GNAP is
       designed to allow the end user and the resource owner to be two
       different people, but still works in the optimized case of them
       being the same party.

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   2.  *Intent registration and inline negotiation:*

       OAuth 2.0 uses different “grant types” that start at different
       endpoints for different purposes.  Many of these require
       discovery of several interrelated parameters.

       GNAP requests all start with the same type of request to the same
       endpoint at the AS.  Next steps are negotiated between the client
       instance and AS based on software capabilities, policies
       surrounding requested access, and the overall context of the
       ongoing request.  GNAP defines a continuation API that allows the
       client instance and AS to request and send additional information
       from each other over multiple steps.  This continuation API uses
       the same access token protection that other GNAP-protected APIs
       use.  GNAP allows discovery to optimize the requests but it isn’t
       required thanks to the negotiation capabilities.

       GNAP is able to handle the life-cycle of an authorization
       request, and therefore simplifies the mental model surrounding
       OAuth2.  For instance, there's no need for refresh tokens when
       the API enables proper rotation of access tokens.

   3.  *Client instances:*

       OAuth 2.0 requires all clients to be registered at the AS and to
       use a client_id known to the AS as part of the protocol.  This
       client_id is generally assumed to be assigned by a trusted
       authority during a registration process, and OAuth places a lot
       of trust on the client_id as a result.  Dynamic registration
       allows different classes of clients to get a client_id at
       runtime, even if they only ever use it for one request.

       GNAP allows the client instance to present an unknown key to the
       AS and use that key to protect the ongoing request.  GNAP’s
       client instance identifier mechanism allows for pre-registered
       clients and dynamically registered clients to exist as an
       optimized case without requiring the identifier as part of the
       protocol at all times.

   4.  *Expanded delegation:*

       OAuth 2.0 defines the “scope” parameter for controlling access to
       APIs.  This parameter has been coopted to mean a number of
       different things in different protocols, including flags for
       turning special behavior on and off, including the return of data
       apart from the access token.  The “resource” parameter and RAR
       extensions (as defined in [I-D.ietf-oauth-rar]) expand on the
       “scope” concept in similar but different ways.

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       GNAP defines a rich structure for requesting access, with string
       references as an optimization.  GNAP defines methods for
       requesting directly-returned user information, separate from API
       access.  This information includes identifiers for the current
       user and structured assertions.  The core GNAP protocol makes no
       assumptions or demands on the format or contents of the access
       token, but the RS extension allows a negotiation of token formats
       between the AS and RS.

   5.  *Cryptography-based security:*

       OAuth 2.0 uses shared bearer secrets, including the client_secret
       and access token, and advanced authentication and sender
       constraint have been built on after the fact in inconsistent
       ways.

       In GNAP, all communication between the client instance and AS is
       bound to a key held by the client instance.  GNAP uses the same
       cryptographic mechanisms for both authenticating the client (to
       the AS) and binding the access token (to the RS and the AS).
       GNAP allows extensions to define new cryptographic protection
       mechanisms, as new methods are expected to become available over
       time.  GNAP does not have a notion of “public clients” because
       key information can always be sent and used dynamically.

   6.  *Privacy and usable security:*

       OAuth 2.0's deployment model assumes a strong binding between the
       AS and the RS.

       GNAP is designed to be interoperable with decentralized identity
       standards and to provide a human-centric authorization layer.  In
       addition to the core protocol, GNAP supports various patterns of
       communication between RSs and ASs through extensions.  GNAP tries
       to limit the odds of a consolidation to just a handful of super-
       popular AS services.

Appendix C.  Example Protocol Flows

   The protocol defined in this specification provides a number of
   features that can be combined to solve many different kinds of
   authentication scenarios.  This section seeks to show examples of how
   the protocol would be applied for different situations.

   Some longer fields, particularly cryptographic information, have been
   truncated for display purposes in these examples.

