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Versions: 00 01 02 03 draft-ietf-oauth-token-exchange

OAuth Working Group                                          B. Campbell
Internet-Draft                                                J. Bradley
Intended status: Standards Track                           Ping Identity
Expires: January 24, 2016                                  July 23, 2015


          OAuth 2.0 Token Exchange: an STS for the REST of us
                      draft-campbell-oauth-sts-03

Abstract

   An OAuth 2.0 framework for exchanging security tokens enabling
   authorization servers to act as lightweight HTTP and JSON based
   security token services.

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
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   This Internet-Draft will expire on January 24, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Delegation vs. Impersonation Semantics  . . . . . . . . .   3
   2.  Security Token Request  . . . . . . . . . . . . . . . . . . .   4
   3.  Security Token Response . . . . . . . . . . . . . . . . . . .   6
     3.1.  Successful Security Token Response  . . . . . . . . . . .   6
     3.2.  Error Response  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix C.  Document History . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A security token service (STS) is a service capable of validating and
   issuing security tokens, which enables web service clients to obtain
   appropriate temporary access credentials for resources in
   heterogeneous environments or across security domains.  Clients have
   historically used WS-Trust [WS-Trust] as the protocol to interact
   with an STS for token exchange.  However WS-Trust is a fairly
   heavyweight framework which uses XML, SOAP, WS-Security, XML-
   Signatures, etc. while the trend in modern web development has been
   towards more lightweight services utilizing 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 and securing access to HTTP and RESTful resources but do
   not provide all that is needed to support generic STS interactions.

   This specification defines a lightweight protocol extending OAuth 2.0
   that enables clients to request and obtain security tokens from
   authorization servers acting in the role of an STS.  There is support
   for enabling one party to act on behalf of another as well as
   enabling one party to delegate constrained authority to another.
   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 utilize communication patterns and data formats that should be



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   more palatable to developers accustom to working with RESTful
   systems.

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

   The security tokens obtained from an STS could be used in a variety
   of contexts, the specifics of which are beyond the scope of this
   document.

   The scope of this specification is limited to the definition of a
   framework and basic wire protocol for an STS style token exchange
   utilizing OAuth 2.0.  The syntax, semantics and security
   characteristics of the tokens themselves (both those presented to the
   AS 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, whether signatures and/or encryption of tokens is
   required, etc., 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.

1.1.  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 RFC
   2119 [RFC2119].

1.2.  Terminology

   This specification uses the terms "authorization server" "token
   endpoint", "access token request", "access token response", and
   "client" defined by OAuth 2.0 [RFC6749].

1.3.  Delegation vs. Impersonation Semantics

   When principal A impersonates principal B, A is given all the rights
   that B has within some defined rights context.  Whereas, with
   delegation semantics, principal A still has its own identity separate
   from B and it is explicitly understood that while B may have
   delegated its rights to A, any actions taken are being taken by A and
   not B.  In a sense, A is an agent for B.




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   Delegation semantics are therefore different than impersonation
   semantics, with which it is sometimes confused.  When principal A
   impersonates principal B, then in so far 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.

   A security token with delegation semantics is requested using this
   framework by including both an on_behalf_of token and an act_as token
   in the request.  The on_behalf_of token represents the identity of
   the party on behalf of whom the token is being requested while the
   act_as token represents the identity of the party to whom the access
   rights of the returned token are being delegated.  In this case, the
   token returned to the client will contain claims about both parties.

   A security token with impersonation semantics is requested using this
   framework by including an on_behalf_of token in the request and
   omitting the act_as token.  The on_behalf_of token represents the
   identity of the party on behalf of whom the token is being requested
   the token returned to the client will contain claims about that
   party.

2.  Security Token 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 OAuth 2.0 [RFC6749].

   Client authentication to the authorization server is done using the
   normal mechanisms provided by OAuth 2.0.  Section 2.3.1 of The OAuth
   2.0 Authorization Framework [RFC6749] defines password-based
   authentication of the client, however, client authentication is
   extensible and other mechanisms are allowed.  For example, [RFC7522]
   and [RFC7523] define client authentication using SAML Assertions and
   JSON Web Tokens respectively.  Other mechanisms, such as TLS client
   authentication, are also possible.  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.

