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Versions: (draft-richer-transactional-authz) 00

GNAP                                                      J. Richer, Ed.
Internet-Draft                                       Bespoke Engineering
Intended status: Standards Track                         17 October 2020
Expires: 20 April 2021


              Grant Negotiation and Authorization Protocol
                    draft-ietf-gnap-core-protocol-00

Abstract

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

   This document has been prepared by the GNAP working group design team
   of Kathleen Moriarty, Fabien Imbault, Dick Hardt, Mike Jones, and
   Justin Richer.  This document is intended as a starting point for the
   working group and includes decision points for discussion and
   agreement.  Many of the features in this proposed protocol can be
   accomplished in a number of ways.  Where possible, the editor has
   included notes and discussion from the design team regarding the
   options as understood.

   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.

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 20 April 2021.




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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Sequences . . . . . . . . . . . . . . . . . . . . . . . .   7
       1.3.1.  Redirect-based Interaction  . . . . . . . . . . . . .  10
       1.3.2.  User-code Interaction . . . . . . . . . . . . . . . .  12
       1.3.3.  Asynchronous Authorization  . . . . . . . . . . . . .  14
       1.3.4.  Software-only Authorization . . . . . . . . . . . . .  15
       1.3.5.  Refreshing an Expired Access Token  . . . . . . . . .  16
   2.  Requesting Access . . . . . . . . . . . . . . . . . . . . . .  17
     2.1.  Requesting Resources  . . . . . . . . . . . . . . . . . .  19
       2.1.1.  Requesting a Single Access Token  . . . . . . . . . .  19
       2.1.2.  Requesting Resources By Reference . . . . . . . . . .  21
       2.1.3.  Requesting Multiple Access Tokens . . . . . . . . . .  23
       2.1.4.  Signaling Token Behavior  . . . . . . . . . . . . . .  25
     2.2.  Requesting User Information . . . . . . . . . . . . . . .  27
     2.3.  Identifying the RC  . . . . . . . . . . . . . . . . . . .  28
       2.3.1.  Identifying the RC Instance . . . . . . . . . . . . .  30
       2.3.2.  Identifying the RC Key  . . . . . . . . . . . . . . .  31
       2.3.3.  Providing Displayable RC Information  . . . . . . . .  32
       2.3.4.  Authenticating the RC . . . . . . . . . . . . . . . .  33
     2.4.  Identifying the User  . . . . . . . . . . . . . . . . . .  33
       2.4.1.  Identifying the User by Reference . . . . . . . . . .  34
     2.5.  Interacting with the User . . . . . . . . . . . . . . . .  35
       2.5.1.  Redirect to an Arbitrary URL  . . . . . . . . . . . .  37
       2.5.2.  Open an Application-specific URL  . . . . . . . . . .  37
       2.5.3.  Receive a Callback After Interaction  . . . . . . . .  38
       2.5.4.  Display a Short User Code . . . . . . . . . . . . . .  42
       2.5.5.  Indicate Desired Interaction Locales  . . . . . . . .  42
       2.5.6.  Extending Interaction Modes . . . . . . . . . . . . .  42
     2.6.  Declaring RC Capabilities . . . . . . . . . . . . . . . .  43
     2.7.  Referencing an Existing Grant Request . . . . . . . . . .  43



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     2.8.  Requesting OpenID Connect Claims  . . . . . . . . . . . .  43
     2.9.  Extending The Grant Request . . . . . . . . . . . . . . .  44
   3.  Grant Response  . . . . . . . . . . . . . . . . . . . . . . .  45
     3.1.  Request Continuation  . . . . . . . . . . . . . . . . . .  46
     3.2.  Access Tokens . . . . . . . . . . . . . . . . . . . . . .  47
       3.2.1.  Single Access Token . . . . . . . . . . . . . . . . .  48
       3.2.2.  Multiple Access Tokens  . . . . . . . . . . . . . . .  50
     3.3.  Interaction Modes . . . . . . . . . . . . . . . . . . . .  51
       3.3.1.  Redirection to an arbitrary URL . . . . . . . . . . .  51
       3.3.2.  Launch of an application URL  . . . . . . . . . . . .  52
       3.3.3.  Post-interaction Callback to an RC URL  . . . . . . .  52
       3.3.4.  Display of a Short User Code  . . . . . . . . . . . .  53
       3.3.5.  Extending Interaction Mode Responses  . . . . . . . .  54
     3.4.  Returning User Information  . . . . . . . . . . . . . . .  54
     3.5.  Returning Dynamically-bound Reference Handles . . . . . .  56
     3.6.  Error Response  . . . . . . . . . . . . . . . . . . . . .  58
     3.7.  Extending the Response  . . . . . . . . . . . . . . . . .  58
   4.  Interaction at the AS . . . . . . . . . . . . . . . . . . . .  58
     4.1.  Interaction at a Redirected URI . . . . . . . . . . . . .  59
     4.2.  Interaction at the User Code URI  . . . . . . . . . . . .  59
     4.3.  Interaction through an Application URI  . . . . . . . . .  60
     4.4.  Post-Interaction Completion . . . . . . . . . . . . . . .  60
       4.4.1.  Completing Interaction with a Browser Redirect to the
               Callback URI  . . . . . . . . . . . . . . . . . . . .  61
       4.4.2.  Completing Interaction with a Direct HTTP Request
               Callback  . . . . . . . . . . . . . . . . . . . . . .  62
       4.4.3.  Calculating the interaction hash  . . . . . . . . . .  62
   5.  Continuing a Grant Request  . . . . . . . . . . . . . . . . .  64
     5.1.  Continuing After a Completed Interaction  . . . . . . . .  66
     5.2.  Continuing During Pending Interaction . . . . . . . . . .  67
     5.3.  Modifying an Existing Request . . . . . . . . . . . . . .  69
     5.4.  Getting the Current State of a Grant Request  . . . . . .  74
     5.5.  Canceling a Grant Request . . . . . . . . . . . . . . . .  75
   6.  Token Management  . . . . . . . . . . . . . . . . . . . . . .  75
     6.1.  Rotating the Access Token . . . . . . . . . . . . . . . .  76
     6.2.  Revoking the Access Token . . . . . . . . . . . . . . . .  78
   7.  Using Access Tokens . . . . . . . . . . . . . . . . . . . . .  79
   8.  Binding Keys  . . . . . . . . . . . . . . . . . . . . . . . .  80
     8.1.  Detached JWS  . . . . . . . . . . . . . . . . . . . . . .  81
     8.2.  Attached JWS  . . . . . . . . . . . . . . . . . . . . . .  84
     8.3.  Mutual TLS  . . . . . . . . . . . . . . . . . . . . . . .  86
     8.4.  Demonstration of Proof-of-Possession (DPoP) . . . . . . .  88
     8.5.  HTTP Signing  . . . . . . . . . . . . . . . . . . . . . .  89
     8.6.  OAuth Proof of Possession (PoP) . . . . . . . . . . . . .  91
   9.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . .  92
   10. Resource Servers  . . . . . . . . . . . . . . . . . . . . . .  93
     10.1.  Introspecting a Token  . . . . . . . . . . . . . . . . .  94
     10.2.  Deriving a downstream token  . . . . . . . . . . . . . .  95



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     10.3.  Registering a Resource Handle  . . . . . . . . . . . . .  97
     10.4.  Requesting a Resources With Insufficient Access  . . . .  98
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  98
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  99
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  99
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  99
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  99
   Appendix A.  Document History . . . . . . . . . . . . . . . . . . 101
   Appendix B.  Component Data Models  . . . . . . . . . . . . . . . 105
   Appendix C.  Example Protocol Flows . . . . . . . . . . . . . . . 105
     C.1.  Redirect-Based User Interaction . . . . . . . . . . . . . 106
     C.2.  Secondary Device Interaction  . . . . . . . . . . . . . . 110
   Appendix D.  No User Involvement  . . . . . . . . . . . . . . . . 113
     D.1.  Asynchronous Authorization  . . . . . . . . . . . . . . . 114
     D.2.  Applying OAuth 2 Scopes and Client IDs  . . . . . . . . . 117
   Appendix E.  JSON Structures and Polymorphism . . . . . . . . . . 118
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 119

1.  Protocol

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

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

   This protocol solves many of the same use cases as OAuth 2.0
   [RFC6749], OpenID Connect [OIDC], and the family of protocols that
   have grown up around that ecosystem.  However, GNAP is not an
   extension of OAuth 2.0 and is not intended to be directly compatible
   with OAuth 2.0.  GNAP seeks to provide functionality and solve use
   cases that OAuth 2.0 cannot easily or cleanly address.  Even so, GNAP
   and OAuth 2.0 will exist in parallel for many deployments, and
   considerations have been taken to facilitate the mapping and
   transition from legacy systems to GNAP.  Some examples of these can
   be found in Appendix D.2.

1.1.  Roles

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




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

   Resource Client (RC, aka "client")  Requests tokens from the AS and
      uses tokens at the RS.  An instance of the RC software is
      identified by its key, which can be known to the AS prior to the
      first request.  The AS determines which policies apply to a given
      RC, including what it can request and on whose behalf.

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

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

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

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

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

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





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   A single role need not be deployed as a monolithic service.  For
   example, An RC could have components that are installed on the RQ's
   device as well as a back-end system that it communicates with.  If
   both of these components participate in the delegation protocol, they
   are both considered part of the RC.

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

   [[ Editor's note: The names for the roles are an area of ongoing
   discussion within the working group, as is the appropriate precision
   of what activities and expectations a particular role covers.  In
   particular, the AS might be formally decomposed into delegation
   components, that the client talks to, and interaction components,
   that the user talks to.  Several alternative names have been proposed
   for different roles and components, including:

   *  Grant Server (for Authorization Server)

   *  Grant Client (for Resource Client)

   *  Operator (for Requesting Party)

   ]]

1.2.  Elements

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

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

   Grant  The process by which a the RC requests and is given delegated
      access to the RS by the AS through the authority of the RO.




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

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

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

   [[ Editor's note: What other core elements need an introduction here?
   These aren't roles to be taken on by different parties, nor are they
   descriptions of the possible configurations of parties, but these are
   still important moving parts within the protocol. ]]

1.3.  Sequences

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

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

   In some circumstances, the information needed at a given stage is
   communicated out-of-band or is pre-configured between the components
   or entities performing the roles.  For example, one entity can fulfil
   multiple roles, and so explicit communication between the roles is
   not necessary within the protocol flow.














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        +------------+             +------------+
        | Requesting | ~ ~ ~ ~ ~ ~ |  Resource  |
        | Party (RQ) |             | Owner (RO) |
        +------------+             +------------+
            +                            +
            +                            +
           (A)                          (B)
            +                            +
            +                            +
        +--------+                       +       +------------+
        |Resource|--------------(1)------+------>|  Resource  |
        | Client |                       +       |   Server   |
        |  (RC)  |       +---------------+       |    (RS)    |
        |        |--(2)->| Authorization |       |            |
        |        |<-(3)--|     Server    |       |            |
        |        |       |      (AS)     |       |            |
        |        |--(4)->|               |       |            |
        |        |<-(5)--|               |       |            |
        |        |--------------(6)------------->|            |
        |        |       |               |<~(7)~~|            |
        |        |<-------------(8)------------->|            |
        |        |--(9)->|               |       |            |
        |        |<-(10)-|               |       |            |
        |        |--------------(11)------------>|            |
        |        |       |               |<~(12)~|            |
        |        |-(13)->|               |       |            |
        |        |       |               |       |            |
        +--------+       +---------------+       +------------+

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

   *  (A) The RQ interacts with the RC to indicate a need for resources
      on behalf of the RO.  This could identify the RS the RC needs to
      call, the resources needed, or the RO that is needed to approve
      the request.  Note that the RO and RQ are often the same entity in
      practice.

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

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




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

   *  (B) If interaction is required, the AS interacts with the RO
      (Section 4) to gather authorization.  The interactive component of
      the AS can function using a variety of possible mechanisms
      including web page redirects, applications, challenge/response
      protocols, or other methods.  The RO approves the request for the
      RC being operated by the RQ.  Note that the RO and RQ are often
      the same entity in practice.

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

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

   *  (6) The RC uses the access token (Section 7) to call the RS.

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

   *  (8) The RC to call the RS (Section 7) using the access token until
      the RS or RC determine that the token is no longer valid.

   *  (9) When the token no longer works, the RC fetches an updated
      access token (Section 6.1) based on the rights granted in (5).

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

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

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

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

   The following sections and Appendix C contain specific guidance on
   how to use the GNAP protocol in different situations and deployments.




