Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)
draft-ietf-ace-oauth-authz-27

ACE Working Group                                               L. Seitz
Internet-Draft                                                      RISE
Intended status: Standards Track                             G. Selander
Expires: May 24, 2020                                           Ericsson
                                                           E. Wahlstroem

                                                              S. Erdtman
                                                              Spotify AB
                                                           H. Tschofenig
                                                                Arm Ltd.
                                                       November 21, 2019


  Authentication and Authorization for Constrained Environments (ACE)
               using the OAuth 2.0 Framework (ACE-OAuth)
                     draft-ietf-ace-oauth-authz-27

Abstract

   This specification defines a framework for authentication and
   authorization in Internet of Things (IoT) environments called ACE-
   OAuth.  The framework is based on a set of building blocks including
   OAuth 2.0 and CoAP, thus transforming a well-known and widely used
   authorization solution into a form suitable for IoT devices.
   Existing specifications are used where possible, but extensions are
   added and profiles are defined to better serve the IoT use cases.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 24, 2020.








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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  CoAP  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Protocol Interactions . . . . . . . . . . . . . . . . . . . .  11
   5.  Framework . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Discovering Authorization Servers . . . . . . . . . . . .  16
       5.1.1.  Unauthorized Resource Request Message . . . . . . . .  16
       5.1.2.  AS Request Creation Hints . . . . . . . . . . . . . .  17
         5.1.2.1.  The Client-Nonce Parameter  . . . . . . . . . . .  19
     5.2.  Authorization Grants  . . . . . . . . . . . . . . . . . .  20
     5.3.  Client Credentials  . . . . . . . . . . . . . . . . . . .  20
     5.4.  AS Authentication . . . . . . . . . . . . . . . . . . . .  21
     5.5.  The Authorization Endpoint  . . . . . . . . . . . . . . .  21
     5.6.  The Token Endpoint  . . . . . . . . . . . . . . . . . . .  21
       5.6.1.  Client-to-AS Request  . . . . . . . . . . . . . . . .  22
       5.6.2.  AS-to-Client Response . . . . . . . . . . . . . . . .  25
       5.6.3.  Error Response  . . . . . . . . . . . . . . . . . . .  27
       5.6.4.  Request and Response Parameters . . . . . . . . . . .  28
         5.6.4.1.  Grant Type  . . . . . . . . . . . . . . . . . . .  28
         5.6.4.2.  Token Type  . . . . . . . . . . . . . . . . . . .  29
         5.6.4.3.  Profile . . . . . . . . . . . . . . . . . . . . .  29
         5.6.4.4.  Client-Nonce  . . . . . . . . . . . . . . . . . .  30
       5.6.5.  Mapping Parameters to CBOR  . . . . . . . . . . . . .  30
     5.7.  The Introspection Endpoint  . . . . . . . . . . . . . . .  31
       5.7.1.  Introspection Request . . . . . . . . . . . . . . . .  32
       5.7.2.  Introspection Response  . . . . . . . . . . . . . . .  33
       5.7.3.  Error Response  . . . . . . . . . . . . . . . . . . .  34
       5.7.4.  Mapping Introspection parameters to CBOR  . . . . . .  35
     5.8.  The Access Token  . . . . . . . . . . . . . . . . . . . .  35



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       5.8.1.  The Authorization Information Endpoint  . . . . . . .  36
         5.8.1.1.  Verifying an Access Token . . . . . . . . . . . .  37
         5.8.1.2.  Protecting the Authorization       Information
                   Endpoint  . . . . . . . . . . . . . . . . . . . .  39
       5.8.2.  Client Requests to the RS . . . . . . . . . . . . . .  39
       5.8.3.  Token Expiration  . . . . . . . . . . . . . . . . . .  40
       5.8.4.  Key Expiration  . . . . . . . . . . . . . . . . . . .  41
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  42
     6.1.  Protecting Tokens . . . . . . . . . . . . . . . . . . . .  42
     6.2.  Communication Security  . . . . . . . . . . . . . . . . .  43
     6.3.  Long-Term Credentials . . . . . . . . . . . . . . . . . .  43
     6.4.  Unprotected AS Request Creation Hints . . . . . . . . . .  44
     6.5.  Minimal security requirements        for communication  .  45
     6.6.  Token Freshness and Expiration  . . . . . . . . . . . . .  46
     6.7.  Combining profiles  . . . . . . . . . . . . . . . . . . .  46
     6.8.  Unprotected Information . . . . . . . . . . . . . . . . .  47
     6.9.  Identifying audiences . . . . . . . . . . . . . . . . . .  47
     6.10. Denial of service against or with      Introspection  . .  48
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  49
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  50
     8.1.  ACE Authorization Server Request Creation Hints . . . . .  50
     8.2.  OAuth Extensions Error Registration . . . . . . . . . . .  50
     8.3.  OAuth Error Code CBOR Mappings Registry . . . . . . . . .  51
     8.4.  OAuth Grant Type CBOR Mappings  . . . . . . . . . . . . .  51
     8.5.  OAuth Access Token Types  . . . . . . . . . . . . . . . .  52
     8.6.  OAuth Access Token Type CBOR Mappings . . . . . . . . . .  52
       8.6.1.  Initial Registry Contents . . . . . . . . . . . . . .  52
     8.7.  ACE Profile Registry  . . . . . . . . . . . . . . . . . .  52
     8.8.  OAuth Parameter Registration  . . . . . . . . . . . . . .  53
     8.9.  OAuth Parameters CBOR Mappings Registry . . . . . . . . .  53
     8.10. OAuth Introspection Response Parameter Registration . . .  54
     8.11. OAuth Token Introspection Response CBOR Mappings Registry  54
     8.12. JSON Web Token Claims . . . . . . . . . . . . . . . . . .  55
     8.13. CBOR Web Token Claims . . . . . . . . . . . . . . . . . .  55
     8.14. Media Type Registrations  . . . . . . . . . . . . . . . .  56
     8.15. CoAP Content-Format Registry  . . . . . . . . . . . . . .  57
     8.16. Expert Review Instructions  . . . . . . . . . . . . . . .  57
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  58
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  59
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  59
     10.2.  Informative References . . . . . . . . . . . . . . . . .  61
   Appendix A.  Design Justification . . . . . . . . . . . . . . . .  64
   Appendix B.  Roles and Responsibilities . . . . . . . . . . . . .  67
   Appendix C.  Requirements on Profiles . . . . . . . . . . . . . .  70
   Appendix D.  Assumptions on AS knowledge about C and RS . . . . .  70
   Appendix E.  Deployment Examples  . . . . . . . . . . . . . . . .  71
     E.1.  Local Token Validation  . . . . . . . . . . . . . . . . .  71
     E.2.  Introspection Aided Token Validation  . . . . . . . . . .  75



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   Appendix F.  Document Updates . . . . . . . . . . . . . . . . . .  79
     F.1.  Version -21 to 22 . . . . . . . . . . . . . . . . . . . .  80
     F.2.  Version -20 to 21 . . . . . . . . . . . . . . . . . . . .  80
     F.3.  Version -19 to 20 . . . . . . . . . . . . . . . . . . . .  80
     F.4.  Version -18 to -19  . . . . . . . . . . . . . . . . . . .  80
     F.5.  Version -17 to -18  . . . . . . . . . . . . . . . . . . .  80
     F.6.  Version -16 to -17  . . . . . . . . . . . . . . . . . . .  80
     F.7.  Version -15 to -16  . . . . . . . . . . . . . . . . . . .  81
     F.8.  Version -14 to -15  . . . . . . . . . . . . . . . . . . .  81
     F.9.  Version -13 to -14  . . . . . . . . . . . . . . . . . . .  81
     F.10. Version -12 to -13  . . . . . . . . . . . . . . . . . . .  81
     F.11. Version -11 to -12  . . . . . . . . . . . . . . . . . . .  82
     F.12. Version -10 to -11  . . . . . . . . . . . . . . . . . . .  82
     F.13. Version -09 to -10  . . . . . . . . . . . . . . . . . . .  82
     F.14. Version -08 to -09  . . . . . . . . . . . . . . . . . . .  82
     F.15. Version -07 to -08  . . . . . . . . . . . . . . . . . . .  82
     F.16. Version -06 to -07  . . . . . . . . . . . . . . . . . . .  83
     F.17. Version -05 to -06  . . . . . . . . . . . . . . . . . . .  83
     F.18. Version -04 to -05  . . . . . . . . . . . . . . . . . . .  83
     F.19. Version -03 to -04  . . . . . . . . . . . . . . . . . . .  84
     F.20. Version -02 to -03  . . . . . . . . . . . . . . . . . . .  84
     F.21. Version -01 to -02  . . . . . . . . . . . . . . . . . . .  84
     F.22. Version -00 to -01  . . . . . . . . . . . . . . . . . . .  85
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  85

1.  Introduction

   Authorization is the process for granting approval to an entity to
   access a generic resource [RFC4949].  The authorization task itself
   can best be described as granting access to a requesting client, for
   a resource hosted on a device, the resource server (RS).  This
   exchange is mediated by one or multiple authorization servers (AS).
   Managing authorization for a large number of devices and users can be
   a complex task.

   While prior work on authorization solutions for the Web and for the
   mobile environment also applies to the Internet of Things (IoT)
   environment, many IoT devices are constrained, for example, in terms
   of processing capabilities, available memory, etc.  For web
   applications on constrained nodes, this specification RECOMMENDS the
   use of CoAP [RFC7252] as replacement for HTTP.

   A detailed treatment of constraints can be found in [RFC7228], and
   the different IoT deployments present a continuous range of device
   and network capabilities.  Taking energy consumption as an example:
   At one end there are energy-harvesting or battery powered devices
   which have a tight power budget, on the other end there are mains-
   powered devices, and all levels in between.



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   Hence, IoT devices may be very different in terms of available
   processing and message exchange capabilities and there is a need to
   support many different authorization use cases [RFC7744].

   This specification describes a framework for authentication and
   authorization in constrained environments (ACE) built on re-use of
   OAuth 2.0 [RFC6749], thereby extending authorization to Internet of
   Things devices.  This specification contains the necessary building
   blocks for adjusting OAuth 2.0 to IoT environments.

   More detailed, interoperable specifications can be found in profiles.
   Implementations may claim conformance with a specific profile,
   whereby implementations utilizing the same profile interoperate while
   implementations of different profiles are not expected to be
   interoperable.  Some devices, such as mobile phones and tablets, may
   implement multiple profiles and will therefore be able to interact
   with a wider range of low end devices.  Requirements on profiles are
   described at contextually appropriate places throughout this
   specification, and also summarized in Appendix C.

2.  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.

   Certain security-related terms such as "authentication",
   "authorization", "confidentiality", "(data) integrity", "message
   authentication code", and "verify" are taken from [RFC4949].

   Since exchanges in this specification are described as RESTful
   protocol interactions, HTTP [RFC7231] offers useful terminology.

   Terminology for entities in the architecture is defined in OAuth 2.0
   [RFC6749] such as client (C), resource server (RS), and authorization
   server (AS).

   Note that the term "endpoint" is used here following its OAuth
   definition, which is to denote resources such as token and
   introspection at the AS and authz-info at the RS (see Section 5.8.1
   for a definition of the authz-info endpoint).  The CoAP [RFC7252]
   definition, which is "An entity participating in the CoAP protocol"
   is not used in this specification.

   The specifications in this document is called the "framework" or "ACE
   framework".  When referring to "profiles of this framework" it refers



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   to additional specifications that define the use of this
   specification with concrete transport and communication security
   protocols (e.g., CoAP over DTLS).

   We use the term "Access Information" for parameters other than the
   access token provided to the client by the AS to enable it to access
   the RS (e.g. public key of the RS, profile supported by RS).

   We use the term "Authorization Information" to denote all
   information, including the claims of relevant access tokens, that an
   RS uses to determine whether an access request should be granted.

3.  Overview

   This specification defines the ACE framework for authorization in the
   Internet of Things environment.  It consists of a set of building
   blocks.

   The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys
   widespread deployment.  Many IoT devices can support OAuth 2.0
   without any additional extensions, but for certain constrained
   settings additional profiling is needed.

   Another building block is the lightweight web transfer protocol CoAP
   [RFC7252], for those communication environments where HTTP is not
   appropriate.  CoAP typically runs on top of UDP, which further
   reduces overhead and message exchanges.  While this specification
   defines extensions for the use of OAuth over CoAP, other underlying
   protocols are not prohibited from being supported in the future, such
   as HTTP/2 [RFC7540], MQTT [MQTT5.0], BLE [BLE] and QUIC
   [I-D.ietf-quic-transport].  Note that this document specifies
   protocol exchanges in terms of RESTful verbs such as GET and POST.
   Future profiles using protocols that do not support these verbs MUST
   specify how the corresponding protocol messages are transmitted
   instead.

   A third building block is CBOR [RFC7049], for encodings where JSON
   [RFC8259] is not sufficiently compact.  CBOR is a binary encoding
   designed for small code and message size, which may be used for
   encoding of self contained tokens, and also for encoding payloads
   transferred in protocol messages.

   A fourth building block is the CBOR-based secure message format COSE
   [RFC8152], which enables object-level layer security as an
   alternative or complement to transport layer security (DTLS [RFC6347]
   or TLS [RFC8446]).  COSE is used to secure self-contained tokens such
   as proof-of-possession (PoP) tokens, which are an extension to the
   OAuth bearer tokens.  The default token format is defined in CBOR web



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   token (CWT) [RFC8392].  Application layer security for CoAP using
   COSE can be provided with OSCORE [RFC8613].

   With the building blocks listed above, solutions satisfying various
   IoT device and network constraints are possible.  A list of
   constraints is described in detail in [RFC7228] and a description of
   how the building blocks mentioned above relate to the various
   constraints can be found in Appendix A.

   Luckily, not every IoT device suffers from all constraints.  The ACE
   framework nevertheless takes all these aspects into account and
   allows several different deployment variants to co-exist, rather than
   mandating a one-size-fits-all solution.  It is important to cover the
   wide range of possible interworking use cases and the different
   requirements from a security point of view.  Once IoT deployments
   mature, popular deployment variants will be documented in the form of
   ACE profiles.

3.1.  OAuth 2.0

   The OAuth 2.0 authorization framework enables a client to obtain
   scoped access to a resource with the permission of a resource owner.
   Authorization information, or references to it, is passed between the
   nodes using access tokens.  These access tokens are issued to clients
   by an authorization server with the approval of the resource owner.
   The client uses the access token to access the protected resources
   hosted by the resource server.

   A number of OAuth 2.0 terms are used within this specification:

   The token and introspection Endpoints:
      The AS hosts the token endpoint that allows a client to request
      access tokens.  The client makes a POST request to the token
      endpoint on the AS and receives the access token in the response
      (if the request was successful).
      In some deployments, a token introspection endpoint is provided by
      the AS, which can be used by the RS if it needs to request
      additional information regarding a received access token.  The RS
      makes a POST request to the introspection endpoint on the AS and
      receives information about the access token in the response.  (See
      "Introspection" below.)



   Access Tokens:
      Access tokens are credentials needed to access protected
      resources.  An access token is a data structure representing
      authorization permissions issued by the AS to the client.  Access



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      tokens are generated by the AS and consumed by the RS.  The access
      token content is opaque to the client.

      Access tokens can have different formats, and various methods of
      utilization e.g., cryptographic properties) based on the security
      requirements of the given deployment.



   Refresh Tokens:
      Refresh tokens are credentials used to obtain access tokens.
      Refresh tokens are issued to the client by the authorization
      server and are used to obtain a new access token when the current
      access token becomes invalid or expires, or to obtain additional
      access tokens with identical or narrower scope (such access tokens
      may have a shorter lifetime and fewer permissions than authorized
      by the resource owner).  Issuing a refresh token is optional at
      the discretion of the authorization server.  If the authorization
      server issues a refresh token, it is included when issuing an
      access token (i.e., step (B) in Figure 1).

      A refresh token in OAuth 2.0 is a string representing the
      authorization granted to the client by the resource owner.  The
      string is usually opaque to the client.  The token denotes an
      identifier used to retrieve the authorization information.  Unlike
      access tokens, refresh tokens are intended for use only with
      authorization servers and are never sent to resource servers.  In
      this framework, refresh tokens are encoded in binary instead of
      strings, if used.


   Proof of Possession Tokens:
      A token may be bound to a cryptographic key, which is then used to
      bind the token to a request authorized by the token.  Such tokens
      are called proof-of-possession tokens (or PoP tokens).

      The proof-of-possession (PoP) security concept used here assumes
      that the AS acts as a trusted third party that binds keys to
      tokens.  In the case of access tokens, these so called PoP keys
      are then used by the client to demonstrate the possession of the
      secret to the RS when accessing the resource.  The RS, when
      receiving an access token, needs to verify that the key used by
      the client matches the one bound to the access token.  When this
      specification uses the term "access token" it is assumed to be a
      PoP access token token unless specifically stated otherwise.

      The key bound to the token (the PoP key) may use either symmetric
      or asymmetric cryptography.  The appropriate choice of the kind of



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      cryptography depends on the constraints of the IoT devices as well
      as on the security requirements of the use case.



      Symmetric PoP key:
         The AS generates a random symmetric PoP key.  The key is either
         stored to be returned on introspection calls or encrypted and
         included in the token.  The PoP key is also encrypted for the
         token recipient and sent to the recipient together with the
         token.



