Grant Negotiation and Authorization Protocol
draft-ietf-gnap-core-protocol-03

Document Type Active Internet-Draft (gnap WG)
Authors Justin Richer  , Aaron Parecki  , Fabien Imbault 
Last updated 2021-01-06
Replaces draft-richer-transactional-authz
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GNAP                                                      J. Richer, Ed.
Internet-Draft                                       Bespoke Engineering
Intended status: Standards Track                              A. Parecki
Expires: 10 July 2021                                               Okta
                                                              F. Imbault
                                                                acert.io
                                                          6 January 2021

              Grant Negotiation and Authorization Protocol
                    draft-ietf-gnap-core-protocol-03

Abstract

   GNAP defines a mechanism for delegating authorization to a piece of
   software, and conveying that delegation to the software.  This
   delegation can include access to a set of APIs as well as 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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   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 10 July 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Sequences . . . . . . . . . . . . . . . . . . . . . . . .   7
       1.4.1.  Redirect-based Interaction  . . . . . . . . . . . . .  10
       1.4.2.  User-code Interaction . . . . . . . . . . . . . . . .  12
       1.4.3.  Asynchronous Authorization  . . . . . . . . . . . . .  15
       1.4.4.  Software-only Authorization . . . . . . . . . . . . .  16
       1.4.5.  Refreshing an Expired Access Token  . . . . . . . . .  17
   2.  Requesting Access . . . . . . . . . . . . . . . . . . . . . .  18
     2.1.  Requesting Resources  . . . . . . . . . . . . . . . . . .  20
       2.1.1.  Requesting a Single Access Token  . . . . . . . . . .  20
       2.1.2.  Requesting Resources By Reference . . . . . . . . . .  24
       2.1.3.  Requesting Multiple Access Tokens . . . . . . . . . .  26
       2.1.4.  Signaling Token Behavior  . . . . . . . . . . . . . .  28
     2.2.  Requesting User Information . . . . . . . . . . . . . . .  29
     2.3.  Identifying the Client Instance . . . . . . . . . . . . .  30
       2.3.1.  Identifying the Client Instance . . . . . . . . . . .  32
       2.3.2.  Identifying the Client Instance Key . . . . . . . . .  33
       2.3.3.  Providing Displayable Client Instance Information . .  34
       2.3.4.  Authenticating the Client Instance  . . . . . . . . .  34
     2.4.  Identifying the User  . . . . . . . . . . . . . . . . . .  35
       2.4.1.  Identifying the User by Reference . . . . . . . . . .  36
     2.5.  Interacting with the User . . . . . . . . . . . . . . . .  36
       2.5.1.  Redirect to an Arbitrary URL  . . . . . . . . . . . .  38
       2.5.2.  Open an Application-specific URL  . . . . . . . . . .  38
       2.5.3.  Receive a Callback After Interaction  . . . . . . . .  39
       2.5.4.  Display a Short User Code . . . . . . . . . . . . . .  41
       2.5.5.  Indicate Desired Interaction Locales  . . . . . . . .  41
       2.5.6.  Extending Interaction Modes . . . . . . . . . . . . .  41
     2.6.  Declaring Client Capabilities . . . . . . . . . . . . . .  42
     2.7.  Referencing an Existing Grant Request . . . . . . . . . .  42
     2.8.  Extending The Grant Request . . . . . . . . . . . . . . .  42
   3.  Grant Response  . . . . . . . . . . . . . . . . . . . . . . .  42
     3.1.  Request Continuation  . . . . . . . . . . . . . . . . . .  44
     3.2.  Access Tokens . . . . . . . . . . . . . . . . . . . . . .  45

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       3.2.1.  Single Access Token . . . . . . . . . . . . . . . . .  45
       3.2.2.  Multiple Access Tokens  . . . . . . . . . . . . . . .  48
     3.3.  Interaction Modes . . . . . . . . . . . . . . . . . . . .  49
       3.3.1.  Redirection to an arbitrary URL . . . . . . . . . . .  49
       3.3.2.  Launch of an application URL  . . . . . . . . . . . .  50
       3.3.3.  Post-interaction Callback to a Client Instance
               Accessible URL  . . . . . . . . . . . . . . . . . . .  50
       3.3.4.  Display of a Short User Code  . . . . . . . . . . . .  51
       3.3.5.  Extending Interaction Mode Responses  . . . . . . . .  52
     3.4.  Returning User Information  . . . . . . . . . . . . . . .  52
     3.5.  Returning Dynamically-bound Reference Handles . . . . . .  53
     3.6.  Error Response  . . . . . . . . . . . . . . . . . . . . .  55
     3.7.  Extending the Response  . . . . . . . . . . . . . . . . .  55
   4.  Interaction at the AS . . . . . . . . . . . . . . . . . . . .  55
     4.1.  Interaction at a Redirected URI . . . . . . . . . . . . .  56
     4.2.  Interaction at the User Code URI  . . . . . . . . . . . .  56
     4.3.  Interaction through an Application URI  . . . . . . . . .  57
     4.4.  Post-Interaction Completion . . . . . . . . . . . . . . .  57
       4.4.1.  Completing Interaction with a Browser Redirect to the
               Callback URI  . . . . . . . . . . . . . . . . . . . .  57
       4.4.2.  Completing Interaction with a Direct HTTP Request
               Callback  . . . . . . . . . . . . . . . . . . . . . .  58
       4.4.3.  Calculating the interaction hash  . . . . . . . . . .  59
   5.  Continuing a Grant Request  . . . . . . . . . . . . . . . . .  60
     5.1.  Continuing After a Completed Interaction  . . . . . . . .  62
     5.2.  Continuing During Pending Interaction . . . . . . . . . .  63
     5.3.  Modifying an Existing Request . . . . . . . . . . . . . .  64
     5.4.  Getting the Current State of a Grant Request  . . . . . .  69
     5.5.  Canceling a Grant Request . . . . . . . . . . . . . . . .  70
   6.  Token Management  . . . . . . . . . . . . . . . . . . . . . .  70
     6.1.  Rotating the Access Token . . . . . . . . . . . . . . . .  71
     6.2.  Revoking the Access Token . . . . . . . . . . . . . . . .  72
   7.  Using Access Tokens . . . . . . . . . . . . . . . . . . . . .  73
   8.  Binding Keys  . . . . . . . . . . . . . . . . . . . . . . . .  74
     8.1.  Detached JWS  . . . . . . . . . . . . . . . . . . . . . .  75
     8.2.  Attached JWS  . . . . . . . . . . . . . . . . . . . . . .  77
     8.3.  Mutual TLS  . . . . . . . . . . . . . . . . . . . . . . .  81
     8.4.  Demonstration of Proof-of-Possession (DPoP) . . . . . . .  83
     8.5.  HTTP Signing  . . . . . . . . . . . . . . . . . . . . . .  84
     8.6.  OAuth Proof of Possession (PoP) . . . . . . . . . . . . .  85
   9.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . .  87
   10. Resource Servers  . . . . . . . . . . . . . . . . . . . . . .  88
     10.1.  Introspecting a Token  . . . . . . . . . . . . . . . . .  88
     10.2.  Deriving a downstream token  . . . . . . . . . . . . . .  90
     10.3.  Registering a Resource Handle  . . . . . . . . . . . . .  91
     10.4.  Requesting Resources With Insufficient Access  . . . . .  93
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  93
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  94

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   13. Security Considerations . . . . . . . . . . . . . . . . . . .  94
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  94
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  94
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  96
   Appendix B.  Component Data Models  . . . . . . . . . . . . . . .  97
   Appendix C.  Example Protocol Flows . . . . . . . . . . . . . . .  97
     C.1.  Redirect-Based User Interaction . . . . . . . . . . . . .  97
     C.2.  Secondary Device Interaction  . . . . . . . . . . . . . . 101
   Appendix D.  No User Involvement  . . . . . . . . . . . . . . . . 104
     D.1.  Asynchronous Authorization  . . . . . . . . . . . . . . . 105
     D.2.  Applying OAuth 2 Scopes and Client IDs  . . . . . . . . . 108
   Appendix E.  JSON Structures and Polymorphism . . . . . . . . . . 109
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 110

1.  Introduction

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

   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 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.  Even so, GNAP
   and OAuth 2.0 will 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 D.2.

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.

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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.

   Authorization Server (AS)  Manages the requested delegations for the
      RO.  The AS issues tokens and directly delegated information to an
      instance of the client.  The AS is defined by its grant endpoint,
      a single URL that accepts a POST request with a JSON payload.  The
      AS could also have other endpoints, including interaction
      endpoints and user code endpoints, and these are introduced to the
      RC as needed during the delegation process.

   Client  Requests tokens and directly delegated information from the
      AS, and uses tokens at the RS.  For some kinds of client software,
      there could be many instances of a single piece of client
      software.  This specification differentiates between a specific
      instance (the client instance) and the software running the
      instance (the client software).  A client instance is identified
      by its unique key, which can be known to the AS prior to the first
      request or introduced to the AS as part of the protocol.  The AS
      determines which policies apply to a given client instance,
      including what it can request and on whose behalf.

   Resource Server (RS, aka "API")  Accepts tokens from the client
      instance issued by the AS and serves delegated resources on behalf
      of the RO.  There could be multiple RSs protected by the AS that
      the client instance will call.

   Resource Owner (RO)  Authorizes the request from the client instance
      to the RS, often interactively at the AS.

   Requesting Party (RQ, aka "user")  Operates and interacts with the
      client instance.

   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 RQ 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 RQ

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   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 RQ'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.

   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 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.

   [[ See issue #29 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/29) ]]

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.

   Access Token  A credential representing a set of access rights
      delegated to the client instance.  The access token is created by
      the AS, consumed and verified by the RS, and issued to and carried
      by the client instance.  The contents and format of the access
      token are opaque to the client.

   Grant  The process by which the client instance requests and is given

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      delegated access to the RS by the AS through the authority of the
      RO.

   Cryptographic Key  A cryptographic element binding a request to a
      holder of the key.  Access tokens and client instances can be
      associated with specific keys.

   Resource  A protected API served by the RS and accessed by the client
      instance.  Access to this resource is delegated by the RO as part
      of the grant process.

   Subject Information  Information about the RO that is returned
      directly to the client instance from the AS without the client
      instance making a separate call to an RS.  Access to this
      information is delegated by the RO as part of the grant process.

1.4.  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 fulfil
   multiple roles, and so explicit communication between the roles is
   not necessary within the protocol flow.

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        +------------+             +------------+
        | Requesting | ~ ~ ~ ~ ~ ~ |  Resource  |
        | Party (RQ) |             | 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 RQ 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 RQ
      are often the same entity in practice.

   *  (1) The client instance attempts to call the RS (Section 10.4) to
      determine what access is needed.  The RS informs the client
      instance that access can be granted through the AS.  Note that for
      most situations, the client instance already knows which AS to
      talk to and which kinds of access it needs.

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

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   *  (3) The AS processes the request and determines what is needed to
      fulfill the request.  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 RQ.  Note that the RO and RQ
      are often the same entity in practice.

   *  (4) The client instance continues the grant at the AS (Section 5).

   *  (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) to call
      the RS.

   *  (7) The RS determines if the token is sufficient for the request
      by examining the token, potentially calling the AS (Section 10.1).
      Note that the RS could also examine the token directly, call an
      internal data store, execute a policy engine request, or any
      number of alternative methods for validating the token and its
      fitness for the request.

   *  (8) The client instance calls the RS (Section 7) 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 fetches an
      updated access token (Section 6.1) based on the rights granted in
      (5).

   *  (10) The AS issues a new access token (Section 3.2) to the client
      instance.

   *  (11) The client instance uses the new access token (Section 7) to
      call the RS.

   *  (12) The RS determines if the new token is sufficient for the
      request by examining the token, potentially calling the AS
      (Section 10.1).

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   *  (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.

1.4.1.  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 requesting party (RQ) and the resource owner (RO).  Since
   the client instance is capable of directing the user to an arbitrary
   URL and receiving responses from the user's browser, interaction here
   is handled through front-channel redirects using the user's browser.
   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.

    +--------+                                  +--------+         +------+
    | Client |                                  |   AS   |         |  RO  |
    |Instance|                                  |        |         |  +   |
    |        |< (1) + Start Session + + + + + + + + + + + + + + + +|  RQ  |
    |        |                                  |        |         |(User)|
    |        |--(2)--- Request Access --------->|        |         |      |
    |        |                                  |        |         |      |
    |        |<-(3)-- Interaction Needed -------|        |         |      |
    |        |                                  |        |         |      |
    |        |+ (4) + Redirect for Interaction + + + + + + + + + > |      |
    |        |                                  |        |         |      |
    |        |                                  |        |<+ (5) +>|      |
    |        |                                  |        |  AuthN  |      |
    |        |                                  |        |         |      |
    |        |                                  |        |<+ (6) +>|      |
    |        |                                  |        |  AuthZ  |      |
    |        |                                  |        |         |      |
    |        |< (7) + Redirect for Continuation + + + + + + + + + +|      |
    |        |                                  |        |         +------+
    |        |--(8)--- Continue Request ------->|        |
    |        |                                  |        |
    |        |<-(9)----- Grant Access ----------|        |
    |        |                                  |        |
    +--------+                                  +--------+

   1.  The client instance establishes a verifiable session to the user,
       in the role of the RQ.

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   2.  The client instance requests access to the resource (Section 2).
       The client instance indicates that it can redirect to an
       arbitrary URL (Section 2.5.1) and receive a callback from the
       browser (Section 2.5.3).  The client instance stores verification
       information for its callback in the session created in (1).