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1.3.1.  Redirect-based Interaction

   In this example flow, the RC is a web application that wants access
   to resources on behalf of the current user, who acts as both the
   requesting party (RQ) and the resource owner (RO).  Since the RC is
   capable of directing the user to an arbitrary URL and receiving
   responses from the user's browser, interaction here is handled
   through front-channel redirects using the user's browser.  The RC
   uses a persistent session with the user to ensure the same user that
   is starting the interaction is the user that returns from the
   interaction.

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

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

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






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   3.  The AS determines that interaction is needed and responds
       (Section 3) with a URL to send the user to (Section 3.3.1) and
       information needed to verify the callback (Section 3.3.3) in (7).
       The AS also includes information the RC will need to continue the
       request (Section 3.1) in (8).  The AS associates this
       continuation information with an ongoing request that will be
       referenced in (4), (6), and (8).

   4.  The RC stores the verification and continuation information from
       (3) in the session from (1).  The RC then redirects the user to
       the URL (Section 4.1) given by the AS in (3).  The user's browser
       loads the interaction redirect URL.  The AS loads the pending
       request based on the incoming URL generated in (3).

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

   6.  As the RO, the user authorizes the pending request from the RC.

   7.  When the AS is done interacting with the user, the AS redirects
       the user back (Section 4.4.1) to the RC using the callback URL
       provided in (2).  The callback URL is augmented with an
       interaction reference that the AS associates with the ongoing
       request created in (2) and referenced in (4).  The callback URL
       is also augmented with a hash of the security information
       provided in (2) and (3).  The RC loads the verification
       information from (2) and (3) from the session created in (1).
       The RC calculates a hash (Section 4.4.3) based on this
       information and continues only if the hash validates.  Note that
       the RC needs to ensure that the parameters for the incoming
       request match those that it is expecting from the session created
       in (1).  The RC also needs to be prepared for the RQ never being
       returned to the RC and handle time outs appropriately.

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

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

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






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1.3.2.  User-code Interaction

   In this example flow, the RC is a device that is capable of
   presenting a short, human-readable code to the user and directing the
   user to enter that code at a known URL.  The RC is not capable of
   presenting an arbitrary URL to the user, nor is it capable of
   accepting incoming HTTP requests from the user's browser.  The RC
   polls the AS while it is waiting for the RO to authorize the request.
   The user's interaction is assumed to occur on a secondary device.  In
   this example it is assumed that the user is both the RQ and RO,
   though the user is not assumed to be interacting with the RC through
   the same web browser used for interaction at the AS.

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

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

   2.   The AS determines that interaction is needed and responds
        (Section 3) with a user code to communicate to the user
        (Section 3.3.4).  This could optionally include a URL to direct



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        the user to, but this URL should be static and so could be
        configured in the RC's documentation.  The AS also includes
        information the RC will need to continue the request
        (Section 3.1) in (8) and (10).  The AS associates this
        continuation information with an ongoing request that will be
        referenced in (4), (6), (8), and (10).

   3.   The RC stores the continuation information from (2) for use in
        (8) and (10).  The RC then communicates the code to the user
        (Section 4.1) given by the AS in (2).

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

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

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

   7.   As the RO, the user authorizes the pending request from the RC.

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

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

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

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





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

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

1.3.3.  Asynchronous Authorization

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

    +--------+                                  +--------+         +------+
    |   RC   |                                  |   AS   |         |  RO  |
    |        |--(1)--- Request Access --------->|        |         |      |
    |        |                                  |        |         |      |
    |        |<-(2)-- Not Yet Granted (Wait) ---|        |         |      |
    |        |                                  |        |<+ (3) +>|      |
    |        |                                  |        |  AuthN  |      |
    |        |--(6)--- Continue Request (A) --->|        |         |      |
    |        |                                  |        |<+ (4) +>|      |
    |        |<-(7)-- Not Yet Granted (Wait) ---|        |  AuthZ  |      |
    |        |                                  |        |         |      |
    |        |                                  |        |<+ (5) +>|      |
    |        |                                  |        |Completed|      |
    |        |                                  |        |         |      |
    |        |--(8)--- Continue Request (B) --->|        |         +------+
    |        |                                  |        |
    |        |<-(9)------ Grant Access ---------|        |
    |        |                                  |        |
    +--------+                                  +--------+

   1.  The RC requests access to the resource (Section 2).  The RC does
       not send any interactions modes to the server, indicating that it
       does not expect to interact with the RO.  The RC can also signal
       which RO it requires authorization from, if known, by using the
       user request section (Section 2.4).

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



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

   4.  The RO authorizes the pending request from the RC.

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

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

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

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

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

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

1.3.4.  Software-only Authorization

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










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

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

   2.  The AS determines that the request is been authorized, the AS
       grants access to the information in the form of access tokens
       (Section 3.2) and direct subject information (Section 3.4) to the
       RC.

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

1.3.5.  Refreshing an Expired Access Token

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

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

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





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

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

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

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

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

2.  Requesting Access

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

   resources  Describes the rights that the RC is requesting for one or
      more access tokens to be used at RS's.  Section 2.1

   subject  Describes the information about the RO that the RC is
      requesting to be returned directly in the response from the AS.
      Section 2.2

   client  Describes the RC that is making this request, including the
      key that the RC will use to protect this request and any
      continuation requests at the AS and any user-facing information
      about the RC used in interactions at the AS.  Section 2.3

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

   interact  Describes the modes that the RC has for allowing the RO to
      interact with the AS and modes for the RC to receive updates when
      interaction is complete.  Section 2.5




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   capabilities  Identifies named extension capabilities that the RC can
      use, signaling to the AS which extensions it can use.  Section 2.6

   existing_grant  Identifies a previously-existing grant that the RC is
      extending with this request.  Section 2.7

   claims  Identifies the identity claims to be returned as part of an
      OpenID Connect claims request.  Section 2.8

   Additional members of this request object can be defined by
   extensions to this protocol as described in Section 2.9

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

   {
       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "dolphin-metadata"
       ],
       "client": {
         "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
         },
         "key": {
           "proof": "jwsd",
           "jwk": {
                       "kty": "RSA",
                       "e": "AQAB",
                       "kid": "xyz-1",
                       "alg": "RS256",
                       "n": "kOB5rR4Jv0GMeL...."
           }



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         }
       },
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "capabilities": ["ext1", "ext2"],
       "subject": {
           "sub_ids": ["iss-sub", "email"],
           "assertions": ["id_token"]
       }
   }

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

2.1.  Requesting Resources

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

2.1.1.  Requesting a Single Access Token

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

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

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

   The value of this field is under the control of the AS.  This field
   MUST be compared using an exact byte match of the string value
   against known types by the AS.  The AS MUST ensure that there is no



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   collision between different authorization data types that it
   supports.  The AS MUST NOT do any collation or normalization of data
   types during comparison.  It is RECOMMENDED that designers of
   general-purpose APIs use a URI for this field to avoid collisions
   between multiple API types protected by a single AS.

   While it is expected that many APIs will have its own properties, a
   set of common properties are defined here.  Specific API
   implementations SHOULD NOT re-use these fields with different
   semantics or syntax.  The available values for these properties are
   determined by the API being protected at the RS.

   [[ Editor's note: this will align with OAuth 2 RAR, but the details
   of exactly how it aligns are TBD.  Since RAR needs to work in the
   confines of OAuth 2, RAR has to define how to interact with "scope",
   "resource", and other existing OAuth 2 mechanisms that don't exist in
   GNAP. ]].

   actions  The types of actions the RC will take at the RS as an array
      of strings.  For example, an RC asking for a combination of "read"
      and "write" access.

   locations  The location of the RS as an array of strings.  These
      strings are typically URIs identifying the location of the RS.

   datatypes  The kinds of data available to the RC at the RS's API as
      an array of strings.  For example, an RC asking for access to raw
      "image" data and "metadata" at a photograph API.

   identifier  A string identifier indicating a specific resource at the
      RS.  For example, a patient identifier for a medical API or a bank
      account number for a financial API.

   The following non-normative example shows the use of both common and
   API-specific fields as part of two different access "type" values.
















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       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           {
               "type": "financial-transaction",
               "actions": [
                   "withdraw"
               ],
               "identifier": "account-14-32-32-3",
               "currency": "USD"
           }
       ]

   If this request is approved, the resulting access token
   (Section 3.2.1) will include the sum of both of the requested types
   of access.

2.1.2.  Requesting Resources By Reference

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

   [[ Editor's note: we could describe more about how the expansion
   would work.  For example, expand into an object where the value of
   the "type" field is the value of the string.  Or we could leave it
   open and flexible, since it's really up to the AS/RS to interpret. ]]

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





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   This value is opaque to the RC and MAY be any valid JSON string, and
   therefore could include spaces, unicode characters, and properly
   escaped string sequences.  However, in some situations the value is
   intended to be seen and understood be the RC developer.  In such
   cases, the API designer choosing any such human-readable strings
   SHOULD take steps to ensure the string values are not easily confused
   by a developer

   This functionality is similar in practice to OAuth 2's "scope"
   parameter [RFC6749], where a single string represents the set of
   access rights requested by the RC.  As such, the reference string
   could contain any valid OAuth 2 scope value as in Appendix D.2.  Note
   that the reference string here is not bound to the same character
   restrictions as in OAuth 2's "scope" definition.

   A single "resources" array MAY include both object-type and string-
   type resource items.

       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "read",
           "dolphin-metadata",
           {
               "type": "financial-transaction",
               "actions": [
                   "withdraw"
               ],
               "identifier": "account-14-32-32-3",
               "currency": "USD"
           },
           "some other thing"
       ]




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   [[ Editor's note: passing resource requests by reference really is
   akin to a "scope", and we have many years of experience showing us
   that the simplicity of giving a developer a set of strings to send is
   a simple and powerful pattern.  We could always require objects and
   just use the "type" field as a scope value, but that's a lot of
   complexity to pay for the simple case.  Client developers will always
   know which kind they need to send, because they're picking from the
   API's documentation. ]]

2.1.3.  Requesting Multiple Access Tokens

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

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































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

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









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2.1.4.  Signaling Token Behavior

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

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

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

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

   split_token  The RC is capable of receiving multiple access tokens
      (Section 3.2.2) in response to any single token request
      (Section 2.1.1), or receiving a different number of tokens than
      specified in the multiple token request (Section 2.1.3).  The
      labels of the returned additional tokens are chosen by the AS.
      The client MUST be able to tell from the token response where and
      how it can use the each access tokens. [[ Editor's note: This
      functionality is controversial at best as it requires
      significantly more complexity on the client in order to solve one
      class of AS/RS deployment choices. ]]

   bind_token  The RC wants the issued access token to be bound to the
      key the RC used (Section 2.3.2) to make the request.  The
      resulting access token MUST be bound using the same "proof"
      mechanism used by the client with a "key" value of "true",
      indicating the client's presented key is to be used for binding.
      [[ Editor's note: should there be a different flag and mechanism
      for the client to explicitly indicate which binding method it
      wants to use, especially if the client wants to use a different
      method at the AS than the RS? ]]

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

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




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

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

   [[ Editor's note: while these reference values are "reserved", the
   ultimate decider for what a reference means is the AS, which means an
   AS could arguably decide that one of these values means something
   else.  Also, this kind of reservation potentially steps on API
   namespaces, which OAuth 2 is careful not to do but common extensions
   like OIDC do with their own scope definitions.  However, in OIDC,
   several "scope" values have behavior similar to what's defined here,
   particularly "openid" turns on ID tokens in the response and
   "offline_access" signals for the return of a refresh token, and these
   can be used outside of OpenID Connect itself.  However, to keep these
   flags out of the general API namespace, we could use a different
   syntax for sending them.  In particular, they could be defined under
   a GNAP-specific "type" object, where all the flags are fields on the
   object.













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   resources: [
       {
           type: "gnap-flags",
           flag1: true,
           flag2: false,
           flag3: true ...
       },
       "reference1",
       "scope2", ...
   ]

   Alternatively, all the flags could be sent in an array separate from
   the rest of the request.

   resources: [
       "reference1",
       "scope2",
       ["flag1", "flag2", "flag3"] ...
   ]

   This whole thing might also belong in an extension, as it's advanced
   behavior signaling for very specific cases.  However, it seems other
   extensions would be likely to extend this kind of thing, like OIDC
   did with "offline_access". ]]

2.2.  Requesting User Information

   If the RC is requesting information about the RO from the AS, it
   sends a "subject" field as a JSON object.  This object MAY contain
   the following fields (or additional fields defined in a registry TBD
   (Section 12)).