      Asymmetric PoP key:
         An asymmetric key pair is generated on the token's recipient
         and the public key is sent to the AS (if it does not already
         have knowledge of the recipient's public key).  Information
         about the public key, which is the PoP key in this case, is
         either stored to be returned on introspection calls or included
         inside the token and sent back to the requesting party.  The
         consumer of the token can identify the public key from the
         information in the token, which allows the recipient of the
         token to use the corresponding private key for the proof of
         possession.

      The token is either a simple reference, or a structured
      information object (e.g., CWT [RFC8392]) protected by a
      cryptographic wrapper (e.g., COSE [RFC8152]).  The choice of PoP
      key does not necessarily imply a specific credential type for the
      integrity protection of the token.



   Scopes and Permissions:
      In OAuth 2.0, the client specifies the type of permissions it is
      seeking to obtain (via the scope parameter) in the access token
      request.  In turn, the AS may use the scope response parameter to
      inform the client of the scope of the access token issued.  As the
      client could be a constrained device as well, this specification
      defines the use of CBOR encoding, see Section 5, for such requests
      and responses.

      The values of the scope parameter in OAuth 2.0 are expressed as a
      list of space-delimited, case-sensitive strings, with a semantic
      that is well-known to the AS and the RS.  More details about the
      concept of scopes is found under Section 3.3 in [RFC6749].




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   Claims:
      Information carried in the access token or returned from
      introspection, called claims, is in the form of name-value pairs.
      An access token may, for example, include a claim identifying the
      AS that issued the token (via the "iss" claim) and what audience
      the access token is intended for (via the "aud" claim).  The
      audience of an access token can be a specific resource or one or
      many resource servers.  The resource owner policies influence what
      claims are put into the access token by the authorization server.

      While the structure and encoding of the access token varies
      throughout deployments, a standardized format has been defined
      with the JSON Web Token (JWT) [RFC7519] where claims are encoded
      as a JSON object.  In [RFC8392], an equivalent format using CBOR
      encoding (CWT) has been defined.



   Introspection:
      Introspection is a method for a resource server to query the
      authorization server for the active state and content of a
      received access token.  This is particularly useful in those cases
      where the authorization decisions are very dynamic and/or where
      the received access token itself is an opaque reference rather
      than a self-contained token.  More information about introspection
      in OAuth 2.0 can be found in [RFC7662].

3.2.  CoAP

   CoAP is an application layer protocol similar to HTTP, but
   specifically designed for constrained environments.  CoAP typically
   uses datagram-oriented transport, such as UDP, where reordering and
   loss of packets can occur.  A security solution needs to take the
   latter aspects into account.

   While HTTP uses headers and query strings to convey additional
   information about a request, CoAP encodes such information into
   header parameters called 'options'.

   CoAP supports application-layer fragmentation of the CoAP payloads
   through blockwise transfers [RFC7959].  However, blockwise transfer
   does not increase the size limits of CoAP options, therefore data
   encoded in options has to be kept small.

   Transport layer security for CoAP can be provided by DTLS or TLS
   [RFC6347][RFC8446] [I-D.ietf-tls-dtls13].  CoAP defines a number of
   proxy operations that require transport layer security to be
   terminated at the proxy.  One approach for protecting CoAP



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   communication end-to-end through proxies, and also to support
   security for CoAP over a different transport in a uniform way, is to
   provide security at the application layer using an object-based
   security mechanism such as COSE [RFC8152].

   One application of COSE is OSCORE [RFC8613], which provides end-to-
   end confidentiality, integrity and replay protection, and a secure
   binding between CoAP request and response messages.  In OSCORE, the
   CoAP messages are wrapped in COSE objects and sent using CoAP.

   This framework RECOMMENDS the use of CoAP as replacement for HTTP for
   use in constrained environments.  For communication security this
   framework does not make an explicit protocol recommendation, since
   the choice depends on the requirements of the specific application.
   DTLS [RFC6347], [I-D.ietf-tls-dtls13] and OSCORE [RFC8613] are
   mentioned as examples, other protocols fulfilling the requirements
   from Section 6.5 are also applicable.

4.  Protocol Interactions

   The ACE framework is based on the OAuth 2.0 protocol interactions
   using the token endpoint and optionally the introspection endpoint.
   A client obtains an access token, and optionally a refresh token,
   from an AS using the token endpoint and subsequently presents the
   access token to a RS to gain access to a protected resource.  In most
   deployments the RS can process the access token locally, however in
   some cases the RS may present it to the AS via the introspection
   endpoint to get fresh information.  These interactions are shown in
   Figure 1.  An overview of various OAuth concepts is provided in
   Section 3.1.

   The OAuth 2.0 framework defines a number of "protocol flows" via
   grant types, which have been extended further with extensions to
   OAuth 2.0 (such as [RFC7521] and [RFC8628]).  What grant types works
   best depends on the usage scenario and [RFC7744] describes many
   different IoT use cases but there are two preferred grant types,
   namely the Authorization Code Grant (described in Section 4.1 of
   [RFC7521]) and the Client Credentials Grant (described in Section 4.4
   of [RFC7521]).  The Authorization Code Grant is a good fit for use
   with apps running on smart phones and tablets that request access to
   IoT devices, a common scenario in the smart home environment, where
   users need to go through an authentication and authorization phase
   (at least during the initial setup phase).  The native apps
   guidelines described in [RFC8252] are applicable to this use case.
   The Client Credential Grant is a good fit for use with IoT devices
   where the OAuth client itself is constrained.  In such a case, the
   resource owner has pre-arranged access rights for the client with the




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   authorization server, which is often accomplished using a
   commissioning tool.

   The consent of the resource owner, for giving a client access to a
   protected resource, can be provided dynamically as in the traditional
   OAuth flows, or it could be pre-configured by the resource owner as
   authorization policies at the AS, which the AS evaluates when a token
   request arrives.  The resource owner and the requesting party (i.e.,
   client owner) are not shown in Figure 1.

   This framework supports a wide variety of communication security
   mechanisms between the ACE entities, such as client, AS, and RS.  It
   is assumed that the client has been registered (also called enrolled
   or onboarded) to an AS using a mechanism defined outside the scope of
   this document.  In practice, various techniques for onboarding have
   been used, such as factory-based provisioning or the use of
   commissioning tools.  Regardless of the onboarding technique, this
   provisioning procedure implies that the client and the AS exchange
   credentials and configuration parameters.  These credentials are used
   to mutually authenticate each other and to protect messages exchanged
   between the client and the AS.

   It is also assumed that the RS has been registered with the AS,
   potentially in a similar way as the client has been registered with
   the AS.  Established keying material between the AS and the RS allows
   the AS to apply cryptographic protection to the access token to
   ensure that its content cannot be modified, and if needed, that the
   content is confidentiality protected.

   The keying material necessary for establishing communication security
   between C and RS is dynamically established as part of the protocol
   described in this document.

   At the start of the protocol, there is an optional discovery step
   where the client discovers the resource server and the resources this
   server hosts.  In this step, the client might also determine what
   permissions are needed to access the protected resource.  A generic
   procedure is described in Section 5.1; profiles MAY define other
   procedures for discovery.

   In Bluetooth Low Energy, for example, advertisements are broadcasted
   by a peripheral, including information about the primary services.
   In CoAP, as a second example, a client can make a request to "/.well-
   known/core" to obtain information about available resources, which
   are returned in a standardized format as described in [RFC6690].






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   +--------+                               +---------------+
   |        |---(A)-- Token Request ------->|               |
   |        |                               | Authorization |
   |        |<--(B)-- Access Token ---------|    Server     |
   |        |    + Access Information       |               |
   |        |    + Refresh Token (optional) +---------------+
   |        |                                      ^ |
   |        |            Introspection Request  (D)| |
   | Client |                  (optional)          | |
   |        |                         Response     | |(E)
   |        |                         (optional)   | v
   |        |                               +--------------+
   |        |---(C)-- Token + Request ----->|              |
   |        |                               |   Resource   |
   |        |<--(F)-- Protected Resource ---|    Server    |
   |        |                               |              |
   +--------+                               +--------------+

                      Figure 1: Basic Protocol Flow.

   Requesting an Access Token (A):
      The client makes an access token request to the token endpoint at
      the AS.  This framework assumes the use of PoP access tokens (see
      Section 3.1 for a short description) wherein the AS binds a key to
      an access token.  The client may include permissions it seeks to
      obtain, and information about the credentials it wants to use
      (e.g., symmetric/asymmetric cryptography or a reference to a
      specific credential).



   Access Token Response (B):
      If the AS successfully processes the request from the client, it
      returns an access token and optionally a refresh token (note that
      only certain grant types support refresh tokens).  It can also
      return additional parameters, referred to as "Access Information".
      In addition to the response parameters defined by OAuth 2.0 and
      the PoP access token extension, this framework defines parameters
      that can be used to inform the client about capabilities of the
      RS, e.g. the profiles the RS supports.  More information about
      these parameters can be found in Section 5.6.4.



   Resource Request (C):
      The client interacts with the RS to request access to the
      protected resource and provides the access token.  The protocol to
      use between the client and the RS is not restricted to CoAP.



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      HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also
      viable candidates.

      Depending on the device limitations and the selected protocol,
      this exchange may be split up into two parts:

         (1) the client sends the access token containing, or
         referencing, the authorization information to the RS, that may
         be used for subsequent resource requests by the client, and

         (2) the client makes the resource access request, using the
         communication security protocol and other Access Information
         obtained from the AS.

      The Client and the RS mutually authenticate using the security
      protocol specified in the profile (see step B) and the keys
      obtained in the access token or the Access Information.  The RS
      verifies that the token is integrity protected and originated by
      the AS.  It then compares the claims contained in the access token
      with the resource request.  If the RS is online, validation can be
      handed over to the AS using token introspection (see messages D
      and E) over HTTP or CoAP.



   Token Introspection Request (D):
      A resource server may be configured to introspect the access token
      by including it in a request to the introspection endpoint at that
      AS.  Token introspection over CoAP is defined in Section 5.7 and
      for HTTP in [RFC7662].

      Note that token introspection is an optional step and can be
      omitted if the token is self-contained and the resource server is
      prepared to perform the token validation on its own.



   Token Introspection Response (E):
      The AS validates the token and returns the most recent parameters,
      such as scope, audience, validity etc. associated with it back to
      the RS.  The RS then uses the received parameters to process the
      request to either accept or to deny it.



   Protected Resource (F):
      If the request from the client is authorized, the RS fulfills the
      request and returns a response with the appropriate response code.



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      The RS uses the dynamically established keys to protect the
      response, according to the communication security protocol used.

5.  Framework

   The following sections detail the profiling and extensions of OAuth
   2.0 for constrained environments, which constitutes the ACE
   framework.

   Credential Provisioning
      For IoT, it cannot be assumed that the client and RS are part of a
      common key infrastructure, so the AS provisions credentials or
      associated information to allow mutual authentication between
      client and RS.  The resulting security association between client
      and RS may then also be used to bind these credentials to the
      access tokens the client uses.



   Proof-of-Possession
      The ACE framework, by default, implements proof-of-possession for
      access tokens, i.e., that the token holder can prove being a
      holder of the key bound to the token.  The binding is provided by
      the "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession] indicating
      what key is used for proof-of-possession.  If a client needs to
      submit a new access token, e.g., to obtain additional access
      rights, they can request that the AS binds this token to the same
      key as the previous one.



   ACE Profiles
      The client or RS may be limited in the encodings or protocols it
      supports.  To support a variety of different deployment settings,
      specific interactions between client and RS are defined in an ACE
      profile.  In ACE framework the AS is expected to manage the
      matching of compatible profile choices between a client and an RS.
      The AS informs the client of the selected profile using the
      "ace_profile" parameter in the token response.

   OAuth 2.0 requires the use of TLS both to protect the communication
   between AS and client when requesting an access token; between client
   and RS when accessing a resource and between AS and RS if
   introspection is used.  In constrained settings TLS is not always
   feasible, or desirable.  Nevertheless it is REQUIRED that the
   communications named above are encrypted, integrity protected and
   protected against message replay.  It is also REQUIRED that the
   communicating endpoints perform mutual authentication.  Furthermore



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   it MUST be assured that responses are bound to the requests in the
   sense that the receiver of a response can be certain that the
   response actually belongs to a certain request.  Note that setting up
   such a secure communication may require some unprotected messages to
   be exchanged first (e.g. sending the token from the client to the
   RS).

   Profiles MUST specify a communication security protocol that provides
   the features required above.

   In OAuth 2.0 the communication with the Token and the Introspection
   endpoints at the AS is assumed to be via HTTP and may use Uri-query
   parameters.  When profiles of this framework use CoAP instead, it is
   REQUIRED to use of the following alternative instead of Uri-query
   parameters: The sender (client or RS) encodes the parameters of its
   request as a CBOR map and submits that map as the payload of the POST
   request.

   Profiles that use CBOR encoding of protocol message parameters at the
   outermost encoding layer MUST use the media format 'application/
   ace+cbor'.  If CoAP is used for communication, the Content-Format
   MUST be abbreviated with the ID: 19 (see Section 8.15).

   The OAuth 2.0 AS uses a JSON structure in the payload of its
   responses both to client and RS.  If CoAP is used, it is REQUIRED to
   use CBOR [RFC7049] instead of JSON.  Depending on the profile, the
   CBOR payload MAY be enclosed in a non-CBOR cryptographic wrapper.

5.1.  Discovering Authorization Servers

   In order to determine the AS in charge of a resource hosted at the
   RS, C MAY send an initial Unauthorized Resource Request message to
   RS.  RS then denies the request and sends the address of its AS back
   to C.

   Instead of the initial Unauthorized Resource Request message, other
   discovery methods may be used, or the client may be pre-provisioned
   with an RS-to-AS mapping.

5.1.1.  Unauthorized Resource Request Message

   An Unauthorized Resource Request message is a request for any
   resource hosted by RS for which the client does not have
   authorization granted.  RSes MUST treat any request for a protected
   resource as an Unauthorized Resource Request message when any of the
   following hold:

   o  The request has been received on an unprotected channel.



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   o  The RS has no valid access token for the sender of the request
      regarding the requested action on that resource.

   o  The RS has a valid access token for the sender of the request, but
      that token does not authorize the requested action on the
      requested resource.

   Note: These conditions ensure that the RS can handle requests
   autonomously once access was granted and a secure channel has been
   established between C and RS.  The authz-info endpoint, as part of
   the process for authorizing to protected resources, is not itself a
   protected resource and MUST NOT be protected as specified above (cf.
   Section 5.8.1).

   Unauthorized Resource Request messages MUST be denied with an
   "unauthorized_client" error response.  In this response, the Resource
   Server SHOULD provide proper AS Request Creation Hints to enable the
   Client to request an access token from RS's AS as described in
   Section 5.1.2.

   The handling of all client requests (including unauthorized ones) by
   the RS is described in Section 5.8.2.

5.1.2.  AS Request Creation Hints

   The AS Request Creation Hints message is sent by an RS as a response
   to an Unauthorized Resource Request message (see Section 5.1.1) to
   help the sender of the Unauthorized Resource Request message acquire
   a valid access token.  The AS Request Creation Hints message is a
   CBOR map, with a MANDATORY element "AS" specifying an absolute URI
   (see Section 4.3 of [RFC3986]) that identifies the appropriate AS for
   the RS.

   The message can also contain the following OPTIONAL parameters:

   o  A "audience" element containing a suggested audience that the
      client should request at the AS.

   o  A "kid" element containing the key identifier of a key used in an
      existing security association between the client and the RS.  The
      RS expects the client to request an access token bound to this
      key, in order to avoid having to re-establish the security
      association.

   o  A "cnonce" element containing a client-nonce.  See
      Section 5.1.2.1.





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   o  A "scope" element containing the suggested scope that the client
      should request towards the AS.

   Figure 2 summarizes the parameters that may be part of the AS Request
   Creation Hints.

           /-----------+----------+---------------------\
           | Name      | CBOR Key | Value Type          |
           |-----------+----------+---------------------|
           | AS        |     1    | text string         |
           | kid       |     2    | byte string         |
           | audience  |     5    | text string         |
           | scope     |     9    | text or byte string |
           | cnonce    |    39    | byte string         |
           \-----------+----------+---------------------/

                    Figure 2: AS Request Creation Hints

   Note that the schema part of the AS parameter may need to be adapted
   to the security protocol that is used between the client and the AS.
   Thus the example AS value "coap://as.example.com/token" might need to
   be transformed to "coaps://as.example.com/token".  It is assumed that
   the client can determine the correct schema part on its own depending
   on the way it communicates with the AS.

   Figure 3 shows an example for an AS Request Creation Hints message
   payload using CBOR [RFC7049] diagnostic notation, using the parameter
   names instead of the CBOR keys for better human readability.

       4.01 Unauthorized
       Content-Format: application/ace+cbor
       Payload :
       {
        "AS" : "coaps://as.example.com/token",
        "audience" : "coaps://rs.example.com"
        "scope" : "rTempC",
        "cnonce" : h'e0a156bb3f'
       }

            Figure 3: AS Request Creation Hints payload example

   In this example, the attribute AS points the receiver of this message
   to the URI "coaps://as.example.com/token" to request access
   permissions.  The originator of the AS Request Creation Hints payload
   (i.e., RS) uses a local clock that is loosely synchronized with a
   time scale common between RS and AS (e.g., wall clock time).
   Therefore, it has included a parameter "nonce" (see Section 5.1.2.1).