   3.  The AS determines that interaction is needed and responds
       (Section 3) with a URL to send the user to (Section 3.3.1) and
       information needed to verify the callback (Section 3.3.3) 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).

   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 URL (Section 4.1) given
       by the AS in (3).  The user's browser loads the interaction
       redirect URL.  The AS loads the pending request based on the
       incoming URL 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 4.4.1) to the client instance using the
       callback URL provided in (2).  The callback URL is augmented with
       an interaction reference that the AS associates with the ongoing
       request created in (2) and referenced in (4).  The callback URL
       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 4.4.3) 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 RQ never being
       returned to the client instance and handle time outs
       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.

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   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.

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

1.4.2.  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 URL.  The client instance is
   not capable of presenting an arbitrary URL 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 RQ and RO, though the user is not assumed to be interacting
   with the client instance through the same web browser used for
   interaction at the AS.

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

   1.   The client instance requests access to the resource (Section 2).
        The client instance indicates that it can display a user code
        (Section 2.5.4).

   2.   The AS determines that interaction is needed and responds
        (Section 3) with a user code to communicate to the user
        (Section 3.3.4).  This could optionally include a URL to direct
        the user to, but this URL should be static and so could be
        configured in the client instance's documentation.  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).

   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) given by the AS in (2).

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   4.   The user's directs their browser to the user code URL.  This URL
        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 URL.

   5.   The RQ 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.

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

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1.4.3.  Asynchronous Authorization

   In this example flow, the RQ 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 ---------|        |
    |        |                                  |        |
    +--------+                                  +--------+

   1.  The client instance requests access to the resource (Section 2).
       The client instance does not send any interactions 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 user request section
       (Section 2.4).

   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).

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   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
       resources 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 (3) 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.  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.

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

1.4.4.  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 a 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.

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       +--------+                                  +--------+
       | Client |                                  |   AS   |
       |Instance|--(1)--- Request Access --------->|        |
       |        |                                  |        |
       |        |<-(2)---- Grant Access -----------|        |
       |        |                                  |        |
       +--------+                                  +--------+

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

   2.  The AS determines that the request is 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.

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

1.4.5.  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 new access
   token by rotating the expired access token at the AS using the
   token's management URL.

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

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

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   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 presents the token (Section 7) to the RS.
       The RS validates the token and returns an appropriate response
       for the API.

   4.  When the access token is expired, the RS responds to the client
       instance with an error.

   5.  The client instance calls the token management URI returned in
       (2) to rotate the access token (Section 6.1).  The client
       instance presents the access token as well as the appropriate
       key.

   6.  The AS validates the rotation request including the signature and
       keys presented in (5) and returns a new access token
       (Section 3.2.1).  The response includes a new access token and
       can also include updated token management information, which the
       client instance will store in place of the values returned in
       (2).

2.  Requesting Access

   To start a request, the client instance sends JSON [RFC8259] document
   with an object as its root.  Each member of the request object
   represents a different aspect of the client instance's request.  Each
   field is described in detail in a section below.

   resources (object / array of objects/strings)  Describes the rights
      that the client instance is requesting for one or more access
      tokens to be used at RS's.  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.  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 at the AS.  Section 2.3

   user (object / string)  Identifies the RQ to the AS in a manner that
      the AS can verify, either directly or by interacting with the RQ
      to determine their status as the RO.  Section 2.4

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   interact (object)  Describes the modes that the client instance has
      for allowing the RO to interact with the AS and modes for the
      client instance to receive updates when interaction is complete.
      Section 2.5

   capabilities (array of strings)  Identifies named extension
      capabilities that the client instance can use, signaling to the AS
      which extensions it can use.  Section 2.6

   existing_grant (string)  Identifies a previously-existing grant that
      the client instance is extending with this request.  Section 2.7

   Additional members of this request object can be defined by
   extensions to this protocol as described in Section 2.8

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

   {
       "resources": [
           {
               "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": "jwsd",
           "jwk": {
                       "kty": "RSA",
                       "e": "AQAB",
                       "kid": "xyz-1",

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                       "alg": "RS256",
                       "n": "kOB5rR4Jv0GMeL...."
           }
         }
       },
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "capabilities": ["ext1", "ext2"],
       "subject": {
           "sub_ids": ["iss_sub", "email"],
           "assertions": ["id_token"]
       }
   }

   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 Resources

   If the client instance is requesting one or more access tokens for
   the purpose of accessing an API, the client instance MUST include a
   "resources" field.  This field MUST be an array (for a single access
   token (Section 2.1.1)) or an object (for multiple access tokens
   (Section 2.1.3)), as described in the following sections.

2.1.1.  Requesting a Single Access Token

   When requesting an access token, the client instance MUST send a
   "resources" field containing a JSON array.  The elements of the JSON
   array represent rights of access that the client instance is
   requesting in the access token.  The requested access is the union of
   all elements within the array.

   The client instance declares what access it wants to associate with
   the resulting access token using objects that describe multiple
   dimensions of access.  Each object contains a "type" property that
   determines the type of API that the client instance is calling.

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

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   The value of this 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 its own properties, a
   set of common properties are defined here.  Specific API
   implementations SHOULD NOT re-use these fields with different
   semantics or syntax.  The available values for these properties are
   determined by the API being protected at the RS.

   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.

   The following non-normative example is asking for 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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       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "delete"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           }
       ]

   The access requested for a given object when using these fields is
   the cross-product of all fields of the object.  That is to say, the
   object represents a request for all "action" values listed within the
   object to be used at all "locations" values listed within the object
   for all "datatype" values listed within the object.  Assuming the
   request above was granted, the RC 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
   "action" against one "location" and a different "action" against a
   different "location", the RC can include multiple separate objects in
   the "resources" array.  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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       "resources": [
           {
               "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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       "resources": [
           {
               "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
   (Section 3.2.1)'s access rights will be the union of the requested
   types of access for each of the two APIs, just as above.

2.1.2.  Requesting Resources By Reference

   Instead of sending an object describing the requested resource
   (Section 2.1.1), a client instance MAY send a string known to the AS
   or RS representing the access being requested.  Each string SHOULD
   correspond to a specific expanded object representation at the AS.

       "resources": [
           "read", "dolphin-metadata", "some other thing"
       ]

   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

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   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'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 scope value as in
   Appendix D.2.  Note that the reference string here is not bound to
   the same character restrictions as in OAuth 2's "scope" definition.

   A single "resources" array MAY include both object-type and string-
   type resource items.  In this non-normative example, the RC 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".

       "resources": [
           {
               "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"
       ]

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   The requested access is the union of all elements of the array,
   including both objects and reference strings.

2.1.3.  Requesting Multiple Access Tokens

   When requesting multiple access tokens, the resources field is a JSON
   object.  The names of the JSON object fields are token identifiers
   chosen by the client instance, and MAY be any valid string.  The
   values of the JSON object fields are JSON arrays representing a
   single access token request, as specified in requesting a single
   access token (Section 2.1.1).

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

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

   Any approved access requests are returned in the multiple access
   token response (Section 3.2.2) structure using the token identifiers
   in the request.

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2.1.4.  Signaling Token Behavior

   While the AS is ultimately in control of how tokens are returned and
   bound to the client instance, sometimes the client instance has
   context about what it can support that can affect the AS's response.
   This specification defines several flags that are passed as resource
   reference strings (Section 2.1.2).

   Each flag applies only to the single resource request in which it
   appears.

   Support of all flags is optional, such as any other resource
   reference value.

   multi_token  The client instance wishes to support multiple
      simultaneous access tokens through the token rotation process.
      When the client instance rotates an access token (Section 6.1),
      the AS does not invalidate the previous access token.  The old
      access token continues to remain valid until such time as it
      expires or is revoked through other means.

   split_token  The client instance is capable of receiving multiple
      access tokens (Section 3.2.2) in response to any single token
      request (Section 2.1.1), or receiving a different number of tokens
      than specified in the multiple token request (Section 2.1.3).  The
      labels of the returned additional tokens are chosen by the AS.
      The client instance MUST be able to tell from the token response
      where and how it can use each of the access tokens.  [[ See issue
      #37 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/37)
      ]]

   bind_token  The client instance wants the issued access token to be
      bound to the key the client instance used (Section 2.3.2) to make
      the request.  The resulting access token MUST be bound using the
      same "proof" mechanism used by the client instance with a "key"
      value of "true", indicating the client instance's presented key is
      to be used for binding.  [[ See issue #38 (https://github.com/
      ietf-wg-gnap/gnap-core-protocol/issues/38) ]]

   The AS MUST respond with any applied flags in the token response
   (Section 3.2) "resources" section.

   In this non-normative example, the requested access token is to be
   bound to the client instance's key and should be kept during
   rotation.

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       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "read",
           "bind_token",
           "multi_token"
       ]

   Additional flags can be registered in a registry TBD (Section 12).

   [[ See issue #39 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/39) ]]

2.2.  Requesting User 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 (or additional fields defined in a
   registry TBD (Section 12)).

   sub_ids (array of strings)  An array of subject identifier subject
      types requested for the RO, as defined by
      [I-D.ietf-secevent-subject-identifiers].

   assertions (array of strings)  An array of requested assertion
      formats.  Possible values include "id_token" for an [OIDC] ID
      Token and "saml2" for a SAML 2 assertion.  Additional assertion
      values are defined by a registry TBD (Section 12).  [[ See issue
      #41 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/41)
      ]]

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   "subject": {
      "sub_ids": [ "iss_sub", "email" ],
      "assertions": [ "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.

   Subject identifiers 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.

   The AS SHOULD NOT re-use subject identifiers for multiple different
   ROs.

   [[ See issue #42 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/42) ]]

   Note: the "sub_ids" and "assertions" request fields are independent
   of each other, and a returned assertion MAY omit a requested subject
   identifier.

   [[ See issue #43 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/43) ]]

2.3.  Identifying the Client Instance

   When sending a non-continuation request to the AS, the client
   instance MUST identify itself by including the "client" field of the
   request and by signing the request as described in Section 8.  Note
   that for a continuation request (Section 5), the client instance is
   identified by its association with the request being continued and so
   this field is not sent under those circumstances.

   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 2.3.2.  This field is
      REQUIRED.

   class_id (string)  An identifier string that the AS can use to

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      identify the client software comprising this client instance.  The
      contents and format of this field are up to the AS.  This field is
      OPTIONAL.

   display (object)  An object containing additional information that
      the AS MAY display to the RO during interaction, authorization,
      and management.  This field is OPTIONAL.

"client": {
    "key": {
        "proof": "httpsig",
        "jwk": {
                    "kty": "RSA",
                    "e": "AQAB",
                    "kid": "xyz-1",
                    "alg": "RS256",
                    "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
        },
        "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
    },
    "class_id": "web-server-1234",
    "display": {
        "name": "My Client Display Name",
        "uri": "https://example.net/client"
    }
}

   Additional fields are defined in a registry TBD (Section 12).

   The client instance MUST prove possession of any presented key by the
   "proof" mechanism associated with the key in the request.  Proof
   types are defined in a registry TBD (Section 12) and an initial set
   of methods is described in Section 8.

   Note that the AS MAY know the client instance's public key ahead of
   time, and the AS MAY apply different policies to the request
   depending on what has been registered against that key.  If the same
   public key is sent by value on subsequent access requests, the AS
   SHOULD 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.

   [[ See issue #44 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/44) ]]

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2.3.1.  Identifying the Client Instance

   If the client instance has an instance identifier that the AS can use
   to determine appropriate key information, the client instance can
   send this value in the "instance_id" field.  The instance identifier
   MAY be assigned to a client instance at runtime through the
   Section 3.5 or MAY be obtained in another fashion, such as a static
   registration process at the AS.

   instance_id (string)  An identifier string that the AS can use to
      identify the particular instance of this client software.  The
      content and structure of this identifier is opaque to the client
      instance.

   "client": {
       "instance_id": "client-541-ab"
   }

   If there are no additional fields to send, the client instance MAY
   send the instance identifier as a direct reference value in lieu of
   the object.

   "client": "client-541-ab"

   When the AS receives a request with an instance identifier, the AS
   MUST ensure that the key used to sign the request (Section 8) is
   associated with the instance identifier.

   If the "instance_id" field is sent, it MUST NOT be accompanied by
   other fields unless such fields are explicitly marked safe for
   inclusion alongside the instance identifier.

   [[ See issue #45 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/45) ]]

   If the AS does not recognize the instance identifier, the request
   MUST be rejected with an error.

   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.
   The client instance MUST NOT send a symmetric key by value in the
   request, as doing so would expose the key directly instead of proving
   possession of it.

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2.3.2.  Identifying the Client Instance Key

   The client instance key MUST be a public key in at least one
   supported format and MUST be applicable to the proofing mechanism
   used in the request.  If the key is sent in multiple formats, all the
   keys MUST be the same.  The key presented in this field MUST be the
   key used to sign the request.

   proof (string)  The form of proof that the client instance will use
      when presenting the key to the AS.  The valid values of this field
      and the processing requirements for each are detailed in
      Section 8.  This field is REQUIRED.

   jwk (object)  Value of the public key as a JSON Web Key. MUST contain
      an "alg" field which is used to validate the signature.  MUST
      contain the "kid" field to identify the key in the signed object.

   cert (string)  PEM serialized value of the certificate used to sign
      the request, with optional internal whitespace.

   cert#S256 (string)  The certificate thumbprint calculated as per
      OAuth-MTLS [RFC8705] in base64 URL encoding.