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

   assertions  An array of requested assertion formats.  Possible values
      include "id_token" for an [OIDC] ID Token and "saml2" for a SAML 2
      assertion.  Additional assertion values are defined by a registry
      TBD (Section 12).  [[ Editor's note: These values are lifted from
      [RFC8693]'s "token type identifiers" list, but is there a better
      source?]]

   "subject": {
      "sub_ids": [ "iss-sub", "email" ],
      "assertions": [ "id_token", "saml2" ]
   }





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

   Subject identifiers requested by the RC serve only to identify the RO
   in the context of the AS and can't be used as communication channels
   by the RC, as discussed in Section 3.4.  One method of requesting
   communication channels and other identity claims are discussed in
   Section 2.8.

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

   [[ Editor's Note: What we're really saying here is that "even if the
   AS gives you an email address to identify the user, that isn't a
   claim that this is a valid email address for that current user, so
   don't try to email them."  In order to get a workable email address,
   or anything that you can use to contact them, you'd need a full
   identity protocol and not just this.  Also, subject identifiers are
   asserted by the AS and therefore naturally scoped to the AS.  Would
   changing the name to "as_sub_ids" or "local_sub_ids" help convey that
   point? ]]

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

   [[ Editor's note: we're potentially conflating these two types in the
   same structure, so perhaps these should be split.  There's also a
   difference between user information and authentication event
   information. ]]

2.3.  Identifying the RC

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

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

   key  The public key of the RC to be used in this request as described
      in Section 2.3.2.  This field is REQUIRED.



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   class_id  An identifier string that the AS can use to identify the
      software comprising this instance of the RC.  The contents and
      format of this field are up to the AS.  This field is OPTIONAL.

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

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

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

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

   Note that the AS MAY know the RC's public key ahead of time, and the
   AS MAY apply different policies to the request depending on what has
   been registered against that key.  If the same public key is sent by
   value on subsequent access requests, the AS SHOULD treat these
   requests as coming from the same RC software instance for purposes of
   identification, authentication, and policy application.  If the AS
   does not know the RC's public key ahead of time, the AS MAY accept or
   reject the request based on AS policy, attestations within the client
   request, and other mechanisms.








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   [[ Editor's note: additional client attestation frameworks will
   eventually need to be addressed here.  For example, the organization
   the client represents, or a family of client software deployed in a
   cluster, or the posture of the device the client is installed on.
   These all need to be separable from the client's key and potentially
   the instance identifier. ]]

2.3.1.  Identifying the RC Instance

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

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

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

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

   "client": "client-541-ab"

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

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

   [[ Editor's note: It seems clear that an instance identifier is
   mutually exclusive with most of the fields in the request (eg, we
   don't want an attacker being able to swap out a client's registered
   key just by accessing the identifier).  However, some proposed
   concepts might fit alongside an instance identifier that change at
   runtime, such as device posture or another dynamic attestation.
   Should these be sent in the "client" block alongside the instance
   identifier, should there be a separate top-level block for runtime
   attestations, or some other mechanism? ]]





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

   If the RC instance is identified in this manner, the registered key
   for the RC MAY be a symmetric key known to the AS.  The RC MUST NOT
   send a symmetric key by value in the request, as doing so would
   expose the key directly instead of proving possession of it.

   [[ Editor's note: In many ways, passing an instance identifier is
   analogous to OAuth 2's "client_id" parameter [RFC6749], especially
   when coupled with a confidential client's registration and
   authentication process.  See Appendix D.2 for an example.  Something
   like this is required to make things easier for client developers in
   the common case where the AS already knows the client's key, and to
   allow symmetric keys. ]]

2.3.2.  Identifying the RC Key

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

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

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

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

   cert#256  The certificate thumbprint calculated as per OAuth-MTLS
      [RFC8705] in base64 URL encoding.

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

   [[ Editor's note: we will eventually want to have fetchable keys, I
   would guess.  Things like DID for key identification are going to be
   important. ]]

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




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

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

2.3.3.  Providing Displayable RC Information

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

   name  Display name of the RC software

   uri  User-facing web page of the RC software

   logo_uri  Display image to represent the RC software

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

   [[ Editor's note: would we want to support pushing a display logo by
   value?  On the upside it allows for more dynamic detached clients and
   doesn't require the AS to fetch information.  On the downside, this
   is harder for the AS to enforce a policy about and could lead to
   potential exploits caused by sending binary image files. ]]

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

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





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2.3.4.  Authenticating the RC

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

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

2.4.  Identifying the User

   If the RC knows the identity of the RQ through one or more
   identifiers or assertions, the RC MAY send that information to the AS
   in the "user" field.  The RC MAY pass this information by value or by
   reference.

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

   assertions  An object containing assertions as values keyed on the
      assertion type defined by a registry TBD (Section 12).  Possible
      keys include "id_token" for an [OIDC] ID Token and "saml2" for a
      SAML 2 assertion.  Additional assertion values are defined by a
      registry TBD (Section 12).  [[ Editor's note: These keys are
      lifted from [RFC8693]'s "token type identifiers" list, but is
      there a better source?  Additionally: should this be an array of
      objects with internal typing like the sub_ids?  Do we expect more
      than one assertion per user anyway? ]]





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

   Subject identifiers are hints to the AS in determining the RO and
   MUST NOT be taken as declarative statements that a particular RO is
   present at the RC and acting as the RQ.  Assertions SHOULD be
   validated by the AS. [[ editor's note: is this a MUST?  Assertion
   validation is extremely specific to the kind of assertion in place,
   what other guidance and requirements can we put in place here? ]]

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

   [[ Editor's note: we're potentially conflating identification
   (sub_ids) and provable presence (assertions and a trusted reference
   handle) in the same structure, so perhaps these should be split.  The
   security parameters are pretty different here. ]]

   If the AS trusts the RC to present verifiable assertions, the AS MAY
   decide, based on its policy, to skip interaction with the RO, even if
   the RC provides one or more interaction modes in its request.

2.4.1.  Identifying the User by Reference

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

   If the RC has a reference for the RQ at this AS, the RC MAY pass that
   reference as a string.  The format of this string is opaque to the
   RC.

   "user": "XUT2MFM1XBIKJKSDU8QM"

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

   [[ Editor's note: we might be able to fold this function into an
   unstructured user assertion reference issued by the AS to the RC.  We
   could put it in as an assertion type of "gnap_reference" or something



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   like that.  Downside: it's more verbose and potentially confusing to
   the client developer to have an assertion-like thing that's internal
   to the AS and not an assertion. ]]

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

2.5.  Interacting with the User

   Many times, the AS will require interaction with the RO in order to
   approve a requested delegation to the RC for both resources and
   direct claim information.  Many times the RQ using the RC is the same
   person as the RO, and the RC can directly drive interaction with the
   AS by redirecting the RQ on the same device, or by launching an
   application.  Other times, the RC can provide information to start
   the RO's interaction on a secondary device, or the RC will wait for
   the RO to approve the request asynchronously.  The RC could also be
   signaled that interaction has completed by the AS making callbacks.
   To facilitate all of these modes, the RC declares the means that it
   can interact using the "interact" field.

   The "interact" field is a JSON object with keys that declare
   different interaction modes.  A RC MUST NOT declare an interaction
   mode it does not support.  The RC MAY send multiple modes in the same
   request.  There is no preference order specified in this request.  An
   AS MAY respond to any, all, or none of the presented interaction
   modes (Section 3.3) in a request, depending on its capabilities and
   what is allowed to fulfill the request.  This specification defines
   the following interaction modes:

   redirect  Indicates that the RC can direct the RQ to an arbitrary URL
      at the AS for interaction.  Section 2.5.1

   app  Indicates that the RC can launch an application on the RQ's
      device for interaction.  Section 2.5.2

   callback  Indicates that the RC can receive a callback from the AS
      after interaction with the RO has concluded.  Section 2.5.3

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

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





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

   [[ Editor's note: there need to be more examples (Appendix C) that
   knit together the interaction modes into common flows, like an authz-
   code equivalent.  But it's important for the protocol design that
   these are separate pieces to allow such knitting to take place. ]]

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

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

   In this non-normative example, the RC is indicating that it can
   display a use code (Section 2.5.4) and direct the RQ to an arbitrary
   URL of maximum length (Section 2.5.1.1) 255 characters, but it cannot
   accept a callback.

       "interact": {
           "redirect": 255,
           "user_code": true
       }

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

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











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2.5.1.  Redirect to an Arbitrary URL

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

   "interact": {
      "redirect": true
   }

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

2.5.1.1.  Redirect to an Arbitrary Shortened URL

   If the RC would prefer to redirect to a shortened URL defined by the
   AS at runtime, the RC indicates this by sending the "redirect" field
   with an integer indicating the maximum character length of the
   returned URL.  The AS MAY use this value to decide whether to return
   a shortened form of the response URL.  If the AS cannot shorten its
   response URL enough to fit in the requested size, the AS SHOULD
   return an error. [[ Editor's note: Or maybe just ignore this part of
   the interaction request? ]]

   "interact": {
      "redirect": 255
   }

   If this interaction mode is supported for this RC and request, the AS
   returns a redirect interaction response with short URL Section 3.3.1.

2.5.2.  Open an Application-specific URL

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

   "interact": {
      "app": true
   }






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

   [[ Editor's note: this is similar to the "redirect" above today as
   most apps use captured URLs, but there seems to be a desire for
   splitting the web-based interaction and app-based interaction into
   different URIs.  There's also the possibility of wanting more in the
   payload than can be reasonably put into the URL, or at least having
   separate payloads. ]]

2.5.3.  Receive a Callback After Interaction

   If the RC is capable of receiving a message from the AS indicating
   that the RO has completed their interaction, the RC indicates this by
   sending the "callback" field.  The value of this field is an object
   containing the following members.

   uri  REQUIRED.  Indicates the URI to send the RO to after
      interaction.  This URI MAY be unique per request and MUST be
      hosted by or accessible by the RC.  This URI MUST NOT contain any
      fragment component.  This URI MUST be protected by HTTPS, be
      hosted on a server local to the RO's browser ("localhost"), or use
      an application-specific URI scheme.  If the RC needs any state
      information to tie to the front channel interaction response, it
      MUST use a unique callback URI to link to that ongoing state.  The
      allowable URIs and URI patterns MAY be restricted by the AS based
      on the RC's presented key information.  The callback URI SHOULD be
      presented to the RO during the interaction phase before redirect.
      [[ Editor's note: should we enforce the callback URI to be unique
      per request?  That helps with some fixation attacks, but not with
      others, and it would be problematic for an AS that wants to lock
      down each client instance to a single callback instead of a
      family/pattern of callbacks. ]]

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

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

   hash_method  OPTIONAL.  The hash calculation mechanism to be used for





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      the callback hash in Section 4.4.3.  Can be one of "sha3" or
      "sha2".  If absent, the default value is "sha3".  [[ Editor's
      note: This should be expandable via a registry of cryptographic
      options, and it would be good if we didn't define our own
      identifiers here.  See also note about cryptographic functions in
      Section 4.4.3.  ]]

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

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

   [[ Editor's note: There has been some call for a post-interaction
   redirect that is not tied to the underlying security model -
   specifically, sending the user over to a client-hosted page with
   client-specific instructions on how to continue.  This would be
   something hosted externally to the client instance, so the client
   instance would never see this incoming call.  We could accomplish
   that using this "callback" post-redirect mechanism but with "method":
   "static" or "nonce": false or some other signal to indicate that the
   client won't see the incoming request. ]]

   [[ Editor's note: The callback information could alternatively be
   combined with other methods like "redirect", essentially putting
   everything in the "callback" object into the field for the other
   objects.  However, this would require each method to define its own
   set of rules about how callbacks can be used, and we would want them
   all to be consistent with each other with clear information about how
   the AS is supposed to respond to all of these.

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





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   So if the object is there, you do the redirect on completion, if the
   object isn't there (it's a boolean, like today), you don't redirect
   when you're done.  Previous versions of this specification used this
   structure, but it was abandoned in favor of the current setup to
   allow for different combinations of user interaction methods at the
   same time while still keeping a consistent security model.  OAuth 2's
   "grant_type" model has proved to be limiting in unanticipated ways
   since it requires an entirely new grant type to be invented any time
   there is a new combination of aspects, or it requires each grant type
   to have many of the same optionalities.  Combining these fields back
   into one, in this way, would allow a client to declare that it
   expects a callback in response to one kind of interaction method but
   not others, and include multiple combinations at once.  For example,
   if a client wants to allow a user to redirect to the AS and back on
   the same device, or to use a usercode on a secondary device without a
   callback, and the client wants to offer both modes simultaneously.
   This could alternately be accomplished by allowing the client to
   "bundle" interaction parameters together, if desirable - for example,
   if "interact" were an array, the client would accept any combination
   represented by one object.  This example binds the "callback" only to
   the first "redirect" method, and second (short) "redirect" and
   "user_code" method do not use a callback.

