



OAuth Working Group                                             M. Jones
Internet-Draft                                                A. Nadalin
Intended status: Standards Track                               Microsoft
Expires: January 21, 2020                               B. Campbell, Ed.
                                                           Ping Identity
                                                              J. Bradley
                                                                  Yubico
                                                            C. Mortimore
                                                              Salesforce
                                                           July 20, 2019


                        OAuth 2.0 Token Exchange
                   draft-ietf-oauth-token-exchange-19

Abstract

   This specification defines a protocol for an HTTP- and JSON- based
   Security Token Service (STS) by defining how to request and obtain
   security tokens from OAuth 2.0 authorization servers, including
   security tokens employing impersonation and delegation.

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 January 21, 2020.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Delegation vs. Impersonation Semantics  . . . . . . . . .   4
     1.2.  Requirements Notation and Conventions . . . . . . . . . .   6
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Token Exchange Request and Response . . . . . . . . . . . . .   6
     2.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.1.1.  Relationship Between Resource, Audience and Scope . .   9
     2.2.  Response  . . . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.1.  Successful Response . . . . . . . . . . . . . . . . .   9
       2.2.2.  Error Response  . . . . . . . . . . . . . . . . . . .  11
     2.3.  Example Token Exchange  . . . . . . . . . . . . . . . . .  11
   3.  Token Type Identifiers  . . . . . . . . . . . . . . . . . . .  13
   4.  JSON Web Token Claims and Introspection Response Parameters .  15
     4.1.  "act" (Actor) Claim . . . . . . . . . . . . . . . . . . .  15
     4.2.  "scope" (Scopes) Claim  . . . . . . . . . . . . . . . . .  17
     4.3.  "client_id" (Client Identifier) Claim . . . . . . . . . .  18
     4.4.  "may_act" (Authorized Actor) Claim  . . . . . . . . . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  OAuth URI Registration  . . . . . . . . . . . . . . . . .  20
       7.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  20
     7.2.  OAuth Parameters Registration . . . . . . . . . . . . . .  21
       7.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  21
     7.3.  OAuth Access Token Type Registration  . . . . . . . . . .  22
       7.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .  22
     7.4.  JSON Web Token Claims Registration  . . . . . . . . . . .  22
       7.4.1.  Registry Contents . . . . . . . . . . . . . . . . . .  22
     7.5.  OAuth Token Introspection Response Registration . . . . .  23
       7.5.1.  Registry Contents . . . . . . . . . . . . . . . . . .  23
     7.6.  OAuth Extensions Error Registration . . . . . . . . . . .  23
       7.6.1.  Registry Contents . . . . . . . . . . . . . . . . . .  23
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Appendix A.  Additional Token Exchange Examples . . . . . . . . .  26
     A.1.  Impersonation Token Exchange Example  . . . . . . . . . .  26
       A.1.1.  Token Exchange Request  . . . . . . . . . . . . . . .  26
       A.1.2.  Subject Token Claims  . . . . . . . . . . . . . . . .  26
       A.1.3.  Token Exchange Response . . . . . . . . . . . . . . .  27



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       A.1.4.  Issued Token Claims . . . . . . . . . . . . . . . . .  27
     A.2.  Delegation Token Exchange Example . . . . . . . . . . . .  28
       A.2.1.  Token Exchange Request  . . . . . . . . . . . . . . .  28
       A.2.2.  Subject Token Claims  . . . . . . . . . . . . . . . .  29
       A.2.3.  Actor Token Claims  . . . . . . . . . . . . . . . . .  29
       A.2.4.  Token Exchange Response . . . . . . . . . . . . . . .  29
       A.2.5.  Issued Token Claims . . . . . . . . . . . . . . . . .  30
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  31
   Appendix C.  Document History . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   A security token is a set of information that facilitates the sharing
   of identity and security information in heterogeneous environments or
   across security domains.  Examples of security tokens include JSON
   Web Tokens (JWTs) [JWT] and SAML 2.0 Assertions
   [OASIS.saml-core-2.0-os].  Security tokens are typically signed to
   achieve integrity and sometimes also encrypted to achieve
   confidentiality.  Security tokens are also sometimes described as
   Assertions, such as in [RFC7521].

   A Security Token Service (STS) is a service capable of validating
   security tokens provided to it and issuing new security tokens in
   response, which enables clients to obtain appropriate access
   credentials for resources in heterogeneous environments or across
   security domains.  Web Service clients have used WS-Trust [WS-Trust]
   as the protocol to interact with an STS for token exchange.  While
   WS-Trust uses XML and SOAP, the trend in modern Web development has
   been towards RESTful patterns and JSON.  The OAuth 2.0 Authorization
   Framework [RFC6749] and OAuth 2.0 Bearer Tokens [RFC6750] have
   emerged as popular standards for authorizing third-party
   applications' access to HTTP and RESTful resources.  The conventional
   OAuth 2.0 interaction involves the exchange of some representation of
   resource owner authorization for an access token, which has proven to
   be an extremely useful pattern in practice.  However, its input and
   output are somewhat too constrained as is to fully accommodate a
   security token exchange framework.

   This specification defines a protocol extending OAuth 2.0 that
   enables clients to request and obtain security tokens from
   authorization servers acting in the role of an STS.  Similar to OAuth
   2.0, this specification focuses on client developer simplicity and
   requires only an HTTP client and JSON parser, which are nearly
   universally available in modern development environments.  The STS
   protocol defined in this specification is not itself RESTful (an STS
   doesn't lend itself particularly well to a REST approach) but does




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   utilize communication patterns and data formats that should be
   familiar to developers accustomed to working with RESTful systems.

   A new grant type for a token exchange request and the associated
   specific parameters for such a request to the token endpoint are
   defined by this specification.  A token exchange response is a normal
   OAuth 2.0 response from the token endpoint with a few additional
   parameters defined herein to provide information to the client.

   The entity that makes the request to exchange tokens is considered
   the client in the context of the token exchange interaction.
   However, that does not restrict usage of this profile to traditional
   OAuth clients.  An OAuth resource server, for example, might assume
   the role of the client during token exchange in order to trade an
   access token that it received in a protected resource request for a
   new token that is appropriate to include in a call to a backend
   service.  The new token might be an access token that is more
   narrowly scoped for the downstream service or it could be an entirely
   different kind of token.

   The scope of this specification is limited to the definition of a
   basic request-and-response protocol for an STS-style token exchange
   utilizing OAuth 2.0.  Although a few new JWT claims are defined that
   enable delegation semantics to be expressed, the specific syntax,
   semantics and security characteristics of the tokens themselves (both
   those presented to the authorization server and those obtained by the
   client) are explicitly out of scope and no requirements are placed on
   the trust model in which an implementation might be deployed.
   Additional profiles may provide more detailed requirements around the
   specific nature of the parties and trust involved, such as whether
   signing and/or encryption of tokens is needed or if proof-of-
   possession style tokens will be required or issued; however, such
   details will often be policy decisions made with respect to the
   specific needs of individual deployments and will be configured or
   implemented accordingly.

   The security tokens obtained may be used in a number of contexts, the
   specifics of which are also beyond the scope of this specification.