   Additional key types are defined in a registry TBD (Section 12).

   This non-normative example shows a single key presented in multiple
   formats using a single proofing mechanism.

    "key": {
        "proof": "jwsd",
        "jwk": {
                    "kty": "RSA",
                    "e": "AQAB",
                    "kid": "xyz-1",
                    "alg": "RS256",
                    "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
        },
        "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
    }

   Continuation requests (Section 5) MUST use the same key (or its most
   recent rotation) and proof method as the initial request.

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2.3.3.  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

   uri (string)  User-facing web page of the client software

   logo_uri (string)  Display image to represent the client software

       "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
       }

   [[ See issue #48 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/48) ]]

   Additional display fields are defined by a registry TBD (Section 12).

   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.

2.3.4.  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 D.

   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

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   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 RQ 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.

   sub_ids (array of strings)  An array of subject identifiers for the
      RQ, as defined by [I-D.ietf-secevent-subject-identifiers].

   assertions (object)  An object containing assertions as values keyed
      on the assertion type defined by a registry TBD (Section 12).
      Possible keys include "id_token" for an [OIDC] ID Token and
      "saml2" for a SAML 2 assertion.  Additional assertion values are
      defined by a registry TBD (Section 12).  [[ See issue #41
      (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/41) ]]

   "user": {
      "sub_ids": [ {
        "subject_type": "email",
        "email": "user@example.com"
      } ],
      "assertions": {
        "id_token": "eyj..."
      }
   }

   Subject identifiers are hints to the AS in determining the RO and
   MUST NOT be taken as declarative statements that a particular RO is
   present at the client instance and acting as the RQ.  Assertions
   SHOULD be validated by the AS.  [[ See issue #49 (https://github.com/
   ietf-wg-gnap/gnap-core-protocol/issues/49) ]]

   If the identified RQ does not match the RO present at the AS during
   an interaction step, the AS SHOULD reject the request with an error.

   [[ See issue #50 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/50) ]]

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   If the AS trusts the client instance to present verifiable
   assertions, 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.

2.4.1.  Identifying the User by Reference

   User reference identifiers can be dynamically issued by the AS
   (Section 3.5) to allow the client instance to represent the same RQ
   to the AS over subsequent requests.

   If the client instance has a reference for the RQ 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"

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

   [[ See issue #51 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/51) ]]

   If the AS does not recognize the user reference, it MUST return an
   error.

2.5.  Interacting with the User

   Many times, the AS will require interaction with the RO in order to
   approve a requested delegation to the client instance for both
   resources and direct claim information.  Many times the RQ using the
   client instance is the same person as the RO, and the client instance
   can directly drive interaction with the AS by redirecting the RQ on
   the same device, or by 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 completed by the AS making
   callbacks.  To facilitate all of these modes, the client instance
   declares the means that it can interact using the "interact" field.

   The "interact" field is a JSON object with keys that declare
   different interaction modes.  A 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

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   the presented interaction modes (Section 3.3) in a request, depending
   on its capabilities and what is allowed to fulfill the request.  This
   specification defines the following interaction modes:

   redirect (boolean)  Indicates that the client instance can direct the
      RQ to an arbitrary URL at the AS for interaction.  Section 2.5.1

   app (boolean)  Indicates that the client instance can launch an
      application on the RQ's device for interaction.  Section 2.5.2

   callback (object)  Indicates that the client instance can receive a
      callback from the AS after interaction with the RO has concluded.
      Section 2.5.3

   user_code (boolean)  Indicates that the client instance can
      communicate a human-readable short code to the RQ for use with a
      stable URL at the AS.  Section 2.5.4

   ui_locales (array of strings)  Indicates the RQ's preferred locales
      that the AS can use during interaction, particularly before the RO
      has authenticated.  Section 2.5.5

   The following sections detail requests for interaction modes.
   Additional interaction modes are defined in a registry TBD
   (Section 12).

   In this non-normative example, the client instance is indicating that
   it can redirect (Section 2.5.1) the RQ to an arbitrary URL and can
   receive a callback (Section 2.5.3) through a browser request.

       "interact": {
           "redirect": true,
           "callback": {
               "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 2.5.4) and direct the RQ to an
   arbitrary URL (Section 2.5.1) on a secondary device, but it cannot
   accept a callback.

       "interact": {
           "redirect": true,
           "user_code": true
       }

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   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
   error since the client instance will be unable to complete the
   request without authorization.

   The AS SHOULD apply suitable timeouts to any interaction mechanisms
   provided, including user codes and redirection URLs.  The client
   instance SHOULD apply suitable timeouts to any callback URLs.

2.5.1.  Redirect to an Arbitrary URL

   If the client instance is capable of directing the RQ to a URL
   defined by the AS at runtime, the client instance indicates this by
   sending the "redirect" field with the boolean value "true".  The
   means by which the client instance will activate this URL is out of
   scope of this specification, but common methods include an HTTP
   redirect, launching a browser on the RQ's device, providing a
   scannable image encoding, and printing out a URL to an interactive
   console.

   "interact": {
      "redirect": true
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a redirect interaction response
   Section 3.3.1.

2.5.2.  Open an Application-specific URL

   If the client instance can open a URL associated with an application
   on the RQ's device, the client instance indicates this by sending the
   "app" field with boolean value "true".  The means by which the client
   instance determines the application to open with this URL are out of
   scope of this specification.

   "interact": {
      "app": true
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns an app interaction response with an app URL
   payload Section 3.3.2.

   [[ See issue #54 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/54) ]]

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2.5.3.  Receive a Callback After Interaction

   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 "callback" field.  The value
   of this field is an object containing the following members.

   uri (string)  REQUIRED.  Indicates the URI to send the RO to after
      interaction.  This URI MAY be unique per request and MUST be
      hosted by or accessible by the client instance.  This URI MUST NOT
      contain any fragment component.  This 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.  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.  [[ See
      issue #55 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
      issues/55) ]]

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

   method (string)  REQUIRED.  The callback method that the AS will use
      to contact the client instance.  Valid values include "redirect"
      Section 2.5.3.1 and "push" Section 2.5.3.2, with other values
      defined by a registry TBD (Section 12).

   hash_method (string)  OPTIONAL.  The hash calculation mechanism to be
      used for the callback hash in Section 4.4.3.  Can be one of "sha3"
      or "sha2".  If absent, the default value is "sha3".  [[ See issue
      #56 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/56)
      ]]

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

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   If this interaction mode is supported for this client instance and
   request, the AS returns a nonce for use in validating the callback
   response (Section 3.3.3).  Requests to the callback URI MUST be
   processed as described in Section 4.4, and the AS MUST require
   presentation of an interaction callback reference as described in
   Section 5.1.

   [[ See issue #58 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/58) ]]

   [[ See issue #59 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/59) ]]

2.5.3.1.  Receive an HTTP Callback Through the Browser

   A callback "method" value of "redirect" indicates that the client
   instance will expect a call from the RO's browser using the HTTP
   method GET as described in Section 4.4.1.

   "interact": {
       "callback": {
          "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 4.4.1.

   Since the incoming request to the callback URL is from the RO's
   browser, this method is usually used when the RO and RQ are the same
   entity.  As such, the client instance MUST ensure the RQ is present
   on the request to prevent substitution attacks.

2.5.3.2.  Receive an HTTP Direct Callback

   A callback "method" value of "push" indicates that the client
   instance will expect a call from the AS directly using the HTTP
   method POST as described in Section 4.4.2.

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

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   Requests to the callback URI MUST be processed by the client instance
   as described in Section 4.4.2.

   Since the incoming request to the callback URL is from the AS and not
   from the RO's browser, the client instance MUST NOT require the RQ to
   be present on the incoming HTTP request.

   [[ See issue #60 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/60) ]]

2.5.4.  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 sending the "user_code" field with the
   boolean value "true".  This code is to be entered at a static URL
   that does not change at runtime, as described in Section 3.3.4.

   "interact": {
       "user_code": true
   }

   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 4.2.

2.5.5.  Indicate Desired Interaction Locales

   If the client instance knows the RQ'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": {
       "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.

2.5.6.  Extending Interaction Modes

   Additional interaction modes are defined in a registry TBD
   (Section 12).

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2.6.  Declaring Client Capabilities

   If the client software supports extension capabilities, the client
   instance MAY present them to the AS in the "capabilities" field.
   This field is an array of strings representing specific extensions
   and capabilities, as defined by a registry TBD (Section 12).

   "capabilities": ["ext1", "ext2"]

2.7.  Referencing an Existing Grant Request

   If the client instance has a reference handle from a previously
   granted request, it MAY send that reference in the "existing_grant"
   field.  This field is a single string consisting of the "value" of
   the "access_token" returned in a previous request's continuation
   response (Section 3.1).

   "existing_grant": "80UPRY5NM33OMUKMKSKU"

   The AS MUST dereference the grant associated with the reference and
   process this request in the context of the referenced one.  The AS
   MUST NOT alter the existing grant associated with the reference.

   [[ See issue #62 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/62) ]]

2.8.  Extending The Grant Request

   The request object MAY be extended by registering new items in a
   registry TBD (Section 12).  Extensions SHOULD be orthogonal to other
   parameters.  Extensions MUST document any aspects where the extension
   item affects or influences the values or behavior of other request
   and response objects.

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.
      Section 3.1

   access_token (object)  A single access token that the client instance
      can use to call the RS on behalf of the RO.  Section 3.2.1

   multiple_access_token (object)  Multiple named access tokens that the

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      client instance can use to call the RS on behalf of the RO.
      Section 3.2.2

   interact (object)  Indicates that interaction through some set of
      defined mechanisms needs to take place.  Section 3.3

   subject (object)  Claims about the RO as known and declared by the
      AS.  Section 3.4

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

   user_handle (string)  An identifier this client instance instance can
      use to identify its current RQ when making future requests.
      Section 3.5

   error (object)  An error code indicating that something has gone
      wrong.  Section 3.6

   In this example, the AS is returning an interaction URL
   (Section 3.3.1), a callback nonce (Section 3.3.3), and a continuation
   response (Section 3.1).

{
    "interact": {
        "redirect": "https://server.example.com/interact/4CF492MLVMSW9MKMXKHQ",
        "callback": "MBDOFXG4Y5CVJCX821LH"
    },
    "continue": {
        "access_token": {
            "value": "80UPRY5NM33OMUKMKSKU",
            "key": true
        },
        "uri": "https://server.example.com/tx"
    }
}

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

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{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L"
    },
    "subject": {
        "sub_ids": [ {
           "subject_type": "email",
           "email": "user@example.com",
        } ]
    }
}

3.1.  Request Continuation

   If the AS determines that the request can be continued with
   additional requests, it responds with the "continue" field.  This
   field contains a JSON object with the following properties.

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

   wait (integer)  RECOMMENDED.  The amount of time in integer seconds
      the client instance SHOULD wait after receiving this continuation
      handle and calling the URI.

   access_token (object)  REQUIRED.  A unique access token for
      continuing the request, 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 "key" field of this access token is always the
      boolean value "true".  This access token MUST NOT be usable at
      resources outside of the AS.  The client instance MUST present the
      access token in all requests to the continuation URI as described
      in Section 7.  [[ See issue #66 (https://github.com/ietf-wg-gnap/
      gnap-core-protocol/issues/66) ]]

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

   The client instance can use the values of this field to continue the
   request as described in Section 5.  Note that the client instance
   MUST sign all continuation requests with its key as described in
   Section 8 and MUST present the access token in its continuation
   request.

   This field SHOULD be returned when interaction is expected, to allow
   the client instance to follow up after interaction has been
   concluded.

3.2.  Access Tokens

   If the AS has successfully granted one or more access tokens to the
   client instance, the AS responds with either the "access_token" or
   the "multiple_access_token" field.  The AS MUST NOT respond with both
   the "access_token" and "multiple_access_token" fields.

   [[ See issue #68 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/68) ]]

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.

   value (string)  REQUIRED.  The value of the access token as a string.
      The value is opaque to the client instance.  The value SHOULD be
      limited to ASCII characters to facilitate transmission over HTTP
      headers within other protocols without requiring additional
      encoding.

   manage (string)  OPTIONAL.  The management URI for this access token.

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      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.

   resources (array of objects/strings)  RECOMMENDED.  A description of
      the rights associated with this access token, as defined in
      Section 2.1.1.  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.

   expires_in (integer)  OPTIONAL.  The number of seconds in which the
      access will expire.  The client instance MUST NOT use the access
      token past this time.  An RS MUST NOT accept an 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.

   key (object / string / boolean)  REQUIRED.  The key that the token is
      bound to.  If the boolean value "true" is used, the token is bound
      to the key used by the client instance (Section 2.3.2) in its
      request for access.  If the boolean value "false" is used, the
      token is a bearer token with no key bound to it.  Otherwise, the
      key MUST be an object or string in a format described in
      Section 2.3.2, describing a public key to which the client
      instance can use the associated private key.  The client instance
      MUST be able to dereference or process the key information in
      order to be able to sign the request.

   The following non-normative example shows a single bearer token with
   a management URL that has access to three described resources.

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    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [
            {
                "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 access token bound
   to the client instance's key, which was presented using the detached
   JWS (Section 8.1) binding method.