   "interact": [
     {
       "redirect": true,
       "callback": {
          "method": "redirect",
          "uri": "https://client.example.net/return/123455",
          "nonce": "LKLTI25DK82FX4T4QFZC"
       }
     },
     {
       "redirect": 255,
       "user_code": true
     }
   ]

   It's not clear what a response to such an array would be.  Would the
   AS pick one of these bundles?  Would it be allowed to respond to any
   or all of them?  Could an AS use different URIs for each bundle?
   (This seems likely, at least.)  Would there be a security problem if
   the AS used the same URI for both bundles, since one requires a front
   channel redirect and the other does not?

   ]]





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2.5.3.1.  Receive an HTTP Callback Through the Browser

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

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

   Requests to the callback URI MUST be processed by the RC as described
   in Section 4.4.1.

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

2.5.3.2.  Receive an HTTP Direct Callback

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

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

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

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

   [[ Editor's note: This post-interaction method can be used in
   advanced use cases like asynchronous authorization, or simply to
   signal the client that it should move to the next part of the
   protocol, even when there is no user present at the client.  As such
   it can feel a little odd being inside the "interact" block of the



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   protocol, but it does align with the redirect-based "callback" method
   and it seems they really should be mutually-exclusive.  Additionally,
   should there be a method for simply pushing the updated response
   directly to the client, instead? ]]

2.5.4.  Display a Short User Code

   If the RC is capable of displaying or otherwise communicating a
   short, human-entered code to the RO, the RC indicates this by sending
   the "user_code" field with the boolean value "true".  This code is to
   be entered at a static URL that does not change at runtime, as
   described in Section 3.3.4.

   "interact": {
       "user_code": true
   }

   If this interaction mode is supported for this RC and request, the AS
   returns a user code and interaction URL as specified in Section 4.2.

2.5.5.  Indicate Desired Interaction Locales

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

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

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

2.5.6.  Extending Interaction Modes

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

   [[ Editor's note: we should have guidance in here about how to define
   other interaction modes.  There's already interest in defining
   message-based protocols like DIDCOMM and challenge-response protocols
   like FIDO, for example. ]]







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2.6.  Declaring RC Capabilities

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

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

2.7.  Referencing an Existing Grant Request

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

   "existing_grant": "80UPRY5NM33OMUKMKSKU"

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

   [[ Editor's note: this basic capability is to allow for both step-up
   authorization and downscoped authorization, but by explicitly
   creating a new request and not modifying an existing one.  What's the
   best guidance for how an AS should process this?  What are the use
   cases that help differentiate this from modification of an existing
   request? ]]

2.8.  Requesting OpenID Connect Claims

   If the RC and AS both support OpenID Connect's claims query language
   as defined in [OIDC] Section 5.5, the RC sends the value of the
   OpenID Connect "claims" authorization request parameter as a JSON
   object under the name "claims" in the root of the request.

           "claims": {
                   "id_token" : {
                       "email"          : { "essential" : true },
                       "email_verified" : { "essential" : true }
                   },
                   "userinfo" : {
                       "name"           : { "essential" : true },
                       "picture"        : null
                   }
           }





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   The contents of the "claims" parameter have the same semantics as
   they do in OpenID Connect's "claims" authorization request parameter,
   including all extensions such as [OIDC4IA].  The AS MUST process the
   claims object in the same way that it would with an OAuth 2 based
   authorization request.

   Note that because this is an independent query object, the "claims"
   value can augment or alter other portions of the request, namely the
   "resources" and "subject" fields.  This query language uses the
   fields in the top level of the object to indicate the target for any
   requested claims.  For instance, the "userinfo" target indicates that
   a returned access token would grant access to the given claims at the
   UserInfo Endpoint, while the "id_token" target indicates that the
   claims would be returned in an ID Token as described in Section 3.4.

   [[ Editor's note: in order to use the "claims" parameter as defined
   in OIDC, we have to violate the principle of orthogonality in
   Section 2.9.  An alternative approach would be to split up the
   portions of the claims request, so that "id_token" claims would go
   into the "subject" field and "userinfo" claims would go into the
   "resources" request, but this violates the original field definition
   from OIDC and gets into the territory of defining an identity schema
   request.  This approach would also invalidate extensions to the
   "claims" standard as each "target" would need to have its own
   separate mapping to some part of the GNAP protocol. ]]

   [[ Editor's note: I'm not a fan of GNAP defining how OIDC would work
   at all and would rather that work be done by the OIDF in an
   extension.  However, I think it is important for discussion to see
   this kind of thing in context with the rest of the protocol, for now.
   In the future, I would anticipate this would be defined by the OIDF
   as a relatively small but robust identity layer on top of GNAP. ]]

2.9.  Extending The Grant Request

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

   [[ Editor's note: we should have more guidance and examples on what
   possible top-level extensions would look like. ]]








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3.  Grant Response

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

   continue  Indicates that the RC can continue the request by making an
      additional request using these parameters.  Section 3.1

   access_token  A single access token that the RC can use to call the
      RS on behalf of the RO.  Section 3.2.1

   multiple_access_token  Multiple named access tokens that the RC can
      use to call the RS on behalf of the RO.  Section 3.2.2

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

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

   instance_id  An identifier this RC instance can use to identify
      itself when making future requests.  Section 3.5

   user_handle  An identifier this RC instance can use to identify its
      current RQ when making future requests.  Section 3.5

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

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

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




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

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

3.1.  Request Continuation

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

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

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

   access_token  RECOMMENDED.  A unique access token for continuing the
      request, in the format specified in Section 3.2.1.  This access
      token MUST be bound to the RC's key used in the request and MUST
      NOT be a "bearer" token.  This access token MUST NOT be usable at
      resources outside of the AS.  [[ Editor's note: Is this a
      restriction we want to enforce? ]] If the AS includes an access
      token, the RC MUST present the access token in all requests to the
      continuation URI as described in Section 7.










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

   The RC can use the values of this field to continue the request as
   described in Section 5.  Note that the RC MUST sign all continuation
   requests with its key as described in Section 8.  If the AS includes
   an "access_token", the RC MUST present the access token in its
   continuation request.

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

   [[ Editor's note: The AS can use the optional "access_token" as a
   credential for the client to manage the grant request itself over
   time.  This is in parallel with access token management as well as RS
   access in general.  If the AS uses the access token, the continuation
   URL can be static, and potentially even the same as the initial
   request URL.  If the AS does not use an access token here, it needs
   to use unique URLs in its response and bind the client's key to
   requests to those URLs - or potentially only allow one request per
   client at a time.  The optionality adds a layer of complexity, but
   the client behavior is deterministic in all possible cases and it re-
   uses existing functions and structures instead of inventing something
   special just to talk to the AS.  The optional access token represents
   a design compromise, but the working group can decide to either
   require the access token on all requests or to remove the access
   token functionality and require the security of the continuation
   requests be based on unique URLs. ]]

3.2.  Access Tokens

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

   [[ Editor's note: I really don't like the dichotomy between
   "access_token" and "multiple_access_tokens" and their being mutually
   exclusive, and I think we should design away from this pattern toward
   something less error-prone. ]]



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3.2.1.  Single Access Token

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

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

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

   resources  RECOMMENDED.  A description of the rights associated with
      this access token, as defined in Section 3.2.1.  If included, this
      MUST reflect the rights associated with the issued access token.
      These rights MAY vary from what was requested by the RC.

   expires_in  OPTIONAL.  The number of seconds in which the access will
      expire.  The RC MUST NOT use the access token past this time.  An
      RS MUST NOT accept an access token past this time.  Note that the
      access token MAY be revoked by the AS or RS at any point prior to
      its expiration.

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

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










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    "access_token": {
        "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
        "key": false,
        "manage": "https://server.example.com/token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L",
        "resources": [
            {
                "type": "photo-api",
                "actions": [
                    "read",
                    "write",
                    "dolphin"
                ],
                "locations": [
                    "https://server.example.net/",
                    "https://resource.local/other"
                ],
                "datatypes": [
                    "metadata",
                    "images"
                ]
            },
            "read", "dolphin-metadata"
        ]
    }

   The following non-normative example shows a single access token bound
   to the RC's key, which was presented using the detached JWS
   (Section 8.1) binding method.

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

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

   [[ Editor's note: There has been interest in describing a way for the
   AS to tell the client both how and where to use the token.  This kind
   of directed access token could allow for some interesting deployment
   patterns where the client doesn't know much]]






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3.2.2.  Multiple Access Tokens

   If the RC has requested multiple access tokens and the AS has granted
   at least one of them, the AS responds with the
   "multiple_access_tokens" field.  The value of this field is a JSON
   object, and the property names correspond to the token identifiers
   chosen by the RC in the multiple access token request
   (Section 2.1.3).  The values of the properties of this object are
   access tokens as described in Section 3.2.1.

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

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

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

   The multiple access token response MUST be used when multiple access
   tokens are requested, even if only one access token is issued as a
   result of the request.  The AS MAY refuse to issue one or more of the
   requested access tokens, for any reason.  In such cases the refused
   token is omitted from the response and all of the other issued access
   tokens are included in the response the requested names appropriate
   names.

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

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

   [[ Editor's note: Do we need to specify that the management URIs are
   different if we require the token to be presented? ]]




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3.3.  Interaction Modes

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

   redirect  Redirect to an arbitrary URL.  Section 3.3.1

   app  Launch of an application URL.  Section 3.3.2

   callback  Callback to an RC URL after interaction is completed.
      Section 3.3.3

   user_code  Display a short user code.  Section 3.3.4

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

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

3.3.1.  Redirection to an arbitrary URL

   If the RC indicates that it can redirect to an arbitrary URL
   (Section 2.5.1) and the AS supports this mode for the RC's request,
   the AS responds with the "redirect" field, which is a string
   containing the URL to direct the RQ to.  This URL MUST be unique for
   the request and MUST NOT contain any security-sensitive information.

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

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






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   [[ Editor's note: This is one aspect where the AS might actually be
   two separate roles.  Namely, a delegation server (back end) and
   interaction server (user-facing).]]

   The RC sends the RQ to the URL to interact with the AS.  The RC MUST
   NOT alter the URL in any way.  The means for the RC to send the RQ to
   this URL is out of scope of this specification, but common methods
   include an HTTP redirect, launching the system browser, displaying a
   scannable code, or printing out the URL in an interactive console.

3.3.2.  Launch of an application URL

   If the RC indicates that it can launch an application URL
   (Section 2.5.2) and the AS supports this mode for the RC's request,
   the AS responds with the "app" field, which is a string containing
   the URL to direct the RQ to.  This URL MUST be unique for the request
   and MUST NOT contain any security-sensitive information.

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

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

   [[ Editor's note: This will probably need to be expanded to an object
   to account for other parameters needed in app2app use cases, like
   addresses for distributed storage systems, server keys, and the like.
   Details TBD as people build this out. ]]

3.3.3.  Post-interaction Callback to an RC URL

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

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

   [[ Editor's note: This is fairly parallel to the request but it kinda
   hides the fact that this is a nonce from the AS, not the client. ]]




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

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

3.3.4.  Display of a Short User Code

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

   code  REQUIRED.  A unique short code that the user can type into an
      authorization server.  This string MUST be case-insensitive, MUST
      consist of only easily typeable characters (such as letters or
      numbers).  The time in which this code will be accepted SHOULD be
      short lived, such as several minutes.  It is RECOMMENDED that this
      code be no more than eight characters in length.

   url  RECOMMENDED.  The interaction URL that the RC will direct the RO
      to.  This URL MUST be stable at the AS such that RCs can be
      statically configured with it.

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

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

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



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   could be statically configured with it, perhaps referring the RQ to
   the URL via documentation instead of through an interactive means.
   If the RC is capable of communicating an arbitrary URL to the RQ,
   such as through a scannable code, the RC can use the "redirect"
   (Section 2.5.1) mode for this purpose instead of or in addition to
   the user code mode.

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

   [[ Editor's note: This is one aspect where the AS might actually be
   two separate roles.  Namely, a delegation server (back end) and
   interaction server (user-facing).]]