   The client makes a general security token request to the token
   endpoint with an extension grant type by including the following
   parameters using the "application/x-www-form-urlencoded" format with
   a character encoding of UTF-8 in the HTTP request entity-body:

   grant_type




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      REQUIRED.  The value "urn:ietf:params:oauth:grant-type:security-
      token-request" indicates that it is a security token request.

   aud
      OPTIONAL.  Indicates the location of the service or resource where
      the client intends to use the requested security token.  The value
      MUST be an absolute URI as defined by Section 4.3 of [RFC3986].
      The URI MAY include a query component but MUST NOT include a
      fragment component.  When applicable, the value of this parameter
      also typically informs the audience restrictions on the returned
      security token.

   scope
      OPTIONAL.  A list of space-delimited, case-sensitive strings that
      allow the client to specify the desired scope of requested
      security token in the context of the service or resource where the
      token will be used (possibly indicated by the "aud" parameter).

   requested_security_token_type
      OPTIONAL.  Identifier for the type of the requested security
      token.  For example, a JWT can be requested with the identifier
      "urn:ietf:params:oauth:token-type:jwt", which is defined in JSON
      Web Token [RFC7519].  If the requested type is unspecified, the
      returned token type is at the discretion of the authorization
      server and may be dictated by knowledge of the requirements of the
      service or resource whose location is indicated by the "aud"
      parameter.

   on_behalf_of
      REQUIRED.  The value of this request parameter is a security token
      which represents the identity of the party on behalf of whom the
      request is being made.  Typically the subject of this token will
      be the primary subject of the security token returned in response
      to this request.

   on_behalf_of_token_type
      REQUIRED.  An identifier that indicates the type of the security
      token sent with the "on_behalf_of" parameter.

   act_as
      OPTIONAL.  The value of this request parameter is a security token
      which represents the identity of the party that is authorized to
      use the requested security token.  When this parameter is present,
      it indicates that the client wants a token that contains claims
      about two distinct entities: 1) the entity represented by the
      token in the "on_behalf_of" parameter as the primary subject and
      2) the entity represented by this token as a party who is
      authorized to act on behalf of that subject.



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   act_as_token_type
      REQUIRED when the "act_as" parameter is present in the request but
      MUST NOT be included otherwise.  The value of this parameter is an
      identifier that indicates the type of the security token sent with
      the "act_as" parameter.

3.  Security Token Response

   The authorization server responds to a security token request with a
   normal OAuth 2.0 response from the token endpoint as defined in
   Section 5 of RFC 6749 [RFC6749].  Additional details and explanation
   are provided in the following subsections.

3.1.  Successful Security Token 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
   response using the "application/json" media type as defined by
   [RFC4627] 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 security token request.  The "access_token"
      parameter from Section 5.1 of RFC 6749 [RFC6749] is used here to
      carry the requested security token, which allows this token
      exchange framework to use the existing OAuth 2.0 request and
      response constructs defined for the token endpoint.

   security_token_type
      REQUIRED.  An identifier for the general type of the returned
      security token.  For example, if the security token is a JWT, this
      value of the "security_token_type" is
      "urn:ietf:params:oauth:token-type:jwt".

   token_type
      REQUIRED.  A case insensitive value describing the type of the
      token issued as discussed in Section 7.1 of RFC 6749 [RFC6749].
      Note that this value is different from the value of the
      "security_token_type" and provides the client with information
      about how to utilize the token to access protected resources.  For
      example, a value of "Bearer" as defined in [RFC6750] indicates
      that the security token is a bearer token and the client can
      simply present it as is without any additional proof of



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      eligibility beyond the contents of the token itself.  A value of
      "PoP", on the other hand, indicates that use of the token will
      require demonstrating possession of a cryptographic key associated
      with the security token ([I-D.ietf-oauth-pop-key-distribution]
      describes the "PoP" token type).

   expires_in
      RECOMMENDED.  The validity lifetime, in seconds, of the issued
      security token.  For example, the value 3600 denotes that the
      token will expire in one hour from the time the response was
      generated.

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

   refresh_token
      NOT RECOMMENDED.  Refresh tokens will typically not be issued in
      response to a "urn:ietf:params:oauth:grant-type:security-token-
      request" grant type requests.