1.1.  Delegation vs. Impersonation Semantics

   One common use case for an STS (as alluded to in the previous
   section) is to allow a resource server A to make calls to a backend
   service C on behalf of the requesting user B.  Depending on the local
   site policy and authorization infrastructure, it may be desirable for
   A to use its own credentials to access C along with an annotation of
   some form that A is acting on behalf of B ("delegation"), or for A to
   be granted a limited access credential to C but that continues to



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   identify B as the authorized entity ("impersonation").  Delegation
   and impersonation can be useful concepts in other scenarios involving
   multiple participants as well.

   When principal A impersonates principal B, A is given all the rights
   that B has within some defined rights context and is
   indistinguishable from B in that context.  Thus, when principal A
   impersonates principal B, then insofar as any entity receiving such a
   token is concerned, they are actually dealing with B.  It is true
   that some members of the identity system might have awareness that
   impersonation is going on, but it is not a requirement.  For all
   intents and purposes, when A is impersonating B, A is B within the
   context of the rights authorized by the token.  A's ability to
   impersonate B could be limited in scope or time, or even with a one-
   time-use restriction, whether via the contents of the token or an
   out-of-band mechanism.

   Delegation semantics are different than impersonation semantics,
   though the two are closely related.  With delegation semantics,
   principal A still has its own identity separate from B and it is
   explicitly understood that while B may have delegated some of its
   rights to A, any actions taken are being taken by A representing B.
   In a sense, A is an agent for B.

   Delegation and impersonation are not inclusive of all situations.
   When a principal is acting directly on its own behalf, for example,
   neither delegation nor impersonation are in play.  They are, however,
   the more common semantics operating for token exchange and, as such,
   are given more direct treatment in this specification.

   Delegation semantics are typically expressed in a token by including
   information about both the primary subject of the token as well as
   the actor to whom that subject has delegated some of its rights.
   Such a token is sometimes referred to as a composite token because it
   is composed of information about multiple subjects.  Typically, in
   the request, the "subject_token" represents the identity of the party
   on behalf of whom the token is being requested while the
   "actor_token" represents the identity of the party to whom the access
   rights of the issued token are being delegated.  A composite token
   issued by the authorization server will contain information about
   both parties.  When and if a composite token is issued is at the
   discretion of the authorization server and applicable policy and
   configuration.

   The specifics of representing a composite token and even whether or
   not such a token will be issued depend on the details of the
   implementation and the kind of token.  The representations of
   composite tokens that are not JWTs are beyond the scope of this



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   specification.  The "actor_token" request parameter, however, does
   provide a means for providing information about the desired actor and
   the JWT "act" claim can provide a representation of a chain of
   delegation.

1.2.  Requirements Notation and Conventions

   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.

1.3.  Terminology

   This specification uses the terms "access token type", "authorization
   server", "client", "client identifier", "resource server", "token
   endpoint", "token request", and "token response" defined by OAuth 2.0
   [RFC6749], and the terms "Base64url Encoding", "Claim", and "JWT
   Claims Set" defined by JSON Web Token (JWT) [JWT].

2.  Token Exchange Request and Response

2.1.  Request

   A client requests a security token by making a token request to the
   authorization server's token endpoint using the extension grant type
   mechanism defined in Section 4.5 of [RFC6749].

   Client authentication to the authorization server is done using the
   normal mechanisms provided by OAuth 2.0.  Section 2.3.1 of [RFC6749]
   defines password-based authentication of the client, however, client
   authentication is extensible and other mechanisms are possible.  For
   example, [RFC7523] defines client authentication using bearer JSON
   Web Tokens (JWTs) [JWT].  The supported methods of client
   authentication and whether or not to allow unauthenticated or
   unidentified clients are deployment decisions that are at the
   discretion of the authorization server.  Note that omitting client
   authentication allows for a compromised token to be leveraged via an
   STS into other tokens by anyone possessing the compromised token.
   Thus client authentication allows for additional authorization checks
   by the STS as to which entities are permitted to impersonate or
   receive delegations from other entities.

   The client makes a token exchange request to the token endpoint with
   an extension grant type using the HTTP "POST" method.  The following
   parameters are included in the HTTP request entity-body using the




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   "application/x-www-form-urlencoded" format with a character encoding
   of UTF-8 as described in Appendix B of RFC6749 [RFC6749].

   grant_type
      REQUIRED.  The value "urn:ietf:params:oauth:grant-type:token-
      exchange" indicates that a token exchange is being performed.

   resource
      OPTIONAL.  A URI that indicates the target service or resource
      where the client intends to use the requested security token.
      This enables the authorization server to apply policy as
      appropriate for the target, such as determining the type and
      content of the token to be issued or if and how the token is to be
      encrypted.  In many cases, a client will not have knowledge of the
      logical organization of the systems with which it interacts and
      will only know a URI of the service where it intends to use the
      token.  The "resource" parameter allows the client to indicate to
      the authorization server where it intends to use the issued token
      by providing the location, typically as an https URL, in the token
      exchange request in the same form that will be used to access that
      resource.  The authorization server will typically have the
      capability to map from a resource URI value to an appropriate
      policy.  The value of the "resource" parameter MUST be an absolute
      URI, as specified by Section 4.3 of [RFC3986], which MAY include a
      query component and MUST NOT include a fragment component.
      Multiple "resource" parameters may be used to indicate that the
      issued token is intended to be used at the multiple resources
      listed.  See [I-D.ietf-oauth-resource-indicators] for additional
      background and uses of the "resource" parameter.

   audience
      OPTIONAL.  The logical name of the target service where the client
      intends to use the requested security token.  This serves a
      purpose similar to the "resource" parameter, but with the client
      providing a logical name for the target service.  Interpretation
      of the name requires that the value be something that both the
      client and the authorization server understand.  An OAuth client
      identifier, a SAML entity identifier [OASIS.saml-core-2.0-os], an
      OpenID Connect Issuer Identifier [OpenID.Core], are examples of
      things that might be used as "audience" parameter values.
      However, "audience" values used with a given authorization server
      must be unique within that server, to ensure that they are
      properly interpreted as the intended type of value.  Multiple
      "audience" parameters may be used to indicate that the issued
      token is intended to be used at the multiple audiences listed.
      The "audience" and "resource" parameters may be used together to
      indicate multiple target services with a mix of logical names and
      resource URIs.



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   scope
      OPTIONAL.  A list of space-delimited, case-sensitive strings, as
      defined in Section 3.3 of [RFC6749], that allow the client to
      specify the desired scope of the requested security token in the
      context of the service or resource where the token will be used.
      The values and associated semantics of scope are service specific
      and expected to be described in the relevant service
      documentation.

   requested_token_type
      OPTIONAL.  An identifier, as described in Section 3, for the type
      of the requested security token.  If the requested type is
      unspecified, the issued token type is at the discretion of the
      authorization server and may be dictated by knowledge of the
      requirements of the service or resource indicated by the
      "resource" or "audience" parameter.

   subject_token
      REQUIRED.  A security token that represents the identity of the
      party on behalf of whom the request is being made.  Typically, the
      subject of this token will be the subject of the security token
      issued in response to the request.

   subject_token_type
      REQUIRED.  An identifier, as described in Section 3, that
      indicates the type of the security token in the "subject_token"
      parameter.

   actor_token
      OPTIONAL.  A security token that represents the identity of the
      acting party.  Typically, this will be the party that is
      authorized to use the requested security token and act on behalf
      of the subject.

   actor_token_type
      An identifier, as described in Section 3, that indicates the type
      of the security token in the "actor_token" parameter.  This is
      REQUIRED when the "actor_token" parameter is present in the
      request but MUST NOT be included otherwise.