3.3.5.  Extending Interaction Mode Responses

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

3.4.  Returning User Information

   If information about the RO is requested and the AS grants the RC
   access to that data, the AS returns the approved information in the
   "subject" response field.  This field is an object with the following
   OPTIONAL properties.

   sub_ids  An array of subject identifiers for the RO, as defined by
      [I-D.ietf-secevent-subject-identifiers]. [[ Editor's note: privacy
      considerations are needed around returning identifiers. ]]

   assertions  An object containing assertions as values keyed on the
      assertion type defined by a registry TBD (Section 12).  [[
      Editor's note: should this be an array of objects with internal
      typing like the sub_ids?  Do we expect more than one assertion per
      user anyway? ]]

   updated_at  Timestamp in integer seconds indicating when the
      identified account was last updated.  The RC MAY use this value to
      determine if it needs to request updated profile information
      through an identity API.  The definition of such an identity API
      is out of scope for this specification.







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

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

   Subject identifiers returned by the AS SHOULD uniquely identify the
   RO at the AS.  Some forms of subject identifier are opaque to the RC
   (such as the subject of an issuer and subject pair), while others
   forms (such as email address and phone number) are intended to allow
   the RC to correlate the identifier with other account information at
   the RC.  The RC MUST NOT request or use any returned subject
   identifiers for communication purposes (see Section 2.2).  That is, a
   subject identifier returned in the format of an email address or a
   phone number only identifies the RO to the AS and does not indicate
   that the AS has validated that the represented email address or phone
   number in the identifier is suitable for communication with the
   current user.  To get such information, the RC MUST use an identity
   protocol to request and receive additional identity claims.  While
   Section 2.8 specifies one such method, other identity protocols could
   also be used on top of GNAP to convey this information and the
   details of an identity protocol and associated schema are outside the
   scope of this specification.

   [[ Editor's note: subject identifiers here are naturally scoped to
   the AS; even though using an external identifier like an email
   address or phone number implies a global namespace in use, the
   association of that identifier to the current user is still under the
   view of the AS.  Would changing the name to "as_sub_ids" or
   "local_sub_ids" help convey that point?  Would it also be desirable
   to have an identifier that's globally unique by design?  The
   "iss_sub" type almost gets us there by explicitly calling out the
   issuer URL, but tuples are hard to deal with in practice and so tend
   to get ignored in practice in the OIDC space. ]]








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   [[ Editor's note: This will need substantial privacy considerations,
   as this is releasing information about the current user that could be
   tied to other information at the RC or elsewhere.  To facilitate
   this, should we have another form of identifier that's a globally
   unique identifier of some form?  DIDs could facilitate that kind of
   namespace. ]]

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

3.5.  Returning Dynamically-bound Reference Handles

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

   Some references, such as for the RC instance's identity
   (Section 2.3.1) or the requested resources (Section 2.1.2), can be
   managed statically through an admin console or developer portal
   provided by the AS or RS.  The developer of the RC can include these
   values in their code for a more efficient and compact request.

   If desired, the AS MAY also generate and return some of these
   references dynamically to the RC in its response to facilitate
   multiple interactions with the same software.  The RC SHOULD use
   these references in future requests in lieu of sending the associated
   data value.  These handles are intended to be used on future
   requests.

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

   [[ Editor's note: these constructs used to be objects to allow for
   expansion to future fields, like a management URI or different
   presentation types or expiration, but those weren't used in practice.
   Is that desirable anymore or is collapsing them like this the right
   direction? ]]

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

   instance_id  A string value used to represent the information in the
      "client" object that the RC can use in a future request, as
      described in Section 2.3.1.



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

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

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

   [[ Editor's note: the ability to dynamically return reference handles
   allows for an inline version of dynamic registration without needing
   to go through a discrete registration step, for clients where that
   makes sense.  Currently this is entirely up to the AS to decide when
   to issue these, but maybe the client should signal that it can
   receive these handles as part of the request?  The new "token flags"
   construct in Section 2.1.4 almost gets at that, but for a different
   part of the request structure.  Since the client is the component
   that will know if it's in a position to make use of such reference
   handles in the future (like a mobile app) or if it's just going to
   evaporate at the end of a session (like an SPA).  Ultimately we need
   to deal with a range of dynamism, not just the "pre-registered" vs.
   "non-registered" use cases that OAuth forces us in to. ]]

   [[ Editor's note: The client-bound "instance_id" could serve as the
   hook we would need for RFC7592 style dynamic client management,
   including additional components like key rotation.  If the AS returns
   an object instead of a string here, that could include everything
   that the client would need in order to make REST-style management
   calls, similar to token management.

{
    "client": {
        "instance_id": "7C7C4AZ9KHRS6X63AJAO",
        "manage": "https://example.server.com/client/7C7C4AZ9KHRS6X63AJAO",
        "access_token": {
            "value": "4TB8N6BW7OZB8CDFONP219RP1LT0OS9M2PMHKUR6",
            "key": true
        }
    }
}





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   The client would sign all requests with its key and use the presented
   access token.  A "POST" or "PATCH" request would update client
   information, including having a method for key rotation using nested
   signatures.  A "DELETE" request would un-register the client, etc. ]]

3.6.  Error Response

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

   error  The error code.

   {

     "error": "user_denied"

   }

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

   user_denied  The RO denied the request.

   too_fast  The RC did not respect the timeout in the wait response.

   unknown_request  The request referenced an unknown ongoing access
      request.

   [[ Editor's note: I think we will need a more robust error mechanism,
   and we need to be more clear about what error states are allowed in
   what circumstances.  Additionally, is the "error" parameter exclusive
   with others in the return? ]]

3.7.  Extending the Response

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

   [[ Editor's note: what guidance should we give to designers on this?
   ]]

4.  Interaction at the AS

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



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

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

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

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

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

   [[ Editor's note: there are some privacy and security considerations
   here but for the most part we don't want to be overly prescriptive
   about the UX, I think. ]]

4.1.  Interaction at a Redirected URI

   When the RO is directed to the AS through the "redirect"
   (Section 3.3.1) mode, the AS can interact with the RO through their
   web browser to authenticate the user as an RO and gather their
   consent.  Note that since the RC does not add any parameters to the
   URL, the AS MUST determine the grant request being referenced from
   the URL value itself.  If the URL cannot be associated with a
   currently active request, the AS MUST display an error to the RO and
   MUST NOT attempt to redirect the RO back to any RC even if a callback
   is supplied (Section 2.5.3).

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

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

4.2.  Interaction at the User Code URI

   When the RO is directed to the AS through the "user_code"
   (Section 3.3.4) mode, the AS can interact with the RO through their
   web browser to collect the user code, authenticate the user as an RO,
   and gather their consent.  Note that since the URL itself is static,
   the AS MUST determine the grant request being referenced from the
   user code value itself.  If the user code cannot be associated with a
   currently active request, the AS MUST display an error to the RO and



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   MUST NOT attempt to redirect the RO back to any RC even if a callback
   is supplied (Section 2.5.3).

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

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

4.3.  Interaction through an Application URI

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

   [[ Editor's note: Should we have anything to say about an app sending
   information to a back-end to get details on the pending request? ]]

4.4.  Post-Interaction Completion

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















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   [[ Editor's note: there might be some other kind of push-based
   notification or callback that the client can use, or an out-of-band
   non-HTTP protocol.  The AS would know about this if supported and
   used, but the guidance here should be written in such a way as to not
   be too restrictive in the next steps that it can take.  Still, it's
   important that the AS not expect or even allow clients to poll if the
   client has stated it can take a callback of some form, otherwise that
   sets up a potential session fixation attack vector that the client is
   trying to and able to avoid.  There has also been a call for post-
   interaction that doesn't tie into the security of the protocol, like
   redirecting to a static webpage hosted by the client's company.
   Would this fit here? ]]

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

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

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

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

   When using the "callback" interaction mode (Section 3.3.3) with the
   "redirect" method, the AS signals to the RC that interaction is
   complete and the request can be continued by directing the RO (in
   their browser) back to the RC's callback URL sent in the callback
   request (Section 2.5.3.1).

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

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

   interact_ref  REQUIRED.  The interaction reference generated for this
      interaction.

   The means of directing the RO to this URL are outside the scope of
   this specification, but common options include redirecting the RO
   from a web page and launching the system browser with the target URL.




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

   When receiving the request, the RC MUST parse the query parameters to
   calculate and validate the hash value as described in Section 4.4.3.
   If the hash validates, the RC sends a continuation request to the AS
   as described in Section 5.1 using the interaction reference value
   received here.

4.4.2.  Completing Interaction with a Direct HTTP Request Callback

   When using the "callback" interaction mode (Section 3.3.3) with the
   "push" method, the AS signals to the RC that interaction is complete
   and the request can be continued by sending an HTTP POST request to
   the RC's callback URL sent in the callback request (Section 2.5.3.2).

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

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

   interact_ref  REQUIRED.  The interaction reference generated for this
      interaction.

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

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

   When receiving the request, the RC MUST parse the JSON object and
   validate the hash value as described in Section 4.4.3.  If the hash
   validates, the RC sends a continuation request to the AS as described
   in Section 5.1 using the interaction reference value received here.

4.4.3.  Calculating the interaction hash

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



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   [[ Editor's note: If the client uses a unique callback URL per
   request, that prevents some of the same attacks, but without the same
   cryptographic binding between the interaction and delegation
   channels.  A unique URI would allow the client to differentiate
   inputs, but it would not prevent an attacker from injecting an
   unrelated interaction reference into this channel. ]]

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

   VJLO6A4CAYLBXHTR0KRO
   MBDOFXG4Y5CVJCX821LH
   4IFWWIKYBC2PQ6U56NL1

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

   [[ Editor's note: these hash algorithms should be pluggable, and
   ideally we shouldn't redefine yet another crypto registry for this
   purpose, but I'm not convinced an appropriate one already exists.
   Furthermore, we should be following best practices here whether it's
   a plain hash, a keyed MAC, an HMAC, or some other form of
   cryptographic function.  I'm not sure what the defaults and options
   ought to be, but SHA512 and SHA3 were picked based on what was
   available to early developers. ]]

4.4.3.1.  SHA3-512

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

p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A

4.4.3.2.  SHA2-512

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




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62SbcD3Xs7L40rjgALA-ymQujoh2LB2hPJyX9vlcr1H6ecChZ8BNKkG_HrOKP_Bpj84rh4mC9aE9x7HPBFcIHw

5.  Continuing a Grant Request

   While it is possible for the AS to return a Section 3 with all the
   RC's requested information (including access tokens (Section 3.2) and
   direct user information (Section 3.4)), it's more common that the AS
   and the RC will need to communicate several times over the lifetime
   of an access grant.  This is often part of facilitating interaction
   (Section 4), but it could also be used to allow the AS and RC to
   continue negotiating the parameters of the original grant request
   (Section 2).

   To enable this ongoing negotiation, the AS returns a "continue" field
   in the response (Section 3.1) that contains information the RC needs
   to continue this process with another request, including a URI to
   access as well as an optional access token to use during the
   continued requests.

   When the RC makes any calls to the continuation URL, the RC MUST
   present proof of the most recent key associated with this ongoing
   request by signing the request as described in Section 8.  The key in
   use will be either the key from the initial request (Section 2.3.2)
   or its most recent rotation. [[ Editor's note: we need to have a
   secure way to rotate the key used for the continuation here.  In most
   cases this will be a rotation for the client instance, since a client
   without an instance record would likely just present a new key for a
   new request.  In that case it could go with the client management,
   above - but it doesn't necessarily have to be. ]]

   For example, here the RC makes a POST request and signs with detached
   JWS:

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

   If the AS includes an "access_token" in the "continue" response in
   Section 3.1, the RC MUST include the access token the request as
   described in Section 7.  Note that the access token is always bound
   to the RC's presented key (or its most recent rotation).

   For example, here the RC makes a POST request with the interaction
   reference, includes the access token, and signs with detached JWS:







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

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   The AS MUST be able to tell from the RC's request which specific
   ongoing request is being accessed.  Common methods for doing so
   include using a unique, unguessable URL for each continuation
   response, associating the request with the provided access token, or
   allowing only a single ongoing grant request for a given RC instance
   at a time.  If the AS cannot determine a single active grant request
   to map the continuation request to, the AS MUST return an error.

   The ability to continue an already-started request allows the RC to
   perform several important functions, including presenting additional
   information from interaction, modifying the initial request, and
   getting the current state of the request.

   If a "wait" parameter was included in the continuation response
   (Section 3.1), the RC MUST NOT call the continuation URI prior to
   waiting the number of seconds indicated.  If no "wait" period is
   indicated, the RC SHOULD wait at least 5 seconds [[ Editor's note:
   what's a reasonable amount of time so as not to DOS the server?? ]].
   If the RC does not respect the given wait period, the AS MUST return
   an error.