       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "key": true,
           "resources": [
               "finance", "medical"
           ]
       }

   If the client instance requested multiple access tokens
   (Section 2.1.3), the AS MUST NOT respond with a single access token
   structure unless the client instance sends the "split_token" flag as
   described in Section 2.1.4.

   [[ See issue #69 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/69) ]]

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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
   "multiple_access_tokens" field.  The value of this field is a JSON
   object, and the property names correspond to the token identifiers
   chosen by the client instance in the multiple access token request
   (Section 2.1.3).  The values of the properties of this object are
   access tokens as described in Section 3.2.1.

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

    "multiple_access_tokens": {
        "token1": {
            "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
            "key": false,
            "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L"
        },
        "token2": {
            "value": "UFGLO2FDAFG7VGZZPJ3IZEMN21EVU71FHCARP4J1",
            "key": false
        }
    }

   Each access token corresponds to the named resources arrays in the
   client instance's request (Section 2.1.3).

   The multiple access token response MUST be used when multiple access
   tokens are requested, even if only one access token is issued as a
   result of the request.  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 the requested names appropriate
   names.

   If the client instance requested a single access token
   (Section 2.1.1), the AS MUST NOT respond with the multiple access
   token structure unless the client instance sends the "split_token"
   flag as described in Section 2.1.4.

   Each access token MAY have different proofing mechanisms.  If
   management is allowed, each access token SHOULD have different
   management URIs.

   [[ See issue #70 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/70) ]]

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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 URL.  Section 3.3.1

   app (string)  Launch of an application URL.  Section 3.3.2

   callback (string)  Callback to a client instance accessible URL after
      interaction is completed.  Section 3.3.3

   user_code (object)  Display a short user code.  Section 3.3.4

   Additional interaction mode responses can be defined in a registry
   TBD (Section 12).

   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).

3.3.1.  Redirection to an arbitrary URL

   If the client instance indicates that it can redirect to an arbitrary
   URL (Section 2.5.1) 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 URL to direct the RQ to.  This URL MUST be
   unique for the request and MUST NOT contain any security-sensitive
   information.

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

   The interaction URL returned represents a function of the AS but MAY
   be completely distinct from the URL the client instance uses to
   request access (Section 2), allowing an AS to separate its user-
   interactive functionality from its back-end security functionality.

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   [[ See issue #72 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/72) ]]

   The client instance sends the RQ to the URL to interact with the AS.
   The client instance MUST NOT alter the URL in any way.  The means for
   the client instance to send the RQ to this URL 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 URL in an interactive console.

3.3.2.  Launch of an application URL

   If the client instance indicates that it can launch an application
   URL (Section 2.5.2) 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 URL to direct the RQ to.  This URL MUST be
   unique for the request and MUST NOT contain any security-sensitive
   information.

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

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

   [[ See issue #71 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/71) ]]

3.3.3.  Post-interaction Callback to a Client Instance Accessible URL

   If the client instance indicates that it can receive a
   post-interaction callback on a URL (Section 2.5.3) and the AS
   supports this mode for the client instance's request, the AS responds
   with a "callback" field containing a nonce that the client instance
   will use in validating the callback as defined in Section 4.4.1.

       "interact": {
           "callback": "MBDOFXG4Y5CVJCX821LH"
       }

   When the RO completes interaction at the AS, the AS MUST call the
   client instance's callback URL using the method indicated in the
   callback request (Section 2.5.3) as described in Section 4.4.1.

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   If the AS returns a "callback" nonce, the client instance MUST NOT
   continue a grant request before it receives the associated
   interaction reference on the callback URI.

3.3.4.  Display of a Short User Code

   If the client instance indicates that it can display a short
   user-typeable code (Section 2.5.4) and the AS supports this mode for
   the client instance's request, the AS responds with a "user_code"
   field.  This field is an object that contains the following members.

   code (string)  REQUIRED.  A unique short code that the user can type
      into an authorization server.  This string MUST be case-
      insensitive, MUST consist of only easily typeable characters (such
      as letters or numbers).  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.

   url (string)  RECOMMENDED.  The interaction URL that the client
      instance will direct the RO to.  This URL MUST be stable at the AS
      such that client instance's can be statically configured with it.

       "interact": {
           "user_code": {
               "code": "A1BC-3DFF",
               "url": "https://srv.ex/device"
           }
       }

   The client instance MUST communicate the "code" to the RQ in some
   fashion, such as displaying it on a screen or reading it out audibly.
   The "code" is a one-time-use credential that the AS uses to identify
   the pending request from the client instance.  When the RO enters
   this code (Section 4.2) into the AS, the AS MUST determine the
   pending request that it was associated with.  If the AS does not
   recognize the entered code, the AS MUST display an error to the user.
   If the AS detects too many unrecognized codes entered, it SHOULD
   display an error to the user.

   The client instance SHOULD also communicate the URL if possible to
   facilitate user interaction, but since the URL should be stable, the
   client instance should be able to safely decide to not display this
   value.  As this interaction mode is designed to facilitate
   interaction via a secondary device, it is not expected that the
   client instance redirect the RQ to the URL given here at runtime.
   Consequently, the URL needs to be stable enough that a client
   instance could be statically configured with it, perhaps referring

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   the RQ to the URL via documentation instead of through an interactive
   means.  If the client instance is capable of communicating an
   arbitrary URL to the RQ, such as through a scannable code, the client
   instance can use the "redirect" (Section 2.5.1) mode for this purpose
   instead of or in addition to the user code mode.

   The interaction URL returned represents a function of the AS but MAY
   be completely distinct from the URL the client instance uses to
   request access (Section 2), allowing an AS to separate its user-
   interactive functionality from its back-end security functionality.

   [[ See issue #72 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/72) ]]

3.3.5.  Extending Interaction Mode Responses

   Extensions to this specification can define new interaction mode
   responses in a registry TBD (Section 12).  Extensions MUST document
   the corresponding interaction request.

3.4.  Returning User 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.  This field is an object with the
   following OPTIONAL properties.

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

   assertions (object)  An object containing assertions as values keyed
      on the assertion type defined by a registry TBD (Section 12).  [[
      See issue #41 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
      issues/41) ]]

   updated_at (string)  Timestamp as an ISO8610 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.

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   "subject": {
      "sub_ids": [ {
        "subject_type": "email",
        "email": "user@example.com",
      } ],
      "assertions": {
        "id_token": "eyj..."
      }
   }

   The AS MUST return the "subject" field only in cases where the AS is
   sure that the RO and the RQ are the same party.  This can be
   accomplished through some forms of interaction with the RO
   (Section 4).

   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.

   [[ See issue #75 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/75) ]]

   Extensions to this specification MAY define additional response
   properties in a registry TBD (Section 12).

3.5.  Returning Dynamically-bound Reference Handles

   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.

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   Some references, such as for the client instance's identity
   (Section 2.3.1) or the requested resources (Section 2.1.2), 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 some of these
   references dynamically to the client instance in its response to
   facilitate multiple interactions with the same software.  The client
   instance SHOULD use these references in future requests in lieu of
   sending the associated data value.  These handles are intended to be
   used on future requests.

   Dynamically generated handles are string values that MUST be
   protected by the client instance as secrets.  Handle values MUST be
   unguessable and MUST NOT contain any sensitive information.  Handle
   values are opaque to the client instance.

   All dynamically generated handles are returned as fields in the root
   JSON object of the response.  This specification defines the
   following dynamic handle returns, additional handles can be defined
   in a registry TBD (Section 12).

   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.

   user_handle (string)  A string value used to represent the current
      user.  The client instance can use in a future request, as
      described in Section 2.4.1.

   This non-normative example shows two handles along side an issued
   access token.

   {
       "user_handle": "XUT2MFM1XBIKJKSDU8QM",
       "instance_id": "7C7C4AZ9KHRS6X63AJAO",
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "key": false
       }
   }

   [[ See issue #77 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/77) ]]

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   [[ See issue #78 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/78) ]]

3.6.  Error Response

   If the AS determines that the request cannot be issued for any
   reason, it responds to the client instance with an error message.

   error (string)  The error code.

   {

     "error": "user_denied"

   }

   The error code is one of the following, with additional values
   available in a registry TBD (Section 12):

   user_denied  The RO denied the request.

   too_fast  The client instance did not respect the timeout in the wait
      response.

   unknown_request  The request referenced an unknown ongoing access
      request.

   [[ See issue #79 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/79) ]]

3.7.  Extending the Response

   Extensions to this specification MAY define additional fields for the
   grant response in a registry TBD (Section 12).

4.  Interaction at the AS

   If the client instance indicates that it is capable of driving
   interaction with the RO in its request (Section 2.5), and the AS
   determines that interaction is required and responds to one or more
   of the client instance's interaction modes, the client instance
   SHOULD initiate one of the returned interaction modes in the response
   (Section 3.3).

   When the RO is interacting with the AS, the AS MAY perform whatever
   actions it sees fit, including but not limited to:

   *  authenticate the current user (who may be the RQ) as the RO

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   *  gather consent and authorization from the RO for access to
      requested resources and direct information

   *  allow the RO to modify the parameters of the request (such as
      disallowing some requested resources or specifying an account or
      record)

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

4.1.  Interaction at a Redirected URI

   When the RO is directed to the AS through the "redirect"
   (Section 3.3.1) mode, the AS can interact with the RO through their
   web browser to authenticate the user as an RO and gather their
   consent.  Note that since the client instance does not add any
   parameters to the URL, the AS MUST determine the grant request being
   referenced from the URL value itself.  If the URL 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 callback is supplied (Section 2.5.3).

   The interaction URL MUST be reachable from the RO's browser, though
   note that the RO MAY open the URL on a separate device from the
   client instance itself.  The interaction URL MUST be accessible from
   an HTTP GET request, and MUST be protected by HTTPS or equivalent
   means.

   With this method, it is common for the RO to be the same party as the
   RQ, since the client instance has to communicate the redirection URI
   to the RQ.

4.2.  Interaction at the User Code URI

   When the RO is directed to the AS through the "user_code"
   (Section 3.3.4) mode, the AS can interact with the RO through their
   web browser to collect the user code, authenticate the user as an RO,
   and gather their consent.  Note that since the URL itself is static,
   the AS MUST determine the grant request being referenced from the
   user code value itself.  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 callback is supplied (Section 2.5.3).

   The user code URL MUST be reachable from the RO's browser, though
   note that the RO MAY open the URL on a separate device from the
   client instance itself.  The user code URL MUST be accessible from an
   HTTP GET request, and MUST be protected by HTTPS or equivalent means.

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   While it is common for the RO to be the same party as the RQ, since
   the client instance has to communicate the user code to someone,
   there are cases where the RQ and RO are separate parties and the
   authorization happens asynchronously.

4.3.  Interaction through an Application URI

   When the client instance successfully launches an application through
   the "app" mode (Section 3.3.2), the AS interacts with the RO through
   that application to authenticate the user as the RO and gather their
   consent.  The details of this interaction are out of scope for this
   specification.

4.4.  Post-Interaction Completion

   Upon completing an interaction with the RO, if a "callback"
   (Section 3.3.3) mode is available with the current request, the AS
   MUST follow the appropriate method at the end of interaction to allow
   the client instance to continue.  If this mode is not available, the
   AS SHOULD instruct the RO to return to their client instance upon
   completion.  Note that these steps still take place in most error
   cases, such as when the RO has denied access.  This pattern allows
   the client instance to potentially recover from the error state
   without restarting the request from scratch by modifying its request
   or providing additional information directly to the AS.

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

   The AS MUST calculate a hash value based on the client instance and
   AS nonces and the interaction reference, as described in
   Section 4.4.3.  The client instance will use this value to validate
   the return call from the AS.

   The AS then MUST send the hash and interaction reference based on the
   interaction finalization mode as described in the following sections.

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

   When using the "callback" interaction mode (Section 3.3.3) with the
   "redirect" method, 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 callback URL
   sent in the callback request (Section 2.5.3.1).

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

   hash  REQUIRED.  The interaction hash value as described in
      Section 4.4.3.

   interact_ref  REQUIRED.  The interaction reference generated for this
      interaction.

   The means of directing the RO to this URL 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 URL.

https://client.example.net/return/123455
  ?hash=p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A
  &interact_ref=4IFWWIKYBC2PQ6U56NL1

   When receiving the request, the client instance MUST parse the query
   parameters to calculate and validate the hash value as described in
   Section 4.4.3.  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.

4.4.2.  Completing Interaction with a Direct HTTP Request Callback

   When using the "callback" interaction mode (Section 3.3.3) with the
   "push" method, 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 URL sent in the callback
   request (Section 2.5.3.2).

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

   hash (string)  REQUIRED.  The interaction hash value as described in
      Section 4.4.3.

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

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

{
  "hash": "p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A",
  "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
}

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   When receiving the request, the client instance MUST parse the JSON
   object and validate the hash value as described in Section 4.4.3.  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.

4.4.3.  Calculating the interaction hash

   The "hash" parameter in the request to the client instance's callback
   URL 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.  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 first
   takes the "nonce" value sent by the client instance in the
   interaction section of the initial request (Section 2.5.3), the AS's
   nonce value from the callback response (Section 3.3.3), and the
   "interact_ref" sent to the client instance's callback URL.  These
   three values are concatenated to each other in this order using a
   single newline character as a separator between the fields.  There is
   no padding or whitespace before or after any of the lines, and no
   trailing newline character.

