OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                        November 3, 2013
Expires: May 7, 2014


                        OAuth 2.0 Token Exchange
                  draft-jones-oauth-token-exchange-00

Abstract

   This specification defines how to request and obtain Security Tokens
   from OAuth Authorization Servers, including enabling one party to act
   on behalf of another or enabling one party to delegate authority to
   another.

Status of this Memo

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

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   This Internet-Draft will expire on May 7, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Notation and Conventions . . . . . . . . . . . 3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.3.  On-Behalf-Of vs. Impersonation Semantics  . . . . . . . . . 3
   2.  Security Token Request  . . . . . . . . . . . . . . . . . . . . 4
     2.1.  Act-As Security Token Requests  . . . . . . . . . . . . . . 5
     2.2.  On-Behalf-Of Security Token Requests  . . . . . . . . . . . 5
   3.  Security Token Response . . . . . . . . . . . . . . . . . . . . 6
   4.  Conveying Eligibility to Act As Another Party . . . . . . . . . 6
   5.  Implementation Considerations . . . . . . . . . . . . . . . . . 7
   6.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Document History . . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9































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1.  Introduction

   This specification defines how to request and obtain Security Tokens
   from OAuth Authorization Servers [RFC6749], including enabling one
   party to act on behalf of another or enabling one party to delegate
   authority to another.  This functionality is intentionally parallel
   to the functionality defined by [WS-Trust], including On-Behalf-Of
   and Act-As.

   A Security Token is a set of information that facilitates the sharing
   of identity and security information across security domains.
   Examples of Security Tokens include JSON Web Tokens (JWTs) [JWT] and
   SAML 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 described as
   Assertions in [I-D.ietf-oauth-assertions].

   This specification defines a new Security Token Request Grant Type
   used at the Token Endpoint to convey the parameters for a Security
   Token request and Security Token response parameter used in responses
   to these requests.  The Security Token Request is a JSON Web Token
   (JWT) [JWT] that is signed by the requesting party that contains
   parameters of the request as Claims.

   The Security Tokens obtained could be used in a number of contexts,
   the specifics of which are beyond the scope of this specification.
   Examples include using them with the

1.1.  Requirements Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD 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", "Token Request", and "Token Response" defined by OAuth 2.0
   [RFC6749], and the terms "Claim" and "JWT Claims Set" defined by JSON
   Web Token (JWT) [JWT].

1.3.  On-Behalf-Of vs. Impersonation Semantics

   When principal A acts on behalf of principal B, A is given all the
   rights that B has within some defined rights context.  Whereas, with
   on-behalf-of 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



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   not B. In a sense, A is an agent for B.

   On-behalf-of 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
   Claims 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 acting for B, A is B.


2.  Security Token Request

   A Security Token Request is sent to the Token Endpoint as a Token
   Request message using this Grant Type value:

   security_token_request  Grant Type value indicating that this Token
      Request is a Security Token Request.

   A Token Request parameter of the same name is used to convey the
   information contained in Security Token Request as a JWT:

   security_token_request  Token Request parameter whose value is a JWT
      containing the Security Token Request information.

   An example Security Token Request (with extra line breaks for display
   purposes only) follows:

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

     grant_type=security_token_request&security_token_request=
     eyJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJ ... [omitted for brevity]

   The "security_token_request" parameter value is a JWT with the
   following members:

   iss  REQUIRED.  The issuer of the principal requesting the Security
      Token.

   sub  REQUIRED.  The identifier of the principal requesting the
      Security Token at the issuer.

   security_token_type  OPTIONAL.  An identifier for the type of the
      requested Security Token.  If not present, the default is that a
      JWT is being requested.  A JWT can also be requested with the
      identifier "urn:ietf:params:oauth:token-type:jwt".



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   scopes  OPTIONAL.  An array of strings, each of which represents a
      service context that the requested Security Token is being
      requested to be used for.  The array MUST contain at least one
      scope value.  The definition of these contexts is outside the
      scope of this specification.  (Note: This request element serves
      the same purpose as the WS-Trust AppliesTo RST element.)

   The JWT MUST be signed by the issuer so the identity of the
   requesting party can be validated.