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

   If the AS determines that the RC can make a further continuation
   request, the AS MUST include a new "continue" response (Section 3.1).
   If the continuation was previously bound to an access token, the new
   "continue" response MUST include a bound access token as well, and
   this token SHOULD be a new access token. [[ Editor's note: this used
   to be a MUST, but is it safe to back off that requirement? ]] If the
   AS does not return a new "continue" response, the RC MUST NOT make an
   additional continuation request.  If a RC does so, the AS MUST return
   an error.

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




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5.1.  Continuing After a Completed Interaction

   When the AS responds to the RC's "callback" parameter as in
   Section 4.4.1, this response includes an interaction reference.  The
   RC MUST include that value as the field "interact_ref" in a POST
   request to the continuation URI.

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

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

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

   The Section 3 MAY contain any newly-created access tokens
   (Section 3.2) or newly-released subject claims (Section 3.4).  The
   response MAY contain a new "continue" response (Section 3.1) as
   described above.  The response SHOULD NOT contain any interaction
   responses (Section 3.3). [[ Editor's note: This last one might be
   overly restrictive, since some kinds of interaction could require
   multiple round trips.  We need more examples and experience beyond
   redirect-based interaction here. ]]

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
















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

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

   [[ Editor's note: other interaction methods, such as a challenge-
   response cryptographic protocol, would use a similar construct as
   here, but have different rules.  Would it be reasonable to allow them
   to be combined?  Could this be combined further with the "update"
   method in Section 5.3? ]]

5.2.  Continuing During Pending Interaction

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

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

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








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

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

   [[ Editor's note: Do we want to be more precise about what's expected
   inside the "continue" object?  I think that at least the URI is
   required, access token required IF used, etc.  This is even if they
   haven't changed since last time, and the client will use whatever
   value comes back. ]]

   [[ Editor's note: extensions to this might need to communicate to the
   client what the current state of the user interaction is.  This has
   been done in similar proprietary protocols, but the details of that
   information tend to be highly application specific.  Like "user
   hasn't logged in yet", "user has logged in but is still sitting at
   the page", or "user seems to have wandered off".  We might be able to
   provide a decent framework for hanging this kind of stuff on. ]]

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

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



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5.3.  Modifying an Existing Request

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

   The RC MAY include the "resources" and "subject" fields as described
   in Section 2.1 and Section 2.2.  Inclusion of these fields override
   any values in the initial request, which MAY trigger additional
   requirements and policies by the AS.  For example, if the RC is
   asking for more access, the AS could require additional interaction
   with the RO to gather additional consent.  If the RC is asking for
   more limited access, the AS could determine that sufficient
   authorization has been granted to the RC and return the more limited
   access rights immediately.  [[ Editor's note: We could state
   something like "resources and subject MUST NOT be the same as in the
   initial or previous request" to enforce that this really is a change,
   but is there value in calling that out here?  Somehow we do probably
   want to tell the AS to not let a client simply post the same request
   here to rotate access tokens now that we've got an explicit function
   for that, right? ]]

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

   The RC MAY include the "user" field as described in Section 2.4 to
   present new assertions or information about the RQ. [[ Editor's note:
   This would allow the client to do things like gather the user's
   identifiers post-request, or gather an assertion from an on-device
   element that the AS can verify.  It opens up potential avenues for
   trouble if the user here is different from the RO that's already
   showed up at the AS or race conditions if the RQ's identity changes
   mid-stream.  But that said, this seems important for multi-log-in
   cases and the like, probably. ]]

   The RC MUST NOT include the "client" section of the request. [[
   Editor's note: We do not want the client to be able to get swapped
   out from underneath the user, especially post-consent.  However,
   including this field in a PATCH update request might be the place to
   define key rotation for the grant request itself, but we'd need to be
   very careful of how that works.  And it feels like it might have



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   consequences outside of the request, such as rotating the key for all
   ongoing grants for a given client instance, which isn't really
   desirable here.  We need a lot more discussion and engineering on
   this before including it. ]]

   The RC MAY include post-interaction responses such as described in
   Section 5.1. [[ Editor's note: it seems a little odd to include this
   in a request but I can't see a reason to not allow it. ]]

   Modification requests MUST NOT alter previously-issued access tokens.
   Instead, any access tokens issued from a continuation are considered
   new, separate access tokens.  The AS MAY revoke existing access
   tokens after a modification has occurred. [[ Editor's note: this
   might be subject to the "multi_token" flag, but since we're creating
   a NEW access token and not rotating an existing one, this seems to be
   a different use case. ]]

   Modification requests MAY result in previously-issued access tokens
   being revoked. [[ Editor's note: there is a solid argument to be made
   for always revoking old access tokens here, but we need more
   discussion on the boundaries for such a requirement.  If they stick
   around, it does make a "read" request weird because now we've got
   multiple access tokens sticking around associated with a grant
   request and no good place to put them. ]]

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

   For example, an RC initially requests a set of resources using
   references:

















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

   {
       "resources": [
           "read", "write"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

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

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": ...
   }

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












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

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

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

   {
       "continue": {
           "access_token": {
               "value": "M33OMUK80UPRY5NMKSKU",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": ...
   }

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















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

   {
       "resources": [
           "read"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

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

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
               "key": true
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": ...
   }

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






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   [[ Editor's note: the net result of this is that interaction requests
   are really only meant to be responded to exactly once by the AS.
   This isn't spelled out explicitly, but could be included in
   Section 2.5 and/or Section 3.3. ]]

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

   {
       "resources": [
           "read", "write"
       ],
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return/654321",
               "nonce": "K82FX4T4LKLTI25DQFZC"
           }
       }
   }

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

5.4.  Getting the Current State of a Grant Request

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

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






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

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

   [[ Editor's note: I'm a little dubious about the need for this
   particular function in reality, but including it for completeness
   sake.  There are a lot of questions we need to answer, such as
   whether it's safe to include access tokens and claims in the response
   of this kind of "read" at all, and whether it makes sense to include
   items like interaction nonces in the response.  This discussion
   should be driven by the use cases calling for this "read"
   functionality.  There have been similar functions within proprietary
   protocols where the client calls an endpoint at the AS to figure out
   where the user is in the interaction process at the AS, letting the
   client provide a smarter UI.  It doesn't seem like we could do that
   in depth here since it would be highly application specific, but that
   might be a good example of how to extend a response and give a client
   extra information. ]]

5.5.  Canceling a Grant Request

   If the RC wishes to cancel an ongoing grant request, it makes an HTTP
   DELETE request to the continuation URI.

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

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

6.  Token Management

   If an access token response includes the "manage" parameter as
   described in Section 3.2.1, the RC MAY call this URL to manage the
   access token with any of the actions defined in the following
   sections.  Other actions are undefined by this specification.



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

   If the token is sender-constrained (i.e., not a bearer token), it
   MUST be sent with the appropriate binding for the access token
   (Section 7).

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

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

   [[ Editor's note: Should we allow for "update" to an access token by
   the client posting new information from a "request"?  It seems this
   might make things weird since an access token is generally considered
   an unchanging thing, and the client could always request a new access
   token if they're allowed to continue the grant request post-issuance
   as in Section 5.3.  There's also a possibility of being able to
   "read" a token's state with a GET, much like token introspection but
   using the token's/client's key instead of the RS key.  But would a
   client need to "read" a token state after issuance?  Is there a
   security risk to offering that functionality?  Introspection is
   nearly always relegated to RS calls in practice since the client is
   focused on using the token at the RS.  The client can always read the
   state of the grant itself, separately. ]]

6.1.  Rotating the Access Token

   The RC makes an HTTP POST to the token management URI, sending the
   access token in the appropriate header and signing the request with
   the appropriate key.

   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Detached-JWS: eyj0....

   [[ Editor's note: This could alternatively be an HTTP PUT verb, since
   we are telling the AS that we want to replace the token.  However, we
   are not providing the information we want to replace the token with,
   and in fact that's up to the AS entirely, not the client.  For that
   reason, I think a POST still makes the most sense. ]]





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

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

   If the token is validated and the key is appropriate for the request,
   the AS MUST invalidate the current access token associated with this
   URL, if possible, and return a new access token response as described
   in Section 3.2.1, unless the "multi_token" flag is specified in the
   request. [[ Editor's note: We could also use different verbs to
   signal whether or not the old token should be kept around or not,
   instead of using a token flag to do this. ]] The value of the access
   token MUST NOT be the same as the current value of the access token
   used to access the management API.  The response MAY include an
   updated access token management URL as well, and if so, the RC MUST
   use this new URL to manage the new access token.

   [[ Editor's note: the net result is that the client's always going to
   use the management URL that comes back.  But should we let the server
   omit it from the response if it doesn't change?  That seems like an
   odd optimization that doesn't help the client. ]]























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

   [[ Editor's note: If the client is using its own key as the proof,
   like with a bearer access token, the AS is going to need to know if
   the client's key has been rotated.  We don't have a mechanism for
   rotating the token's key or the client's key yet either - so that
   could occur through this management function as well. ]]

6.2.  Revoking the Access Token

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

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







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   DELETE /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Detached-JWS: eyj0....

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

   204 No Content

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

7.  Using Access Tokens

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

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

   Authorization: Bearer OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0

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

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

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

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






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   [[ Editor's note: I don't actually like the idea of using only one
   header type for differently-bound access tokens.  Perhaps instead
   these values should somehow reflect the key binding types.  Maybe
   there can be multiple fields after the "GNAP" keyword using
   structured headers?  Or a set of derived headers like GNAP-mtls?
   This might also be better as a separate specification, like it was in
   OAuth 2.  However, access tokens should be able to use any key
   binding mechanisms here, plus bearer. ]]

8.  Binding Keys

   Any keys presented by the RC to the AS or RS MUST be validated as
   part of the request in which they are presented.  The type of binding
   used is indicated by the proof parameter of the key section in the
   initial request Section 2.3.2.  Values defined by this specification
   are as follows:

   jwsd  A detached JWS signature header

   jws  Attached JWS payload

   mtls  Mutual TLS certificate verification

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

   httpsig  HTTP Signing signature header

   oauthpop  OAuth PoP key proof authentication header

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

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

   When used in the GNAP delegation protocol, these key binding
   mechanisms allow the AS to ensure that the keys presented by the RC
   in the initial request are in control of the party calling any
   follow-up or continuation requests.  To facilitate this requirement,
   all keys in the initial request Section 2.3.2 MUST be proved in all
   continuation requests Section 5 and token management requests



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   Section 6, modulo any rotations on those keys over time that the AS
   knows about.  The AS MUST validate all keys presented by the RC
   (Section 2.3.2) or referenced in an ongoing request for each call
   within that request.

   [[ Editor's note: We are going to need a way for a client to rotate
   its keys securely, even while an ongoing grant is in effect. ]]

   When used to bind to an access token, the

8.1.  Detached JWS

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

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

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

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

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

   ts  A timestamp of the request in integer seconds

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

   [[ Editor's note: It's not the usual practice to put additional
   information into the header of a JWS, but this keeps us from having
   to normalize the body serialization.  Alternatively, we could add all
   these fields to the body of the request, but then it gets awkward for
   non-body requests like GET/DELETE. ]]





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

   The RC presents the signature in the Detached-JWS HTTP Header field.
   [[ Editor's Note: this is a custom header field, do we need this?  It
   seems like the best place to put this. ]]













































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

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






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   If the request being made does not have a message body, such as an
   HTTP GET, OPTIONS, or DELETE method, the JWS signature is calculated
   over an empty payload.

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

   [[ Editor's note: this is a potentially fragile signature mechanism.
   It doesn't protect arbitrary headers or other specific aspects of the
   request, but it's simple to calculate and useful for body-driven
   requests, like the client to the AS.  Additionally it is potentially
   fragile since a multi-tier system could parse the payload and pass
   the parsed payload downstream with potential transformations, making
   downstream signature validation impossible.  We might want to remove
   this in favor of general-purpose HTTP signing, or at least provide
   guidance on its use. ]]

8.2.  Attached JWS

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

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

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

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

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

   ts  A timestamp of the request in integer seconds

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



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   [[ Editor's note: It's not the usual practice to put additional
   information into the header of a JWS, but this keeps us from having
   to modify the body to use this signature method.  Alternatively, we
   could add all these fields to the body of the request, but then it
   gets awkward for non-body requests like GET/DELETE. ]]

   The payload of the JWS object is the JSON serialized body of the
   request, and the object is signed according to JWS and serialized
   into compact form [RFC7515].