   VJLO6A4CAYLBXHTR0KRO
   MBDOFXG4Y5CVJCX821LH
   4IFWWIKYBC2PQ6U56NL1

   The party then hashes this string with the appropriate algorithm
   based on the "hash_method" parameter of the "callback".  If the
   "hash_method" value is not present in the client instance's request,
   the algorithm defaults to "sha3".

   [[ See issue #56 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/56) ]]

4.4.3.1.  SHA3-512

   The "sha3" hash method consists of hashing the input string with the
   512-bit SHA3 algorithm.  The byte array is then encoded using URL
   Safe Base64 with no padding.  The resulting string is the hash value.

p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A

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4.4.3.2.  SHA2-512

   The "sha2" hash method consists of hashing the input string with the
   512-bit SHA2 algorithm.  The byte array is then encoded using URL
   Safe Base64 with no padding.  The resulting string is the hash value.

62SbcD3Xs7L40rjgALA-ymQujoh2LB2hPJyX9vlcr1H6ecChZ8BNKkG_HrOKP_Bpj84rh4mC9aE9x7HPBFcIHw

5.  Continuing a Grant Request

   While it is possible for the AS to return a Section 3 with all the
   client instance's requested information (including access tokens
   (Section 3.2) and direct user information (Section 3.4)), it's more
   common that the AS and the client instance will need to communicate
   several times over the lifetime of an access grant.  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).

   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 an
   access token to use during the continued requests.

   The access token is initially bound to the same key and method the
   client instance used to make the initial request.  As a consequence,
   when the client instance makes any calls to the continuation URL, the
   client instance MUST present the access token as described in
   Section 7 and present proof of the client instance's key (or its most
   recent rotation) by signing the request as described in Section 8.

   [[ See issue #85 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/85) ]]

   For example, here the client instance makes a POST request to a
   unique URI and signs the request with detached JWS:

   POST /continue/KSKUOMUKM HTTP/1.1
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Host: server.example.com
   Detached-JWS: ejy0...

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   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 URL, the provided access token, and the client
   instance identified by the key signature.  If the AS cannot determine
   a single active grant request to map the continuation request to, the
   AS MUST return an error.

   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 getting the current state of the request.

   All requests to the continuation API are protected by this bound
   access token.  For example, here the client instance makes a POST
   request to a stable continuation endpoint URL with the interaction
   reference (Section 5.1), includes the access token, and signs with
   detached JWS:

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   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 SHOULD wait at least 5
   seconds.  If the client instance does not respect the given wait
   period, the AS MUST return an error. [[ See issue #86
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/86) ]]

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

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   If the AS determines that the client instance can make a further
   continuation request, the AS MUST include a new "continue" response
   (Section 3.1).  The new "continue" response MUST include a bound
   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 NOT make an
   additional continuation request.  If a client instance does so, the
   AS MUST return an error. [[ See issue #87 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/87) ]]

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

5.1.  Continuing After a Completed Interaction

   When the AS responds to the client instance's "callback" parameter as
   in Section 4.4.1, 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
   Detached-JWS: ejy0...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   Since the interaction reference is a one-time-use value as described
   in Section 4.4.1, if the client instance needs to make additional
   continuation calls after this request, the client instance MUST NOT
   include the interaction reference.  If the AS detects a client
   instance submitting the same interaction reference multiple times,
   the AS MUST return an error and SHOULD invalidate the ongoing
   request.

   The 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.  The response SHOULD NOT contain any interaction
   responses (Section 3.3). [[ See issue #89 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/89) ]]

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

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{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L"
    },
    "subject": {
        "sub_ids": [ {
           "subject_type": "email",
           "email": "user@example.com",
        } ]
    }
}

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

   [[ See issue #88 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/88) ]]

5.2.  Continuing During Pending Interaction

   When the client instance does not include a "callback" 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
   Content-type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   The 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).

   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.

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

   [[ See issue #90 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/90) ]]

   [[ See issue #91 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/91) ]]

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

{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L"
    },
    "subject": {
        "sub_ids": [ {
           "subject_type": "email",
           "email": "user@example.com",
        } ]
    }
}

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 claims have 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.

   The client instance MAY include the "resources" 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

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   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.  [[ See issue #92 (https://github.com/ietf-wg-gnap/gnap-
   core-protocol/issues/92) ]]

   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 RO, 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 RQ.
   [[ See issue #93 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/93) ]]

   The client instance MUST NOT include the "client" section of the
   request. [[ See issue #94 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/94) ]]

   The client instance MAY include post-interaction responses such as
   described in Section 5.1. [[ See issue #95 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/95) ]]

   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 existing access
   tokens after a modification has occurred.  [[ See issue #96
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/96) ]]

   If the modified request can be granted immediately by the AS, the
   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
   Detached-JWS: ejy0...

   {
       "resources": [
           "read", "write"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "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",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "key": false,
           "resources": [
               "read", "write"
           ]
       }
   }

   This "continue" field allows the client instance to make an eventual
   continuation call.  In the future, 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.

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   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   {
       "resources": [
           "read"
       ]
       ...
   }

   The AS replaces the previous "resources" from the first request,
   allowing the AS to determine if any previously-granted consent
   already applies.  In this case, the AS would likely determine that
   reducing the breadth of the requested access means that new access
   tokens can be issued to the client instance.  The AS would likely
   revoke previously-issued access tokens that had the greater access
   rights associated with them.

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

   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.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Detached-JWS: ejy0...

   {
       "resources": [
           "read"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "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",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "key": false,
           "resources": [
               "read"
           ]
       }
   }

   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.

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   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.

   [[ See issue #97 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/97) ]]

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   {
       "resources": [
           "read", "write"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "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.

5.4.  Getting the Current State of a Grant Request

   If the client instance needs to get the current state of an ongoing
   grant request, it makes an HTTP GET request to the continuation URI.
   This request MUST NOT alter the grant request in any fashion,
   including causing the issuance of new access tokens or modification
   of interaction parameters.

   The AS MAY include existing access tokens and previously-released
   subject claims in the response.  The AS MUST NOT issue a new access
   token or release a new subject claim in response to this request.

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   GET /continue HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   The response MAY include any fields described Section 3 that are
   applicable to this ongoing request, including the most recently
   issued access tokens, any released subject claims, and any currently
   active interaction modes.  The response MAY contain a new "continue"
   response (Section 3.1) as described above.

   [[ See issue #98 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/98) ]]

5.5.  Canceling a Grant Request

   If the client instance wishes to cancel an ongoing grant request, it
   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
   Detached-JWS: ejy0...

   If the request is successfully cancelled, the AS responds with an
   HTTP 202.  The AS MUST revoke all associated access tokens, if
   possible.

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 URL to
   manage the access token with any of the 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).

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   If the token is a bearer token, the client instance MUST present
   proof of the same key identified in the initial request
   (Section 2.3.2) as described in Section 8.

   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.

   [[ See issue #99 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/99) ]]

6.1.  Rotating the Access Token

   The client instance makes an HTTP POST to the token management URI,
   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
   Detached-JWS: eyj0....

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

   If the access token has expired, the AS SHOULD honor the rotation
   request to the token management URL since it is likely that the
   client instance is attempting to refresh the expired token.  To
   support this, the AS MAY apply different lifetimes for the use of the
   token in management vs. its use at an RS.  An AS MUST NOT honor a
   rotation request for an access token that has been revoked, either by
   the AS or by the client instance through the token management URI
   (Section 6.2).

   If the token is validated and the key is appropriate for the request,
   the AS MUST invalidate the current access token associated with this
   URL, if possible, and return a new access token response as described
   in Section 3.2.1, unless the "multi_token" flag is specified in the
   request.  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 MAY include an updated access token management URL as
   well, and if so, the client instance MUST use this new URL to manage
   the new access token. [[ See issue #101 (https://github.com/ietf-wg-
   gnap/gnap-core-protocol/issues/101) ]]

   [[ See issue #102 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/102) ]]

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{
    "access_token": {
        "value": "FP6A8H6HY37MH13CK76LBZ6Y1UADG6VEUPEER5H2",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [
            {
                "type": "photo-api",
                "actions": [
                    "read",
                    "write",
                    "dolphin"
                ],
                "locations": [
                    "https://server.example.net/",
                    "https://resource.local/other"
                ],
                "datatypes": [
                    "metadata",
                    "images"
                ]
            },
            "read", "dolphin-metadata"
        ]
    }
}

   [[ See issue #103 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/103) ]]

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
   Detached-JWS: eyj0....

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   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 URL as valid, since the end result is
   still the token not being usable.

7.  Using Access Tokens

   The method the client instance uses to send an access token to the RS
   depends on the value of the "key" and "proof" parameters in the
   access token response (Section 3.2.1).

   If the key value is the boolean "false", the access token is a bearer
   token sent using the HTTP Header method defined in [RFC6750].

   Authorization: Bearer OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0

   The form parameter and query parameter methods of [RFC6750] MUST NOT
   be used.

   If the "key" value is the boolean "true", the access token MUST be
   sent to the RS using the same key and proofing mechanism that the
   client instance used in its initial request.

   If the "key" value is an object, the value of the "proof" field
   within the key indicates the particular proofing mechanism to use.
   The access token is sent using the HTTP authorization scheme "GNAP"
   along with a key proof as described in Section 8 for the key bound to
   the access token.  For example, a "jwsd"-bound access token is sent
   as follows:

   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Detached-JWS: eyj0....

   [[ See issue #104 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/104) ]]

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8.  Binding Keys

   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
   section in the initial request Section 2.3.2.  Values defined by this
   specification are as follows:

   jwsd  A detached JWS signature header

   jws  Attached JWS payload

   mtls  Mutual TLS certificate verification

   dpop  OAuth Demonstration of Proof-of-Possession key proof header

   httpsig  HTTP Signing signature header

   oauthpop  OAuth PoP key proof authentication header

   Additional proofing methods are defined by a registry TBD
   (Section 12).

   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 by the AS.  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 recipient 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.

   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 2.3.2), 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.  The AS MUST validate all keys
   presented by the client instance (Section 2.3.2) or referenced in an
   ongoing request for each call within that request.

   [[ See issue #105 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/105) ]]

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   When used to bind to an access token, the access token MUST be
   covered by the signature method.

8.1.  Detached JWS

   This method is indicated by "jwsd" in the "proof" field.  A JWS
   [RFC7515] signature object is created as follows:

   The header of the JWS MUST contain the "kid" field of the key bound
   to this client instance for this request.  The JWS header MUST
   contain an "alg" field appropriate for the key identified by kid and
   MUST NOT be "none".  The "b64" field MUST be set to "false" and the
   "crit" field MUST contain at least "b64" as specified in [RFC7797]

   To protect the request, the JWS header MUST contain the following
   additional fields.

   htm (string)  The HTTP Method used to make this request, as an
      uppercase ASCII string.

   htu (string)  The HTTP URI used for this request, including all path
      and query components.

   ts (integer)  A timestamp of the request in integer seconds

   at_hash (string)  When to bind a request to an access token, the
      access token hash value.  Its value is the base64url encoding of
      the left-most half of the hash of the octets of the ASCII
      representation of the "access_token" value, where the hash
      algorithm used is the hash algorithm used in the "alg" header
      parameter of the JWS's JOSE Header.  For instance, if the "alg" is
      "RS256", hash the "access_token" value with SHA-256, then take the
      left-most 128 bits and base64url encode them.

   [[ See issue #106 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/106) ]]

   The payload of the JWS object is the serialized body of the request,
   and the object is signed according to detached JWS [RFC7797].

   The client instance presents the signature in the Detached-JWS HTTP
   Header field.

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  POST /tx HTTP/1.1
  Host: server.example.com
  Content-Type: application/json
  Detached-JWS: eyJiNjQiOmZhbHNlLCJhbGciOiJSUzI1NiIsImtpZCI6Inh5ei0xIn0.
    .Y287HMtaY0EegEjoTd_04a4GC6qV48GgVbGKOhHdJnDtD0VuUlVjLfwne8AuUY3U7e8
    9zUWwXLnAYK_BiS84M8EsrFvmv8yDLWzqveeIpcN5_ysveQnYt9Dqi32w6IOtAywkNUD
    ZeJEdc3z5s9Ei8qrYFN2fxcu28YS4e8e_cHTK57003WJu-wFn2TJUmAbHuqvUsyTb-nz
    YOKxuCKlqQItJF7E-cwSb_xULu-3f77BEU_vGbNYo5ZBa2B7UHO-kWNMSgbW2yeNNLbL
    C18Kv80GF22Y7SbZt0e2TwnR2Aa2zksuUbntQ5c7a1-gxtnXzuIKa34OekrnyqE1hmVW
    peQ

  {
      "resources": [
          "dolphin-metadata"
      ],
      "interact": {
          "redirect": true,
          "callback": {
              "method": "redirect",
              "uri": "https://client.foo",
              "nonce": "VJLO6A4CAYLBXHTR0KRO"
          }
      },
      "client": {
        "proof": "jwsd",
        "key": {
          "jwk": {
                      "kty": "RSA",
                      "e": "AQAB",
                      "kid": "xyz-1",
                      "alg": "RS256",
                      "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8
  xYJCNaOKNJn_Oz0YhdHbXTeWO5AoyspDWJbN5w_7bdWDxgpD-y6jnD1u9YhBOCWObNPF
  vpkTM8LC7SdXGRKx2k8Me2r_GssYlyRpqvpBlY5-ejCywKRBfctRcnhTTGNztbbDBUyD
  SWmFMVCHe5mXT4cL0BwrZC6S-uu-LAx06aKwQOPwYOGOslK8WPm1yGdkaA1uF_FpS6LS
  63WYPHi_Ap2B7_8Wbw4ttzbMS_doJvuDagW8A1Ip3fXFAHtRAcKw7rdI4_Xln66hJxFe
  kpdfWdiPQddQ6Y1cK2U3obvUg7w"
          }
        }
        "display": {
          "name": "My Client Display Name",
          "uri": "https://example.net/client"
        },
      }
  }

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   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.