   The following is an example of a JWT Claims Set for a Security Token
   Request:

     {
      "iss": "https://server.example.com",
      "sub": "24400320",
      "scopes": ["example"]
     }

2.1.  Act-As Security Token Requests

   This specification defines the ability to request a Security Token
   for the requesting party to use to act as the specified party.  This
   is accomplished using this Token Request parameter:

   act_as  This OPTIONAL request parameter indicates that the requested
      Security Token is expected to contain information about the
      identity represented by the Security Token that is the value of
      this parameter, enabling the requesting party to use the returned
      Security Token to act as this identity.

   The following is an example of a JWT Claims Set for a Security Token
   Request using an "act_as" Claim:

     {
      "iss": "https://server.example.com",
      "sub": "24400320",
      "scopes": ["example"],
      "act_as": "eyJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJ ..."
     }

2.2.  On-Behalf-Of Security Token Requests

   This specification defines the ability to request a Security Token on
   behalf of another party.  This is accomplished using this Token
   Request parameter:





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   on_behalf_of  This OPTIONAL request parameter indicates that the
      Security Token is being requested on behalf of another party.  The
      identity of the party upon whose behalf the request is being made
      is represented by the Security Token that is the value of this
      parameter.  Proof of eligibility to act on behalf of that identity
      MAY be conveyed by including an "actor" Claim identifying the
      requesting party in the Security Token, per Section 4, provided
      the Security Token is a JWT.

   The following is an example of a JWT Claims Set for a Security Token
   Request using an "on_behalf_of" Claim:

     {
      "iss": "https://server.example.com",
      "sub": "24400320",
      "scopes": ["example"],
      "on_behalf_of": "eyJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJ ..."
     }


3.  Security Token Response

   A Security Token Response is returned from the Token Endpoint as a
   Token Response message containing these members:

   security_token  The returned Security Token.

   security_token_type  An identifier for the type of the returned
      Security Token.  If the Security Token is a JWT, this identifier
      is "urn:ietf:params:oauth:token-type:jwt".

   An example successful response is as follows:

     HTTP/1.1 200 OK
     Content-Type: application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
      "security_token": "eyJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJ ...",
      "security_token_type": "urn:ietf:params:oauth:token-type:jwt"
     }


4.  Conveying Eligibility to Act As Another Party

   It is useful to be able to make a statement that one party is
   authorized to act on behalf of another party.  This can be done by



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   having the party being acted for sign a Security Token containing a
   Claim identifying party that will act for it as an authorized actor.
   This statement can also optionally identify scopes in which the actor
   is eligible to act through another Claim.  The following Claims are
   defined for use in JWTs for these purposes:

   actor  Security Token that identifies a party who is asserted as
      being eligible to act for the party identified by the JWT
      containing this Claim.

   scopes  OPTIONAL.  An array of strings, each of which represents a
      service context for which the actor is asserted as being eligible
      to act for the party identified by the JWT containing this Claim.
      The array MUST contain at least one scope value.  The definition
      of these contexts is outside the scope of this specification.


5.  Implementation Considerations

   Implementations of the specification MUST implement support for using
   JWTs as the Security Tokens.  Other Security Token types MAY be
   supported.


6.  Open Issues

   The following decisions need to be made and updates this spec
   performed:

   o  Should we say anything about proof of possession of the target
      party's key in the On-Behalf-Of case beyond specifying the use of
      the "actor" Claim?

   o  Revise the text in the On-Behalf-Of vs. Impersonation Semantics
      section to better align the terminology used with the semantics
      specified.


7.  IANA Considerations

   The "security_token_request" Grant Type is to be registered in the
   OAuth Parameters registry.

   The "scopes", "act_as", and "on_behalf_of" Claims are to be
   registered in the JSON Web Token Claims registry.






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8.  Security Considerations

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

   In addition, on-behalf-of introduces its own unique security issues.
   Any time one principal is delegated the rights of another principal,
   the potential for abuse is always a concern.  That is why use of the
   "scopes" member is suggested.  The scope values restrict the contexts
   in which the delegated rights can be exercised.


9.  References

9.1.  Normative References

   [JWT]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token (work in
              progress), October 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework",
              RFC 6749, October 2012.

9.2.  Informative References

   [I-D.ietf-oauth-assertions]
              Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", draft-ietf-oauth-assertions
              (work in progress), July 2013.

   [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.

   [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>.






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Appendix A.  Document History

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

   -00

   o  Created initial version.


Author's Address

   Michael B. Jones
   Microsoft

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



































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