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

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

   eyJiNjQiOmZhbHNlLCJhbGciOiJSUzI1NiIsImtpZCI6Inh5ei0xIn0.ewogICAgIm
   NsaWVudCI6IHsKICAgICAgICAibmFtZSI6ICJNeSBDbGllbnQgRGlzcGxheSBOYW1l
   IiwKICAgICAgICAidXJpIjogImh0dHBzOi8vZXhhbXBsZS5uZXQvY2xpZW50IgogIC
   AgfSwKICAgICJyZXNvdXJjZXMiOiBbCiAgICAgICAgImRvbHBoaW4tbWV0YWRhdGEi
   CiAgICBdLAogICAgImludGVyYWN0IjogewogICAgICAgICJyZWRpcmVjdCI6IHRydW
   UsCiAgICAgICAgImNhbGxiYWNrIjogewogICAgCQkidXJpIjogImh0dHBzOi8vY2xp
   ZW50LmZvbyIsCiAgICAJCSJub25jZSI6ICJWSkxPNkE0Q0FZTEJYSFRSMEtSTyIKIC
   AgIAl9CiAgICB9LAogICAgImtleXMiOiB7CgkJInByb29mIjogImp3c2QiLAogICAg
   ICAgICJqd2tzIjogewogICAgICAgICAgICAia2V5cyI6IFsKICAgICAgICAgICAgIC
   AgIHsKICAgICAgICAgICAgICAgICAgICAia3R5IjogIlJTQSIsCiAgICAgICAgICAg
   ICAgICAgICAgImUiOiAiQVFBQiIsCiAgICAgICAgICAgICAgICAgICAgImtpZCI6IC
   J4eXotMSIsCiAgICAgICAgICAgICAgICAgICAgImFsZyI6ICJSUzI1NiIsCiAgICAg
   ICAgICAgICAgICAgICAgIm4iOiAia09CNXJSNEp2MEdNZUxhWTZfSXRfcjNPUndkZj
   hjaV9KdGZmWHlhU3g4eFlKQ0NOYU9LTkpuX096MFloZEhiWFRlV081QW95c3BEV0pi
   TjV3XzdiZFdEeGdwRC15NmpuRDF1OVloQk9DV09iTlBGdnBrVE04TEM3U2RYR1JLeD
   JrOE1lMnJfR3NzWWx5UnBxdnBCbFk1LWVqQ3l3S1JCZmN0UmNuaFRUR056dGJiREJV
   eURTV21GTVZDSGU1bVhUNGNMMEJ3clpDNlMtdXUtTEF4MDZhS3dRT1B3WU9HT3NsSz
   hXUG0xeUdka2FBMXVGX0ZwUzZMUzYzV1lQSGlfQXAyQjdfOFdidzR0dHpiTVNfZG9K
   dnVEYWdXOEExSXAzZlhGQUh0UkFjS3c3cmRJNF9YbG42NmhKeEZla3BkZldkaVBRZG
   RRNlkxY0syVTNvYnZVZzd3IgogICAgICAgICAgICAgICAgfQogICAgICAgICAgICBd
   CiAgICAgICAgfQogICAgfQp9.Y287HMtaY0EegEjoTd_04a4GC6qV48GgVbGKOhHdJ
   nDtD0VuUlVjLfwne8AuUY3U7e89zUWwXLnAYK_BiS84M8EsrFvmv8yDLWzqveeIpcN
   5_ysveQnYt9Dqi32w6IOtAywkNUDZeJEdc3z5s9Ei8qrYFN2fxcu28YS4e8e_cHTK5
   7003WJu-wFn2TJUmAbHuqvUsyTb-nzYOKxuCKlqQItJF7E-cwSb_xULu-3f77BEU_v
   GbNYo5ZBa2B7UHO-kWNMSgbW2yeNNLbLC18Kv80GF22Y7SbZt0e2TwnR2Aa2zksuUb
   ntQ5c7a1-gxtnXzuIKa34OekrnyqE1hmVWpeQ







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   If the request being made does not have a message body, such as an
   HTTP GET, OPTIONS, or DELETE method, the JWS signature is calculated
   over an empty payload and passed in the "Detached-JWS" header as
   described in Section 8.1.

   [[ Editor's note: A downside to this method is that it requires the
   content type to be something other than application/json, and it
   doesn't work against an RS without additional profiling since it
   takes over the request body - plus we have to specify different
   delivery locations for a GET vs. a POST, for example.  Additionally
   it is potentially fragile like a detached JWS since a multi-tier
   system could parse the payload and pass the parsed payload downstream
   with potential transformations.  We might want to remove this in
   favor of general-purpose HTTP signing, or at least provide guidance
   on its use. ]]

8.3.  Mutual TLS

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

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



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 907/p6BW/LV1NCgYB1QtFSfGxowqb9FRIMD2kvMSmO0EMxgwZ6k6spa+jk0IsI3k
 lwLW9b+Tfn/daUbIDctxeJneq2anQyU2znBgQl6KILDSF4eaOqlBut/KNZHHazJh

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





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   [[ Editor's note: This method requires no changes to the HTTP message
   itself, since the security relies on the TLS layer.  However, the
   application level will need to validate that the certificate key used
   in the request is the one expected for the specific request. ]]

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

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

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

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

 {
     "resources": [
         "dolphin-metadata"
     ],
     "interact": {
         "redirect": true,
         "callback": {
             "method": "redirect",
             "uri": "https://client.foo",
             "nonce": "VJLO6A4CAYLBXHTR0KRO"
         }



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     },
     "client": {
       "display": {
         "name": "My Client Display Name",
         "uri": "https://example.net/client"
       },
       "proof": "dpop",
       "key": {
         "jwk": {
                     "kty": "RSA",
                     "e": "AQAB",
                     "kid": "xyz-1",
                     "alg": "RS256",
                     "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xYJ
 CCNaOKNJn_Oz0YhdHbXTeWO5AoyspDWJbN5w_7bdWDxgpD-y6jnD1u9YhBOCWObNPFvpkTM
 8LC7SdXGRKx2k8Me2r_GssYlyRpqvpBlY5-ejCywKRBfctRcnhTTGNztbbDBUyDSWmFMVCH
 e5mXT4cL0BwrZC6S-uu-LAx06aKwQOPwYOGOslK8WPm1yGdkaA1uF_FpS6LS63WYPHi_Ap2
 B7_8Wbw4ttzbMS_doJvuDagW8A1Ip3fXFAHtRAcKw7rdI4_Xln66hJxFekpdfWdiPQddQ6Y
 1cK2U3obvUg7w"
         }
       }
     }
 }

   [[ Editor's note: this method requires duplication of the key in the
   header and the request body, which is redundant and potentially
   awkward.  The signature also doesn't protect the body of the request.
   ]]

8.5.  HTTP Signing

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

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



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   CIthXpdH5u4VX3TBe6GJDO6Mkzc6vB+67OWzPwhYTplUiFFV6UZCsDEeu
   Sa/Ue1yLEAMg=="]}
   Digest: SHA=oZz2O3kg5SEFAhmr0xEBbc4jEfo=

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

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









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8.6.  OAuth Proof of Possession (PoP)

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

   *  The at (access token) field MUST be omitted unless this method is
      being used in conjunction with an access token as in Section 7.
      [[ Editor's note: this is in contradiction to the referenced spec
      which makes this field mandatory. ]]

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

   *  All components of the URL MUST be calculated and supplied

   *  The m (method) field MUST be supplied

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

   {
       "resources": [



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

   [[ Editor's note: This is a stale draft from the OAuth working group,
   but it does at least provide some basic functionality for protecting
   HTTP messages with a signature.  This work is likely to be subsumed
   by the general-purpose HTTP message signature mechanism in
   Section 8.5. ]]

9.  Discovery

   By design, the protocol minimizes the need for any pre-flight
   discovery.  To begin a request, the RC only needs to know the
   endpoint of the AS and which keys it will use to sign the request.
   Everything else can be negotiated dynamically in the course of the
   protocol.




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   However, the AS can have limits on its allowed functionality.  If the
   RC wants to optimize its calls to the AS before making a request, it
   MAY send an HTTP OPTIONS request to the grant request endpoint to
   retrieve the server's discovery information.  The AS MUST respond
   with a JSON document containing the following information:

   grant_request_endpoint  REQUIRED.  The full URL of the AS's grant
      request endpoint.  This MUST match the URL the RC used to make the
      discovery request.

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

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

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

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

   assertions  OPTIONAL.  A list of the AS's supported assertion
      formats.  The values of this list correspond to possible values of
      the subject assertion section (Section 2.2) of the request.

   The information returned from this method is for optimization
   purposes only.  The AS MAY deny any request, or any portion of a
   request, even if it lists a capability as supported.  For example, a
   given RC can be registered with the "mtls" key proofing mechanism,
   but the AS also returns other proofing methods, then the AS will deny
   a request from that RC using a different proofing mechanism.

10.  Resource Servers

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

   [[ Editor's note: This section is for discussion of possible advanced
   functionality.  It seems like it should be a separate document or set
   of documents, and it's not even close to being well-baked.  This also
   adds additional endpoints to the AS, as this is separate from the
   token request process, and therefore would require RS-facing



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   discovery or configuration information to make it work.  Also-also,
   it does presume the RS can sign requests in the same way that a
   client does, but hopefully we can be more consistent with this than
   RFC7662 was able to do. ]]

10.1.  Introspecting a Token

   When the RS receives an access token, it can call the introspection
   endpoint at the AS to get token information. [[ Editor's note: this
   isn't super different from the token management URIs, but the RS has
   no way to get that URI, and it's bound to the RS's keys instead of
   the RC's or token's keys. ]]

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

   1.  The RC calls the RS with its access token.

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

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

   4.  The RS fulfills the request from the RC.

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

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

   {
       "access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
   }

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



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   Content-type: application/json

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

10.2.  Deriving a downstream token

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

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

   1.  The RC calls RS1 with an access token.

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

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

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



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   5.  RS2 fulfills the call from RS1.

   6.  RS1 fulfills the call from RC.

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

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

   [[ Editor's note: this is similar to Section 2.7 but based on the
   access token and not the grant.  We might be able to re-use that
   function: the fact that the keys presented are not the ones used for
   the access token should indicate that it's a different party and a
   different kind of request, but there might be some subtle security
   issues there. ]]

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

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

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




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10.3.  Registering a Resource Handle

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

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

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

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

   }

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

   Content-type: application/json

   {
       "resource_handle": "FWWIKYBQ6U56NL1"
   }

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





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   [[ Editor's note: It's not an exact match here because the
   "resource_handle" returned now represents a collection of objects
   instead of a single one.  Perhaps we should let this return a list of
   strings instead?  Or use a different syntax than the resource
   request?  Also, this borrows heavily from UMA 2's "distributed
   authorization" model and, like UMA, might be better suited to an
   extension than the core protocol. ]]

10.4.  Requesting a Resources With Insufficient Access

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

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

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

   [[ Editor's note: this borrows heavily from UMA 2's "distributed
   authorization" model and, like UMA, might be better suited to an
   extension than the core protocol. ]]

11.  Acknowledgements

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









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   In particular, the author would like to thank Aaron Parecki and Mike
   Jones for insights into how to integrate identity and authentication
   systems into the core protocol, and to Dick Hardt for the use cases,
   diagrams, and insights provided in the XAuth proposal that have been
   incorporated here.  The author would like to especially thank Mike
   Varley and the team at SecureKey for feedback and development of
   early versions of the XYZ protocol that fed into this standards work.

12.  IANA Considerations

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

13.  Security Considerations

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

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

14.  Privacy Considerations

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

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

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

15.  Normative References

   [BCP195]   "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", 2015, <http://www.rfc-editor.org/info/bcp195>.

   [I-D.ietf-httpbis-message-signatures]
              Backman, A., Richer, J., and M. Sporny, "Signing HTTP
              Messages", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-message-signatures-00, 10 April 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
              message-signatures-00.txt>.

   [I-D.ietf-oauth-dpop]
              Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstration of
              Proof-of-Possession at the Application Layer (DPoP)", Work



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              in Progress, Internet-Draft, draft-ietf-oauth-dpop-01, 1
              May 2020, <http://www.ietf.org/internet-drafts/draft-ietf-
              oauth-dpop-01.txt>.

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

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

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://openiD.net/specs/openiD-connect-core-1_0.html>.

   [OIDC4IA]  Lodderstedt, T. and D. Fett, "OpenID Connect for Identity
              Assurance 1.0", October 2019, <https://openid.net/specs/
              openid-connect-4-identity-assurance-1_0.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.