   In processing the request, the authorization server MUST perform the
   appropriate validation procedures for the indicated token type and,
   if the actor token is present, also perform the appropriate
   validation procedures for its indicated token type.  The validity
   criteria and details of any particular token are beyond the scope of
   this document and are specific to the respective type of token and
   its content.




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   In the absence of one-time-use or other semantics specific to the
   token type, the act of performing a token exchange has no impact on
   the validity of the subject token or actor token.  Furthermore, the
   exchange is a one-time event and does not create a tight linkage
   between the input and output tokens, so that (for example) while the
   expiration time of the output token may be influenced by that of the
   input token, renewal or extension of the input token is not expected
   to be reflected in the output token's properties.  It may still be
   appropriate or desirable to propagate token revocation events.
   However, doing so is not a general property of the STS protocol and
   would be specific to a particular implementation, token type or
   deployment.

2.1.1.  Relationship Between Resource, Audience and Scope

   When requesting a token, the client can indicate the desired target
   service(s) where it intends to use that token by way of the
   "audience" and "resource" parameters, as well as indicating the
   desired scope of the requested token using the "scope" parameter.
   The semantics of such a request are that the client is asking for a
   token with the requested scope that is usable at all the requested
   target services.  Effectively, the requested access rights of the
   token are the cartesian product of all the scopes at all the target
   services.

   An authorization server may be unwilling or unable to fulfill any
   token request but the likelihood of an unfulfillable request is
   significantly higher when very broad access rights are being
   solicited.  As such, in the absence of specific knowledge about the
   relationship of systems in a deployment, clients should exercise
   discretion in the breadth of the access requested, particularly the
   number of target services.  An authorization server can use the
   "invalid_target" error code, defined in Section 2.2.2, to inform a
   client that it requested access to too many target services
   simultaneously.

2.2.  Response

   The authorization server responds to a token exchange request with a
   normal OAuth 2.0 response from the token endpoint, as specified in
   Section 5 of [RFC6749].  Additional details and explanation are
   provided in the following subsections.

2.2.1.  Successful Response

   If the request is valid and meets all policy and other criteria of
   the authorization server, a successful token response is constructed
   by adding the following parameters to the entity-body of the HTTP



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   response using the "application/json" media type, as specified by
   [RFC8259], and an HTTP 200 status code.  The parameters are
   serialized into a JavaScript Object Notation (JSON) structure by
   adding each parameter at the top level.  Parameter names and string
   values are included as JSON strings.  Numerical values are included
   as JSON numbers.  The order of parameters does not matter and can
   vary.

   access_token
      REQUIRED.  The security token issued by the authorization server
      in response to the token exchange request.  The "access_token"
      parameter from Section 5.1 of [RFC6749] is used here to carry the
      requested token, which allows this token exchange protocol to use
      the existing OAuth 2.0 request and response constructs defined for
      the token endpoint.  The identifier "access_token" is used for
      historical reasons and the issued token need not be an OAuth
      access token.

   issued_token_type
      REQUIRED.  An identifier, as described in Section 3, for the
      representation of the issued security token.

   token_type
      REQUIRED.  A case-insensitive value specifying the method of using
      the access token issued, as specified in Section 7.1 of [RFC6749].
      It provides the client with information about how to utilize the
      access token to access protected resources.  For example, a value
      of "Bearer", as specified in [RFC6750], indicates that the issued
      security token is a bearer token and the client can simply present
      it as is without any additional proof of eligibility beyond the
      contents of the token itself.  Note that the meaning of this
      parameter is different from the meaning of the "issued_token_type"
      parameter, which declares the representation of the issued
      security token; the term "token type" is more typically used with
      the aforementioned meaning as the structural or syntactical
      representation of the security token, as it is in all
      "*_token_type" parameters in this specification.  If the issued
      token is not an access token or usable as an access token, then
      the "token_type" value "N_A" is used to indicate that an OAuth 2.0
      "token_type" identifier is not applicable in that context.

   expires_in
      RECOMMENDED.  The validity lifetime, in seconds, of the token
      issued by the authorization server.  Oftentimes the client will
      not have the inclination or capability to inspect the content of
      the token and this parameter provides a consistent and token-type-
      agnostic indication of how long the token can be expected to be




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      valid.  For example, the value 1800 denotes that the token will
      expire in thirty minutes from the time the response was generated.

   scope
      OPTIONAL, if the scope of the issued security token is identical
      to the scope requested by the client; otherwise, REQUIRED.

   refresh_token
      OPTIONAL.  A refresh token will typically not be issued when the
      exchange is of one temporary credential (the subject_token) for a
      different temporary credential (the issued token) for use in some
      other context.  A refresh token can be issued in cases where the
      client of the token exchange needs the ability to access a
      resource even when the original credential is no longer valid
      (e.g., user-not-present or offline scenarios where there is no
      longer any user entertaining an active session with the client).
      Profiles or deployments of this specification should clearly
      document the conditions under which a client should expect a
      refresh token in response to "urn:ietf:params:oauth:grant-
      type:token-exchange" grant type requests.

2.2.2.  Error Response

   If the request itself is not valid or if either the "subject_token"
   or "actor_token" are invalid for any reason, or are unacceptable
   based on policy, the authorization server MUST construct an error
   response, as specified in Section 5.2 of [RFC6749].  The value of the
   "error" parameter MUST be the "invalid_request" error code.

   If the authorization server is unwilling or unable to issue a token
   for any target service indicated by the "resource" or "audience"
   parameters, the "invalid_target" error code SHOULD be used in the
   error response.

   The authorization server MAY include additional information regarding
   the reasons for the error using the "error_description" as discussed
   in Section 5.2 of [RFC6749].

   Other error codes may also be used, as appropriate.

2.3.  Example Token Exchange

   The following example demonstrates a hypothetical token exchange in
   which an OAuth resource server assumes the role of the client during
   the exchange.  It trades an access token, which it received in a
   protected resource request, for a new token that it will use to call
   to a backend service (extra line breaks and indentation in the
   examples are for display purposes only).



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   Figure 1 shows the resource server receiving a protected resource
   request containing an OAuth access token in the Authorization header,
   as specified in Section 2.1 of [RFC6750].

    GET /resource HTTP/1.1
    Host: frontend.example.com
    Authorization: Bearer accVkjcJyb4BWCxGsndESCJQbdFMogUC5PbRDqceLTC

                   Figure 1: Protected Resource Request

   In Figure 2, the resource server assumes the role of client for the
   token exchange and the access token from the request in Figure 1 is
   sent to the authorization server using a request as specified in
   Section 2.1.  The value of the "subject_token" parameter carries the
   access token and the value of the "subject_token_type" parameter
   indicates that it is an OAuth 2.0 access token.  The resource server,
   acting in the role of the client, uses its identifier and secret to
   authenticate to the authorization server using the HTTP Basic
   authentication scheme.  The "resource" parameter indicates the
   location of the backend service, https://backend.example.com/api,
   where the issued token will be used.