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   Figure 4 illustrates the mandatory to use binary encoding of the
   message payload shown in Figure 3.

   a4                                   # map(4)
      01                                # unsigned(1) (=AS)
      78 1c                             # text(28)
         636f6170733a2f2f61732e657861
         6d706c652e636f6d2f746f6b656e   # "coaps://as.example.com/token"
      05                                # unsigned(5) (=audience)
      76                                # text(22)
         636f6170733a2f2f72732e657861
         6d706c652e636f6d               # "coaps://rs.example.com"
      09                                # unsigned(9) (=scope)
      66                                # text(6)
         7254656d7043                   # "rTempC"
      18 27                             # unsigned(39) (=cnonce)
      45                                # bytes(5)
         e0a156bb3f                     #

        Figure 4: AS Request Creation Hints example encoded in CBOR

5.1.2.1.  The Client-Nonce Parameter

   If the RS does not synchronize its clock with the AS, it could be
   tricked into accepting old access tokens, that are either expired or
   have been compromised.  In order to ensure some level of token
   freshness in that case, the RS can use the "cnonce" (client-nonce)
   parameter.  The processing requirements for this parameter are as
   follows:

   o  A RS sending a "cnonce" parameter in an an AS Request Creation
      Hints message MUST store information to validate that a given
      cnonce is fresh.  How this is implemented internally is out of
      scope for this specification.  Expiration of client-nonces should
      be based roughly on the time it would take a client to obtain an
      access token after receiving the AS Request Creation Hints
      message, with some allowance for unexpected delays.

   o  A client receiving a "cnonce" parameter in an AS Request Creation
      Hints message MUST include this in the parameters when requesting
      an access token at the AS, using the "cnonce" parameter from
      Section 5.6.4.4.

   o  If an AS grants an access token request containing a "cnonce"
      parameter, it MUST include this value in the access token, using
      the "cnonce" claim specified in Section 5.8.





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   o  A RS that is using the client-nonce mechanism and that receives an
      access token MUST verify that this token contains a cnonce claim,
      with a client-nonce value that is fresh according to the
      information stored at the first step above.  If the cnonce claim
      is not present or if the cnonce claim value is not fresh, the RS
      MUST discard the access token.  If this was an interaction with
      the authz-info endpoint the RS MUST also respond with an error
      message using a response code equivalent to the CoAP code 4.01
      (Unauthorized).

5.2.  Authorization Grants

   To request an access token, the client obtains authorization from the
   resource owner or uses its client credentials as a grant.  The
   authorization is expressed in the form of an authorization grant.

   The OAuth framework [RFC6749] defines four grant types.  The grant
   types can be split up into two groups, those granted on behalf of the
   resource owner (password, authorization code, implicit) and those for
   the client (client credentials).  Further grant types have been added
   later, such as [RFC7521] defining an assertion-based authorization
   grant.

   The grant type is selected depending on the use case.  In cases where
   the client acts on behalf of the resource owner, the authorization
   code grant is recommended.  If the client acts on behalf of the
   resource owner, but does not have any display or has very limited
   interaction possibilities, it is recommended to use the device code
   grant defined in [RFC8628].  In cases where the client acts
   autonomously the client credentials grant is recommended.

   For details on the different grant types, see section 1.3 of
   [RFC6749].  The OAuth 2.0 framework provides an extension mechanism
   for defining additional grant types, so profiles of this framework
   MAY define additional grant types, if needed.

5.3.  Client Credentials

   Authentication of the client is mandatory independent of the grant
   type when requesting an access token from the token endpoint.  In the
   case of the client credentials grant type, the authentication and
   grant coincide.

   Client registration and provisioning of client credentials to the
   client is out of scope for this specification.

   The OAuth framework defines one client credential type in section
   2.3.1 of [RFC6749]: client id and client secret.



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   [I-D.erdtman-ace-rpcc] adds raw-public-key and pre-shared-key to the
   client credentials types.  Profiles of this framework MAY extend with
   an additional client credentials type using client certificates.

5.4.  AS Authentication

   The client credential grant does not, by default, authenticate the AS
   that the client connects to.  In classic OAuth, the AS is
   authenticated with a TLS server certificate.

   Profiles of this framework MUST specify how clients authenticate the
   AS and how communication security is implemented.  By default, server
   side TLS certificates, as defined by OAuth 2.0, are required.

5.5.  The Authorization Endpoint

   The OAuth 2.0 authorization endpoint is used to interact with the
   resource owner and obtain an authorization grant, in certain grant
   flows.  The primary use case for the ACE-OAuth framework is for
   machine-to-machine interactions that do not involve the resource
   owner in the authorization flow; therefore, this endpoint is out of
   scope here.  Future profiles may define constrained adaptation
   mechanisms for this endpoint as well.  Non-constrained clients
   interacting with constrained resource servers can use the
   specification in section 3.1 of [RFC6749] and the attack
   countermeasures suggested in section 4.2 of [RFC6819].

5.6.  The Token Endpoint

   In standard OAuth 2.0, the AS provides the token endpoint for
   submitting access token requests.  This framework extends the
   functionality of the token endpoint, giving the AS the possibility to
   help the client and RS to establish shared keys or to exchange their
   public keys.  Furthermore, this framework defines encodings using
   CBOR, as a substitute for JSON.

   The endpoint may, however, be exposed over HTTPS as in classical
   OAuth or even other transports.  A profile MUST define the details of
   the mapping between the fields described below, and these transports.
   If HTTPS is used, JSON or CBOR payloads may be supported.  If JSON
   payloads are used, the semantics of Section 4 of the OAuth 2.0
   specification MUST be followed (with additions as described below).
   If CBOR payload is supported, the semantics described below MUST be
   followed.

   For the AS to be able to issue a token, the client MUST be
   authenticated and present a valid grant for the scopes requested.
   Profiles of this framework MUST specify how the AS authenticates the



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   client and how the communication between client and AS is protected,
   fulfilling the requirements specified in Section 5.

   The default name of this endpoint in an url-path is '/token', however
   implementations are not required to use this name and can define
   their own instead.

   The figures of this section use CBOR diagnostic notation without the
   integer abbreviations for the parameters or their values for
   illustrative purposes.  Note that implementations MUST use the
   integer abbreviations and the binary CBOR encoding, if the CBOR
   encoding is used.

5.6.1.  Client-to-AS Request

   The client sends a POST request to the token endpoint at the AS.  The
   profile MUST specify how the communication is protected.  The content
   of the request consists of the parameters specified in the relevant
   subsection of section 4 of the OAuth 2.0 specification [RFC6749],
   depending on the grant type, with the following exceptions and
   additions:

   o  The parameter "grant_type" is OPTIONAL in the context of this
      framework (as opposed to REQUIRED in RFC6749).  If that parameter
      is missing, the default value "client_credentials" is implied.

   o  The "audience" parameter from [I-D.ietf-oauth-token-exchange] is
      OPTIONAL to request an access token bound to a specific audience.

   o  The "cnonce" parameter defined in Section 5.6.4.4 is REQUIRED if
      the RS provided a client-nonce in the "AS Request Creation Hints"
      message Section 5.1.2

   o  The "scope" parameter MAY be encoded as a byte string instead of
      the string encoding specified in section 3.3 of [RFC6749], in
      order allow compact encoding of complex scopes.  The syntax of
      such a binary encoding is explicitly not specified here and left
      to profiles or applications, specifically note that a binary
      encoded scope does not necessarily use the space character '0x20'
      to delimit scope-tokens.

   o  The client can send an empty (null value) "ace_profile" parameter
      to indicate that it wants the AS to include the "ace_profile"
      parameter in the response.  See Section 5.6.4.3.

   o  A client MUST be able to use the parameters from
      [I-D.ietf-ace-oauth-params] in an access token request to the




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      token endpoint and the AS MUST be able to process these additional
      parameters.

   The default behavior, is that the AS generates a symmetric proof-of-
   possession key for the client.  In order to use an asymmetric key
   pair or to re-use a key previously established with the RS, the
   client is supposed to use the "req_cnf" parameter from
   [I-D.ietf-ace-oauth-params].

   If CBOR is used then these parameters MUST be encoded as a CBOR map.

   When HTTP is used as a transport then the client makes a request to
   the token endpoint by sending the parameters using the "application/
   x-www-form-urlencoded" format with a character encoding of UTF-8 in
   the HTTP request entity-body, as defined in section 3.2 of [RFC6749].

   The following examples illustrate different types of requests for
   proof-of-possession tokens.

   Figure 5 shows a request for a token with a symmetric proof-of-
   possession key.  The content is displayed in CBOR diagnostic
   notation, without abbreviations for better readability.

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "client_id" : "myclient",
     "audience" : "tempSensor4711"
    }

    Figure 5: Example request for an access token bound to a symmetric
                                   key.

   Figure 6 shows a request for a token with an asymmetric proof-of-
   possession key.  Note that in this example OSCORE [RFC8613] is used
   to provide object-security, therefore the Content-Format is
   "application/oscore" wrapping the "application/ace+cbor" type
   content.  The OSCORE option has a decoded interpretation appended in
   parentheses for the reader's convenience.  Also note that in this
   example the audience is implicitly known by both client and AS.
   Furthermore note that this example uses the "req_cnf" parameter from
   [I-D.ietf-ace-oauth-params].






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   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   OSCORE: 0x09, 0x05, 0x44, 0x6C
     (h=0, k=1, n=001, partialIV= 0x05, kid=[0x44, 0x6C])
   Content-Format: "application/oscore"
   Payload:
     0x44025d1 ... (full payload omitted for brevity) ... 68b3825e

   Decrypted payload:
   {
     "client_id" : "myclient",
     "req_cnf" : {
       "COSE_Key" : {
         "kty" : "EC",
         "kid" : h'11',
         "crv" : "P-256",
         "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
         "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
       }
     }
   }

        Figure 6: Example token request bound to an asymmetric key.

   Figure 7 shows a request for a token where a previously communicated
   proof-of-possession key is only referenced using the "req_cnf"
   parameter from [I-D.ietf-ace-oauth-params].

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "client_id" : "myclient",
     "audience" : "valve424",
     "scope" : "read",
     "req_cnf" : {
       "kid" : b64'6kg0dXJM13U'
     }
   }W

       Figure 7: Example request for an access token bound to a key
                                reference.

   Refresh tokens are typically not stored as securely as proof-of-
   possession keys in requesting clients.  Proof-of-possession based



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   refresh token requests MUST NOT request different proof-of-possession
   keys or different audiences in token requests.  Refresh token
   requests can only use to request access tokens bound to the same
   proof-of-possession key and the same audience as access tokens issued
   in the initial token request.

5.6.2.  AS-to-Client Response

   If the access token request has been successfully verified by the AS
   and the client is authorized to obtain an access token corresponding
   to its access token request, the AS sends a response with the
   response code equivalent to the CoAP response code 2.01 (Created).
   If client request was invalid, or not authorized, the AS returns an
   error response as described in Section 5.6.3.

   Note that the AS decides which token type and profile to use when
   issuing a successful response.  It is assumed that the AS has prior
   knowledge of the capabilities of the client and the RS (see
   Appendix D).  This prior knowledge may, for example, be set by the
   use of a dynamic client registration protocol exchange [RFC7591].  If
   the client has requested a specific proof-of-possession key using the
   "req_cnf" parameter from [I-D.ietf-ace-oauth-params], this may also
   influence which profile the AS selects, as it needs to support the
   use of the key type requested the client.

   The content of the successful reply is the Access Information.  When
   using CBOR payloads, the content MUST be encoded as a CBOR map,
   containing parameters as specified in Section 5.1 of [RFC6749], with
   the following additions and changes:

   ace_profile:
      OPTIONAL unless the request included an empty ace_profile
      parameter in which case it is MANDATORY.  This indicates the
      profile that the client MUST use towards the RS.  See
      Section 5.6.4.3 for the formatting of this parameter.  If this
      parameter is absent, the AS assumes that the client implicitly
      knows which profile to use towards the RS.

   token_type:
      This parameter is OPTIONAL, as opposed to 'required' in [RFC6749].
      By default implementations of this framework SHOULD assume that
      the token_type is "PoP".  If a specific use case requires another
      token_type (e.g., "Bearer") to be used then this parameter is
      REQUIRED.

   Furthermore [I-D.ietf-ace-oauth-params] defines additional parameters
   that the AS MUST be able to use when responding to a request to the
   token endpoint.



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   Figure 8 summarizes the parameters that can currently be part of the
   Access Information.  Future extensions may define additional
   parameters.

           /-------------------+-------------------------------\
           | Parameter name    | Specified in                  |
           |-------------------+-------------------------------|
           | access_token      |  RFC 6749                     |
           | token_type        |  RFC 6749                     |
           | expires_in        |  RFC 6749                     |
           | refresh_token     |  RFC 6749                     |
           | scope             |  RFC 6749                     |
           | state             |  RFC 6749                     |
           | error             |  RFC 6749                     |
           | error_description |  RFC 6749                     |
           | error_uri         |  RFC 6749                     |
           | ace_profile       | [this document]               |
           | cnf               | [I-D.ietf-ace-oauth-params]   |
           | rs_cnf            | [I-D.ietf-ace-oauth-params]   |
           \-------------------+-------------------------------/

                  Figure 8: Access Information parameters

   Figure 9 shows a response containing a token and a "cnf" parameter
   with a symmetric proof-of-possession key, which is defined in
   [I-D.ietf-ace-oauth-params].  Note that the key identifier 'kid' is
   only used to simplify indexing and retrieving the key, and no
   assumptions should be made that it is unique in the domains of either
   the client or the RS.






















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   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "access_token" : b64'SlAV32hkKG ...
      (remainder of CWT omitted for brevity;
      CWT contains COSE_Key in the "cnf" claim)',
     "ace_profile" : "coap_dtls",
     "expires_in" : "3600",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

       Figure 9: Example AS response with an access token bound to a
                              symmetric key.

5.6.3.  Error Response

   The error responses for CoAP-based interactions with the AS are
   generally equivalent to the ones for HTTP-based interactions as
   defined in Section 5.2 of [RFC6749], with the following exceptions:

   o  When using CBOR the raw payload before being processed by the
      communication security protocol MUST be encoded as a CBOR map.

   o  A response code equivalent to the CoAP code 4.00 (Bad Request)
      MUST be used for all error responses, except for invalid_client
      where a response code equivalent to the CoAP code 4.01
      (Unauthorized) MAY be used under the same conditions as specified
      in Section 5.2 of [RFC6749].

   o  The Content-Format (for CoAP-based interactions) or media type
      (for HTTP-based interactions) "application/ace+cbor" MUST be used
      for the error response.

   o  The parameters "error", "error_description" and "error_uri" MUST
      be abbreviated using the codes specified in Figure 12, when a CBOR
      encoding is used.

   o  The error code (i.e., value of the "error" parameter) MUST be
      abbreviated as specified in Figure 10, when a CBOR encoding is
      used.




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           /------------------------+-------------\
           | Name                   | CBOR Values |
           |------------------------+-------------|
           | invalid_request        |      1      |
           | invalid_client         |      2      |
           | invalid_grant          |      3      |
           | unauthorized_client    |      4      |
           | unsupported_grant_type |      5      |
           | invalid_scope          |      6      |
           | unsupported_pop_key    |      7      |
           | incompatible_profiles  |      8      |
           \------------------------+-------------/

           Figure 10: CBOR abbreviations for common error codes

   In addition to the error responses defined in OAuth 2.0, the
   following behavior MUST be implemented by the AS:

   o  If the client submits an asymmetric key in the token request that
      the RS cannot process, the AS MUST reject that request with a
      response code equivalent to the CoAP code 4.00 (Bad Request)
      including the error code "unsupported_pop_key" defined in
      Figure 10.

   o  If the client and the RS it has requested an access token for do
      not share a common profile, the AS MUST reject that request with a
      response code equivalent to the CoAP code 4.00 (Bad Request)
      including the error code "incompatible_profiles" defined in
      Figure 10.

5.6.4.  Request and Response Parameters

   This section provides more detail about the new parameters that can
   be used in access token requests and responses, as well as
   abbreviations for more compact encoding of existing parameters and
   common parameter values.

5.6.4.1.  Grant Type

   The abbreviations specified in the registry defined in Section 8.4
   MUST be used in CBOR encodings instead of the string values defined
   in [RFC6749], if CBOR payloads are used.









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           /--------------------+------------+------------------------\
           | Name               | CBOR Value | Original Specification |
           |--------------------+------------+------------------------|
           | password           |      0     |       RFC6749          |
           | authorization_code |      1     |       RFC6749          |
           | client_credentials |      2     |       RFC6749          |
           | refresh_token      |      3     |       RFC6749          |
           \--------------------+------------+------------------------/

           Figure 11: CBOR abbreviations for common grant types

5.6.4.2.  Token Type

   The "token_type" parameter, defined in section 5.1 of [RFC6749],
   allows the AS to indicate to the client which type of access token it
   is receiving (e.g., a bearer token).

   This document registers the new value "PoP" for the OAuth Access
   Token Types registry, specifying a proof-of-possession token.  How
   the proof-of-possession by the client to the RS is performed MUST be
   specified by the profiles.

   The values in the "token_type" parameter MUST use the CBOR
   abbreviations defined in the registry specified by Section 8.6, if a
   CBOR encoding is used.

   In this framework the "pop" value for the "token_type" parameter is
   the default.  The AS may, however, provide a different value.

5.6.4.3.  Profile

   Profiles of this framework MUST define the communication protocol and
   the communication security protocol between the client and the RS.
   The security protocol MUST provide encryption, integrity and replay
   protection.  It MUST also provide a binding between requests and
   responses.  Furthermore profiles MUST define a list of allowed proof-
   of-possession methods, if they support proof-of-possession tokens.