   When the server (AS or RS) receives the Detached-JWS header, it MUST
   parse its contents as a detached JWS object.  The HTTP Body is used
   as the payload for purposes of validating the JWS, with no
   transformations.

8.2.  Attached JWS

   This method is indicated by "jws" in the "proof" field.  A JWS
   [RFC7515] signature object is created as follows:

   The header of the JWS MUST contain the "kid" field of the key bound
   to this client instance for this request.  The JWS header MUST
   contain an "alg" field appropriate for the key identified by kid and
   MUST NOT be "none".

   To protect the request, the JWS header MUST contain the following
   additional fields.

   htm (string)  The HTTP Method used to make this request, as an
      uppercase ASCII string.

   htu (string)  The HTTP URI used for this request, including all path
      and query components.

   ts (integer)  A timestamp of the request in integer seconds

   at_hash (string)  When to bind a request to an access token, the
      access token hash value.  Its value is the base64url encoding of
      the left-most half of the hash of the octets of the ASCII
      representation of the "access_token" value, where the hash
      algorithm used is the hash algorithm used in the "alg" header
      parameter of the JWS's JOSE Header.  For instance, if the "alg" is
      "RS256", hash the "access_token" value with SHA-256, then take the
      left-most 128 bits and base64url encode them.

   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 client instance presents the JWS as the body of the request along
   with a content type of "application/jose".  The AS MUST extract the
   payload of the JWS and treat it as the request body for further
   processing.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/jose

   eyJhbGciOiJSUzI1NiIsImtpZCI6IktBZ05wV2JSeXk5T
   WYycmlrbDQ5OExUaE1ydmtiWldIVlNRT0JDNFZIVTQiLC
   JodG0iOiJwb3N0IiwiaHR1IjoiL3R4IiwidHMiOjE2MDM
   4MDA3ODN9.eyJjYXBhYmlsaXRpZXMiOltdLCJjbGllbnQ
   iOnsia2V5Ijp7Imp3ayI6eyJrdHkiOiJSU0EiLCJlIjoi
   QVFBQiIsImtpZCI6IktBZ05wV2JSeXk5TWYycmlrbDQ5O
   ExUaE1ydmtiWldIVlNRT0JDNFZIVTQiLCJuIjoibGxXbU
   hGOFhBMktOTGRteE9QM2t4RDlPWTc2cDBTcjM3amZoejk
   0YTkzeG0yRk5xb1NQY1JaQVBkMGxxRFM4TjNVaWE1M2RC
   MjNaNTlPd1k0YnBNX1ZmOEdKdnZwdExXbnhvMVB5aG1Qc
   i1lY2RTQ1JRZFRjX1pjTUY0aFJWNDhxcWx2dUQwbXF0Y0
   RiSWtTQkR2Y2NKbVpId2ZUcERIaW5UOHR0dmNWUDhWa0F
   NQXE0a1ZhenhPcE1vSVJzb3lFcF9lQ2U1cFN3cUhvMGRh
   Q1dOS1ItRXBLbTZOaU90ZWRGNE91bXQ4TkxLVFZqZllnR
   khlQkRkQ2JyckVUZDR2Qk13RHRBbmpQcjNDVkN3d3gyYk
   FRVDZTbHhGSjNmajJoaHlJcHE3cGM4clppYjVqTnlYS3d
   mQnVrVFZZWm96a3NodC1Mb2h5QVNhS3BZVHA4THROWi13
   In0sInByb29mIjoiandzIn0sIm5hbWUiOiJNeSBGaXN0I
   ENsaWVudCIsInVyaSI6Imh0dHA6Ly9sb2NhbGhvc3QvY2
   xpZW50L2NsaWVudElEIn0sImludGVyYWN0Ijp7ImNhbGx
   iYWNrIjp7Im1ldGhvZCI6InJlZGlyZWN0Iiwibm9uY2Ui
   OiJkOTAyMTM4ODRiODQwOTIwNTM4YjVjNTEiLCJ1cmkiO
   iJodHRwOi8vbG9jYWxob3N0L2NsaWVudC9yZXF1ZXN0LW
   RvbmUifSwicmVkaXJlY3QiOnRydWV9LCJyZXNvdXJjZXM
   iOnsiYWN0aW9ucyI6WyJyZWFkIiwicHJpbnQiXSwibG9j
   YXRpb25zIjpbImh0dHA6Ly9sb2NhbGhvc3QvcGhvdG9zI
   l0sInR5cGUiOiJwaG90by1hcGkifSwic3ViamVjdCI6ey
   JzdWJfaWRzIjpbImlzcy1zdWIiLCJlbWFpbCJdfX0.LUy
   Z8_fERmxbYARq8kBYMwzcd8GnCAKAlo2ZSYLRRNAYWPrp
   2XGLJOvg97WK1idf_LB08OJmLVsCXxCvn9mgaAkYNL_Zj
   HcusBvY1mNo0E1sdTEr31CVKfC-6WrZCscb8YqE4Ayhh0
   Te8kzSng3OkLdy7xN4xeKuHzpF7yGsM52JZ0cBcTo6WrY
   EfGdr08AWQJ59ht72n3jTsmYNy9A6I4Wrvfgj3TNxmwYo
   jpBAicfjnzA1UVcNm9F_xiSz1_y2tdH7j5rVqBMQife-k
   9Ewk95vr3lurthenliYSNiUinVfoW1ybnaIBcTtP1_YCx
   g_h1y-B5uZEvYNGCuoCqa6IQ

   This example's JWS header decodes to:

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   {
     "alg": "RS256",
     "kid": "KAgNpWbRyy9Mf2rikl498LThMrvkbZWHVSQOBC4VHU4",
     "htm": "post",
     "htu": "/tx",
     "ts": 1603800783
   }

   And the JWS body decodes to:

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   {
     "capabilities": [],
     "client": {
       "key": {
         "jwk": {
           "kty": "RSA",
           "e": "AQAB",
           "kid": "KAgNpWbRyy9Mf2rikl498LThMrvkbZWHVSQOBC4VHU4",
           "n": "llWmHF8XA2KNLdmxOP3kxD9OY76p0Sr37jfhz94a93xm2FNqoSPc
           RZAPd0lqDS8N3Uia53dB23Z59OwY4bpM_Vf8GJvvptLWnxo1PyhmPr-ecd
           SCRQdTc_ZcMF4hRV48qqlvuD0mqtcDbIkSBDvccJmZHwfTpDHinT8ttvcV
           P8VkAMAq4kVazxOpMoIRsoyEp_eCe5pSwqHo0daCWNKR-EpKm6NiOtedF4
           Oumt8NLKTVjfYgFHeBDdCbrrETd4vBMwDtAnjPr3CVCwwx2bAQT6SlxFJ3
           fj2hhyIpq7pc8rZib5jNyXKwfBukTVYZozksht-LohyASaKpYTp8LtNZ-w"
         },
         "proof": "jws"
       },
       "name": "My Fist Client",
       "uri": "http://localhost/client/clientID"
     },
     "interact": {
       "callback": {
         "method": "redirect",
         "nonce": "d90213884b840920538b5c51",
         "uri": "http://localhost/client/request-done"
       },
       "redirect": true
     },
     "resources": {
       "actions": [
         "read",
         "print"
       ],
       "locations": [
         "http://localhost/photos"
       ],
       "type": "photo-api"
     },
     "subject": {
       "sub_ids": [
         "iss_sub",
         "email"
       ]
     }
   }

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   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 8.1.

   [[ See issue #109 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/109) ]]

8.3.  Mutual TLS

   This method is indicated by "mtls" in the "proof" field.  The client
   instance presents its TLS client certificate during TLS negotiation
   with the server (either AS or RS).  The AS or RS takes the thumbprint
   of the TLS client certificate presented during mutual TLS negotiation
   and compares that thumbprint to the thumbprint presented by the
   client instance application as described in [RFC8705] section 3.

POST /tx HTTP/1.1
Host: server.example.com
Content-Type: application/json
SSL_CLIENT_CERT: MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFADCBmjE3MDUGA1UEAwwuQmVz
 cG9rZSBFbmdpbmVlcmluZyBSb290IENlcnRpZmljYXRlIEF1dGhvcml0eTELMAkG
 A1UECAwCTUExCzAJBgNVBAYTAlVTMRkwFwYJKoZIhvcNAQkBFgpjYUBic3BrLmlv
 MRwwGgYDVQQKDBNCZXNwb2tlIEVuZ2luZWVyaW5nMQwwCgYDVQQLDANNVEkwHhcN
 MTkwNDEwMjE0MDI5WhcNMjQwNDA4MjE0MDI5WjB8MRIwEAYDVQQDDAlsb2NhbGhv
 c3QxCzAJBgNVBAgMAk1BMQswCQYDVQQGEwJVUzEgMB4GCSqGSIb3DQEJARYRdGxz
 Y2xpZW50QGJzcGsuaW8xHDAaBgNVBAoME0Jlc3Bva2UgRW5naW5lZXJpbmcxDDAK
 BgNVBAsMA01USTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMmaXQHb
 s/wc1RpsQ6Orzf6rN+q2ijaZbQxD8oi+XaaN0P/gnE13JqQduvdq77OmJ4bQLokq
 sd0BexnI07Njsl8nkDDYpe8rNve5TjyUDCfbwgS7U1CluYenXmNQbaYNDOmCdHww
 UjV4kKREg6DGAx22Oq7+VHPTeeFgyw4kQgWRSfDENWY3KUXJlb/vKR6lQ+aOJytk
 vj8kVZQtWupPbvwoJe0na/ISNAOhL74w20DWWoDKoNltXsEtflNljVoi5nqsmZQc
 jfjt6LO0T7O1OX3Cwu2xWx8KZ3n/2ocuRqKEJHqUGfeDtuQNt6Jz79v/OTr8puLW
 aD+uyk6NbtGjoQsCAwEAAaOBiTCBhjAJBgNVHRMEAjAAMAsGA1UdDwQEAwIF4DBs
 BgNVHREEZTBjgglsb2NhbGhvc3SCD3Rsc2NsaWVudC5sb2NhbIcEwKgBBIERdGxz
 Y2xpZW50QGJzcGsuaW+GF2h0dHA6Ly90bHNjbGllbnQubG9jYWwvhhNzc2g6dGxz
 Y2xpZW50LmxvY2FsMA0GCSqGSIb3DQEBCwUAA4IBAQCKKv8WlLrT4Z5NazaUrYtl
 TF+2v0tvZBQ7qzJQjlOqAcvxry/d2zyhiRCRS/v318YCJBEv4Iq2W3I3JMMyAYEe
 2573HzT7rH3xQP12yZyRQnetdiVM1Z1KaXwfrPDLs72hUeELtxIcfZ0M085jLboX
 hufHI6kqm3NCyCCTihe2ck5RmCc5l2KBO/vAHF0ihhFOOOby1v6qbPHQcxAU6rEb
 907/p6BW/LV1NCgYB1QtFSfGxowqb9FRIMD2kvMSmO0EMxgwZ6k6spa+jk0IsI3k
 lwLW9b+Tfn/daUbIDctxeJneq2anQyU2znBgQl6KILDSF4eaOqlBut/KNZHHazJh

{
    "resources": [
        "dolphin-metadata"
    ],
    "interact": {

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        "redirect": true,
        "callback": {
            "method": "redirect",
            "uri": "https://client.foo",
            "nonce": "VJLO6A4CAYLBXHTR0KRO"
        }
    },
    "client": {
      "display": {
        "name": "My Client Display Name",
        "uri": "https://example.net/client"
      },
      "key": {
        "proof": "mtls",
        "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFADCBmjE3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"
    }
}

   [[ See issue #110 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/110) ]]

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8.4.  Demonstration of Proof-of-Possession (DPoP)

   This method is indicated by "dpop" in the "proof" field.  The client
   instance creates a Demonstration of Proof-of-Possession signature
   header as described in [I-D.ietf-oauth-dpop] section 2.  In addition
   to the required fields, the DPoP body MUST also contain a digest of
   the request body:

   digest (string)  Digest of the request body as the value of the
      Digest header defined in [RFC3230].