    POST /as/token.oauth2 HTTP/1.1
    Host: as.example.com
    Authorization: Basic cnMwODpsb25nLXNlY3VyZS1yYW5kb20tc2VjcmV0
    Content-Type: application/x-www-form-urlencoded

    grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange
    &resource=https%3A%2F%2Fbackend.example.com%2Fapi
    &subject_token=accVkjcJyb4BWCxGsndESCJQbdFMogUC5PbRDqceLTC
    &subject_token_type=
     urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token

                     Figure 2: Token Exchange Request

   The authorization server validates the client credentials and the
   "subject_token" presented in the token exchange request.  From the
   "resource" parameter, the authorization server is able to determine
   the appropriate policy to apply to the request and issues a token
   suitable for use at https://backend.example.com.  The "access_token"
   parameter of the response shown in Figure 3 contains the new token,
   which is itself a bearer OAuth access token that is valid for one
   minute.  The token happens to be a JWT; however, its structure and
   format are opaque to the client so the "issued_token_type" indicates
   only that it is an access token.






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    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6IjllciJ9.eyJhdWQiOiJo
       dHRwczovL2JhY2tlbmQuZXhhbXBsZS5jb20iLCJpc3MiOiJodHRwczovL2FzLmV
       4YW1wbGUuY29tIiwiZXhwIjoxNDQxOTE3NTkzLCJpYXQiOjE0NDE5MTc1MzMsIn
       N1YiI6ImJkY0BleGFtcGxlLmNvbSIsInNjb3BlIjoiYXBpIn0.40y3ZgQedw6rx
       f59WlwHDD9jryFOr0_Wh3CGozQBihNBhnXEQgU85AI9x3KmsPottVMLPIWvmDCM
       y5-kdXjwhw",
     "issued_token_type":
         "urn:ietf:params:oauth:token-type:access_token",
     "token_type":"Bearer",
     "expires_in":60
    }

                     Figure 3: Token Exchange Response

   The resource server can then use the newly acquired access token in
   making a request to the backend server as illustrated in Figure 4.

    GET /api HTTP/1.1
    Host: backend.example.com
    Authorization: Bearer eyJhbGciOiJFUzI1NiIsImtpZCI6IjllciJ9.eyJhdWQ
       iOiJodHRwczovL2JhY2tlbmQuZXhhbXBsZS5jb20iLCJpc3MiOiJodHRwczovL2
       FzLmV4YW1wbGUuY29tIiwiZXhwIjoxNDQxOTE3NTkzLCJpYXQiOjE0NDE5MTc1M
       zMsInN1YiI6ImJkY0BleGFtcGxlLmNvbSIsInNjb3BlIjoiYXBpIn0.40y3ZgQe
       dw6rxf59WlwHDD9jryFOr0_Wh3CGozQBihNBhnXEQgU85AI9x3KmsPottVMLPIW
       vmDCMy5-kdXjwhw

               Figure 4: Backend Protected Resource Request

   Additional examples can be found in Appendix A.

3.  Token Type Identifiers

   Several parameters in this specification utilize an identifier as the
   value to describe the token in question.  Specifically, they are the
   "requested_token_type", "subject_token_type", "actor_token_type"
   parameters of the request and the "issued_token_type" member of the
   response.  Token type identifiers are URIs.  Token Exchange can work
   with both tokens issued by other parties and tokens from the given
   authorization server.  For the former the token type identifier
   indicates the syntax (e.g., JWT or SAML 2.0) so the authorization
   server can parse it; for the latter it indicates what the given
   authorization server issued it for (e.g., access_token or
   refresh_token).



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   The following token type identifiers are defined by this
   specification.  Other URIs MAY be used to indicate other token types.

   urn:ietf:params:oauth:token-type:access_token
      Indicates that the token is an OAuth 2.0 access token issued by
      the given authorization server.

   urn:ietf:params:oauth:token-type:refresh_token
      Indicates that the token is an OAuth 2.0 refresh token issued by
      the given authorization server.

   urn:ietf:params:oauth:token-type:id_token
      Indicates that the token is an ID Token, as defined in Section 2
      of [OpenID.Core].

   urn:ietf:params:oauth:token-type:saml1
      Indicates that the token is a base64url-encoded SAML 1.1
      [OASIS.saml-core-1.1] assertion.

   urn:ietf:params:oauth:token-type:saml2
      Indicates that the token is a base64url-encoded SAML 2.0
      [OASIS.saml-core-2.0-os] assertion.

   The value "urn:ietf:params:oauth:token-type:jwt", which is defined in
   Section 9 of [JWT], indicates that the token is a JWT.

   The distinction between an access token and a JWT is subtle.  An
   access token represents a delegated authorization decision, whereas
   JWT is a token format.  An access token can be formatted as a JWT but
   doesn't necessarily have to be.  And a JWT might well be an access
   token but not all JWTs are access tokens.  The intent of this
   specification is that "urn:ietf:params:oauth:token-type:access_token"
   be an indicator that the token is a typical OAuth access token issued
   by the authorization server in question, opaque to the client, and
   usable the same manner as any other access token obtained from that
   authorization server.  (It could well be a JWT, but the client isn't
   and needn't be aware of that fact.)  Whereas,
   "urn:ietf:params:oauth:token-type:jwt" is to indicate specifically
   that a JWT is being requested or sent (perhaps in a cross-domain use-
   case where the JWT is used as an authorization grant to obtain an
   access token from a different authorization server as is facilitated
   by [RFC7523]).

   Note that for tokens which are binary in nature, the URI used for
   conveying them needs to be associated with the semantics of a base64
   or other encoding suitable for usage with HTTP and OAuth.





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4.  JSON Web Token Claims and Introspection Response Parameters

   It is useful to have defined mechanisms to express delegation within
   a token as well as to express authorization to delegate or
   impersonate.  Although the token exchange protocol described herein
   can be used with any type of token, this section defines claims to
   express such semantics specifically for JWTs and in an OAuth 2.0
   Token Introspection [RFC7662] response.  Similar definitions for
   other types of tokens are possible but beyond the scope of this
   specification.

   Note that the claims not established herein but used in examples and
   descriptions, such as "iss", "sub", "exp", etc., are defined by
   [JWT].

4.1.  "act" (Actor) Claim

   The "act" (actor) claim provides a means within a JWT to express that
   delegation has occurred and identify the acting party to whom
   authority has been delegated.  The "act" claim value is a JSON object
   and members in the JSON object are claims that identify the actor.
   The claims that make up the "act" claim identify and possibly provide
   additional information about the actor.  For example, the combination
   of the two claims "iss" and "sub" might be necessary to uniquely
   identify an actor.

   However, claims within the "act" claim pertain only to the identity
   of the actor and are not relevant to the validity of the containing
   JWT in the same manner as the top-level claims.  Consequently, non-
   identity claims (e.g., "exp", "nbf", and "aud") are not meaningful
   when used within an "act" claim, and therefore are not used.




















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   Figure 5 illustrates the "act" (actor) claim within a JWT Claims Set.
   The claims of the token itself are about user@example.com while the
   "act" claim indicates that admin@example.com is the current actor.

    {
      "aud":"https://consumer.example.com",
      "iss":"https://issuer.example.com",
      "exp":1443904177,
      "nbf":1443904077,
      "sub":"user@example.com",
      "act":
      {
        "sub":"admin@example.com"
      }
    }

                           Figure 5: Actor Claim

   A chain of delegation can be expressed by nesting one "act" claim
   within another.  The outermost "act" claim represents the current
   actor while nested "act" claims represent prior actors.  The least
   recent actor is the most deeply nested.  The nested "act" claims
   serve as a history trail that connects the initial request and
   subject through the various delegation steps undertaken before
   reaching the current actor.  In this sense, the current actor is
   considered to include the entire authorization/delegation history,
   leading naturally to the nested structure described here.