   A profile MUST specify an identifier that MUST be used to uniquely
   identify itself in the "ace_profile" parameter.  The textual
   representation of the profile identifier is just intended for human
   readability and MUST NOT be used in parameters and claims.  Profiles
   MUST register their identifier in the registry defined in
   Section 8.7.

   Profiles MAY define additional parameters for both the token request
   and the Access Information in the access token response in order to
   support negotiation or signaling of profile specific parameters.



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   Clients that want the AS to provide them with the "ace_profile"
   parameter in the access token response can indicate that by sending a
   ace_profile parameter with a null value in the access token request.

5.6.4.4.  Client-Nonce

   This parameter MUST be sent from the client to the AS, if it
   previously received a "cnonce" parameter in the AS Request Creation
   Hints Section 5.1.2.  The parameter is encoded as a byte string and
   copies the value from the cnonce parameter in the AS Request Creation
   Hints.

5.6.5.  Mapping Parameters to CBOR

   If CBOR encoding is used, all OAuth parameters in access token
   requests and responses MUST be mapped to CBOR types as specified in
   the registry defined by Section 8.9, using the given integer
   abbreviation for the map keys.

   Note that we have aligned the abbreviations corresponding to claims
   with the abbreviations defined in [RFC8392].

   Note also that abbreviations from -24 to 23 have a 1 byte encoding
   size in CBOR.  We have thus chosen to assign abbreviations in that
   range to parameters we expect to be used most frequently in
   constrained scenarios.

























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           /-------------------+----------+---------------------\
           | Name              | CBOR Key | Value Type          |
           |-------------------+----------+---------------------|
           | access_token      | 1        | byte string         |
           | expires_in        | 2        | unsigned integer    |
           | audience          | 5        | text string         |
           | scope             | 9        | text or byte string |
           | client_id         | 24       | text string         |
           | client_secret     | 25       | byte string         |
           | response_type     | 26       | text string         |
           | redirect_uri      | 27       | text string         |
           | state             | 28       | text string         |
           | code              | 29       | byte string         |
           | error             | 30       | unsigned integer    |
           | error_description | 31       | text string         |
           | error_uri         | 32       | text string         |
           | grant_type        | 33       | unsigned integer    |
           | token_type        | 34       | unsigned integer    |
           | username          | 35       | text string         |
           | password          | 36       | text string         |
           | refresh_token     | 37       | byte string         |
           | ace_profile       | 38       | unsigned integer    |
           | cnonce            | 39       | byte string         |
           \-------------------+----------+---------------------/

       Figure 12: CBOR mappings used in token requests and responses

5.7.  The Introspection Endpoint

   Token introspection [RFC7662] can be OPTIONALLY provided by the AS,
   and is then used by the RS and potentially the client to query the AS
   for metadata about a given token, e.g., validity or scope.  Analogous
   to the protocol defined in [RFC7662] for HTTP and JSON, this section
   defines adaptations to more constrained environments using CBOR and
   leaving the choice of the application protocol to the profile.

   Communication between the requesting entity and the introspection
   endpoint at the AS MUST be integrity protected and encrypted.  The
   communication security protocol MUST also provide a binding between
   requests and responses.  Furthermore the two interacting parties MUST
   perform mutual authentication.  Finally the AS SHOULD verify that the
   requesting entity has the right to access introspection information
   about the provided token.  Profiles of this framework that support
   introspection MUST specify how authentication and communication
   security between the requesting entity and the AS is implemented.






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   The default name of this endpoint in an url-path is '/introspect',
   however implementations are not required to use this name and can
   define their own instead.

   The figures of this section uses CBOR diagnostic notation without the
   integer abbreviations for the parameters or their values for better
   readability.

   Note that supporting introspection is OPTIONAL for implementations of
   this framework.

5.7.1.  Introspection Request

   The requesting entity sends a POST request to the introspection
   endpoint at the AS.  The profile MUST specify how the communication
   is protected.  If CBOR is used, the payload MUST be encoded as a CBOR
   map with a "token" entry containing the access token.  Further
   optional parameters representing additional context that is known by
   the requesting entity to aid the AS in its response MAY be included.

   For CoAP-based interaction, all messages MUST use the content type
   "application/ace+cbor", while for HTTP-based interactions the
   equivalent media type "application/ace+cbor" MUST be used.

   The same parameters are required and optional as in Section 2.1 of
   [RFC7662].

   For example, Figure 13 shows a RS calling the token introspection
   endpoint at the AS to query about an OAuth 2.0 proof-of-possession
   token.  Note that object security based on OSCORE [RFC8613] is
   assumed in this example, therefore the Content-Format is
   "application/oscore".  Figure 14 shows the decoded payload.

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "introspect"
   OSCORE: 0x09, 0x05, 0x25
   Content-Format: "application/oscore"
   Payload:
   ... COSE content ...

                 Figure 13: Example introspection request.









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   {
     "token" : b64'7gj0dXJQ43U',
     "token_type_hint" : "PoP"
   }

                        Figure 14: Decoded payload.

5.7.2.  Introspection Response

   If the introspection request is authorized and successfully
   processed, the AS sends a response with the response code equivalent
   to the CoAP code 2.01 (Created).  If the introspection request was
   invalid, not authorized or couldn't be processed the AS returns an
   error response as described in Section 5.7.3.

   In a successful response, the AS encodes the response parameters in a
   map including with the same required and optional parameters as in
   Section 2.2 of [RFC7662] with the following addition:

   ace_profile  OPTIONAL.  This indicates the profile that the RS MUST
      use with the client.  See Section 5.6.4.3 for more details on the
      formatting of this parameter.

   cnonce  OPTIONAL.  A client-nonce provided to the AS by the client.
      The RS MUST verify that this corresponds to the client-nonce
      previously provided to the client in the AS Request Creation
      Hints.  See Section 5.1.2 and Section 5.6.4.4.

   exi  OPTIONAL.  The "expires-in" claim associated to this access
      token.  See Section 5.8.3.

   Furthermore [I-D.ietf-ace-oauth-params] defines more parameters that
   the AS MUST be able to use when responding to a request to the
   introspection endpoint.

   For example, Figure 15 shows an AS response to the introspection
   request in Figure 13.  Note that this example contains the "cnf"
   parameter defined in [I-D.ietf-ace-oauth-params].













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   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "active" : true,
     "scope" : "read",
     "ace_profile" : "coap_dtls",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

                Figure 15: Example introspection response.

5.7.3.  Error Response

   The error responses for CoAP-based interactions with the AS are
   equivalent to the ones for HTTP-based interactions as defined in
   Section 2.3 of [RFC7662], with the following differences:

   o  If content is sent and CBOR is used the payload MUST be encoded as
      a CBOR map and the Content-Format "application/ace+cbor" MUST be
      used.

   o  If the credentials used by the requesting entity (usually the RS)
      are invalid the AS MUST respond with the response code equivalent
      to the CoAP code 4.01 (Unauthorized) and use the required and
      optional parameters from Section 5.2 in [RFC6749].

   o  If the requesting entity does not have the right to perform this
      introspection request, the AS MUST respond with a response code
      equivalent to the CoAP code 4.03 (Forbidden).  In this case no
      payload is returned.

   o  The parameters "error", "error_description" and "error_uri" MUST
      be abbreviated using the codes specified in Figure 12.

   o  The error codes MUST be abbreviated using the codes specified in
      the registry defined by Section 8.3.

   Note that a properly formed and authorized query for an inactive or
   otherwise invalid token does not warrant an error response by this
   specification.  In these cases, the authorization server MUST instead




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   respond with an introspection response with the "active" field set to
   "false".

5.7.4.  Mapping Introspection parameters to CBOR

   If CBOR is used, the introspection request and response parameters
   MUST be mapped to CBOR types as specified in the registry defined by
   Section 8.11, using the given integer abbreviation for the map key.

   Note that we have aligned abbreviations that correspond to a claim
   with the abbreviations defined in [RFC8392] and the abbreviations of
   parameters with the same name from Section 5.6.5.

       /-------------------+----------+-------------------------\
       | Parameter name    | CBOR Key | Value Type              |
       |-------------------+----------+-------------------------|
       | iss               | 1        | text string             |
       | sub               | 2        | text string             |
       | aud               | 3        | text string             |
       | exp               | 4        | integer or              |
       |                   |          |   floating-point number |
       | nbf               | 5        | integer or              |
       |                   |          |   floating-point number |
       | iat               | 6        | integer or              |
       |                   |          |   floating-point number |
       | cti               | 7        | byte string             |
       | scope             | 9        | text or byte string     |
       | active            | 10       | True or False           |
       | token             | 11       | byte string             |
       | client_id         | 24       | text string             |
       | error             | 30       | unsigned integer        |
       | error_description | 31       | text string             |
       | error_uri         | 32       | text string             |
       | token_type_hint   | 33       | text string             |
       | token_type        | 34       | text string             |
       | username          | 35       | text string             |
       | ace_profile       | 38       | unsigned integer        |
       | cnonce            | 39       | byte string             |
       | exi               | 40       | unsigned integer        |
       \-------------------+----------+-------------------------/

        Figure 16: CBOR Mappings to Token Introspection Parameters.

5.8.  The Access Token

   This framework RECOMMENDS the use of CBOR web token (CWT) as
   specified in [RFC8392].




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   In order to facilitate offline processing of access tokens, this
   document uses the "cnf" claim from
   [I-D.ietf-ace-cwt-proof-of-possession] and the "scope" claim from
   [I-D.ietf-oauth-token-exchange] for JWT- and CWT-encoded tokens.  In
   addition to string encoding specified for the "scope" claim, a binary
   encoding MAY be used.  The syntax of such an encoding is explicitly
   not specified here and left to profiles or applications, specifically
   note that a binary encoded scope does not necessarily use the space
   character '0x20' to delimit scope-tokens.

   If the AS needs to convey a hint to the RS about which profile it
   should use to communicate with the client, the AS MAY include an
   "ace_profile" claim in the access token, with the same syntax and
   semantics as defined in Section 5.6.4.3.

   If the client submitted a client-nonce parameter in the access token
   request Section 5.6.4.4, the AS MUST include the value of this
   parameter in the "cnonce" claim specified here.  The "cnonce" claim
   uses binary encoding.

5.8.1.  The Authorization Information Endpoint

   The access token, containing authorization information and
   information about the proof-of-possession method used by the client,
   needs to be transported to the RS so that the RS can authenticate and
   authorize the client request.

   This section defines a method for transporting the access token to
   the RS using a RESTful protocol such as CoAP.  Profiles of this
   framework MAY define other methods for token transport.

   The method consists of an authz-info endpoint, implemented by the RS.
   A client using this method MUST make a POST request to the authz-info
   endpoint at the RS with the access token in the payload.  The RS
   receiving the token MUST verify the validity of the token.  If the
   token is valid, the RS MUST respond to the POST request with 2.01
   (Created).  Section Section 5.8.1.1 outlines how an RS MUST proceed
   to verify the validity of an access token.

   The RS MUST be prepared to store at least one access token for future
   use.  This is a difference to how access tokens are handled in OAuth
   2.0, where the access token is typically sent along with each
   request, and therefore not stored at the RS.

   This specification RECOMMENDS that an RS stores only one token per
   proof-of-possession key, meaning that an additional token linked to
   the same key will overwrite any existing token at the RS.  The reason
   is that this greatly simplifies (constrained) implementations, with



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   respect to required storage and resolving a request to the applicable
   token.

   If the payload sent to the authz-info endpoint does not parse to a
   token, the RS MUST respond with a response code equivalent to the
   CoAP code 4.00 (Bad Request).

   The RS MAY make an introspection request to validate the token before
   responding to the POST request to the authz-info endpoint, e.g. if
   the token is an opaque reference.  Some transport protocols may
   provide a way to indicate that the RS is busy and the client should
   retry after an interval; this type of status update would be
   appropriate while the RS is waiting for an introspection response.

   Profiles MUST specify whether the authz-info endpoint is protected,
   including whether error responses from this endpoint are protected.
   Note that since the token contains information that allow the client
   and the RS to establish a security context in the first place, mutual
   authentication may not be possible at this point.

   The default name of this endpoint in an url-path is '/authz-info',
   however implementations are not required to use this name and can
   define their own instead.

5.8.1.1.  Verifying an Access Token

   When an RS receives an access token, it MUST verify it before storing
   it.  The details of token verification depends on various aspects,
   including the token encoding, the type of token, the security
   protection applied to the token, and the claims.  The token encoding
   matters since the security wrapper differs between the token
   encodings.  For example, a CWT token uses COSE while a JWT token uses
   JOSE.  The type of token also has an influence on the verification
   procedure since tokens may be self-contained whereby token
   verification may happen locally at the RS while a token-by-reference
   requires further interaction with the authorization server, for
   example using token introspection, to obtain the claims associated
   with the token reference.  Self-contained tokens MUST, at a minimum,
   be integrity protected but they MAY also be encrypted.

   For self-contained tokens the RS MUST process the security protection
   of the token first, as specified by the respective token format.  For
   CWT the description can be found in [RFC8392] and for JWT the
   relevant specification is [RFC7519].  This MUST include a
   verification that security protection (and thus the token) was
   generated by an AS that has the right to issue access tokens for this
   RS.




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   In case the token is communicated by reference the RS needs to obtain
   the claims first.  When the RS uses token introspection the relevant
   specification is [RFC7662] with CoAP transport specified in
   Section 5.7.

   Errors may happen during this initial processing stage:

   o  If token or claim verification fails, the RS MUST discard the
      token and, if this was an interaction with authz-info, return an
      error message with a response code equivalent to the CoAP code
      4.01 (Unauthorized).

   o  If the claims cannot be obtained the RS MUST discard the token
      and, in case of an interaction via the authz-info endpoint, return
      an error message with a response code equivalent to the CoAP code
      4.00 (Bad Request).

   Next, the RS MUST verify claims, if present, contained in the access
   token.  Errors are returned when claim checks fail, in the order of
   priority of this list:

   iss  The issuer claim must identify an AS that has the authority to
      issue access tokens for the receiving RS.  If that is not the case
      the RS MUST discard the token.  If this was an interaction with
      authz-info, the RS MUST also respond with a response code
      equivalent to the CoAP code 4.01 (Unauthorized).

   exp  The expiration date must be in the future.  If that is not the
      case the RS MUST discard the token.  If this was an interaction
      with authz-info the RS MUST also respond with a response code
      equivalent to the CoAP code 4.01 (Unauthorized).  Note that the RS
      has to terminate access rights to the protected resources at the
      time when the tokens expire.

   aud  The audience claim must refer to an audience that the RS
      identifies with.  If that is not the case the RS MUST discard the
      token.  If this was an interaction with authz-info, the RS MUST
      also respond with a response code equivalent to the CoAP code 4.03
      (Forbidden).

   scope  The RS must recognize value of the scope claim.  If that is
      not the case the RS MUST discard the token.  If this was an
      interaction with authz-info, the RS MUST also respond with a
      response code equivalent to the CoAP code 4.00 (Bad Request).  The
      RS MAY provide additional information in the error response, to
      clarify what went wrong.





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   Additional processing may be needed for other claims in a way
   specific to a profile or the underlying application.

   Note that the Subject (sub) claim cannot always be verified when the
   token is submitted to the RS since the client may not have
   authenticated yet.  Also note that a counter for the expires_in (exi)
   claim MUST be initialized when the RS first verifies this token.

   Also note that profiles of this framework may define access token
   transport mechanisms that do not allow for error responses.
   Therefore the error messages specified here only apply if the token
   was sent to the authz-info endpoint.

   When sending error responses, the RS MAY use the error codes from
   Section 3.1 of [RFC6750], to provide additional details to the
   client.

5.8.1.2.  Protecting the Authorization Information Endpoint

   As this framework can be used in RESTful environments, it is
   important to make sure that attackers cannot perform unauthorized
   requests on the authz-info endpoints, other than submitting access
   tokens.

   Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT
   on the authz-info endpoint and on it's children (if any).

   The POST method SHOULD NOT be allowed on children of the authz-info
   endpoint.

   The RS SHOULD implement rate limiting measures to mitigate attacks
   aiming to overload the processing capacity of the RS by repeatedly
   submitting tokens.  For CoAP-based communication the RS could use the
   mechanisms from [RFC8516] to indicate that it is overloaded.

5.8.2.  Client Requests to the RS

   Before sending a request to a RS, the client MUST verify that the
   keys used to protect this communication are still valid.  See
   Section 5.8.4 for details on how the client determines the validity
   of the keys used.

   If an RS receives a request from a client, and the target resource
   requires authorization, the RS MUST first verify that it has an
   access token that authorizes this request, and that the client has
   performed the proof-of-possession binding that token to the request.





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   The response code MUST be 4.01 (Unauthorized) in case the client has
   not performed the proof-of-possession, or if RS has no valid access
   token for the client.  If RS has an access token for the client but
   the token does not authorize access for the resource that was
   requested, RS MUST reject the request with a 4.03 (Forbidden).  If RS
   has an access token for the client but it does not cover the action
   that was requested on the resource, RS MUST reject the request with a
   4.05 (Method Not Allowed).

   Note: The use of the response codes 4.03 and 4.05 is intended to
   prevent infinite loops where a dumb Client optimistically tries to
   access a requested resource with any access token received from AS.
   As malicious clients could pretend to be C to determine C's
   privileges, these detailed response codes must be used only when a
   certain level of security is already available which can be achieved
   only when the Client is authenticated.

   Note: The RS MAY use introspection for timely validation of an access
   token, at the time when a request is presented.

   Note: Matching the claims of the access token (e.g., scope) to a
   specific request is application specific.

   If the request matches a valid token and the client has performed the
   proof-of-possession for that token, the RS continues to process the
   request as specified by the underlying application.