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C.1.  Redirect-Based User Interaction

   In this scenario, the user is the RO and has access to a web browser,
   and the client instance can take front-channel callbacks on the same
   device as the user.  This combination is analogous to the OAuth 2.0
   Authorization Code grant type.

   The client instance initiates the request to the AS.  Here the client
   instance identifies itself using its public key.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               {
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               }
           ],
       },
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-1",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8..."
           }
         }

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       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

   The AS processes the request and determines that the RO needs to
   interact.  The AS returns the following response giving the client
   instance the information it needs to connect.  The AS has also
   indicated to the client instance that it can use the given instance
   identifier to identify itself in future requests (Section 2.3.1).

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "interact": {
         "redirect":
           "https://server.example.com/interact/4CF492MLVMSW9MKM",
         "finish": "MBDOFXG4Y5CVJCX821LH"
       }
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue"
       },
       "instance_id": "7C7C4AZ9KHRS6X63AJAO"
   }

   The client instance saves the response and redirects the user to the
   interaction start mode's "redirect" URI by sending the following HTTP
   message to the user's browser.

   HTTP 303 Found
   Location: https://server.example.com/interact/4CF492MLVMSW9MKM

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   The user's browser fetches the AS's interaction URI.  The user logs
   in, is identified as the RO for the resource being requested, and
   approves the request.  Since the AS has a callback parameter that was
   sent in the initial request's interaction finish method, the AS
   generates the interaction reference, calculates the hash, and
   redirects the user back to the client instance with these additional
   values added as query parameters.

   NOTE: '\' line wrapping per RFC 8792

   HTTP 302 Found
   Location: https://client.example.net/return/123455\
     ?hash=x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY\
     &interact_ref=4IFWWIKYBC2PQ6U56NL1

   The client instance receives this request from the user's browser.
   The client instance ensures that this is the same user that was sent
   out by validating session information and retrieves the stored
   pending request.  The client instance uses the values in this to
   validate the hash parameter.  The client instance then calls the
   continuation URI using the associated continuation access token and
   presents the interaction reference in the request body.  The client
   instance signs the request as above.

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   The AS retrieves the pending request by looking up the pending grant
   request associated with the presented continuation access token.
   Seeing that the grant is approved, the AS issues an access token and
   returns this to the client instance.

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   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [{
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           }]
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue"
       }
   }

C.2.  Secondary Device Interaction

   In this scenario, the user does not have access to a web browser on
   the device and must use a secondary device to interact with the AS.
   The client instance can display a user code or a printable QR code.
   The client instance is not able to accept callbacks from the AS and
   needs to poll for updates while waiting for the user to authorize the
   request.

   The client instance initiates the request to the AS.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "dolphin-metadata", "some other thing"
           ],
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "start": ["redirect", "user_code"]
       }
   }

   The AS processes this and determines that the RO needs to interact.
   The AS supports both redirect URIs and user codes for interaction, so
   it includes both.  Since there is no interaction finish mode, the AS
   does not include a nonce, but does include a "wait" parameter on the
   continuation section because it expects the client instance to poll
   for results.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "interact": {
           "redirect": "https://srv.ex/MXKHQ",
           "user_code": {
               "code": "A1BC3DFF"
           }
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue/VGJKPTKC50",
           "wait": 60
       }
   }

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   The client instance saves the response and displays the user code
   visually on its screen along with the static device URI.  The client
   instance also displays the short interaction URI as a QR code to be
   scanned.

   If the user scans the code, they are taken to the interaction
   endpoint and the AS looks up the current pending request based on the
   incoming URI.  If the user instead goes to the static page and enters
   the code manually, the AS looks up the current pending request based
   on the value of the user code.  In both cases, the user logs in, is
   identified as the RO for the resource being requested, and approves
   the request.  Once the request has been approved, the AS displays to
   the user a message to return to their device.

   Meanwhile, the client instance periodically polls the AS every 60
   seconds at the continuation URI.  The client instance signs the
   request using the same key and method that it did in the first
   request.