   For the purpose of applying access control policy, the consumer of a
   token MUST only consider the token's top-level claims and the party
   identified as the current actor by the "act" claim.  Prior actors
   identified by any nested "act" claims are informational only and are
   not to be considered in access control decisions.


















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   The following example in Figure 6 illustrates nested "act" (actor)
   claims within a JWT Claims Set.  The claims of the token itself are
   about user@example.com while the "act" claim indicates that the
   system https://service16.example.com is the current actor and
   https://service77.example.com was a prior actor.  Such a token might
   come about as the result of service16 receiving a token in a call
   from service77 and exchanging it for a token suitable to call
   service26 while the authorization server notes the situation in the
   newly issued token.

    {
      "aud":"https://service26.example.com",
      "iss":"https://issuer.example.com",
      "exp":1443904100,
      "nbf":1443904000,
      "sub":"user@example.com",
      "act":
      {
        "sub":"https://service16.example.com",
        "act":
        {
          "sub":"https://service77.example.com"
        }
      }
    }

                       Figure 6: Nested Actor Claim

   When included as a top-level member of an OAuth token introspection
   response, "act" has the same semantics and format as the claim of the
   same name.

4.2.  "scope" (Scopes) Claim

   The value of the "scope" claim is a JSON string containing a space-
   separated list of scopes associated with the token, in the format
   described in Section 3.3 of [RFC6749].














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   Figure 7 illustrates the "scope" claim within a JWT Claims Set.

    {
      "aud":"https://consumer.example.com",
      "iss":"https://issuer.example.com",
      "exp":1443904177,
      "nbf":1443904077,
      "sub":"dgaf4mvfs75Fci_FL3heQA",
      "scope":"email profile phone address"
    }

                          Figure 7: Scopes Claim

   OAuth 2.0 Token Introspection [RFC7662] already defines the "scope"
   parameter to convey the scopes associated with the token.

4.3.  "client_id" (Client Identifier) Claim

   The "client_id" claim carries the client identifier of the OAuth 2.0
   [RFC6749] client that requested the token.

   The following example in Figure 8 illustrates the "client_id" claim
   within a JWT Claims Set indicating an OAuth 2.0 client with
   "s6BhdRkqt3" as its identifier.

    {
      "aud":"https://consumer.example.com",
      "iss":"https://issuer.example.com",
      "exp":1443904177,
      "sub":"user@example.com",
      "client_id":"s6BhdRkqt3"
    }

                     Figure 8: Client Identifier Claim

   OAuth 2.0 Token Introspection [RFC7662] already defines the
   "client_id" parameter as the client identifier for the OAuth 2.0
   client that requested the token.

4.4.  "may_act" (Authorized Actor) Claim

   The "may_act" claim makes a statement that one party is authorized to
   become the actor and act on behalf of another party.  The claim might
   be used, for example, when a "subject_token" is presented to the
   token endpoint in a token exchange request and "may_act" claim in the
   subject token can be used by the authorization server to determine
   whether the client (or party identified in the "actor_token") is
   authorized to engage in the requested delegation or impersonation.



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   The claim value is a JSON object and members in the JSON object are
   claims that identify the party that is asserted as being eligible to
   act for the party identified by the JWT containing the claim.  The
   claims that make up the "may_act" claim identify and possibly provide
   additional information about the authorized actor.  For example, the
   combination of the two claims "iss" and "sub" are sometimes necessary
   to uniquely identify an authorized actor, while the "email" claim
   might be used to provide additional useful information about that
   party.

   However, claims within the "may_act" claim pertain only to the
   identity of that party and are not relevant to the validity of the
   containing JWT in the same manner as top-level claims.  Consequently,
   claims such as "exp", "nbf", and "aud" are not meaningful when used
   within a "may_act" claim, and therefore are not used.

   Figure 9 illustrates the "may_act" claim within a JWT Claims Set.
   The claims of the token itself are about user@example.com while the
   "may_act" claim indicates that admin@example.com is authorized to act
   on behalf of user@example.com.

    {
      "aud":"https://consumer.example.com",
      "iss":"https://issuer.example.com",
      "exp":1443904177,
      "nbf":1443904077,
      "sub":"user@example.com",
      "may_act":
      {
        "sub":"admin@example.com"
      }
    }

                     Figure 9: Authorized Actor Claim

   When included as a top-level member of an OAuth token introspection
   response, "may_act" has the same semantics and format as the claim of
   the same name.

5.  Security Considerations

   Much of the guidance from Section 10 of [RFC6749], the Security
   Considerations in The OAuth 2.0 Authorization Framework, is also
   applicable here.  Furthermore, [RFC6819] provides additional security
   considerations for OAuth and [I-D.ietf-oauth-security-topics] has
   updated security guidance based on deployment experience and new
   threats that have emerged since OAuth 2.0 was originally published.




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   All of the normal security issues that are discussed in [JWT],
   especially in relationship to comparing URIs and dealing with
   unrecognized values, also apply here.

   In addition, both delegation and impersonation introduce unique
   security issues.  Any time one principal is delegated the rights of
   another principal, the potential for abuse is a concern.  The use of
   the "scope" claim (in addition to other typical constraints such as a
   limited token lifetime) is suggested to mitigate potential for such
   abuse, as it restricts the contexts in which the delegated rights can
   be exercised.

6.  Privacy Considerations

   Tokens employed in the context of the functionality described herein
   may contain privacy-sensitive information and, to prevent disclosure
   of such information to unintended parties, MUST only be transmitted
   over encrypted channels, such as Transport Layer Security (TLS).  In
   cases where it is desirable to prevent disclosure of certain
   information to the client, the token MUST be encrypted to its
   intended recipient.  Deployments SHOULD determine the minimally
   necessary amount of data and only include such information in issued
   tokens.  In some cases, data minimization may include representing
   only an anonymous or pseudonymous user.

7.  IANA Considerations

7.1.  OAuth URI Registration

   This specification registers the following values in the IANA "OAuth
   URI" registry [IANA.OAuth.Parameters] established by [RFC6755].

7.1.1.  Registry Contents

   o  URN: urn:ietf:params:oauth:grant-type:token-exchange
   o  Common Name: Token exchange grant type for OAuth 2.0
   o  Change controller: IESG
   o  Specification Document: Section 2.1 of [[ this specification ]]

   o  URN: urn:ietf:params:oauth:token-type:access_token
   o  Common Name: Token type URI for an OAuth 2.0 access token
   o  Change controller: IESG
   o  Specification Document: Section 3 of [[this specification]]

   o  URN: urn:ietf:params:oauth:token-type:refresh_token
   o  Common Name: Token type URI for an OAuth 2.0 refresh token
   o  Change controller: IESG
   o  Specification Document: Section 3 of [[this specification]]



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   o  URN: urn:ietf:params:oauth:token-type:id_token
   o  Common Name: Token type URI for an ID Token
   o  Change controller: IESG
   o  Specification Document: Section 3 of [[this specification]]

   o  URN: urn:ietf:params:oauth:token-type:saml1
   o  Common Name: Token type URI for a base64url-encoded SAML 1.1
      assertion
   o  Change Controller: IESG
   o  Specification Document: Section 3 of [[this specification]]

   o  URN: urn:ietf:params:oauth:token-type:saml2
   o  Common Name: Token type URI for a base64url-encoded SAML 2.0
      assertion
   o  Change Controller: IESG
   o  Specification Document: Section 3 of [[this specification]]

7.2.  OAuth Parameters Registration

   This specification registers the following values in the IANA "OAuth
   Parameters" registry [IANA.OAuth.Parameters] established by
   [RFC6749].