5.8.3.  Token Expiration

   Depending on the capabilities of the RS, there are various ways in
   which it can verify the expiration of a received access token.  Here
   follows a list of the possibilities including what functionality they
   require of the RS.

   o  The token is a CWT and includes an "exp" claim and possibly the
      "nbf" claim.  The RS verifies these by comparing them to values
      from its internal clock as defined in [RFC7519].  In this case the
      RS's internal clock must reflect the current date and time, or at
      least be synchronized with the AS's clock.  How this clock
      synchronization would be performed is out of scope for this
      specification.

   o  The RS verifies the validity of the token by performing an
      introspection request as specified in Section 5.7.  This requires
      the RS to have a reliable network connection to the AS and to be
      able to handle two secure sessions in parallel (C to RS and RS to
      AS).




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   o  In order to support token expiration for devices that have no
      reliable way of synchronizing their internal clocks, this
      specification defines the following approach: The claim "exi"
      ("expires in") can be used, to provide the RS with the lifetime of
      the token in seconds from the time the RS first receives the
      token.  Processing this claim requires that the RS does the
      following:

      *  For each token the RS receives, that contains an "exi" claim:
         Keep track of the time it received that token and revisit that
         list regularly to expunge expired tokens.

      *  Keep track of the identifiers of tokens containing the "exi"
         claim that have expired (in order to avoid accepting them
         again).

   If a token that authorizes a long running request such as a CoAP
   Observe [RFC7641] expires, the RS MUST send an error response with
   the response code equivalent to the CoAP code 4.01 (Unauthorized) to
   the client and then terminate processing the long running request.

5.8.4.  Key Expiration

   The AS provides the client with key material that the RS uses.  This
   can either be a common symmetric PoP-key, or an asymmetric key used
   by the RS to authenticate towards the client.  Since there is
   currently no expiration metadata associated to those keys, the client
   has no way of knowing if these keys are still valid.  This may lead
   to situations where the client sends requests containing sensitive
   information to the RS using a key that is expired and possibly in the
   hands of an attacker, or accepts responses from the RS that are not
   properly protected and could possibly have been forged by an
   attacker.

   In order to prevent this, the client must assume that those keys are
   only valid as long as the related access token is.  Since the access
   token is opaque to the client, one of the following methods MUST be
   used to inform the client about the validity of an access token:

   o  The client knows a default validity time for all tokens it is
      using (i.e. how long a token is valid after being issued).  This
      information could be provisioned to the client when it is
      registered at the AS, or published by the AS in a way that the
      client can query.

   o  The AS informs the client about the token validity using the
      "expires_in" parameter in the Access Information.




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   A client that is not able to obtain information about the expiration
   of a token MUST NOT use this token.

6.  Security Considerations

   Security considerations applicable to authentication and
   authorization in RESTful environments provided in OAuth 2.0 [RFC6749]
   apply to this work.  Furthermore [RFC6819] provides additional
   security considerations for OAuth which apply to IoT deployments as
   well.  If the introspection endpoint is used, the security
   considerations from [RFC7662] also apply.

   The following subsections address issues specific to this document
   and it's use in constrained environments.

6.1.  Protecting Tokens

   A large range of threats can be mitigated by protecting the contents
   of the access token by using a digital signature or a keyed message
   digest (MAC) or an Authenticated Encryption with Associated Data
   (AEAD) algorithm.  Consequently, the token integrity protection MUST
   be applied to prevent the token from being modified, particularly
   since it contains a reference to the symmetric key or the asymmetric
   key used for proof-of-possession.  If the access token contains the
   symmetric key, this symmetric key MUST be encrypted by the
   authorization server so that only the resource server can decrypt it.
   Note that using an AEAD algorithm is preferable over using a MAC
   unless the token needs to be publicly readable.

   If the token is intended for multiple recipients (i.e. an audience
   that is a group), integrity protection of the token with a symmetric
   key, shared between the AS and the recipients, is not sufficient,
   since any of the recipients could modify the token undetected by the
   other recipients.  Therefore a token with a multi-recipient audience
   MUST be protected with an asymmetric signature.

   It is important for the authorization server to include the identity
   of the intended recipient (the audience), typically a single resource
   server (or a list of resource servers), in the token.  Using a single
   shared secret as proof-of-possession key with multiple resource
   servers is NOT RECOMMENDED since the benefit from using the proof-of-
   possession concept is then significantly reduced.

   If clients are capable of doing so, they should frequently request
   fresh access tokens, as this allows the AS to keep the lifetime of
   the tokens short.  This allows the AS to use shorter proof-of-
   possession key sizes, which translate to a performance benefit for




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   the client and for the resource server.  Shorter keys also lead to
   shorter messages (particularly with asymmetric keying material).

   When authorization servers bind symmetric keys to access tokens, they
   SHOULD scope these access tokens to a specific permission.

   In certain situations it may be necessary to revoke an access token
   that is still valid.  Client-initiated revocation is specified in
   [RFC7009] for OAuth 2.0.  Other revocation mechanisms are currently
   not specified, as the underlying assumption in OAuth is that access
   tokens are issued with a relatively short lifetime.  This may not
   hold true for disconnected constrained devices, needing access tokens
   with relatively long lifetimes, and would therefore necessitate
   further standardization work that is out of scope for this document.

6.2.  Communication Security

   The authorization server MUST offer confidentiality protection for
   any interactions with the client.  This step is extremely important
   since the client may obtain the proof-of-possession key from the
   authorization server for use with a specific access token.  Not using
   confidentiality protection exposes this secret (and the access token)
   to an eavesdropper thereby completely negating proof-of-possession
   security.  Profiles MUST specify how communication security according
   to the requirements in Section 5 is provided.

   Additional protection for the access token can be applied by
   encrypting it, for example encryption of CWTs is specified in
   Section 5.1 of [RFC8392].  Such additional protection can be
   necessary if the token is later transferred over an insecure
   connection (e.g. when it is sent to the authz-info endpoint).

   Developers MUST ensure that the ephemeral credentials (i.e., the
   private key or the session key) are not leaked to third parties.  An
   adversary in possession of the ephemeral credentials bound to the
   access token will be able to impersonate the client.  Be aware that
   this is a real risk with many constrained environments, since
   adversaries can often easily get physical access to the devices.
   This risk can also be mitigated to some extent by making sure that
   keys are refreshed more frequently.

6.3.  Long-Term Credentials

   Both clients and RSs have long-term credentials that are used to
   secure communications, and authenticate to the AS.  These credentials
   need to be protected against unauthorized access.  In constrained
   devices, deployed in publicly accessible places, such protection can




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   be difficult to achieve without specialized hardware (e.g. secure key
   storage memory).

   If credentials are lost or compromised, the operator of the affected
   devices needs to have procedures to invalidate any access these
   credentials give and to revoke tokens linked to such credentials.
   The loss of a credential linked to a specific device MUST NOT lead to
   a compromise of other credentials not linked to that device,
   therefore sharing secret keys between more than two parties is NOT
   RECOMMENDED.

   Operators of clients or RS should have procedures in place to replace
   credentials that are suspected to have been compromised or that have
   been lost.

   Operators also need to have procedures for decommissioning devices,
   that include securely erasing credentials and other security critical
   material in the devices being decommissioned.

6.4.  Unprotected AS Request Creation Hints

   Initially, no secure channel exists to protect the communication
   between C and RS.  Thus, C cannot determine if the AS Request
   Creation Hints contained in an unprotected response from RS to an
   unauthorized request (see Section 5.1.2) are authentic.  It is
   therefore advisable to provide C with a (possibly hard-coded) list of
   trustworthy authorization servers, possibly including information
   used to authenticate the AS, such as a public key or certificate
   fingerprint.  AS Request Creation Hints referring to a URI not listed
   there would be ignored.

   A compromised RS may use the hints to trick a client into contacting
   an AS that is not supposed to be in charge of that RS.  Since this AS
   must be in the hard-coded list of trusted AS no violation of
   privileges and or exposure of credentials should happen, since a
   trusted AS is expected to refuse requestes for which it is not
   applicable and render a corresponding error response.  However a
   compromised RS may use this to perform a denial of service against a
   specific AS, by redirecting a large number of client requests to that
   AS.

   A compromised client can be made to contact any AS, including
   compromised ones.  This should not affect the RS, since it is
   supposed to keep track of which AS are trusted and have corresponding
   credentials to verify the source of access tokens it receives.






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6.5.  Minimal security requirements for communication

   This section summarizes the minimal requirements for the
   communication security of the different protocol interactions.

   C-AS  All communication between the client and the Authorization
      Server MUST be encrypted, integrity and replay protected.
      Furthermore responses from the AS to the client MUST be bound to
      the client's request to avoid attacks where the attacker swaps the
      intended response for an older one valid for a previous request.
      This requires that the client and the Authorization Server have
      previously exchanged either a shared secret or their public keys
      in order to negotiate a secure communication.  Furthermore the
      client MUST be able to determine whether an AS has the authority
      to issue access tokens for a certain RS.  This can for example be
      done through pre-configured lists, or through an online lookup
      mechanism that in turn also must be secured.

   RS-AS  The communication between the Resource Server and the
      Authorization Server via the introspection endpoint MUST be
      encrypted, integrity and replay protected.  Furthermore responses
      from the AS to the RS MUST be bound to the RS's request.  This
      requires that the RS and the Authorization Server have previously
      exchanged either a shared secret, or their public keys in order to
      negotiate a secure communication.  Furthermore the RS MUST be able
      to determine whether an AS has the authority to issue access
      tokens itself.  This is usually configured out of band, but could
      also be performed through an online lookup mechanism provided that
      it is also secured in the same way.

   C-RS  The initial communication between the client and the Resource
      Server can not be secured in general, since the RS is not in
      possession of on access token for that client, which would carry
      the necessary parameters.  Certain security mechanisms (e.g.  DTLS
      with server-side authentication via a certificate or a raw public
      key) can be possible and are RECOMMEND if supported by both
      parties.  After the client has successfully transmitted the access
      token to the RS, a secure communication protocol MUST be
      established between client and RS for the actual resource request.
      This protocol MUST provide confidentiality, integrity and replay
      protection as well as a binding between requests and responses.
      This requires that the client learned either the RS's public key
      or received a symmetric proof-of-possession key bound to the
      access token from the AS.  The RS must have learned either the
      client's public key or a shared symmetric key from the claims in
      the token or an introspection request.  Since ACE does not provide
      profile negotiation between C and RS, the client MUST have learned




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      what profile the RS supports (e.g. from the AS or pre-configured)
      and initiate the communication accordingly.

6.6.  Token Freshness and Expiration

   An RS that is offline faces the problem of clock drift.  Since it
   cannot synchronize its clock with the AS, it may be tricked into
   accepting old access tokens that are no longer valid or have been
   compromised.  In order to prevent this, an RS may use the nonce-based
   mechanism defined in Section 5.1.2 to ensure freshness of an Access
   Token subsequently presented to this RS.

   Another problem with clock drift is that evaluating the standard
   token expiration claim "exp" can give unpredictable results.

   The expiration mechanism implemented by the "exi" claim, based on the
   first time the RS sees the token was defined to provide a more
   predictable alternative.  The "exi" approach has some drawbacks that
   need to be considered:

      A malicious client may hold back tokens with the "exi" claim in
      order to prolong their lifespan.

      If an RS loses state (e.g. due to an unscheduled reboot), it may
      loose the current values of counters tracking the "exi" claims of
      tokens it is storing.

      The RS needs to keep state about expired tokens that used "exi" in
      order to be sure not to accept it again.  Attacker may use this to
      deplete the RS's storage resources.

   The first drawback is inherent to the deployment scenario and the
   "exi" solution.  It can therefore not be mitigated without requiring
   the the RS be online at times.  The second drawback can be mitigated
   by regularly storing the value of "exi" counters to persistent
   memory.  The third problem can be mitigated by the AS, by assigning
   identifiers (e.g. 'cti') to the tokens, that include a RS identifier
   and a sequence number.  The RS would then just have to store the
   highest sequence number of an expired token it has seen, thus
   limiting the necessary state.

6.7.  Combining profiles

   There may be use cases were different profiles of this framework are
   combined.  For example, an MQTT-TLS profile is used between the
   client and the RS in combination with a CoAP-DTLS profile for
   interactions between the client and the AS.  The security of a




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   profile MUST NOT depend on the assumption that the profile is used
   for all the different types of interactions in this framework.

6.8.  Unprotected Information

   Communication with the authz-info endpoint, as well as the various
   error responses defined in this framework, all potentially include
   sending information over an unprotected channel.  These messages may
   leak information to an adversary.  For example error responses for
   requests to the Authorization Information endpoint can reveal
   information about an otherwise opaque access token to an adversary
   who has intercepted this token.

   As far as error messages are concerned, this framework is written
   under the assumption that, in general, the benefits of detailed error
   messages outweigh the risk due to information leakage.  For
   particular use cases, where this assessment does not apply, detailed
   error messages can be replaced by more generic ones.

   In some scenarios it may be possible to protect the communication
   with the authz-info endpoint (e.g. through DTLS with only server-side
   authentication).  In cases where this is not possible this framework
   RECOMMENDS to use encrypted CWTs or tokens that are opaque references
   and need to be subjected to introspection by the RS.

   If the initial unauthorized resource request message (see
   Section 5.1.1) is used, the client MUST make sure that it is not
   sending sensitive content in this request.  While GET and DELETE
   requests only reveal the target URI of the resource, POST and PUT
   requests would reveal the whole payload of the intended operation.

   Since the client is not authenticated at the point when it is
   submitting an access token to the authz-info endpoint, attackers may
   be pretending to be a client and trying to trick an RS to use an
   obsole profile that in turn specifies a vulnerable security mechanism
   via the authz-info endpoint.  Such an attack would require a valid
   access token containing a "profile" claim requesting the use of said
   obsolete profile.  Resource Owners should update the configuration of
   their RS's to prevent them from using such obsolete profiles.

6.9.  Identifying audiences

   The audience claim as defined in [RFC7519] and the equivalent
   "audience" parameter from [I-D.ietf-oauth-token-exchange] are
   intentionally vague on how to match the audience value to a specific
   RS.  This is intended to allow application specific semantics to be
   used.  This section attempts to give some general guidance for the
   use of audiences in constrained environments.



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   URLs are not a good way of identifying mobile devices that can switch
   networks and thus be associated with new URLs.  If the audience
   represents a single RS, and asymmetric keys are used, the RS can be
   uniquely identified by a hash of its public key.  If this approach is
   used this framework RECOMMENDS to apply the procedure from section 3
   of [RFC6920].

   If the audience addresses a group of resource servers, the mapping of
   group identifier to individual RS has to be provisioned to each RS
   before the group-audience is usable.  Managing dynamic groups could
   be an issue, if any RS is not always reachable when the groups'
   memberships change.  Furthermore, issuing access tokens bound to
   symmetric proof-of-possession keys that apply to a group-audience is
   problematic, as an RS that is in possession of the access token can
   impersonate the client towards the other RSs that are part of the
   group.  It is therefore NOT RECOMMENDED to issue access tokens bound
   to a group audience and symmetric proof-of possession keys.

   Even the client must be able to determine the correct values to put
   into the "audience" parameter, in order to obtain a token for the
   intended RS.  Errors in this process can lead to the client
   inadvertently obtaining a token for the wrong RS.  The correct values
   for "audience" can either be provisioned to the client as part of its
   configuration, or dynamically looked up by the client in some
   directory.  In the latter case the integrity and correctness of the
   directory data must be assured.  Note that the "audience" hint
   provided by the RS as part of the "AS Request Creation Hints"
   Section 5.1.2 is not typically source authenticated and integrity
   protected, and should therefore not be treated a trusted value.

6.10.  Denial of service against or with Introspection

   The optional introspection mechanism provided by OAuth and supported
   in the ACE framework allows for two types of attacks that need to be
   considered by implementers.

   First, an attacker could perform a denial of service attack against
   the introspection endpoint at the AS in order to prevent validation
   of access tokens.  To maintain the security of the system, an RS that
   is configured to use introspection MUST NOT allow access based on a
   token for which it couldn't reach the introspection endpoint.

   Second, an attacker could use the fact that an RS performs
   introspection to perform a denial of service attack against that RS
   by repeatedly sending tokens to its authz-info endpoint that require
   an introspection call.  RS can mitigate such attacks by implementing
   rate limits on how many introspection requests they perform in a
   given time interval for a certain client IP address submitting tokens



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   to /authz-info.  When that limit has been reached, incoming requests
   from that address are rejected for a certain amount of time.  A
   general rate limit on the introspection requests should also be
   considered, to mitigate distributed attacks.

7.  Privacy Considerations

   Implementers and users should be aware of the privacy implications of
   the different possible deployments of this framework.

   The AS is in a very central position and can potentially learn
   sensitive information about the clients requesting access tokens.  If
   the client credentials grant is used, the AS can track what kind of
   access the client intends to perform.  With other grants this can be
   prevented by the Resource Owner.  To do so, the resource owner needs
   to bind the grants it issues to anonymous, ephemeral credentials that
   do not allow the AS to link different grants and thus different
   access token requests by the same client.

   The claims contained in a token can reveal privacy sensitive
   information about the client and the RS to any party having access to
   them (whether by processing the content of a self-contained token or
   by introspection).  The AS SHOULD be configured to minimize the
   information about clients and RSs disclosed in the tokens it issues.