   POST /continue/VGJKPTKC50 HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The AS retrieves the pending request based on the pending grant
   request associated with the continuation access token and determines
   that it has not yet been authorized.  The AS indicates to the client
   instance that no access token has yet been issued but it can continue
   to call after another 60 second timeout.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "G7YQT4KQQ5TZY9SLSS5E"
           },
           "uri": "https://server.example.com/continue/ATWHO4Q1WV",
           "wait": 60
       }
   }

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   Note that the continuation URI and access token have been rotated
   since they were used by the client instance to make this call.  The
   client instance polls the continuation URI after a 60 second timeout
   using this new information.

   POST /continue/ATWHO4Q1WV HTTP/1.1
   Host: server.example.com
   Authorization: GNAP G7YQT4KQQ5TZY9SLSS5E
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The AS retrieves the pending request based on the URI and access
   token, determines that it has been approved, and issues an access
   token for the client to use at the RS.

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               "dolphin-metadata", "some other thing"
           ]
       }
   }

C.3.  No User Involvement

   In this scenario, the client instance is requesting access on its own
   behalf, with no user to interact with.

   The client instance creates a request to the AS, identifying itself
   with its public key and using MTLS to make the request.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json

   {
       "access_token": {
           "access": [
               "backend service", "nightly-routine-3"
           ],
       },
       "client": {
         "key": {
           "proof": "mtls",
           "cert#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
         }
       }
   }

   The AS processes this and determines that the client instance can ask
   for the requested resources and issues an access token.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token",
           "access": [
               "backend service", "nightly-routine-3"
           ]
       }
   }

C.4.  Asynchronous Authorization

   In this scenario, the client instance is requesting on behalf of a
   specific RO, but has no way to interact with the user.  The AS can
   asynchronously reach out to the RO for approval in this scenario.

   The client instance starts the request at the AS by requesting a set
   of resources.  The client instance also identifies a particular user.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "read", "dolphin-metadata",
               {
                   "type": "financial-transaction",
                   "actions": [
                       "withdraw"
                   ],
                   "identifier": "account-14-32-32-3",
                   "currency": "USD"
               },
               "some other thing"
           ],
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "user": {
           "sub_ids": [ {
               "format": "opaque",
               "id": "J2G8G8O4AZ"
           } ]
     }
   }

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   The AS processes this and determines that the RO needs to interact.
   The AS determines that it can reach the identified user
   asynchronously and that the identified user does have the ability to
   approve this request.  The AS indicates to the client instance that
   it can poll for continuation.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   The AS reaches out to the RO and prompts them for consent.  In this
   example, the AS has an application that it can push notifications in
   to for the specified account.

   Meanwhile, the client instance periodically polls the AS every 60
   seconds at the continuation URI.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS retrieves the pending request based on the continuation access
   token and determines that it has not yet been authorized.  The AS
   indicates to the client instance that no access token has yet been
   issued but it can continue to call after another 60 second timeout.

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "BI9QNW6V9W3XFJK4R02D"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   Note that the continuation access token value has been rotated since
   it was used by the client instance to make this call.  The client
   instance polls the continuation URI after a 60 second timeout using
   the new token.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP BI9QNW6V9W3XFJK4R02D
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS retrieves the pending request based on the handle and
   determines that it has been approved and it issues an access token.

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               "dolphin-metadata", "some other thing"
           ]
       }
   }

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C.5.  Applying OAuth 2.0 Scopes and Client IDs

   While GNAP is not designed to be directly compatible with OAuth 2.0
   [RFC6749], considerations have been made to enable the use of OAuth
   2.0 concepts and constructs more smoothly within GNAP.

   In this scenario, the client developer has a client_id and set of
   scope values from their OAuth 2.0 system and wants to apply them to
   the new protocol.  Traditionally, the OAuth 2.0 client developer
   would put their client_id and scope values as parameters into a
   redirect request to the authorization endpoint.