 POST /tx HTTP/1.1
 Host: server.example.com
 Content-Type: application/json
 DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IlJTMjU2IiwiandrIjp7Imt0eSI6Il
 JTQSIsImUiOiJBUUFCIiwia2lkIjoieHl6LWNsaWVudCIsImFsZyI6IlJTMjU2Iiwibi
 I6Inp3Q1RfM2J4LWdsYmJIcmhlWXBZcFJXaVk5SS1uRWFNUnBablJySWpDczZiX2VteV
 RrQmtEREVqU3lzaTM4T0M3M2hqMS1XZ3hjUGRLTkdaeUlvSDNRWmVuMU1LeXloUXBMSk
 cxLW9MTkxxbTdwWFh0ZFl6U2RDOU8zLW9peXk4eWtPNFlVeU5aclJSZlBjaWhkUUNiT1
 9PQzhRdWdtZzlyZ05ET1NxcHBkYU5lYXMxb3Y5UHhZdnhxcnoxLThIYTdna0QwMFlFQ1
 hIYUIwNXVNYVVhZEhxLU9fV0l2WVhpY2c2STVqNlM0NFZOVTY1VkJ3dS1BbHluVHhRZE
 1BV1AzYll4VlZ5NnAzLTdlVEpva3ZqWVRGcWdEVkRaOGxVWGJyNXlDVG5SaG5oSmd2Zj
 NWakRfbWFsTmU4LXRPcUs1T1NEbEhUeTZnRDlOcWRHQ20tUG0zUSJ9fQ.eyJodHRwX21
 ldGhvZCI6IlBPU1QiLCJodHRwX3VyaSI6Imh0dHA6XC9cL2hvc3QuZG9ja2VyLmludGV
 ybmFsOjk4MzRcL2FwaVwvYXNcL3RyYW5zYWN0aW9uIiwiaWF0IjoxNTcyNjQyNjEzLCJ
 qdGkiOiJIam9IcmpnbTJ5QjR4N2pBNXl5RyJ9.aUhftvfw2NoW3M7durkopReTvONng1
 fOzbWjAlKNSLL0qIwDgfG39XUyNvwQ23OBIwe6IuvTQ2UBBPklPAfJhDTKd8KHEAfidN
 B-LzUOzhDetLg30yLFzIpcEBMLCjb0TEsmXadvxuNkEzFRL-Q-QCg0AXSF1h57eAqZV8
 SYF4CQK9OUV6fIWwxLDd3cVTx83MgyCNnvFlG_HDyim1Xx-rxV4ePd1vgDeRubFb6QWj
 iKEO7vj1APv32dsux67gZYiUpjm0wEZprjlG0a07R984KLeK1XPjXgViEwEdlirUmpVy
 T9tyEYqGrTfm5uautELgMls9sgSyE929woZ59elg

 {
     "resources": [
         "dolphin-metadata"
     ],
     "interact": {
         "redirect": true,
         "callback": {
             "method": "redirect",
             "uri": "https://client.foo",
             "nonce": "VJLO6A4CAYLBXHTR0KRO"
         }
     },
     "client": {
       "display": {
         "name": "My Client Display Name",
         "uri": "https://example.net/client"

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       },
       "proof": "dpop",
       "key": {
         "jwk": {
                     "kty": "RSA",
                     "e": "AQAB",
                     "kid": "xyz-1",
                     "alg": "RS256",
                     "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xYJ
 CCNaOKNJn_Oz0YhdHbXTeWO5AoyspDWJbN5w_7bdWDxgpD-y6jnD1u9YhBOCWObNPFvpkTM
 8LC7SdXGRKx2k8Me2r_GssYlyRpqvpBlY5-ejCywKRBfctRcnhTTGNztbbDBUyDSWmFMVCH
 e5mXT4cL0BwrZC6S-uu-LAx06aKwQOPwYOGOslK8WPm1yGdkaA1uF_FpS6LS63WYPHi_Ap2
 B7_8Wbw4ttzbMS_doJvuDagW8A1Ip3fXFAHtRAcKw7rdI4_Xln66hJxFekpdfWdiPQddQ6Y
 1cK2U3obvUg7w"
         }
       }
     }
 }

8.5.  HTTP Signing

   This method is indicated by "httpsig" in the "proof" field.  The
   client instance creates an HTTP Signature header as described in
   [I-D.ietf-httpbis-message-signatures] section 4.  The client instance
   MUST calculate and present the Digest header as defined in [RFC3230]
   and include this header in the signature.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 716
   Signature: keyId="xyz-client", algorithm="rsa-sha256",
    headers="(request-target) digest content-length",
    signature="TkehmgK7GD/z4jGkmcHS67cjVRgm3zVQNlNrrXW32Wv7d
   u0VNEIVI/dMhe0WlHC93NP3ms91i2WOW5r5B6qow6TNx/82/6W84p5jqF
   YuYfTkKYZ69GbfqXkYV9gaT++dl5kvZQjVk+KZT1dzpAzv8hdk9nO87Xi
   rj7qe2mdAGE1LLc3YvXwNxuCQh82sa5rXHqtNT1077fiDvSVYeced0UEm
   rWwErVgr7sijtbTohC4FJLuJ0nG/KJUcIG/FTchW9rd6dHoBnY43+3Dzj
   CIthXpdH5u4VX3TBe6GJDO6Mkzc6vB+67OWzPwhYTplUiFFV6UZCsDEeu
   Sa/Ue1yLEAMg=="]}
   Digest: SHA=oZz2O3kg5SEFAhmr0xEBbc4jEfo=

   {
       "resources": [
           "dolphin-metadata"
       ],
       "interact": {
           "redirect": true,

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           "callback": {
               "method": "push",
               "uri": "https://client.foo",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
         },
         "proof": "httpsig",
         "key": {
           "jwk": {
                       "kty": "RSA",
                       "e": "AQAB",
                       "kid": "xyz-1",
                       "alg": "RS256",
                       "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_J
   tffXyaSx8xYJCCNaOKNJn_Oz0YhdHbXTeWO5AoyspDWJbN5w_7bdWDxgpD-
   y6jnD1u9YhBOCWObNPFvpkTM8LC7SdXGRKx2k8Me2r_GssYlyRpqvpBlY5-
   ejCywKRBfctRcnhTTGNztbbDBUyDSWmFMVCHe5mXT4cL0BwrZC6S-uu-LAx
   06aKwQOPwYOGOslK8WPm1yGdkaA1uF_FpS6LS63WYPHi_Ap2B7_8Wbw4ttz
   bMS_doJvuDagW8A1Ip3fXFAHtRAcKw7rdI4_Xln66hJxFekpdfWdiPQddQ6
   Y1cK2U3obvUg7w"
           }
         }
       }
   }

   When used to present an access token as in Section 7, the
   Authorization header MUST be included in the signature.

8.6.  OAuth Proof of Possession (PoP)

   This method is indicated by "oauthpop" in the "proof" field.  The
   client instance creates an HTTP Authorization PoP header as described
   in [I-D.ietf-oauth-signed-http-request] section 4, with the following
   additional requirements:

   *  The "at" (access token) field MUST be omitted unless this method
      is being used in conjunction with an access token as in Section 7.
      [[ See issue #112 (https://github.com/ietf-wg-gnap/gnap-core-
      protocol/issues/112) ]]

   *  The "b" (body hash) field MUST be calculated and supplied, unless
      there is no entity body (such as a GET, OPTIONS, or DELETE
      request).

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   *  All components of the URL MUST be calculated and supplied

   *  The m (method) field MUST be supplied

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   PoP: eyJhbGciOiJSUzI1NiIsImp3ayI6eyJrdHkiOiJSU0EiLCJlIjoi
   QVFBQiIsImtpZCI6Inh5ei1jbGllbnQiLCJhbGciOiJSUzI1NiIsIm4iO
   iJ6d0NUXzNieC1nbGJiSHJoZVlwWXBSV2lZOUktbkVhTVJwWm5ScklqQ3
   M2Yl9lbXlUa0JrRERFalN5c2kzOE9DNzNoajEtV2d4Y1BkS05HWnlJb0g
   zUVplbjFNS3l5aFFwTEpHMS1vTE5McW03cFhYdGRZelNkQzlPMy1vaXl5
   OHlrTzRZVXlOWnJSUmZQY2loZFFDYk9fT0M4UXVnbWc5cmdORE9TcXBwZ
   GFOZWFzMW92OVB4WXZ4cXJ6MS04SGE3Z2tEMDBZRUNYSGFCMDV1TWFVYW
   RIcS1PX1dJdllYaWNnNkk1ajZTNDRWTlU2NVZCd3UtQWx5blR4UWRNQVd
   QM2JZeFZWeTZwMy03ZVRKb2t2allURnFnRFZEWjhsVVhicjV5Q1RuUmhu
   aEpndmYzVmpEX21hbE5lOC10T3FLNU9TRGxIVHk2Z0Q5TnFkR0NtLVBtM
   1EifX0.eyJwIjoiXC9hcGlcL2FzXC90cmFuc2FjdGlvbiIsImIiOiJxa0
   lPYkdOeERhZVBTZnc3NnFjamtqSXNFRmxDb3g5bTU5NFM0M0RkU0xBIiw
   idSI6Imhvc3QuZG9ja2VyLmludGVybmFsIiwiaCI6W1siQWNjZXB0Iiwi
   Q29udGVudC1UeXBlIiwiQ29udGVudC1MZW5ndGgiXSwiVjQ2OUhFWGx6S
   k9kQTZmQU5oMmpKdFhTd3pjSGRqMUloOGk5M0h3bEVHYyJdLCJtIjoiUE
   9TVCIsInRzIjoxNTcyNjQyNjEwfQ.xyQ47qy8bu4fyK1T3Ru1Sway8wp6
   5rfAKnTQQU92AUUU07I2iKoBL2tipBcNCC5zLH5j_WUyjlN15oi_lLHym
   fPdzihtt8_Jibjfjib5J15UlifakjQ0rHX04tPal9PvcjwnyZHFcKn-So
   Y3wsARn-gGwxpzbsPhiKQP70d2eG0CYQMA6rTLslT7GgdQheelhVFW29i
   27NcvqtkJmiAG6Swrq4uUgCY3zRotROkJ13qo86t2DXklV-eES4-2dCxf
   cWFkzBAr6oC4Qp7HnY_5UT6IWkRJt3efwYprWcYouOVjtRan3kEtWkaWr
   G0J4bPVnTI5St9hJYvvh7FE8JirIg

   {
       "resources": [
           "dolphin-metadata"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.foo",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
         },
         "proof": "oauthpop",

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         "key": {
           "jwk": {
                       "kty": "RSA",
                       "e": "AQAB",
                       "kid": "xyz-1",
                       "alg": "RS256",
                       "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_J
   tffXyaSx8xYJCCNaOKNJn_Oz0YhdHbXTeWO5AoyspDWJbN5w_7bdWDxgpD-
   y6jnD1u9YhBOCWObNPFvpkTM8LC7SdXGRKx2k8Me2r_GssYlyRpqvpBlY5-
   ejCywKRBfctRcnhTTGNztbbDBUyDSWmFMVCHe5mXT4cL0BwrZC6S-uu-LAx
   06aKwQOPwYOGOslK8WPm1yGdkaA1uF_FpS6LS63WYPHi_Ap2B7_8Wbw4ttz
   bMS_doJvuDagW8A1Ip3fXFAHtRAcKw7rdI4_Xln66hJxFekpdfWdiPQddQ6
   Y1cK2U3obvUg7w"
           }
         }
       }
   }

   [[ See issue #113 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/113) ]]

9.  Discovery

   By design, the protocol minimizes the need for any pre-flight
   discovery.  To begin a request, the client instance only needs to
   know the endpoint of the AS 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 containing the following information:

   grant_request_endpoint (string)  REQUIRED.  The full URL of the AS's
      grant request endpoint.  This MUST match the URL the client
      instance used to make the discovery request.

   capabilities (array of strings)  OPTIONAL.  A list of the AS's
      capabilities.  The values of this result MAY be used by the client
      instance in the capabilities section (Section 2.6) of the request.

   interaction_methods (array of strings)  OPTIONAL.  A list of the AS's
      interaction methods.  The values of this list correspond to the
      possible fields in the interaction section (Section 2.5) of the
      request.

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   key_proofs (array strings)  OPTIONAL.  A list of the AS's supported
      key proofing mechanisms.  The values of this list correspond to
      possible values of the "proof" field of the key section
      (Section 2.3.2) of the request.

   sub_ids (array of strings)  OPTIONAL.  A list of the AS's supported
      identifiers.  The values of this list correspond to possible
      values of the subject identifier section (Section 2.2) of the
      request.

   assertions (array of strings)  OPTIONAL.  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.

   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, then the
   AS will deny a request from that client instance using a different
   proofing mechanism.

10.  Resource Servers

   In some deployments, a resource server will need to be able to call
   the AS for a number of functions.

   [[ See issue #114 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/114) ]]

10.1.  Introspecting a Token

   When the RS receives an access token, it can call the introspection
   endpoint at the AS to get token information.  [[ See issue #115
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/115) ]]

   +--------+       +------+       +------+
   | Client |--(1)->|  RS  |       |  AS  |
   |Instance|       |      |--(2)->|      |
   |        |       |      |<-(3)--|      |
   |        |       |      |       +------+
   |        |<-(4)--|      |
   +--------+       +------+

   1.  The client instance calls the RS with its access token.

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   2.  The RS introspects the access token value at the AS.  The RS
       signs the request with its own key (not the client instance's key
       or the token's key).

   3.  The AS validates the token value and the client instance's
       request and returns the introspection response for the token.

   4.  The RS fulfills the request from the client instance.

   The RS signs the request with its own key and sends the access token
   as the body of the request.

   POST /introspect HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Detached-JWS: ejy0...

   {
       "access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
   }

   The AS responds with a data structure describing the token's current
   state and any information the RS would need to validate the token's
   presentation, such as its intended proofing mechanism and key
   material.

   Content-type: application/json

   {
       "active": true,
       "resources": [
           "dolphin-metadata", "some other thing"
       ],
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
                       "kty": "RSA",
                       "e": "AQAB",
                       "kid": "xyz-1",
                       "alg": "RS256",
                       "n": "kOB5rR4Jv0GMeL...."
           }
         }
       }
   }

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10.2.  Deriving a downstream token

   Some architectures require an RS to act as a client instance and
   request a derived access token for a secondary RS.  This internal
   token is issued in the context of the incoming access token.