   If tokens are only integrity protected and not encrypted, they may
   reveal information to attackers listening on the wire, or able to
   acquire the access tokens in some other way.  In the case of CWTs the
   token may, e.g., reveal the audience, the scope and the confirmation
   method used by the client.  The latter may reveal the identity of the
   device or application running the client.  This may be linkable to
   the identity of the person using the client (if there is a person and
   not a machine-to-machine interaction).

   Clients using asymmetric keys for proof-of-possession should be aware
   of the consequences of using the same key pair for proof-of-
   possession towards different RSs.  A set of colluding RSs or an
   attacker able to obtain the access tokens will be able to link the
   requests, or even to determine the client's identity.

   An unprotected response to an unauthorized request (see
   Section 5.1.2) may disclose information about RS and/or its existing
   relationship with C.  It is advisable to include as little
   information as possible in an unencrypted response.  If means of
   encrypting communication between C and RS already exist, more
   detailed information may be included with an error response to
   provide C with sufficient information to react on that particular
   error.



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8.  IANA Considerations

   This document creates several registries with a registration policy
   of "Expert Review"; guidelines to the experts are given in
   Section 8.16.

8.1.  ACE Authorization Server Request Creation Hints

   This specification establishes the IANA "ACE Authorization Server
   Request Creation Hints" registry.  The registry has been created to
   use the "Expert Review" registration procedure [RFC8126].  It should
   be noted that, in addition to the expert review, some portions of the
   registry require a specification, potentially a Standards Track RFC,
   be supplied as well.

   The columns of the registry are:

   Name  The name of the parameter

   CBOR Key  CBOR map key for the parameter.  Different ranges of values
      use different registration policies [RFC8126].  Integer values
      from -256 to 255 are designated as Standards Action.  Integer
      values from -65536 to -257 and from 256 to 65535 are designated as
      Specification Required.  Integer values greater than 65535 are
      designated as Expert Review.  Integer values less than -65536 are
      marked as Private Use.

   Value Type  The CBOR data types allowable for the values of this
      parameter.

   Reference  This contains a pointer to the public specification of the
      request creation hint abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 2.
   The Reference column for all of these entries will be this document.

8.2.  OAuth Extensions Error Registration

   This specification registers the following error values in the OAuth
   Extensions Error registry [IANA.OAuthExtensionsErrorRegistry].

   o  Error name: "unsupported_pop_key"
   o  Error usage location: token error response
   o  Related protocol extension: The ACE framework [this document]
   o  Change Controller: IESG
   o  Specification document(s): Section 5.6.3 of [this document]

   o  Error name: "incompatible_profiles"



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   o  Error usage location: token error response
   o  Related protocol extension: The ACE framework [this document]
   o  Change Controller: IESG
   o  Specification document(s): Section 5.6.3 of [this document]

8.3.  OAuth Error Code CBOR Mappings Registry

   This specification establishes the IANA "OAuth Error Code CBOR
   Mappings" registry.  The registry has been created to use the "Expert
   Review" registration procedure [RFC8126], except for the value range
   designated for private use.

   The columns of the registry are:

   Name  The OAuth Error Code name, refers to the name in Section 5.2.
      of [RFC6749], e.g., "invalid_request".
   CBOR Value  CBOR abbreviation for this error code.  Integer values
      less than -65536 are marked as "Private Use", all other values use
      the registration policy "Expert Review" [RFC8126].
   Reference  This contains a pointer to the public specification of the
      error code abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 10.
   The Reference column for all of these entries will be this document.

8.4.  OAuth Grant Type CBOR Mappings

   This specification establishes the IANA "OAuth Grant Type CBOR
   Mappings" registry.  The registry has been created to use the "Expert
   Review" registration procedure [RFC8126], except for the value range
   designated for private use.

   The columns of this registry are:

   Name  The name of the grant type as specified in Section 1.3 of
      [RFC6749].
   CBOR Value  CBOR abbreviation for this grant type.  Integer values
      less than -65536 are marked as "Private Use", all other values use
      the registration policy "Expert Review" [RFC8126].
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.
   Original Specification  This contains a pointer to the public
      specification of the grant type, if one exists.

   This registry will be initially populated by the values in Figure 11.
   The Reference column for all of these entries will be this document.





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8.5.  OAuth Access Token Types

   This section registers the following new token type in the "OAuth
   Access Token Types" registry [IANA.OAuthAccessTokenTypes].

   o  Type name: "PoP"
   o  Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see
      section 3.3 of [I-D.ietf-ace-oauth-params].
   o  HTTP Authentication Scheme(s): N/A
   o  Change Controller: IETF
   o  Specification document(s): [this document]

8.6.  OAuth Access Token Type CBOR Mappings

   This specification established the IANA "OAuth Access Token Type CBOR
   Mappings" registry.  The registry has been created to use the "Expert
   Review" registration procedure [RFC8126], except for the value range
   designated for private use.

   The columns of this registry are:

   Name  The name of token type as registered in the OAuth Access Token
      Types registry, e.g., "Bearer".
   CBOR Value  CBOR abbreviation for this token type.  Integer values
      less than -65536 are marked as "Private Use", all other values use
      the registration policy "Expert Review" [RFC8126].
   Reference  This contains a pointer to the public specification of the
      OAuth token type abbreviation, if one exists.
   Original Specification  This contains a pointer to the public
      specification of the OAuth token type, if one exists.

8.6.1.  Initial Registry Contents

   o  Name: "Bearer"
   o  Value: 1
   o  Reference: [this document]
   o  Original Specification: [RFC6749]

   o  Name: "PoP"
   o  Value: 2
   o  Reference: [this document]
   o  Original Specification: [this document]

8.7.  ACE Profile Registry

   This specification establishes the IANA "ACE Profile" registry.  The
   registry has been created to use the "Expert Review" registration
   procedure [RFC8126].  It should be noted that, in addition to the



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   expert review, some portions of the registry require a specification,
   potentially a Standards Track RFC, be supplied as well.

   The columns of this registry are:

   Name  The name of the profile, to be used as value of the profile
      attribute.
   Description  Text giving an overview of the profile and the context
      it is developed for.
   CBOR Value  CBOR abbreviation for this profile name.  Different
      ranges of values use different registration policies [RFC8126].
      Integer values from -256 to 255 are designated as Standards
      Action.  Integer values from -65536 to -257 and from 256 to 65535
      are designated as Specification Required.  Integer values greater
      than 65535 are designated as "Expert Review".  Integer values less
      than -65536 are marked as Private Use.
   Reference  This contains a pointer to the public specification of the
      profile abbreviation, if one exists.

   This registry will be initially empty and will be populated by the
   registrations from the ACE framework profiles.

8.8.  OAuth Parameter Registration

   This specification registers the following parameter in the "OAuth
   Parameters" registry [IANA.OAuthParameters]:

   o  Name: "ace_profile"
   o  Parameter Usage Location: token response
   o  Change Controller: IESG
   o  Reference: Section 5.6.4.3 of [this document]

8.9.  OAuth Parameters CBOR Mappings Registry

   This specification establishes the IANA "OAuth Parameters CBOR
   Mappings" registry.  The registry has been created to use the "Expert
   Review" registration procedure [RFC8126], except for the value range
   designated for private use.

   The columns of this registry are:

   Name  The OAuth Parameter name, refers to the name in the OAuth
      parameter registry, e.g., "client_id".
   CBOR Key  CBOR map key for this parameter.  Integer values less than
      -65536 are marked as "Private Use", all other values use the
      registration policy "Expert Review" [RFC8126].
   Value Type  The allowable CBOR data types for values of this
      parameter.



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   Reference  This contains a pointer to the public specification of the
      OAuth parameter abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 12.
   The Reference column for all of these entries will be this document.

8.10.  OAuth Introspection Response Parameter Registration

   This specification registers the following parameter in the OAuth
   Token Introspection Response registry
   [IANA.TokenIntrospectionResponse].

   o  Name: "ace_profile"
   o  Description: The communication and communication security profile
      used between client and RS, as defined in ACE profiles.
   o  Change Controller: IESG
   o  Reference: Section 5.7.2 of [this document]

8.11.  OAuth Token Introspection Response CBOR Mappings Registry

   This specification establishes the IANA "OAuth Token Introspection
   Response CBOR Mappings" registry.  The registry has been created to
   use the "Expert Review" registration procedure [RFC8126], except for
   the value range designated for private use.

   The columns of this registry are:

   Name  The OAuth Parameter name, refers to the name in the OAuth
      parameter registry, e.g., "client_id".
   CBOR Key  CBOR map key for this parameter.  Integer values less than
      -65536 are marked as "Private Use", all other values use the
      registration policy "Expert Review" [RFC8126].
   Value Type  The allowable CBOR data types for values of this
      parameter.
   Reference  This contains a pointer to the public specification of the
      introspection response parameter abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 16.
   The Reference column for all of these entries will be this document.

   Note that the mappings of parameters corresponding to claim names
   intentionally coincide with the CWT claim name mappings from
   [RFC8392].








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8.12.  JSON Web Token Claims

   This specification registers the following new claims in the JSON Web
   Token (JWT) registry of JSON Web Token Claims
   [IANA.JsonWebTokenClaims]:

   o  Claim Name: "ace_profile"
   o  Claim Description: The profile a token is supposed to be used
      with.
   o  Change Controller: IESG
   o  Reference: Section 5.8 of [this document]

   o  Claim Name: "exi"
   o  Claim Description: "Expires in".  Lifetime of the token in seconds
      from the time the RS first sees it.  Used to implement a weaker
      from of token expiration for devices that cannot synchronize their
      internal clocks.
   o  Change Controller: IESG
   o  Reference: Section 5.8.3 of [this document]

   o  Claim Name: "cnonce"
   o  Claim Description: "client-nonce".  A nonce previously provided to
      the AS by the RS via the client.  Used to verify token freshness
      when the RS cannot synchronize its clock with the AS.
   o  Change Controller: IESG
   o  Reference: Section 5.8 of [this document]

8.13.  CBOR Web Token Claims

   This specification registers the following new claims in the "CBOR
   Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims].

   o  Claim Name: "scope"
   o  Claim Description: The scope of an access token as defined in
      [RFC6749].
   o  JWT Claim Name: scope
   o  Claim Key: TBD (suggested: 9)
   o  Claim Value Type(s): byte string or text string
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.2 of
      [I-D.ietf-oauth-token-exchange]

   o  Claim Name: "ace_profile"
   o  Claim Description: The profile a token is supposed to be used
      with.
   o  JWT Claim Name: ace_profile
   o  Claim Key: TBD (suggested: 38)
   o  Claim Value Type(s): integer



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   o  Change Controller: IESG
   o  Specification Document(s): Section 5.8 of [this document]

   o  Claim Name: "exi"
   o  Claim Description: The expiration time of a token measured from
      when it was received at the RS in seconds.
   o  JWT Claim Name: exi
   o  Claim Key: TBD (suggested: 40)
   o  Claim Value Type(s): integer
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.8.3 of [this document]

   o  Claim Name: "cnonce"
   o  Claim Description: The client-nonce sent to the AS by the RS via
      the client.
   o  JWT Claim Name: cnonce
   o  Claim Key: TBD (suggested: 39)
   o  Claim Value Type(s): byte string
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.8 of [this document]

8.14.  Media Type Registrations

   This specification registers the 'application/ace+cbor' media type
   for messages of the protocols defined in this document carrying
   parameters encoded in CBOR.  This registration follows the procedures
   specified in [RFC6838].

   Type name: application

   Subtype name: ace+cbor

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as CBOR map containing the
   protocol parameters defined in [this document].

   Security considerations: See Section 6 of this document.

   Interoperability considerations: n/a

   Published specification: [this document]

   Applications that use this media type: The type is used by
   authorization servers, clients and resource servers that support the
   ACE framework as specified in [this document].



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   Additional information:

   Magic number(s): n/a

   File extension(s): .ace

   Macintosh file type code(s): n/a

   Person & email address to contact for further information:
   <iesg@ietf.org>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Ludwig Seitz <ludwig.setiz@ri.se>

   Change controller: IESG

8.15.  CoAP Content-Format Registry

   This specification registers the following entry to the "CoAP
   Content-Formats" registry:

   Media Type: application/ace+cbor

   Encoding

   ID: 19

   Reference: [this document]

8.16.  Expert Review Instructions

   All of the IANA registries established in this document are defined
   to use a registration policy of Expert Review.  This section gives
   some general guidelines for what the experts should be looking for,
   but they are being designated as experts for a reason, so they should
   be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   o  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered, and that the point is likely to be used in
      deployments.  The zones tagged as private use are intended for




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      testing purposes and closed environments; code points in other
      ranges should not be assigned for testing.
   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on.
   o  Since a high degree of overlap is expected between these
      registries and the contents of the OAuth parameters
      [IANA.OAuthParameters] registries, experts should require new
      registrations to maintain alignment with parameters from OAuth
      that have comparable functionality.  Deviation from this alignment
      should only be allowed if there are functional differences, that
      are motivated by the use case and that cannot be easily or
      efficiently addressed by comparable OAuth parameters.

9.  Acknowledgments

   This document is a product of the ACE working group of the IETF.

   Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and
   UMA in IoT scenarios, Robert Taylor for his discussion input, and
   Malisa Vucinic for his input on the predecessors of this proposal.

   Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from
   where large parts of the security considerations where copied.

   Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for
   contributing their work on AS discovery from draft-gerdes-ace-dcaf-
   authorize (see Section 5.1).

   Thanks to Jim Schaad and Mike Jones for their comprehensive reviews.

   Thanks to Benjamin Kaduk for his input on various questions related
   to this work.

   Thanks to Cigdem Sengul for some very useful review comments.

   Ludwig Seitz and Goeran Selander worked on this document as part of
   the CelticPlus project CyberWI, with funding from Vinnova.  Ludwig
   Seitz was also received further funding for this work by Vinnova in
   the context of the CelticNext project Critisec.







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10.  References

10.1.  Normative References

   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
              possession-11 (work in progress), October 2019.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-06 (work in progress), November 2019.

   [I-D.ietf-oauth-token-exchange]
              Jones, M., Nadalin, A., Campbell, B., Bradley, J., and C.
              Mortimore, "OAuth 2.0 Token Exchange", draft-ietf-oauth-
              token-exchange-19 (work in progress), July 2019.

   [IANA.CborWebTokenClaims]
              IANA, "CBOR Web Token (CWT) Claims",
              <https://www.iana.org/assignments/cwt/
              cwt.xhtml#claims-registry>.

   [IANA.JsonWebTokenClaims]
              IANA, "JSON Web Token Claims",
              <https://www.iana.org/assignments/jwt/jwt.xhtml#claims>.

   [IANA.OAuthAccessTokenTypes]
              IANA, "OAuth Access Token Types",
              <https://www.iana.org/assignments/oauth-parameters/
              oauth-parameters.xhtml#token-types>.

   [IANA.OAuthExtensionsErrorRegistry]
              IANA, "OAuth Extensions Error Registry",
              <https://www.iana.org/assignments/oauth-parameters/
              oauth-parameters.xhtml#extensions-error>.

   [IANA.OAuthParameters]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters/
              oauth-parameters.xhtml#parameters>.

   [IANA.TokenIntrospectionResponse]
              IANA, "OAuth Token Introspection Response",
              <https://www.iana.org/assignments/oauth-parameters/
              oauth-parameters.xhtml#token-introspection-response>.



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   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [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>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.






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   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [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>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

10.2.  Informative References

   [BLE]      Bluetooth SIG, "Bluetooth Core Specification v5.1",
              Section 4.4, January 2019,
              <https://www.bluetooth.com/specifications/
              bluetooth-core-specification/>.

   [I-D.erdtman-ace-rpcc]
              Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared-
              Key as OAuth client credentials", draft-erdtman-ace-
              rpcc-02 (work in progress), October 2017.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-24 (work
              in progress), November 2019.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-34 (work in progress), November
              2019.




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   [Margi10impact]
              Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr,
              M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold,
              "Impact of Operating Systems on Wireless Sensor Networks
              (Security) Applications and Testbeds", Proceedings of
              the 19th International Conference on Computer
              Communications and Networks (ICCCN), August 2010.

   [MQTT5.0]  Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT
              Version 5.0", OASIS Standard, March 2019,
              <https://docs.oasis-open.org/mqtt/mqtt/v5.0/
              mqtt-v5.0.html>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,
              <https://www.rfc-editor.org/info/rfc6819>.

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/info/rfc7009>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/info/rfc7521>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.






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   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/info/rfc7591>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7744]  Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M.,
              and S. Kumar, "Use Cases for Authentication and
              Authorization in Constrained Environments", RFC 7744,
              DOI 10.17487/RFC7744, January 2016,
              <https://www.rfc-editor.org/info/rfc7744>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
              <https://www.rfc-editor.org/info/rfc8252>.

   [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>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8516]  Keranen, A., ""Too Many Requests" Response Code for the
              Constrained Application Protocol", RFC 8516,
              DOI 10.17487/RFC8516, January 2019,
              <https://www.rfc-editor.org/info/rfc8516>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.



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   [RFC8628]  Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Device Authorization Grant", RFC 8628,
              DOI 10.17487/RFC8628, August 2019,
              <https://www.rfc-editor.org/info/rfc8628>.

Appendix A.  Design Justification

   This section provides further insight into the design decisions of
   the solution documented in this document.  Section 3 lists several
   building blocks and briefly summarizes their importance.  The
   justification for offering some of those building blocks, as opposed
   to using OAuth 2.0 as is, is given below.