   NOTE: '\' line wrapping per RFC 8792

   HTTP 302 Found
   Location: https://server.example.com/authorize\
     ?client_id=7C7C4AZ9KHRS6X63AJAO\
     &scope=read%20write%20dolphin\
     &redirect_uri=https://client.example.net/return\
     &response_type=code\
     &state=123455

   Now the developer wants to make an analogous request to the AS using
   GNAP.  To do so, the client instance makes an HTTP POST and places
   the OAuth 2.0 values in the appropriate places.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write", "dolphin"
           ],
           "flags": [ "bearer" ]
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return?state=123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

   The client_id can be used to identify the client instance's keys that
   it uses for authentication, the scopes represent resources that the
   client instance is requesting, and the redirect_uri and state value
   are pre-combined into a finish URI that can be unique per request.
   The client instance additionally creates a nonce to protect the
   callback, separate from the state parameter that it has added to its
   return URI.

   From here, the protocol continues as above.

Appendix D.  Interoperability Profiles

   The GNAP specification has many different modes, options, and
   mechanisms, allowing it to solve a wide variety of problems in a wide
   variety of deployments.  The wide applicability of GNAP makes it
   difficult, if not impossible, to define a set of mandatory-to-
   implement features, since one environment's required feature would be
   impossible to do in another environment.  While this is a large
   problem in many systems, GNAP's back-and-forth negotiation process
   allows parties to declare at runtime everything that they support and
   then have the other party select from that the subset of items that
   they also support, leading to functional compatibility in many parts
   of the protocol even in an open world scenario.

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   In addition, GNAP defines a set of interoperability profiles which
   gather together core requirements to fix options into common
   configurations that are likely to be useful to large populations of
   similar applications.

   Conformant AS implementations of these profiles MUST implement at
   least the features as specified in the profile and MAY implement
   additional features or profiles.  Conformant client implementations
   of these profiles MUST implement at least the features as specified,
   except where a subset of the features allows the protocol to function
   (such as using polling instead of a push finish method for the
   Secondary Device profile).

D.1.  Web-based Redirection

   Implementations conformant to the Web-based Redirection profile of
   GNAP MUST implement all of the following features:

   *  _Interaction Start Methods_: redirect

   *  _Interaction Finish Methods_: redirect

   *  _Interaction Hash Algorithms_: sha-256

   *  _Key Proofing Methods_: httpsig with no additional parameters

   *  _Key Formats_: jwks with signature algorithm included in the key's
      alg parameter

   *  _JOSE Signature Algorithm_: PS256

   *  _Subject Identifier Formats_: opaque

   *  _Assertion Formats_: id_token

D.2.  Secondary Device

   Implementations conformant to the Secondary Device profile of GNAP
   MUST implement all of the following features:

   *  _Interaction Start Methods_: user_code and user_code_uri

   *  _Interaction Finish Methods_: push

   *  _Interaction Hash Algorithms_: sha-256

   *  _Key Proofing Methods_: httpsig with no additional parameters

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   *  _Key Formats_: jwks with signature algorithm included in the key's
      alg parameter

   *  _JOSE Signature Algorithm_: PS256

   *  _Subject Identifier Formats_: opaque

   *  _Assertion Formats_: id_token

Appendix E.  Guidance for Extensions

   Extensions to this specification have a variety of places to alter
   the protocol, including many fields and objects that can have
   additional values in a registry (Section 11) established by this
   specification.  Extensions that add new fields, especially to the
   grant request and response, should endeavor to have any new fields be
   as orthogonal as possible to existing fields.  That is to say, if
   functionality is sufficiently close to an existing field, the
   extension should attempt to use that field instead of defining a new
   one, in order to avoid confusion by developers.