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   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

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

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

Appendix A.  Document History

   *  -14

      -  Editorial clarification from design team meetings.

      -  Added "at_hash" to both JWS methods for use with an access
         token.

      -  Allow attached-JWS method to defer to detached-JWS method for
         presentation on a non-body request.

   *  -13

      -  Clarified that "subject" request and response aren't for
         identity claims, just identifiers.

      -  Reworked continuation definition in terms of endpoint actions.





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      -  Defined concrete methods for updating an ongoing grant request
         using PATCH.

      -  Defined method for reading current status of grant request
         using GET.

      -  Expanded editorial comments and strawman examples for
         alternatives.

   *  -12

      -  Collapsed "key" and "display" fields into "client" field.

      -  Changed continuation to use optional access token.

      -  Defined flags for special behavior of tokens.

      -  Defined "key": true to mean access token is bound to client's
         key.

      -  Defined "key": false to indicate an access token.

      -  Added "Elements" section to list and discuss non-role parts of
         the protocol.

   *  -11

      -  Updated based on Design Team feedback and reviews.

      -  Removed oidc_ prefix from several values and used RFC8693
         assertion types.

      -  Changed "client" to "RC" throughout.

      -  Changed "user" to "RQ" or "RO" as appropriate.

      -  Added expansions for request and response section
         introductions.

      -  Added refresh examples.

      -  Added diagrams to RS examples.

      -  Added ui_locales hint to interaction block.

      -  Added section on JSON polymorphism.





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      -  Added numerous editorial notes to describe why elements are in
         place.

      -  Added discussion on composition of roles.

      -  Added requirements for signature methods to cover all aspects
         of request and mechanisms to do so.

   *  -10

      -  Switched to xml2rfc v3 and markdown source.

      -  Updated based on Design Team feedback and reviews.

      -  Added acknowledgements list.

      -  Added sequence diagrams and explanations.

      -  Collapsed "short_redirect" into regular redirect request.

      -  Separated pass-by-reference into subsections.

      -  Collapsed "callback" and "pushback" into a single mode-switched
         method.

      -  Add OIDC Claims request object example.

   *  -09

      -  Major document refactoring based on request and response
         capabilities.

      -  Changed from "claims" language to "subject identifier"
         language.

      -  Added "pushback" interaction capability.

      -  Removed DIDCOMM interaction (better left to extensions).

      -  Excised "transaction" language in favor of "Grant" where
         appropriate.

      -  Added token management URLs.

      -  Added separate continuation URL to use continuation handle
         with.

      -  Added RS-focused functionality section.



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      -  Added notion of extending a grant request based on a previous
         grant.

      -  Simplified returned handle structures.

   *  -08

      -  Added attached JWS signature method.

      -  Added discovery methods.

   *  -07

      -  Marked sections as being controlled by a future registry TBD.

   *  -06

      -  Added multiple resource requests and multiple access token
         response.

   *  -05

      -  Added "claims" request and response for identity support.

      -  Added "capabilities" request for inline discovery support.

   *  -04

      -  Added crypto agility for callback return hash.

      -  Changed "interaction_handle" to "interaction_ref".

   *  -03

      -  Removed "state" in favor of "nonce".

      -  Created signed return parameter for front channel return.

      -  Changed "client" section to "display" section, as well as
         associated handle.

      -  Changed "key" to "keys".

      -  Separated key proofing from key presentation.

      -  Separated interaction methods into booleans instead of "type"
         field.




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   *  -02

      -  Minor editorial cleanups.

   *  -01

      -  Made JSON multimodal for handle requests.

      -  Major updates to normative language and references throughout
         document.

      -  Allowed interaction to split between how the user gets to the
         AS and how the user gets back.

   *  -00

      -  Initial submission.

Appendix B.  Component Data Models

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

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

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

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

Appendix C.  Example Protocol Flows

   The protocol defined in this specification provides a number of
   features that can be combined to solve many different kinds of
   authentication scenarios.  This section seeks to show examples of how
   the protocol would be applied for different situations.

   Some longer fields, particularly cryptographic information, have been
   truncated for display purposes in these examples.








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C.1.  Redirect-Based User Interaction

   In this scenario, the user is the RO and has access to a web browser,
   and the client can take front-channel callbacks on the same device as
   the user.  This combination is analogous to the OAuth 2 Authorization
   Code grant type.

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










































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

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







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   The AS processes the request and determines that the RO needs to
   interact.  The AS returns the following response giving the client
   the information it needs to connect.  The AS has also indicated to
   the client that it can use the given instance identifier to identify
   itself in future requests (Section 2.3.1).

Content-type: application/json

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

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

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

   The user's browser fetches the AS's interaction URL.  The user logs
   in, is identified as the RO for the resource being requested, and
   approves the request.  Since the AS has a callback parameter, the AS
   generates the interaction reference, calculates the hash, and
   redirects the user back to the client with these additional values
   added as query parameters.

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










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   The client receives this request from the user's browser.  The client
   ensures that this is the same user that was sent out by validating
   session information and retrieves the stored pending request.  The
   client uses the values in this to validate the hash parameter.  The
   client then calls the continuation URL and presents the handle and
   interaction reference in the request body.  The client signs the
   request as above.

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

   {
       "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

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































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Content-type: application/json

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

C.2.  Secondary Device Interaction

   In this scenario, the user does not have access to a web browser on
   the device and must use a secondary device to interact with the AS.
   The client can display a user code or a printable QR code.  The
   client prefers a short URL if one is available, with a maximum of 255
   characters in length.  The is not able to accept callbacks from the
   AS and needs to poll for updates while waiting for the user to
   authorize the request.

   The client initiates the request to the AS.








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

   {
       "resources": [
           "dolphin-metadata", "some other thing"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "redirect": 255,
           "user_code": true
       }
   }

   The AS processes this and determines that the RO needs to interact.
   The AS supports both long and short redirect URIs for interaction, so
   it includes both.  Since there is no "callback" the AS does not
   include a nonce, but does include a "wait" parameter on the
   continuation section because it expects the client to poll for
   results.

Content-type: application/json

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

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

   If the user scans the code, they are taken to the interaction
   endpoint and the AS looks up the current pending request based on the
   incoming URL.  If the user instead goes to the static page and enters
   the code manually, the AS looks up the current pending request based
   on the value of the user code.  In both cases, the user logs in, is



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   identified as the RO for the resource being requested, and approves
   the request.  Once the request has been approved, the AS displays to
   the user a message to return to their device.

   Meanwhile, the client periodically polls the AS every 60 seconds at
   the continuation URL.  The client signs the request using the same
   key and method that it did in the first request.

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

   The AS retrieves the pending request based on the handle and
   determines that it has not yet been authorized.  The AS indicates to
   the client that no access token has yet been issued but it can
   continue to call after another 60 second timeout.

Content-type: application/json

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

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

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

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














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Content-type: application/json

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

Appendix D.  No User Involvement

   In this scenario, the client is requesting access on its own behalf,
   with no user to interact with.

   The client creates a request to the AS, identifying itself with its
   public key and using MTLS to make the request.

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

   {
       "resources": [
           "backend service", "nightly-routine-3"
       ],
       "client": {
         "key": {
           "proof": "mtls",
           "cert#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
         }
       }
   }

   The AS processes this and determines that the client can ask for the
   requested resources and issues an access token.












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   Content-type: application/json

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

D.1.  Asynchronous Authorization

   In this scenario, the client is requesting on behalf of a specific
   RO, but has no way to interact with the user.  The AS can
   asynchronously reach out to the RO for approval in this scenario.

   The client starts the request at the AS by requesting a set of
   resources.  The client also identifies a particular user.






























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

   {
       "resources": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "read", "dolphin-metadata",
           {
               "type": "financial-transaction",
               "actions": [
                   "withdraw"
               ],
               "identifier": "account-14-32-32-3",
               "currency": "USD"
           },
           "some other thing"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "user": {
           "sub_ids": [ {
               "subject_type": "email",
               "email": "user@example.com"
           } ]
      }
   }

   The AS processes this and determines that the RO needs to interact.
   The AS determines that it can reach the identified user
   asynchronously and that the identified user does have the ability to
   approve this request.  The AS indicates to the client that it can
   poll for continuation.



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   Content-type: application/json

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

   The AS reaches out to the RO and prompts them for consent.  In this
   example, the AS has an application that it can push notifications in
   to for the specified account.

   Meanwhile, the client periodically polls the AS every 60 seconds at
   the continuation URL.

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

   The AS retrieves the pending request based on the handle and
   determines that it has not yet been authorized.  The AS indicates to
   the client that no access token has yet been issued but it can
   continue to call after another 60 second timeout.

   Content-type: application/json

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

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





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

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

Content-type: application/json

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

D.2.  Applying OAuth 2 Scopes and Client IDs

   While the GNAP protocol is not designed to be directly compatible
   with OAuth 2 [RFC6749], considerations have been made to enable the
   use of OAuth 2 concepts and constructs more smoothly within the GNAP
   protocol.

   In this scenario, the client developer has a "client_id" and set of
   "scope" values from their OAuth 2 system and wants to apply them to
   the new protocol.  Traditionally, the OAuth 2 client developer would
   put their "client_id" and "scope" values as parameters into a
   redirect request to the authorization endpoint.

   HTTP 302 Found
   Location: https://server.example.com/authorize
     ?client_id=7C7C4AZ9KHRS6X63AJAO
     &scope=read%20write%20dolphin
     &redirect_uri=https://client.example.net/return
     &response_type=code
     &state=123455

   Now the developer wants to make an analogous request to the AS using
   the new protocol.  To do so, the client makes an HTTP POST and places
   the OAuth 2 values in the appropriate places.






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

   {
       "resources": [
           "read", "write", "dolphin"
       ],
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "redirect": true,
           "callback": {
               "method": "redirect",
               "uri": "https://client.example.net/return?state=123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

   The client_id can be used to identify the client's keys that it uses
   for authentication, the scopes represent resources that the client is
   requesting, and the "redirect_uri" and "state" value are pre-combined
   into a "callback" URI that can be unique per request.  The client
   additionally creates a nonce to protect the callback, separate from
   the state parameter that it has added to its return URL.

   From here, the protocol continues as above.

Appendix E.  JSON Structures and Polymorphism

   The GNAP protocol makes use of polymorphism within the JSON [RFC8259]
   structures used for the protocol.  Each portion of this protocol is
   defined in terms of the JSON data type that its values can take,
   whether it's a string, object, array, boolean, or number.  For some
   fields, different data types offer different descriptive capabilities
   and are used in different situations for the same field.  Each data
   type provides a different syntax to express the same underlying
   semantic protocol element, which allows for optimization and
   simplification in many common cases.

   Even though JSON is often used to describe strongly typed structures,
   JSON on its own is naturally polymorphic.  In JSON, the named members
   of an object have no type associated with them, and any data type can
   be used as the value for any member.  In practice, each member has a
   semantic type that needs to make sense to the parties creating and
   consuming the object.  Within this protocol, each object member is
   defined in terms of its semantic content, and this semantic content



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   might have expressions in different concrete data types for different
   specific purposes.  Since each object member has exactly one value in
   JSON, each data type for an object member field is naturally mutually
   exclusive with other data types within a single JSON object.

   For example, a resource request for a single access token is composed
   of an array of resource request descriptions while a request for
   multiple access tokens is composed of an object whose member values
   are all arrays.  Both of these represent requests for access, but the
   difference in syntax allows the RC and AS to differentiate between
   the two request types in the same request.

   Another form of polymorphism in JSON comes from the fact that the
   values within JSON arrays need not all be of the same JSON data type.
   However, within this protocol, each element within the array needs to
   be of the same kind of semantic element for the collection to make
   sense, even when the data types are different from each other.

   For example, each aspect of a resource request can be described using
   an object with multiple dimensional components, or the aspect can be
   requested using a string.  In both cases, the resource request is
   being described in a way that the AS needs to interpret, but with
   different levels of specificity and complexity for the RC to deal
   with.  An API designer can provide a set of common access scopes as
   simple strings but still allow RC developers to specify custom access
   when needed for more complex APIs.

   Extensions to this specification can use different data types for
   defined fields, but each extension needs to not only declare what the
   data type means, but also provide justification for the data type
   representing the same basic kind of thing it extends.  For example,
   an extension declaring an "array" representation for a field would
   need to explain how the array represents something akin to the non-
   array element that it is replacing.

Author's Address

   Justin Richer (editor)
   Bespoke Engineering

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









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