   Common IoT constraints are:

   Low Power Radio:

      Many IoT devices are equipped with a small battery which needs to
      last for a long time.  For many constrained wireless devices, the
      highest energy cost is associated to transmitting or receiving
      messages (roughly by a factor of 10 compared to AES)
      [Margi10impact].  It is therefore important to keep the total
      communication overhead low, including minimizing the number and
      size of messages sent and received, which has an impact of choice
      on the message format and protocol.  By using CoAP over UDP and
      CBOR encoded messages, some of these aspects are addressed.
      Security protocols contribute to the communication overhead and
      can, in some cases, be optimized.  For example, authentication and
      key establishment may, in certain cases where security
      requirements allow, be replaced by provisioning of security
      context by a trusted third party, using transport or application
      layer security.

   Low CPU Speed:

      Some IoT devices are equipped with processors that are
      significantly slower than those found in most current devices on
      the Internet.  This typically has implications on what timely
      cryptographic operations a device is capable of performing, which
      in turn impacts, e.g., protocol latency.  Symmetric key
      cryptography may be used instead of the computationally more
      expensive public key cryptography where the security requirements
      so allow, but this may also require support for trusted-third-
      party-assisted secret key establishment using transport- or
      application-layer security.
   Small Amount of Memory:





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      Microcontrollers embedded in IoT devices are often equipped with
      only a small amount of RAM and flash memory, which places
      limitations on what kind of processing can be performed and how
      much code can be put on those devices.  To reduce code size, fewer
      and smaller protocol implementations can be put on the firmware of
      such a device.  In this case, CoAP may be used instead of HTTP,
      symmetric-key cryptography instead of public-key cryptography, and
      CBOR instead of JSON.  An authentication and key establishment
      protocol, e.g., the DTLS handshake, in comparison with assisted
      key establishment, also has an impact on memory and code
      footprints.

   User Interface Limitations:

      Protecting access to resources is both an important security as
      well as privacy feature.  End users and enterprise customers may
      not want to give access to the data collected by their IoT device
      or to functions it may offer to third parties.  Since the
      classical approach of requesting permissions from end users via a
      rich user interface does not work in many IoT deployment
      scenarios, these functions need to be delegated to user-controlled
      devices that are better suitable for such tasks, such as smart
      phones and tablets.

   Communication Constraints:

      In certain constrained settings an IoT device may not be able to
      communicate with a given device at all times.  Devices may be
      sleeping, or just disconnected from the Internet because of
      general lack of connectivity in the area, for cost reasons, or for
      security reasons, e.g., to avoid an entry point for Denial-of-
      Service attacks.

      The communication interactions this framework builds upon (as
      shown graphically in Figure 1) may be accomplished using a variety
      of different protocols, and not all parts of the message flow are
      used in all applications due to the communication constraints.
      Deployments making use of CoAP are expected, but this framework is
      not limited to them.  Other protocols such as HTTP, or even
      protocols such as Bluetooth Smart communication that do not
      necessarily use IP, could also be used.  The latter raises the
      need for application layer security over the various interfaces.

   In the light of these constraints we have made the following design
   decisions:

   CBOR, COSE, CWT:




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      This framework RECOMMENDS the use of CBOR [RFC7049] as data
      format.  Where CBOR data needs to be protected, the use of COSE
      [RFC8152] is RECOMMENDED.  Furthermore, where self-contained
      tokens are needed, this framework RECOMMENDS the use of CWT
      [RFC8392].  These measures aim at reducing the size of messages
      sent over the wire, the RAM size of data objects that need to be
      kept in memory and the size of libraries that devices need to
      support.

   CoAP:

      This framework RECOMMENDS the use of CoAP [RFC7252] instead of
      HTTP.  This does not preclude the use of other protocols
      specifically aimed at constrained devices, like, e.g., Bluetooth
      Low Energy (see Section 3.2).  This aims again at reducing the
      size of messages sent over the wire, the RAM size of data objects
      that need to be kept in memory and the size of libraries that
      devices need to support.

   Access Information:

      This framework defines the name "Access Information" for data
      concerning the RS that the AS returns to the client in an access
      token response (see Section 5.6.2).  This aims at enabling
      scenarios where a powerful client, supporting multiple profiles,
      needs to interact with a RS for which it does not know the
      supported profiles and the raw public key.

   Proof-of-Possession:

      This framework makes use of proof-of-possession tokens, using the
      "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession].  A request
      parameter "cnf" and a Response parameter "cnf", both having a
      value space semantically and syntactically identical to the "cnf"
      claim, are defined for the token endpoint, to allow requesting and
      stating confirmation keys.  This aims at making token theft
      harder.  Token theft is specifically relevant in constrained use
      cases, as communication often passes through middle-boxes, which
      could be able to steal bearer tokens and use them to gain
      unauthorized access.


   Authz-Info endpoint:

      This framework introduces a new way of providing access tokens to
      a RS by exposing a authz-info endpoint, to which access tokens can
      be POSTed.  This aims at reducing the size of the request message
      and the code complexity at the RS.  The size of the request



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      message is problematic, since many constrained protocols have
      severe message size limitations at the physical layer (e.g., in
      the order of 100 bytes).  This means that larger packets get
      fragmented, which in turn combines badly with the high rate of
      packet loss, and the need to retransmit the whole message if one
      packet gets lost.  Thus separating sending of the request and
      sending of the access tokens helps to reduce fragmentation.

   Client Credentials Grant:

      This framework RECOMMENDS the use of the client credentials grant
      for machine-to-machine communication use cases, where manual
      intervention of the resource owner to produce a grant token is not
      feasible.  The intention is that the resource owner would instead
      pre-arrange authorization with the AS, based on the client's own
      credentials.  The client can then (without manual intervention)
      obtain access tokens from the AS.

   Introspection:

      This framework RECOMMENDS the use of access token introspection in
      cases where the client is constrained in a way that it can not
      easily obtain new access tokens (i.e. it has connectivity issues
      that prevent it from communicating with the AS).  In that case
      this framework RECOMMENDS the use of a long-term token, that could
      be a simple reference.  The RS is assumed to be able to
      communicate with the AS, and can therefore perform introspection,
      in order to learn the claims associated with the token reference.
      The advantage of such an approach is that the resource owner can
      change the claims associated to the token reference without having
      to be in contact with the client, thus granting or revoking access
      rights.


Appendix B.  Roles and Responsibilities

   Resource Owner

      *  Make sure that the RS is registered at the AS.  This includes
         making known to the AS which profiles, token_type, scopes, and
         key types (symmetric/asymmetric) the RS supports.  Also making
         it known to the AS which audience(s) the RS identifies itself
         with.
      *  Make sure that clients can discover the AS that is in charge of
         the RS.
      *  If the client-credentials grant is used, make sure that the AS
         has the necessary, up-to-date, access control policies for the
         RS.



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   Requesting Party

      *  Make sure that the client is provisioned the necessary
         credentials to authenticate to the AS.
      *  Make sure that the client is configured to follow the security
         requirements of the Requesting Party when issuing requests
         (e.g., minimum communication security requirements, trust
         anchors).
      *  Register the client at the AS.  This includes making known to
         the AS which profiles, token_types, and key types (symmetric/
         asymmetric) the client.

   Authorization Server

      *  Register the RS and manage corresponding security contexts.
      *  Register clients and authentication credentials.
      *  Allow Resource Owners to configure and update access control
         policies related to their registered RSs.
      *  Expose the token endpoint to allow clients to request tokens.
      *  Authenticate clients that wish to request a token.
      *  Process a token request using the authorization policies
         configured for the RS.
      *  Optionally: Expose the introspection endpoint that allows RS's
         to submit token introspection requests.
      *  If providing an introspection endpoint: Authenticate RSs that
         wish to get an introspection response.
      *  If providing an introspection endpoint: Process token
         introspection requests.
      *  Optionally: Handle token revocation.
      *  Optionally: Provide discovery metadata.  See [RFC8414]
      *  Optionally: Handle refresh tokens.

   Client

      *  Discover the AS in charge of the RS that is to be targeted with
         a request.
      *  Submit the token request (see step (A) of Figure 1).

         +  Authenticate to the AS.
         +  Optionally (if not pre-configured): Specify which RS, which
            resource(s), and which action(s) the request(s) will target.
         +  If raw public keys (rpk) or certificates are used, make sure
            the AS has the right rpk or certificate for this client.
      *  Process the access token and Access Information (see step (B)
         of Figure 1).






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         +  Check that the Access Information provides the necessary
            security parameters (e.g., PoP key, information on
            communication security protocols supported by the RS).
         +  Safely store the proof-of-possession key.
         +  If provided by the AS: Safely store the refresh token.
      *  Send the token and request to the RS (see step (C) of
         Figure 1).

         +  Authenticate towards the RS (this could coincide with the
            proof of possession process).
         +  Transmit the token as specified by the AS (default is to the
            authz-info endpoint, alternative options are specified by
            profiles).
         +  Perform the proof-of-possession procedure as specified by
            the profile in use (this may already have been taken care of
            through the authentication procedure).
      *  Process the RS response (see step (F) of Figure 1) of the RS.

   Resource Server

      *  Expose a way to submit access tokens.  By default this is the
         authz-info endpoint.
      *  Process an access token.

         +  Verify the token is from a recognized AS.
         +  Check the token's integrity.
         +  Verify that the token applies to this RS.
         +  Check that the token has not expired (if the token provides
            expiration information).
         +  Store the token so that it can be retrieved in the context
            of a matching request.

         Note: The order proposed here is not normative, any process
         that arrives at an equivalent result can be used.  A noteworthy
         consideration is whether one can use cheap operations early on
         to quickly discard non-applicable or invalid tokens, before
         performing expensive cryptographic operations (e.g. doing an
         expiration check before verifying a signature).

      *  Process a request.

         +  Set up communication security with the client.
         +  Authenticate the client.
         +  Match the client against existing tokens.
         +  Check that tokens belonging to the client actually authorize
            the requested action.
         +  Optionally: Check that the matching tokens are still valid,
            using introspection (if this is possible.)



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      *  Send a response following the agreed upon communication
         security mechanism(s).
      *  Safely store credentials such as raw public keys for
         authentication or proof-of-possession keys linked to access
         tokens.

Appendix C.  Requirements on Profiles

   This section lists the requirements on profiles of this framework,
   for the convenience of profile designers.

   o  Optionally define new methods for the client to discover the
      necessary permissions and AS for accessing a resource, different
      from the one proposed in Section 5.1.  Section 4
   o  Optionally specify new grant types.  Section 5.2
   o  Optionally define the use of client certificates as client
      credential type.  Section 5.3
   o  Specify the communication protocol the client and RS the must use
      (e.g., CoAP).  Section 5 and Section 5.6.4.3
   o  Specify the security protocol the client and RS must use to
      protect their communication (e.g., OSCORE or DTLS).  This must
      provide encryption, integrity and replay protection.
      Section 5.6.4.3
   o  Specify how the client and the RS mutually authenticate.
      Section 4
   o  Specify the proof-of-possession protocol(s) and how to select one,
      if several are available.  Also specify which key types (e.g.,
      symmetric/asymmetric) are supported by a specific proof-of-
      possession protocol.  Section 5.6.4.2
   o  Specify a unique ace_profile identifier.  Section 5.6.4.3
   o  If introspection is supported: Specify the communication and
      security protocol for introspection.  Section 5.7
   o  Specify the communication and security protocol for interactions
      between client and AS.  This must provide encryption, integrity
      protection, replay protection and a binding between requests and
      responses.  Section 5 and Section 5.6
   o  Specify how/if the authz-info endpoint is protected, including how
      error responses are protected.  Section 5.8.1
   o  Optionally define other methods of token transport than the authz-
      info endpoint.  Section 5.8.1

Appendix D.  Assumptions on AS knowledge about C and RS

   This section lists the assumptions on what an AS should know about a
   client and a RS in order to be able to respond to requests to the
   token and introspection endpoints.  How this information is
   established is out of scope for this document.




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   o  The identifier of the client or RS.
   o  The profiles that the client or RS supports.
   o  The scopes that the RS supports.
   o  The audiences that the RS identifies with.
   o  The key types (e.g., pre-shared symmetric key, raw public key, key
      length, other key parameters) that the client or RS supports.
   o  The types of access tokens the RS supports (e.g., CWT).
   o  If the RS supports CWTs, the COSE parameters for the crypto
      wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the
      RS supports.
   o  The expiration time for access tokens issued to this RS (unless
      the RS accepts a default time chosen by the AS).
   o  The symmetric key shared between client and AS (if any).
   o  The symmetric key shared between RS and AS (if any).
   o  The raw public key of the client or RS (if any).
   o  Whether the RS has synchronized time (and thus is able to use the
      'exp' claim) or not.

Appendix E.  Deployment Examples

   There is a large variety of IoT deployments, as is indicated in
   Appendix A, and this section highlights a few common variants.  This
   section is not normative but illustrates how the framework can be
   applied.

   For each of the deployment variants, there are a number of possible
   security setups between clients, resource servers and authorization
   servers.  The main focus in the following subsections is on how
   authorization of a client request for a resource hosted by a RS is
   performed.  This requires the security of the requests and responses
   between the clients and the RS to be considered.

   Note: CBOR diagnostic notation is used for examples of requests and
   responses.

E.1.  Local Token Validation

   In this scenario, the case where the resource server is offline is
   considered, i.e., it is not connected to the AS at the time of the
   access request.  This access procedure involves steps A, B, C, and F
   of Figure 1.

   Since the resource server must be able to verify the access token
   locally, self-contained access tokens must be used.

   This example shows the interactions between a client, the
   authorization server and a temperature sensor acting as a resource
   server.  Message exchanges A and B are shown in Figure 17.



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      A: The client first generates a public-private key pair used for
      communication security with the RS.
      The client sends a CoAP POST request to the token endpoint at the
      AS.  The security of this request can be transport or application
      layer.  It is up the the communication security profile to define.
      In the example it is assumed that both client and AS have
      performed mutual authentication e.g. via DTLS.  The request
      contains the public key of the client and the Audience parameter
      set to "tempSensorInLivingRoom", a value that the temperature
      sensor identifies itself with.  The AS evaluates the request and
      authorizes the client to access the resource.
      B: The AS responds with a 2.05 Content response containing the
      Access Information, including the access token.  The PoP access
      token contains the public key of the client, and the Access
      Information contains the public key of the RS.  For communication
      security this example uses DTLS RawPublicKey between the client
      and the RS.  The issued token will have a short validity time,
      i.e., "exp" close to "iat", in order to mitigate attacks using
      stolen client credentials.  The token includes the claim such as
      "scope" with the authorized access that an owner of the
      temperature device can enjoy.  In this example, the "scope" claim,
      issued by the AS, informs the RS that the owner of the token, that
      can prove the possession of a key is authorized to make a GET
      request against the /temperature resource and a POST request on
      the /firmware resource.  Note that the syntax and semantics of the
      scope claim are application specific.
      Note: In this example it is assumed that the client knows what
      resource it wants to access, and is therefore able to request
      specific audience and scope claims for the access token.

            Authorization
     Client    Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   and mutual authentication
       |         |
   A:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"token"
       |         | Content-Format: application/ace+cbor
       |         | Payload: <Request-Payload>
       |         |
   B:  |<--------+ Header: 2.05 Content
       |  2.05   | Content-Format: application/ace+cbor
       |         | Payload: <Response-Payload>
       |         |

      Figure 17: Token Request and Response Using Client Credentials.




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   The information contained in the Request-Payload and the Response-
   Payload is shown in Figure 18 Note that the parameter "rs_cnf" from
   [I-D.ietf-ace-oauth-params] is used to inform the client about the
   resource server's public key.

   Request-Payload :
   {
     "audience" : "tempSensorInLivingRoom",
     "client_id" : "myclient",
     "req_cnf" : {
       "COSE_Key" : {
         "kid" : b64'1Bg8vub9tLe1gHMzV76e8',
         "kty" : "EC",
         "crv" : "P-256",
         "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
         "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
       }
     }
   }

   Response-Payload :
   {
     "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...',
     "rs_cnf" : {
       "COSE_Key" : {
         "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
         "kty" : "EC",
         "crv" : "P-256",
         "x"   : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
         "y"   : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
       }
     }
   }

             Figure 18: Request and Response Payload Details.

   The content of the access token is shown in Figure 19.














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   {
     "aud" : "tempSensorInLivingRoom",
     "iat" : "1563451500",
     "exp" : "1563453000",
     "scope" :  "temperature_g firmware_p",
     "cnf" : {
       "COSE_Key" : {
         "kid" : b64'1Bg8vub9tLe1gHMzV76e8',
         "kty" : "EC",
         "crv" : "P-256",
         "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
         "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
       }
     }
   }

        Figure 19: Access Token including Public Key of the Client.

   Messages C and F are shown in Figure 20 - Figure 21.

      C: The client then sends the PoP access token to the authz-info
      endpoint at the RS.  This is a plain CoAP POST request, i.e., no
      transport or application layer security is used between client and
      RS since the token is integrity protected between the AS and RS.
      The RS verifies that the PoP access token was created by a known
      and trusted AS, that it applies to this RS, and that it is valid.
      The RS caches the security context together with authorization
      information about this client contained in the PoP access token.


              Resource
    Client     Server
       |         |
   C:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"authz-info"
       |         | Payload: 0INDoQEKoQVN ...
       |         |
       |<--------+ Header: 2.04 Changed
       |  2.04   |
       |         |

                Figure 20: Access Token provisioning to RS
      The client and the RS runs the DTLS handshake using the raw public
      keys established in step B and C.
      The client sends a CoAP GET request to /temperature on RS over
      DTLS.  The RS verifies that the request is authorized, based on
      previously established security context.




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      F: The RS responds over the same DTLS channel with a CoAP 2.05
      Content response, containing a resource representation as payload.