7.2.1.  Registry Contents

   o  Parameter name: resource
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: audience
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: requested_token_type
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: subject_token
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: subject_token_type
   o  Parameter usage location: token request
   o  Change controller: IESG



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   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: actor_token
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: actor_token_type
   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): Section 2.1 of [[ this specification ]]

   o  Parameter name: issued_token_type
   o  Parameter usage location: token response
   o  Change controller: IESG
   o  Specification document(s): Section 2.2.1 of [[ this specification
      ]]

7.3.  OAuth Access Token Type Registration

   This specification registers the following access token type in the
   IANA "OAuth Access Token Types" registry [IANA.OAuth.Parameters]
   established by [RFC6749].

7.3.1.  Registry Contents

   o  Type name: N_A
   o  Additional Token Endpoint Response Parameters: (none)
   o  HTTP Authentication Scheme(s): (none)
   o  Change controller: IESG
   o  Specification document(s): Section 2.2.1 of [[ this specification
      ]]

7.4.  JSON Web Token Claims Registration

   This specification registers the following Claims in the IANA "JSON
   Web Token Claims" registry [IANA.JWT.Claims] established by [JWT].

7.4.1.  Registry Contents

   o  Claim Name: "act"
   o  Claim Description: Actor
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1 of [[ this specification ]]

   o  Claim Name: "scope"
   o  Claim Description: Scope Values
   o  Change Controller: IESG



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   o  Specification Document(s): Section 4.2 of [[ this specification ]]

   o  Claim Name: "client_id"
   o  Claim Description: Client Identifier
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.3 of [[ this specification ]]

   o  Claim Name: "may_act"
   o  Claim Description: Authorized Actor - the party that is authorized
      to become the actor
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.4 of [[ this specification ]]

7.5.  OAuth Token Introspection Response Registration

   This specification registers the following values in the IANA "OAuth
   Token Introspection Response" registry [IANA.OAuth.Parameters]
   established by [RFC7662].

7.5.1.  Registry Contents

   o  Claim Name: "act"
   o  Claim Description: Actor
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1 of [[ this specification ]]

   o  Claim Name: "may_act"
   o  Claim Description: Authorized Actor - the party that is authorized
      to become the actor
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.4 of [[ this specification ]]

7.6.  OAuth Extensions Error Registration

   This specification registers the following values in the IANA "OAuth
   Extensions Error" registry [IANA.OAuth.Parameters] established by
   [RFC6749].

7.6.1.  Registry Contents

   o  Error Name: "invalid_target"
   o  Error Usage Location: token error response
   o  Related Protocol Extension: OAuth 2.0 Token Exchange
   o  Change Controller: IETF
   o  Specification Document(s): Section 2.2.2 of [[ this specification
      ]]





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8.  References

8.1.  Normative References

   [IANA.JWT.Claims]
              IANA, "JSON Web Token Claims",
              <http://www.iana.org/assignments/jwt>.

   [IANA.OAuth.Parameters]
              IANA, "OAuth Parameters",
              <http://www.iana.org/assignments/oauth-parameters>.

   [JWT]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://tools.ietf.org/html/rfc7519>.

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

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

   [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>.

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

   [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>.

8.2.  Informative References







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   [I-D.ietf-oauth-resource-indicators]
              Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", draft-ietf-oauth-resource-
              indicators-02 (work in progress), January 2019.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", draft-ietf-
              oauth-security-topics-13 (work in progress), July 2019.

   [OASIS.saml-core-1.1]
              Maler, E., Mishra, P., and R. Philpott, "Assertions and
              Protocol for the OASIS Security Assertion Markup Language
              (SAML) V1.1", OASIS Standard oasis-sstc-saml-core-1.1,
              September 2003.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

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

   [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>.

   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
              <https://www.rfc-editor.org/info/rfc6755>.

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

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






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   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/info/rfc7523>.

   [WS-Trust]
              Nadalin, A., Goodner, M., Gudgin, M., Barbir, A., and H.
              Granqvist, "WS-Trust 1.4", February 2012,
              <http://docs.oasis-open.org/ws-sx/ws-trust/v1.4/
              ws-trust.html>.

Appendix A.  Additional Token Exchange Examples

   Two example token exchanges are provided in the following sections
   illustrating impersonation and delegation, respectively (with extra
   line breaks and indentation for display purposes only).

A.1.  Impersonation Token Exchange Example

A.1.1.  Token Exchange Request

   In the following token exchange request, a client is requesting a
   token with impersonation semantics (with only a "subject_token" and
   no "actor_token", delegation is impossible).  The client tells the
   authorization server that it needs a token for use at the target
   service with the logical name "urn:example:cooperation-context".

    POST /as/token.oauth2 HTTP/1.1
    Host: as.example.com
    Content-Type: application/x-www-form-urlencoded

    grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange
    &audience=urn%3Aexample%3Acooperation-context
    &subject_token=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.eyJhdWQiOiJodHRwc
      zovL2FzLmV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9vcmlnaW5hbC1pc3N1ZXI
      uZXhhbXBsZS5uZXQiLCJleHAiOjE0NDE5MTA2MDAsIm5iZiI6MTQ0MTkwOTAwMCwic
      3ViIjoiYmRjQGV4YW1wbGUubmV0Iiwic2NvcGUiOiJvcmRlcnMgcHJvZmlsZSBoaXN
      0b3J5In0.PRBg-jXn4cJuj1gmYXFiGkZzRuzbXZ_sDxdE98ddW44ufsbWLKd3JJ1VZ
      hF64pbTtfjy4VXFVBDaQpKjn5JzAw
    &subject_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Ajwt

                     Figure 10: Token Exchange Request

A.1.2.  Subject Token Claims

   The "subject_token" in the prior request is a JWT and the decoded JWT
   Claims Set is shown here.  The JWT is intended for consumption by the
   authorization server within a specific time window.  The subject of



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   the JWT ("bdc@example.net") is the party on behalf of whom the new
   token is being requested.

     {
       "aud":"https://as.example.com",
       "iss":"https://original-issuer.example.net",
       "exp":1441910600,
       "nbf":1441909000,
       "sub":"bdc@example.net",
       "scope":"orders profile history"
     }

                      Figure 11: Subject Token Claims

A.1.3.  Token Exchange Response

   The "access_token" parameter of the token exchange response shown
   below contains the new token that the client requested.  The other
   parameters of the response indicate that the token is a bearer access
   token that expires in an hour.

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6IjcyIn0.eyJhdWQiOiJ1cm4
       6ZXhhbXBsZTpjb29wZXJhdGlvbi1jb250ZXh0IiwiaXNzIjoiaHR0cHM6Ly9hcy5l
       eGFtcGxlLmNvbSIsImV4cCI6MTQ0MTkxMzYxMCwic3ViIjoiYmRjQGV4YW1wbGUub
       mV0Iiwic2NvcGUiOiJvcmRlcnMgcHJvZmlsZSBoaXN0b3J5In0.rMdWpSGNACTvnF
       uOL74sYZ6MVuld2Z2WkGLmQeR9ztj6w2OXraQlkJmGjyiCq24kcB7AI2VqVxl3wSW
       nVKh85A",
     "issued_token_type":
       "urn:ietf:params:oauth:token-type:access_token",
     "token_type":"Bearer",
     "expires_in":3600
    }

                    Figure 12: Token Exchange Response

A.1.4.  Issued Token Claims

   The decoded JWT Claims Set of the issued token is shown below.  The
   new JWT is issued by the authorization server and intended for
   consumption by a system entity known by the logical name
   "urn:example:cooperation-context" any time before its expiration.
   The subject ("sub") of the JWT is the same as the subject the token
   used to make the request, which effectively enables the client to



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   impersonate that subject at the system entity known by the logical
   name of "urn:example:cooperation-context" by using the token.