3.2.  Error Response

   If either the "on_behalf_of" or "act_as" tokens are invalid for any
   reason, or are unacceptable based on policy, the authorization server
   MUST construct an error response as defined in Section 5.2 of OAuth
   2.0 [RFC6749] The value of the "error" parameter MUST be the
   "invalid_grant" error code.  The authorization server MAY include
   additional information regarding the reasons for the error using the
   "error_description" or "error_uri" parameters.

4.  Examples

   [[ TODO: at least two examples, with and without act_as, showing a
   request/response exchange and including some relevant internal
   details of the tokens involved ]]

5.  IANA Considerations

   [[ TODO ]] The "urn:ietf:params:oauth:grant-type:security-token-
   request" Grant Type is to be registered in the IANA
   urn:ietf:params:oauth registry established in [RFC6755].

   [[ TODO ]] Other parameters like "requested_security_token_type",
   "on_behalf_of", "on_behalf_of_token_type", "act_as", etc. need to be
   registered in the appropriate registries.  The "aud" parameter needs
   to be registered too but that may well get done in




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   [I-D.ietf-oauth-pop-key-distribution] and aud may/does have wider
   applicability so perhaps deserves it's own little spec?

6.  Security Considerations

   [[ TODO ]]

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, DOI
              10.17487/RFC4627, July 2006,
              <http://www.rfc-editor.org/info/rfc4627>.

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

7.2.  Informative References

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-01 (work in progress), March 2015.

   [I-D.jones-oauth-token-exchange]
              Jones, M., Nadalin, A., and C. Baker, "OAuth 2.0 Token
              Exchange", draft-jones-oauth-token-exchange-01 (work in
              progress), July 2014.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., Medeiros, B., and C.
              Mortimore, "OpenID Connect Core 1.0", February 2014.





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   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/
              RFC6750, October 2012,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6755>.

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

   [RFC7522]  Campbell, B., Mortimore, C., and M. Jones, "Security
              Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0
              Client Authentication and Authorization Grants", RFC 7522,
              DOI 10.17487/RFC7522, May 2015,
              <http://www.rfc-editor.org/info/rfc7522>.

   [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, <http://www.rfc-editor.org/info/rfc7523>.

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

Appendix A.  Open Issues

   Some, but surely not all, decisions to be made with potential
   associated draft updates:

   o  Are the constructs for expressing delegation and impersonation the
      'right' ones?  Do they provide sufficient flexibility while being
      reasonably understandable and implementable?

   o  Does it really make sense to use the act_as and on_behalf_of
      terms?  They come with some baggage.

   o  More guidance on what delegation should look like in the returned
      token?  I.e. refer to "azp" in [OpenID.Core]?  Or something else?

   o  Do we need to codify if/how the identity of the client end up in
      returned token?  Should it be an AS decision?  A special case of
      delegation/act_as?  Something else?



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   o  Does the error response need a way to convey additional
      information beyond what OAuth 2.0 provides?

   o  Exactly how the presentation of PoP or other non-bearer tokens
      works.  Should a challenge-response mechanism be considered rather
      than trying to stuff the whole PoP into a single request?

Appendix B.  Acknowledgements

   The author wishes to thank Michael Jones for bringing forth the
   concept of OAuth 2.0 Token Exchange with
   [I-D.jones-oauth-token-exchange].  This draft borrows heavily from
   Jones' work while striving to provide a syntax that is more
   consistent with OAuth 2.0 [RFC6749], which will hopefully be more
   familiar to developers and easier to understand and implement.

   The author also wishes to thank John Bradley for his endless patience
   and willingness to share his expertise.

   The following individuals also contributed ideas, feedback, and
   wording that shaped and formed the final specification:

   Chuck Mortimore, Justin Richter, Phil Hunt, and Scott Tomilson.

Appendix C.  Document History

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

   -03

   o  The aud parameter is now OPTIONAL rather than REQUIRED.

   o  Update references for JWT and SAML/JWT assertion frameworks to the
      new RFCs.

   -02

   o  Refreshing draft before -01 expires.

   -01

   o  Add Bradley as an author.

   -00

   o  Gotta start somewhere...





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Authors' Addresses

   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com


   John Bradley
   Ping Identity

   Email: ve7jtb@ve7jtb.com







































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