   +--------+       +-------+       +------+       +-------+
   | Client |--(1)->|  RS1  |       |  AS  |       |  RS2  |
   |Instance|       |       |--(2)->|      |       |       |
   |        |       |       |<-(3)--|      |       |       |
   |        |       |       |       +------+       |       |
   |        |       |       |                      |       |
   |        |       |       |-----------(4)------->|       |
   |        |       |       |<----------(5)--------|       |
   |        |<-(6)--|       |                      |       |
   +--------+       +-------+                      +-------+

   1.  The client instance calls RS1 with an access token.

   2.  RS1 presents that token to the AS to get a derived token for use
       at RS2.  RS1 indicates that it has no ability to interact with
       the RO.  RS1 signs its request with its own key, not the token's
       key or the client instance's key.

   3.  The AS returns a derived token to RS1 for use at RS2.

   4.  RS1 calls RS2 with the token from (3).

   5.  RS2 fulfills the call from RS1.

   6.  RS1 fulfills the call from client instance.

   If the RS needs to derive a token from one presented to it, it can
   request one from the AS by making a token request as described in
   Section 2 and presenting the existing access token's value in the
   "existing_access_token" field.

   The RS MUST identify itself with its own key and sign the request.

   [[ See issue #116 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/116) ]]

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 POST /tx HTTP/1.1
 Host: server.example.com
 Content-type: application/json
 Detached-JWS: ejy0...

 {
     "resources": [
         {
             "actions": [
                 "read",
                 "write",
                 "dolphin"
             ],
             "locations": [
                 "https://server.example.net/",
                 "https://resource.local/other"
             ],
             "datatypes": [
                 "metadata",
                 "images"
             ]
         },
         "dolphin-metadata"
     ],
     "client": "7C7C4AZ9KHRS6X63AJAO",
     "existing_access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
 }

   The AS responds with a token as described in Section 3.

10.3.  Registering a Resource Handle

   If the RS needs to, it can post a set of resources as described in
   Section 2.1.1 to the AS's resource registration endpoint.

   The RS MUST identify itself with its own key and sign the request.

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   POST /resource HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Detached-JWS: ejy0...

   {
       "resources": [
           {
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "dolphin-metadata"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO"

   }

   The AS responds with a handle appropriate to represent the resources
   list that the RS presented.

   Content-type: application/json

   {
       "resource_handle": "FWWIKYBQ6U56NL1"
   }

   The RS MAY make this handle available as part of a response
   (Section 10.4) or as documentation to developers.

   [[ See issue #117 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/117) ]]

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10.4.  Requesting Resources With Insufficient Access

   If the client instance calls an RS without an access token, or with
   an invalid access token, the RS MAY respond to the client instance
   with an authentication header indicating that GNAP needs to be used
   to access the resource.  The address of the GNAP endpoint MUST be
   sent in the "as_uri" parameter.  The RS MAY additionally return a
   resource reference that the client instance MAY use in its resource
   request (Section 2.1).  This resource reference handle SHOULD be
   sufficient for at least the action the client instance was attempting
   to take at the RS.  The RS MAY use the dynamic resource handle
   request (Section 10.3) to register a new resource handle, or use a
   handle that has been pre-configured to represent what the AS is
   protecting.  The content of this handle is opaque to the RS and the
   client instance.

WWW-Authenticate: GNAP as_uri=http://server.example/tx,resource=FWWIKYBQ6U56NL1

   The client instance then makes a call to the "as_uri" as described in
   Section 2, with the value of "resource" as one of the members of a
   "resources" array Section 2.1.1.  The client instance MAY request
   additional resources and other information, and MAY request multiple
   access tokens.

   [[ See issue #118 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/118) ]]

11.  Acknowledgements

   The editors would like to thank the feedback of the following
   individuals for their reviews, implementations, and contributions:
   Aaron Parecki, Annabelle Backman, Dick Hardt, Dmitri Zagidulin,
   Dmitry Barinov, Fabien Imbault, Francis Pouatcha, George Fletcher,
   Haardik Haardik, Hamid Massaoud, Jacky Yuan, Joseph Heenan, Justin
   Richer, Kathleen Moriarty, Mike Jones, Mike Varley, Nat Sakimura,
   Takahiko Kawasaki, Takahiro Tsuchiya.

   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.

12.  IANA Considerations

   [[ TBD: There are a lot of items in the document that are expandable
   through the use of value registries. ]]

13.  Security Considerations

   [[ TBD: There are a lot of security considerations to add. ]]

   All requests have to be over TLS or equivalent as per [BCP195].  Many
   handles act as shared secrets, though they can be combined with a
   requirement to provide proof of a key as well.

14.  Privacy Considerations

   [[ TBD: There are a lot of privacy considerations to add. ]]

   Handles are passed between parties and therefore should not contain
   any private data.

   When user information is passed to the client instance, the AS needs
   to make sure that it has the permission to do so.

15.  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,
              <http://www.rfc-editor.org/info/bcp195>.

   [I-D.ietf-httpbis-message-signatures]
              Backman, A., Richer, J., and M. Sporny, "Signing HTTP
              Messages", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-message-signatures-01, 17 November 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
              message-signatures-01.txt>.

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   [I-D.ietf-oauth-dpop]
              Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof-
              of-Possession at the Application Layer (DPoP)", Work in
              Progress, Internet-Draft, draft-ietf-oauth-dpop-02, 18
              November 2020, <http://www.ietf.org/internet-drafts/draft-
              ietf-oauth-dpop-02.txt>.

   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", Work in Progress,
              Internet-Draft, draft-ietf-oauth-signed-http-request-03, 8
              August 2016, <http://www.ietf.org/internet-drafts/draft-
              ietf-oauth-signed-http-request-03.txt>.

   [I-D.ietf-secevent-subject-identifiers]
              Backman, A. and M. Scurtescu, "Subject Identifiers for
              Security Event Tokens", Work in Progress, Internet-Draft,
              draft-ietf-secevent-subject-identifiers-06, 4 September
              2020, <http://www.ietf.org/internet-drafts/draft-ietf-
              secevent-subject-identifiers-06.txt>.

   [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/info/rfc2119>.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/info/rfc3230>.

   [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/info/rfc5646>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/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/info/rfc6750>.

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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/info/rfc7515>.

   [RFC7797]  Jones, M., "JSON Web Signature (JWS) Unencoded Payload
              Option", RFC 7797, DOI 10.17487/RFC7797, February 2016,
              <https://www.rfc-editor.org/info/rfc7797>.

   [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/info/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/info/rfc8259>.

   [RFC8693]  Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
              and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
              DOI 10.17487/RFC8693, January 2020,
              <https://www.rfc-editor.org/info/rfc8693>.

   [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/info/rfc8705>.

Appendix A.  Document History

   *  -03

      -  Changed "resource client" terminology to separate "client
         instance" and "client software".

   *  -02

      -  Moved all "editor's note" items to GitHub Issues.

      -  Added JSON types to fields.

      -  Changed "GNAP Protocol" to "GNAP".

      -  Editorial fixes.

   *  -01

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      -  "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.  Component Data Models

   While different implementations of this protocol will have different
   realizations of all the components and artifacts enumerated here, the
   nature of the protocol implies some common structures and elements
   for certain components.  This appendix seeks to enumerate those
   common elements.

   TBD: Client has keys, allowed requested resources, identifier(s),
   allowed requested subjects, allowed

   TBD: AS has "grant endpoint", interaction endpoints, store of trusted
   client keys, policies

   TBD: Token has RO, user, client, resource list, RS list,

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.

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
   Authorization Code grant type.

   The client instance initiates the request to the AS.  Here the client
   instance identifies itself using its public key.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Detached-JWS: ejy0...

   {
       "resources": [
           {
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           }
       ],
       "client": {
         "key": {
           "proof": "jwsd",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-1",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
         }
       },
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

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   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).

Content-type: application/json

{
    "interact": {
       "redirect": "https://server.example.com/interact/4CF492MLVMSW9MKMXKHQ",
       "callback": "MBDOFXG4Y5CVJCX821LH"
    }
    "continue": {
        "access_token": {
            "value": "80UPRY5NM33OMUKMKSKU",
            "key": true
        },
        "uri": "https://server.example.com/continue"
    },
    "instance_id": "7C7C4AZ9KHRS6X63AJAO"
}

   The client instance saves the response and redirects the user to the
   interaction_url by sending the following HTTP message to the user's
   browser.

   HTTP 302 Found
   Location: https://server.example.com/interact/4CF492MLVMSW9MKMXKHQ

   The user's browser fetches the AS's interaction URL.  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, 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.

HTTP 302 Found
Location: https://client.example.net/return/123455
  ?hash=p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A
  &interact_ref=4IFWWIKYBC2PQ6U56NL1

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   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 URL and presents the handle and 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
   Detached-JWS: ejy0...

   {
       "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   The AS retrieves the pending request based on the handle and issues a
   bearer access token and returns this to the client instance.

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Content-type: application/json

{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [{
            "actions": [
                "read",
                "write",
                "dolphin"
            ],
            "locations": [
                "https://server.example.net/",
                "https://resource.local/other"
            ],
            "datatypes": [
                "metadata",
                "images"
            ]
        }]
    },
    "continue": {
        "access_token": {
            "value": "80UPRY5NM33OMUKMKSKU",
            "key": true
        },
        "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
   Detached-JWS: ejy0...

   {
       "resources": [
           "dolphin-metadata", "some other thing"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "redirect": true,
           "user_code": true
       }
   }

   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 "callback" 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.

   Content-type: application/json

   {
       "interact": {
           "redirect": "https://srv.ex/MXKHQ",
           "user_code": {
               "code": "A1BC-3DFF",
               "url": "https://srv.ex/device"
           }
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
               "key": true
           },
           "uri": "https://server.example.com/continue/VGJKPTKC50",
           "wait": 60
       }
   }

   The client instance saves the response and displays the user code
   visually on its screen along with the static device URL.  The client
   instance also displays the short interaction URL as a QR code to be
   scanned.

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   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 URL.  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 URL.  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
   Detached-JWS: ejy0...

   The AS retrieves the pending request based on the handle 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.

   Content-type: application/json

   {
       "continue": {
           "access_token": {
               "value": "G7YQT4KQQ5TZY9SLSS5E",
               "key": true
           },
           "uri": "https://server.example.com/continue/ATWHO4Q1WV",
           "wait": 60
       }
   }

   Note that the continuation URL and access token have been rotated
   since they were used by the client instance to make this call.  The
   client instance polls the continuation URL after a 60 second timeout
   using this new information.

   POST /continue/ATWHO4Q1WV HTTP/1.1
   Host: server.example.com
   Authorization: GNAP G7YQT4KQQ5TZY9SLSS5E
   Detached-JWS: ejy0...

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   The AS retrieves the pending request based on the URL and access
   token, determines that it has been approved, and issues an access
   token for the client to use at the RS.

Content-type: application/json

{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [
            "dolphin-metadata", "some other thing"
        ]
    }
}

Appendix D.  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.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-type: application/json

   {
       "resources": [
           "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.

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   Content-type: application/json

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "key": true,
           "manage": "https://server.example.com/token",
           "resources": [
               "backend service", "nightly-routine-3"
           ]
       }
   }

D.1.  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
   Detached-JWS: ejy0...

   {
       "resources": [
           {
               "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": [ {
               "subject_type": "email",
               "email": "user@example.com"
           } ]
      }
   }

   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.

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   Content-type: application/json

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
               "key": true
           },
           "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 URL.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Detached-JWS: ejy0...

   The AS retrieves the pending request based on the handle 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.

   Content-type: application/json

   {
       "continue": {
           "access_token": {
               "value": "BI9QNW6V9W3XFJK4R02D",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   Note that the continuation handle has been rotated since it was used
   by the client instance to make this call.  The client instance polls
   the continuation URL after a 60 second timeout using the new handle.

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   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP BI9QNW6V9W3XFJK4R02D
   Detached-JWS: ejy0...

   The AS retrieves the pending request based on the handle and
   determines that it has been approved and it issues an access token.

Content-type: application/json

{
    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [
            "dolphin-metadata", "some other thing"
        ]
    }
}

D.2.  Applying OAuth 2 Scopes and Client IDs

   While GNAP is not designed to be directly compatible with OAuth 2
   [RFC6749], considerations have been made to enable the use of OAuth 2
   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 system and wants to apply them to
   the new protocol.  Traditionally, the OAuth 2 client developer would
   put their "client_id" and "scope" values as parameters into a
   redirect request to the authorization endpoint.

   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 values in the appropriate places.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-type: application/json
   Detached-JWS: ejy0...

   {
       "resources": [
           "read", "write", "dolphin"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "redirect": true,
           "callback": {
               "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 "callback" 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 URL.

   From here, the protocol continues as above.

Appendix E.  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

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   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 array of resource request descriptions while a request for
   multiple access tokens is composed of an object whose member values
   are all arrays.  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.

   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 RC 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.

Authors' Addresses

   Justin Richer (editor)
   Bespoke Engineering

   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Aaron Parecki
   Okta

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   Email: aaron@parecki.com
   URI:   https://aaronparecki.com

   Fabien Imbault
   acert.io

   Email: fabien.imbault@acert.io
   URI:   https://acert.io/

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