   Most object fields in GNAP are specified with types, and those types
   can allow different but related behavior.  For example, the access
   array can include either strings or objects, as discussed in
   Section 8.  The use of JSON polymorphism (Appendix F) within GNAP
   allows extensions to define new fields by not only choosing a new
   name but also by using an existing name with a new type.  However,
   the extension's definition of a new type for a field needs to fit the
   same kind of item being extended.  For example, a hypothetical
   extension could define a string value for the access_token request
   field, with a URL to download a hosted access token request.  Such an
   extension would be appropriate as the access_token field still
   defines the access tokens being requested.  However, if an extension
   were to define a string value for the access_token request field,
   with the value instead being something unrelated to the access token
   request such as a value or key format, this would not be an
   appropriate means of extension.  (Note that this specific extension
   example would create another form of SSRF attack surface as discussed
   in Section 13.33.)

   For another example, both interaction interaction start modes
   (Section 2.5.1) and key proofing methods (Section 7.3) can be defined
   as either strings or objects.  An extension could take a method
   defined as a string, such as app, and define an object-based version
   with additional parameters.  This extension should still define a
   method to launch an application on the end user's device, just like
   app does when specified as a string.

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   Additionally, the ability to deal with different types for a field is
   not expected to be equal between an AS and client software, with the
   client software being assumed to be both more varied and more
   simplified than the AS.  Furthermore, the nature of the negotiation
   process in GNAP allows the AS more chance of recovery from unknown
   situations and parameters.  As such, any extensions that change the
   type of any field returned to a client instance should only do so
   when the client instance has indicated specific support for that
   extension through some kind of request parameter.

Appendix F.  JSON Structures and Polymorphism

   GNAP makes use of polymorphism within the JSON [RFC8259] structures
   used for the protocol.  Each portion of this protocol is defined in
   terms of the JSON data type that its values can take, whether it's a
   string, object, array, boolean, or number.  For some fields,
   different data types offer different descriptive capabilities and are
   used in different situations for the same field.  Each data type
   provides a different syntax to express the same underlying semantic
   protocol element, which allows for optimization and simplification in
   many common cases.

   Even though JSON is often used to describe strongly typed structures,
   JSON on its own is naturally polymorphic.  In JSON, the named members
   of an object have no type associated with them, and any data type can
   be used as the value for any member.  In practice, each member has a
   semantic type that needs to make sense to the parties creating and
   consuming the object.  Within this protocol, each object member is
   defined in terms of its semantic content, and this semantic content
   might have expressions in different concrete data types for different
   specific purposes.  Since each object member has exactly one value in
   JSON, each data type for an object member field is naturally mutually
   exclusive with other data types within a single JSON object.

   For example, a resource request for a single access token is composed
   of an object of resource request descriptions while a request for
   multiple access tokens is composed of an array whose member values
   are all objects.  Both of these represent requests for access, but
   the difference in syntax allows the client instance and AS to
   differentiate between the two request types in the same request.

   Another form of polymorphism in JSON comes from the fact that the
   values within JSON arrays need not all be of the same JSON data type.
   However, within this protocol, each element within the array needs to
   be of the same kind of semantic element for the collection to make
   sense, even when the data types are different from each other.

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   For example, each aspect of a resource request can be described using
   an object with multiple dimensional components, or the aspect can be
   requested using a string.  In both cases, the resource request is
   being described in a way that the AS needs to interpret, but with
   different levels of specificity and complexity for the client
   instance to deal with.  An API designer can provide a set of common
   access scopes as simple strings but still allow client software
   developers to specify custom access when needed for more complex
   APIs.

   Extensions to this specification can use different data types for
   defined fields, but each extension needs to not only declare what the
   data type means, but also provide justification for the data type
   representing the same basic kind of thing it extends.  For example,
   an extension declaring an "array" representation for a field would
   need to explain how the array represents something akin to the non-
   array element that it is replacing.  See additional discussion in
   Appendix E.

Authors' Addresses

   Justin Richer (editor)
   Bespoke Engineering
   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Fabien Imbault
   acert.io
   Email: fabien.imbault@acert.io
   URI:   https://acert.io/

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