              Resource
    Client     Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   using Raw Public Keys
       |         |
       +-------->| Header: GET (Code=0.01)
       | GET     | Uri-Path: "temperature"
       |         |
       |         |
       |         |
   F:  |<--------+ Header: 2.05 Content
       | 2.05    | Payload: <sensor value>
       |         |

        Figure 21: Resource Request and Response protected by DTLS.

E.2.  Introspection Aided Token Validation

   In this deployment scenario it is assumed that a client is not able
   to access the AS at the time of the access request, whereas the RS is
   assumed to be connected to the back-end infrastructure.  Thus the RS
   can make use of token introspection.  This access procedure involves
   steps A-F of Figure 1, but assumes steps A and B have been carried
   out during a phase when the client had connectivity to AS.

   Since the client is assumed to be offline, at least for a certain
   period of time, a pre-provisioned access token has to be long-lived.
   Since the client is constrained, the token will not be self contained
   (i.e. not a CWT) but instead just a reference.  The resource server
   uses its connectivity to learn about the claims associated to the
   access token by using introspection, which is shown in the example
   below.

   In the example interactions between an offline client (key fob), a RS
   (online lock), and an AS is shown.  It is assumed that there is a
   provisioning step where the client has access to the AS.  This
   corresponds to message exchanges A and B which are shown in
   Figure 22.

   Authorization consent from the resource owner can be pre-configured,
   but it can also be provided via an interactive flow with the resource
   owner.  An example of this for the key fob case could be that the
   resource owner has a connected car, he buys a generic key that he
   wants to use with the car.  To authorize the key fob he connects it



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   to his computer that then provides the UI for the device.  After that
   OAuth 2.0 implicit flow can used to authorize the key for his car at
   the the car manufacturers AS.

   Note: In this example the client does not know the exact door it will
   be used to access since the token request is not send at the time of
   access.  So the scope and audience parameters are set quite wide to
   start with, while tailored values narrowing down the claims to the
   specific RS being accessed can be provided to that RS during an
   introspection step.

      A: The client sends a CoAP POST request to the token endpoint at
      AS.  The request contains the Audience parameter set to "PACS1337"
      (PACS, Physical Access System), a value the that identifies the
      physical access control system to which the individual doors are
      connected.  The AS generates an access token as an opaque string,
      which it can match to the specific client and the targeted
      audience.  It furthermore generates a symmetric proof-of-
      possession key.  The communication security and authentication
      between client and AS is assumed to have been provided at
      transport layer (e.g. via DTLS) using a pre-shared security
      context (psk, rpk or certificate).
      B: The AS responds with a CoAP 2.05 Content response, containing
      as playload the Access Information, including the access token and
      the symmetric proof-of-possession key.  Communication security
      between C and RS will be DTLS and PreSharedKey.  The PoP key is
      used as the PreSharedKey.

   Note: In this example we are using a symmetric key for a multi-RS
   audience, which is not recommended normally (see Section 6.9).
   However in this case the risk is deemed to be acceptable, since all
   the doors are part of the same physical access control system, and
   therefore the risk of a malicious RS impersonating the client towards
   another RS is low.

















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            Authorization
    Client     Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   and mutual authentication
       |         |
   A:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"token"
       |         | Content-Format: application/ace+cbor
       |         | Payload: <Request-Payload>
       |         |
   B:  |<--------+ Header: 2.05 Content
       |         | Content-Format: application/ace+cbor
       |  2.05   | Payload: <Response-Payload>
       |         |

      Figure 22: Token Request and Response using Client Credentials.

   The information contained in the Request-Payload and the Response-
   Payload is shown in Figure 23.

   Request-Payload:
   {
     "client_id" : "keyfob",
     "audience" : "PACS1337"
   }

   Response-Payload:
   {
     "access_token" : b64'VGVzdCB0b2tlbg==',
     "cnf" : {
       "COSE_Key" : {
         "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
         "kty" : "oct",
         "alg" : "HS256",
         "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
       }
     }
   }

           Figure 23: Request and Response Payload for C offline

   The access token in this case is just an opaque byte string
   referencing the authorization information at the AS.

      C: Next, the client POSTs the access token to the authz-info
      endpoint in the RS.  This is a plain CoAP request, i.e., no DTLS
      between client and RS.  Since the token is an opaque string, the



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      RS cannot verify it on its own, and thus defers to respond the
      client with a status code until after step E.
      D: The RS sends the token to the introspection endpoint on the AS
      using a CoAP POST request.  In this example RS and AS are assumed
      to have performed mutual authentication using a pre shared
      security context (psk, rpk or certificate) with the RS acting as
      DTLS client.
      E: The AS provides the introspection response (2.05 Content)
      containing parameters about the token.  This includes the
      confirmation key (cnf) parameter that allows the RS to verify the
      client's proof of possession in step F.  Note that our example in
      Figure 25 assumes a pre-established key (e.g. one used by the
      client and the RS for a previous token) that is now only
      referenced by its key-identifier 'kid'.
      After receiving message E, the RS responds to the client's POST in
      step C with the CoAP response code 2.01 (Created).


              Resource
     Client    Server
       |         |
   C:  +-------->| Header: POST (T=CON, Code=0.02)
       |  POST   | Uri-Path:"authz-info"
       |         | Payload: b64'VGVzdCB0b2tlbg=='
       |         |
       |         |     Authorization
       |         |       Server
       |         |          |
       |      D: +--------->| Header: POST (Code=0.02)
       |         |  POST    | Uri-Path: "introspect"
       |         |          | Content-Format: "application/ace+cbor"
       |         |          | Payload: <Request-Payload>
       |         |          |
       |      E: |<---------+ Header: 2.05 Content
       |         |  2.05    | Content-Format: "application/ace+cbor"
       |         |          | Payload: <Response-Payload>
       |         |          |
       |         |
       |<--------+ Header: 2.01 Created
       |  2.01   |
       |         |

               Figure 24: Token Introspection for C offline
      The information contained in the Request-Payload and the Response-
      Payload is shown in Figure 25.






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   Request-Payload:
   {
     "token" : b64'VGVzdCB0b2tlbg==',
     "client_id" : "FrontDoor",
   }

   Response-Payload:
   {
     "active" : true,
     "aud" : "lockOfDoor4711",
     "scope" : "open, close",
     "iat" : 1563454000,
     "cnf" : {
       "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk'
     }
   }

         Figure 25: Request and Response Payload for Introspection

      The client uses the symmetric PoP key to establish a DTLS
      PreSharedKey secure connection to the RS.  The CoAP request PUT is
      sent to the uri-path /state on the RS, changing the state of the
      door to locked.
      F: The RS responds with a appropriate over the secure DTLS
      channel.

              Resource
     Client    Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   using Pre Shared Key
       |         |
       +-------->| Header: PUT (Code=0.03)
       | PUT     | Uri-Path: "state"
       |         | Payload: <new state for the lock>
       |         |
   F:  |<--------+ Header: 2.04 Changed
       | 2.04    | Payload: <new state for the lock>
       |         |

       Figure 26: Resource request and response protected by OSCORE

Appendix F.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.






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F.1.  Version -21 to 22

   o  Provided section numbers in references to OAuth RFC.
   o  Updated IANA mapping registries to only use "Private Use" and
      "Expert Review".
   o  Made error messages optional for RS at token submission since it
      may not be able to send them depending on the profile.
   o  Corrected errors in examples.

F.2.  Version -20 to 21

   o  Added text about expiration of RS keys.

F.3.  Version -19 to 20

   o  Replaced "req_aud" with "audience" from the OAuth token exchange
      draft.
   o  Updated examples to remove unnecessary elements.

F.4.  Version -18 to -19

   o  Added definition of "Authorization Information".
   o  Explicitly state that ACE allows encoding refresh tokens in binary
      format in addition to strings.
   o  Renamed "AS Information" to "AS Request Creation Hints" and added
      the possibility to specify req_aud and scope as hints.
   o  Added the "kid" parameter to AS Request Creation Hints.
   o  Added security considerations about the integrity protection of
      tokens with multi-RS audiences.
   o  Renamed IANA registries mapping OAuth parameters to reflect the
      mapped registry.
   o  Added JWT claim names to CWT claim registrations.
   o  Added expert review instructions.
   o  Updated references to TLS from 1.2 to 1.3.

F.5.  Version -17 to -18

   o  Added OSCORE options in examples involving OSCORE.
   o  Removed requirement for the client to send application/cwt, since
      the client has no way to know.
   o  Clarified verification of tokens by the RS.
   o  Added exi claim CWT registration.

F.6.  Version -16 to -17

   o  Added references to (D)TLS 1.3.
   o  Added requirement that responses are bound to requests.




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   o  Specify that grant_type is OPTIONAL in C2AS requests (as opposed
      to REQUIRED in OAuth).
   o  Replaced examples with hypothetical COSE profile with OSCORE.
   o  Added requirement for content type application/ace+cbor in error
      responses for token and introspection requests and responses.
   o  Reworked abbreviation space for claims, request and response
      parameters.
   o  Added text that the RS may indicate that it is busy at the authz-
      info resource.
   o  Added section that specifies how the RS verifies an access token.
   o  Added section on the protection of the authz-info endpoint.
   o  Removed the expiration mechanism based on sequence numbers.
   o  Added reference to RFC7662 security considerations.
   o  Added considerations on minimal security requirements for
      communication.
   o  Added security considerations on unprotected information sent to
      authz-info and in the error responses.

F.7.  Version -15 to -16

   o  Added text the RS using RFC6750 error codes.
   o  Defined an error code for incompatible token request parameters.
   o  Removed references to the actors draft.
   o  Fixed errors in examples.

F.8.  Version -14 to -15

   o  Added text about refresh tokens.
   o  Added text about protection of credentials.
   o  Rephrased introspection so that other entities than RS can do it.
   o  Editorial improvements.

F.9.  Version -13 to -14

   o  Split out the 'aud', 'cnf' and 'rs_cnf' parameters to
      [I-D.ietf-ace-oauth-params]
   o  Introduced the "application/ace+cbor" Content-Type.
   o  Added claim registrations from 'profile' and 'rs_cnf'.
   o  Added note on schema part of AS Information Section 5.1.2
   o  Realigned the parameter abbreviations to push rarely used ones to
      the 2-byte encoding size of CBOR integers.

F.10.  Version -12 to -13

   o  Changed "Resource Information" to "Access Information" to avoid
      confusion.
   o  Clarified section about AS discovery.
   o  Editorial changes



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F.11.  Version -11 to -12

   o  Moved the Request error handling to a section of its own.
   o  Require the use of the abbreviation for profile identifiers.
   o  Added rs_cnf parameter in the introspection response, to inform
      RS' with several RPKs on which key to use.
   o  Allowed use of rs_cnf as claim in the access token in order to
      inform an RS with several RPKs on which key to use.
   o  Clarified that profiles must specify if/how error responses are
      protected.
   o  Fixed label number range to align with COSE/CWT.
   o  Clarified the requirements language in order to allow profiles to
      specify other payload formats than CBOR if they do not use CoAP.

F.12.  Version -10 to -11

   o  Fixed some CBOR data type errors.
   o  Updated boilerplate text

F.13.  Version -09 to -10

   o  Removed CBOR major type numbers.
   o  Removed the client token design.
   o  Rephrased to clarify that other protocols than CoAP can be used.
   o  Clarifications regarding the use of HTTP

F.14.  Version -08 to -09

   o  Allowed scope to be byte strings.
   o  Defined default names for endpoints.
   o  Refactored the IANA section for briefness and consistency.
   o  Refactored tables that define IANA registry contents for
      consistency.
   o  Created IANA registry for CBOR mappings of error codes, grant
      types and Authorization Server Information.
   o  Added references to other document sections defining IANA entries
      in the IANA section.

F.15.  Version -07 to -08

   o  Moved AS discovery from the DTLS profile to the framework, see
      Section 5.1.
   o  Made the use of CBOR mandatory.  If you use JSON you can use
      vanilla OAuth.
   o  Made it mandatory for profiles to specify C-AS security and RS-AS
      security (the latter only if introspection is supported).
   o  Made the use of CBOR abbreviations mandatory.




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   o  Added text to clarify the use of token references as an
      alternative to CWTs.
   o  Added text to clarify that introspection must not be delayed, in
      case the RS has to return a client token.
   o  Added security considerations about leakage through unprotected AS
      discovery information, combining profiles and leakage through
      error responses.
   o  Added privacy considerations about leakage through unprotected AS
      discovery.
   o  Added text that clarifies that introspection is optional.
   o  Made profile parameter optional since it can be implicit.
   o  Clarified that CoAP is not mandatory and other protocols can be
      used.
   o  Clarified the design justification for specific features of the
      framework in appendix A.
   o  Clarified appendix E.2.
   o  Removed specification of the "cnf" claim for CBOR/COSE, and
      replaced with references to [I-D.ietf-ace-cwt-proof-of-possession]

F.16.  Version -06 to -07

   o  Various clarifications added.
   o  Fixed erroneous author email.

F.17.  Version -05 to -06

   o  Moved sections that define the ACE framework into a subsection of
      the framework Section 5.
   o  Split section on client credentials and grant into two separate
      sections, Section 5.2, and Section 5.3.
   o  Added Section 5.4 on AS authentication.
   o  Added Section 5.5 on the Authorization endpoint.

F.18.  Version -04 to -05

   o  Added RFC 2119 language to the specification of the required
      behavior of profile specifications.
   o  Added Section 5.3 on the relation to the OAuth2 grant types.
   o  Added CBOR abbreviations for error and the error codes defined in
      OAuth2.
   o  Added clarification about token expiration and long-running
      requests in Section 5.8.3
   o  Added security considerations about tokens with symmetric PoP keys
      valid for more than one RS.
   o  Added privacy considerations section.
   o  Added IANA registry mapping the confirmation types from RFC 7800
      to equivalent COSE types.




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   o  Added appendix D, describing assumptions about what the AS knows
      about the client and the RS.

F.19.  Version -03 to -04

   o  Added a description of the terms "framework" and "profiles" as
      used in this document.
   o  Clarified protection of access tokens in section 3.1.
   o  Clarified uses of the "cnf" parameter in section 6.4.5.
   o  Clarified intended use of Client Token in section 7.4.

F.20.  Version -02 to -03

   o  Removed references to draft-ietf-oauth-pop-key-distribution since
      the status of this draft is unclear.
   o  Copied and adapted security considerations from draft-ietf-oauth-
      pop-key-distribution.
   o  Renamed "client information" to "RS information" since it is
      information about the RS.
   o  Clarified the requirements on profiles of this framework.
   o  Clarified the token endpoint protocol and removed negotiation of
      "profile" and "alg" (section 6).
   o  Renumbered the abbreviations for claims and parameters to get a
      consistent numbering across different endpoints.
   o  Clarified the introspection endpoint.
   o  Renamed token, introspection and authz-info to "endpoint" instead
      of "resource" to mirror the OAuth 2.0 terminology.
   o  Updated the examples in the appendices.

F.21.  Version -01 to -02

   o  Restructured to remove communication security parts.  These shall
      now be defined in profiles.
   o  Restructured section 5 to create new sections on the OAuth
      endpoints token, introspection and authz-info.
   o  Pulled in material from draft-ietf-oauth-pop-key-distribution in
      order to define proof-of-possession key distribution.
   o  Introduced the "cnf" parameter as defined in RFC7800 to reference
      or transport keys used for proof of possession.
   o  Introduced the "client-token" to transport client information from
      the AS to the client via the RS in conjunction with introspection.
   o  Expanded the IANA section to define parameters for token request,
      introspection and CWT claims.
   o  Moved deployment scenarios to the appendix as examples.







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F.22.  Version -00 to -01

   o  Changed 5.1. from "Communication Security Protocol" to "Client
      Information".
   o  Major rewrite of 5.1 to clarify the information exchanged between
      C and AS in the PoP access token request profile for IoT.

      *  Allow the client to indicate preferences for the communication
         security protocol.
      *  Defined the term "Client Information" for the additional
         information returned to the client in addition to the access
         token.
      *  Require that the messages between AS and client are secured,
         either with (D)TLS or with COSE_Encrypted wrappers.
      *  Removed dependency on OSCOAP and added generic text about
         object security instead.
      *  Defined the "rpk" parameter in the client information to
         transmit the raw public key of the RS from AS to client.
      *  (D)TLS MUST use the PoP key in the handshake (either as PSK or
         as client RPK with client authentication).
      *  Defined the use of x5c, x5t and x5tS256 parameters when a
         client certificate is used for proof of possession.
      *  Defined "tktn" parameter for signaling for how to transfer the
         access token.
   o  Added 5.2. the CoAP Access-Token option for transferring access
      tokens in messages that do not have payload.
   o  5.3.2.  Defined success and error responses from the RS when
      receiving an access token.
   o  5.6.:Added section giving guidance on how to handle token
      expiration in the absence of reliable time.
   o  Appendix B Added list of roles and responsibilities for C, AS and
      RS.

Authors' Addresses

   Ludwig Seitz
   RISE
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se









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   Goeran Selander
   Ericsson
   Faroegatan 6
   Kista  164 80
   Sweden

   Email: goran.selander@ericsson.com


   Erik Wahlstroem
   Sweden

   Email: erik@wahlstromstekniska.se


   Samuel Erdtman
   Spotify AB
   Birger Jarlsgatan 61, 4tr
   Stockholm  113 56
   Sweden

   Email: erdtman@spotify.com


   Hannes Tschofenig
   Arm Ltd.
   Absam  6067
   Austria

   Email: Hannes.Tschofenig@arm.com





















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