     {
       "aud":"urn:example:cooperation-context",
       "iss":"https://as.example.com",
       "exp":1441913610,
       "sub":"bdc@example.net",
       "scope":"orders profile history"
     }

                      Figure 13: Issued Token Claims

A.2.  Delegation Token Exchange Example

A.2.1.  Token Exchange Request

   In the following token exchange request, a client is requesting a
   token and providing both a "subject_token" and an "actor_token".  The
   client tells the authorization server that it needs a token for use
   at the target service with the logical name "urn:example:cooperation-
   context".  Policy at the authorization server dictates that the
   issued token be a composite.

    POST /as/token.oauth2 HTTP/1.1
    Host: as.example.com
    Content-Type: application/x-www-form-urlencoded

    grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange
    &audience=urn%3Aexample%3Acooperation-context
    &subject_token=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.eyJhdWQiOiJodHRwc
      zovL2FzLmV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9vcmlnaW5hbC1pc3N1ZXI
      uZXhhbXBsZS5uZXQiLCJleHAiOjE0NDE5MTAwNjAsInNjb3BlIjoic3RhdHVzIGZlZ
      WQiLCJzdWIiOiJ1c2VyQGV4YW1wbGUubmV0IiwibWF5X2FjdCI6eyJzdWIiOiJhZG1
      pbkBleGFtcGxlLm5ldCJ9fQ.4rPRSWihQbpMIgAmAoqaJojAxj-p2X8_fAtAGTXrvM
      xU-eEZHnXqY0_AOZgLdxw5DyLzua8H_I10MCcckF-Q_g
    &subject_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Ajwt
    &actor_token=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.eyJhdWQiOiJodHRwczo
      vL2FzLmV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9vcmlnaW5hbC1pc3N1ZXIuZ
      XhhbXBsZS5uZXQiLCJleHAiOjE0NDE5MTAwNjAsInN1YiI6ImFkbWluQGV4YW1wbGU
      ubmV0In0.7YQ-3zPfhUvzje5oqw8COCvN5uP6NsKik9CVV6cAOf4QKgM-tKfiOwcgZ
      oUuDL2tEs6tqPlcBlMjiSzEjm3yBg
    &actor_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Ajwt

                     Figure 14: Token Exchange Request






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A.2.2.  Subject Token Claims

   The "subject_token" in the prior request is a JWT and the decoded JWT
   Claims Set is shown here.  The JWT is intended for consumption by the
   authorization server before a specific expiration time.  The subject
   of the JWT ("user@example.net") is the party on behalf of whom the
   new token is being requested.

     {
       "aud":"https://as.example.com",
       "iss":"https://original-issuer.example.net",
       "exp":1441910060,
       "scope":"status feed",
       "sub":"user@example.net",
       "may_act":
       {
         "sub":"admin@example.net"
       }
     }

                      Figure 15: Subject Token Claims

A.2.3.  Actor Token Claims

   The "actor_token" in the prior request is a JWT and the decoded JWT
   Claims Set is shown here.  This JWT is also intended for consumption
   by the authorization server before a specific expiration time.  The
   subject of the JWT ("admin@example.net") is the actor that will wield
   the security token being requested.

     {
       "aud":"https://as.example.com",
       "iss":"https://original-issuer.example.net",
       "exp":1441910060,
       "sub":"admin@example.net"
     }

                       Figure 16: Actor Token Claims

A.2.4.  Token Exchange Response

   The "access_token" parameter of the token exchange response shown
   below contains the new token that the client requested.  The other
   parameters of the response indicate that the token is a JWT that
   expires in an hour and that the access token type is not applicable
   since the issued token is not an access token.





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    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6IjcyIn0.eyJhdWQiOiJ1cm4
       6ZXhhbXBsZTpjb29wZXJhdGlvbi1jb250ZXh0IiwiaXNzIjoiaHR0cHM6Ly9hcy5l
       eGFtcGxlLmNvbSIsImV4cCI6MTQ0MTkxMzYxMCwic2NvcGUiOiJzdGF0dXMgZmVlZ
       CIsInN1YiI6InVzZXJAZXhhbXBsZS5uZXQiLCJhY3QiOnsic3ViIjoiYWRtaW5AZX
       hhbXBsZS5uZXQifX0.3paKl9UySKYB5ng6_cUtQ2qlO8Rc_y7Mea7IwEXTcYbNdwG
       9-G1EKCFe5fW3H0hwX-MSZ49Wpcb1SiAZaOQBtw",
     "issued_token_type":"urn:ietf:params:oauth:token-type:jwt",
     "token_type":"N_A",
     "expires_in":3600
    }

                    Figure 17: Token Exchange Response

A.2.5.  Issued Token Claims

   The decoded JWT Claims Set of the issued token is shown below.  The
   new JWT is issued by the authorization server and intended for
   consumption by a system entity known by the logical name
   "urn:example:cooperation-context" any time before its expiration.
   The subject ("sub") of the JWT is the same as the subject of the
   "subject_token" used to make the request.  The actor ("act") of the
   JWT is the same as the subject of the "actor_token" used to make the
   request.  This indicates delegation and identifies
   "admin@example.net" as the current actor to whom authority has been
   delegated to act on behalf of "user@example.net".

     {
       "aud":"urn:example:cooperation-context",
       "iss":"https://as.example.com",
       "exp":1441913610,
       "scope":"status feed",
       "sub":"user@example.net",
       "act":
       {
         "sub":"admin@example.net"
       }
     }

                      Figure 18: Issued Token Claims







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Appendix B.  Acknowledgements

   This specification was developed within the OAuth Working Group,
   which includes dozens of active and dedicated participants.  It was
   produced under the chairmanship of Hannes Tschofenig, Derek Atkins,
   and Rifaat Shekh-Yusef with Kathleen Moriarty, Stephen Farrell, Eric
   Rescorla, Roman Danyliw, and Benjamin Kaduk serving as Security Area
   Directors.  The following individuals contributed ideas, feedback,
   and wording to this specification:

   Caleb Baker, Vittorio Bertocci, Mike Brown, Thomas Broyer, Roman
   Danyliw, William Denniss, Vladimir Dzhuvinov, Eric Fazendin, Phil
   Hunt, Benjamin Kaduk, Jason Keglovitz, Torsten Lodderstedt, Barry
   Leiba, Adam Lewis, James Manger, Nov Matake, Matt Miller, Hilarie
   Orman, Matthew Perry, Eric Rescorla, Justin Richer, Adam Roach,
   Rifaat Shekh-Yusef, Scott Tomilson, and Hannes Tschofenig.

Appendix C.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   -19

   o  Fix-up changes introduced in -18.
   o  Fix invalid JSON in the Nested Actor Claim example.
   o  Reference figure numbers in text when introducing the examples in
      Section 2 and 4.
   o  Editorial updates from additional IESG evaluation comments.
   o  Add an informational reference to ietf-oauth-resource-indicators
   o  Update ietf-oauth-security-topics ref to 13

   -18

   o  Editorial updates based on a few more IESG evaluation comments.

   -17

   o  Editorial improvements and example fixes resulting from IESG
      evaluation comments.
   o  Added a pointer to RFC6749's Appendix B. on the "Use of
      application/x-www-form-urlencoded Media Type" as a way of
      providing a normative citation (by reference) for the media type.
   o  Strengthened some of the wording in the privacy considerations to
      bring it inline with RFC 7519 Sec. 12 and RFC 6749 Sec. 10.8.

   -16

   o  Fixed typo and added an AD to Acknowledgements.



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   -15

   o  Updated the nested actor claim example to (hopefully) be more
      straightforward.
   o  Reworked Privacy Considerations to say to use TLS in transit,
      minimize the amount of information in the token, and encrypt the
      token if disclosure of its information to the client is a concern
      per https://mailarchive.ietf.org/arch/msg/secdir/
      KJhx4aq_U5uk3k6zpYP-CEHbpVM
   o  Moved the Security and Privacy Considerations sections to before
      the IANA Considerations.

   -14

   o  Added text in Section 4.1 about the "act" claim stating that only
      the top-level claims and the current actor are to be considered in
      applying access control decisions.

   -13

   o  Updated the claim name and value syntax for scope to be consistent
      with the treatment of scope in RFC 7662 OAuth 2.0 Token
      Introspection.
   o  Updated the client identifier claim name to be consistent with the
      treatment of client id in RFC 7662 OAuth 2.0 Token Introspection.

   -12

   o  Updated to use the boilerplate from RFC 8174.

   -11

   o  Added new WG chair and AD to the Acknowledgements.
   o  Applied clarifications suggested during AD review by EKR.

   -10

   o  Defined token type URIs for base64url-encoded SAML 1.1 and SAML
      2.0 assertions.
   o  Applied editorial fixes.

   -09

   o  Changed "security tokens obtained could be used in a number of
      contexts" to "security tokens obtained may be used in a number of
      contexts" per a WGLC suggestion.





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   o  Clarified that the validity of the subject or actor token have no
      impact on the validity of the issued token after the exchange has
      occurred per a WGLC comment.
   o  Changed use of invalid_target error code to a SHOULD per a WGLC
      comment.
   o  Clarified text about non-identity claims within the "act" claim
      being meaningless per a WGLC comment.
   o  Added brief Privacy Considerations section per WGLC comments.

   -08

   o  Use the bibxml reference for OpenID.Core rather than defining it
      inline.
   o  Added editor role for Campbell.
   o  Minor clarification of the text for actor_token.

   -07

   o  Fixed typo (desecration -> discretion).
   o  Added an explanation of the relationship between scope, audience
      and resource in the request and added an "invalid_target" error
      code enabling the AS to tell the client that the requested
      audiences/resources were too broad.

   -06

   o  Drop "An STS for the REST of Us" from the title.
   o  Drop "heavyweight" and "lightweight" from the abstract and
      introduction.
   o  Clarifications on the language around xxxxxx_token_type.
   o  Remove the want_composite parameter.
   o  Add a short mention of proof-of-possession style tokens to the
      introduction and remove the respective open issue.

   -05

   o  Defined the JWT claim "cid" to express the OAuth 2.0 client
      identifier of the client that requested the token.
   o  Defined and requested registration for "act" and "may_act" as
      Token introspection response parameters (in addition to being JWT
      claims).
   o  Loosen up the language about refresh_token in the response to
      OPTIONAL from NOT RECOMMENDED based on feedback form real world
      deployment experience.
   o  Add clarifying text about the distinction between JWT and access
      token URIs.
   o  Close out (remove) some of the Open Issues bullets that have been
      resolved.



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   -04

   o  Clarified that the "resource" and "audience" request parameters
      can be used at the same time (via http://www.ietf.org/mail-
      archive/web/oauth/current/msg15335.html).
   o  Clarified subject/actor token validity after token exchange and
      explained a bit more about the recommendation to not issue refresh
      tokens (via http://www.ietf.org/mail-archive/web/oauth/current/
      msg15318.html).
   o  Updated the examples appendix to use an issuer value that doesn't
      imply that the client issued and signed the tokens and used
      "Bearer" and "urn:ietf:params:oauth:token-type:access_token" in
      one of the responses (via http://www.ietf.org/mail-
      archive/web/oauth/current/msg15335.html).
   o  Defined and registered urn:ietf:params:oauth:token-type:id_token,
      since some use cases perform token exchanges for ID Tokens and no
      URI to indicate that a token is an ID Token had previously been
      defined.

   -03

   o  Updated the document editors (adding Campbell, Bradley, and
      Mortimore).
   o  Added to the title.
   o  Added to the abstract and introduction.
   o  Updated the format of the request to use application/x-www-form-
      urlencoded request parameters and the response to use the existing
      token endpoint JSON parameters defined in OAuth 2.0.
   o  Changed the grant type identifier to urn:ietf:params:oauth:grant-
      type:token-exchange.
   o  Added RFC 6755 registration requests for
      urn:ietf:params:oauth:token-type:refresh_token,
      urn:ietf:params:oauth:token-type:access_token, and
      urn:ietf:params:oauth:grant-type:token-exchange.
   o  Added RFC 6749 registration requests for request/response
      parameters.
   o  Removed the Implementation Considerations and the requirement to
      support JWTs.
   o  Clarified many aspects of the text.
   o  Changed "on_behalf_of" to "subject_token",
      "on_behalf_of_token_type" to "subject_token_type", "act_as" to
      "actor_token", and "act_as_token_type" to "actor_token_type".
   o  Added an "audience" request parameter used to indicate the logical
      names of the target services at which the client intends to use
      the requested security token.
   o  Added a "want_composite" request parameter used to indicate the
      desire for a composite token rather than trying to infer it from
      the presence/absence of token(s) in the request.



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   o  Added a "resource" request parameter used to indicate the URLs of
      resources at which the client intends to use the requested
      security token.
   o  Specified that multiple "audience" and "resource" request
      parameter values may be used.
   o  Defined the JWT claim "act" (actor) to express the current actor
      or delegation principal.
   o  Defined the JWT claim "may_act" to express that one party is
      authorized to act on behalf of another party.
   o  Defined the JWT claim "scp" (scopes) to express OAuth 2.0 scope-
      token values.
   o  Added the "N_A" (not applicable) OAuth Access Token Type
      definition for use in contexts in which the token exchange syntax
      requires a "token_type" value, but in which the token being issued
      is not an access token.
   o  Added examples.

   -02

   o  Enabled use of Security Token types other than JWTs for "act_as"
      and "on_behalf_of" request values.
   o  Referenced the JWT and OAuth Assertions RFCs.

   -01

   o  Updated references.

   -00

   o  Created initial working group draft from draft-jones-oauth-token-
      exchange-01.

Authors' Addresses

   Michael B. Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/


   Anthony Nadalin
   Microsoft

   Email: tonynad@microsoft.com






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   Brian Campbell (editor)
   Ping Identity

   Email: brian.d.campbell@gmail.com


   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com


   Chuck Mortimore
   Salesforce

   Email: cmortimore@salesforce.com



































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