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Versions: 00 01 draft-ietf-oauth-saml2-bearer

                                                        B. Campbell, Ed.
Internet-Draft                                       Ping Identity Corp.
Intended status: Standards Track                            C. Mortimore
Expires: January 28, 2011                                 Salesforce.com
                                                           July 27, 2010


            SAML 2.0 Bearer Assertion Profile for OAuth 2.0
                      draft-campbell-oauth-saml-00

Abstract

   This specification defines the use of a SAML 2.0 bearer assertion as
   means for requesting an OAuth 2.0 access token.

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 28, 2011.

Copyright Notice

   Copyright (c) 2010 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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . . . 3
   2.  SAML Assertion Access Token Request . . . . . . . . . . . . . . 3
     2.1.  Client Requests Access Token  . . . . . . . . . . . . . . . 4
     2.2.  Assertion Format and Processing Requirements  . . . . . . . 5
     2.3.  Error Response  . . . . . . . . . . . . . . . . . . . . . . 6
     2.4.  Example (non-normative) . . . . . . . . . . . . . . . . . . 7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . . . 8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9



































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1.  Introduction

   The Security Assertion Markup Language (SAML) 2.0
   [OASIS.saml-core-2.0-os], is an XML-based framework that provides a
   means for a subject to be identified across security domains.  The
   SAML specification, while primarily targeted at providing cross
   domain web browser single sign-on, was also designed to be modular
   and extensible to facilitate use in other contexts.  The Assertion,
   an XML security token, is a fundamental construct of SAML that is
   most often adopted for use in other protocols and specifications.  An
   assertion is generally issued by an identity provider and consumed by
   a service provider who relies on its content to identify the subject
   for security related purposes.

   OAuth 2.0 [I-D.ietf.oauth-v2] provides a method for making
   authenticated HTTP requests to a resource using an access token.
   Tokens are issued to third-party clients by an authorization server
   with the (sometimes implicit) approval of the resource owner.  OAuth
   defines multiple profiles for obtaining access tokens to support a
   wide range of client types and user experiences.  One such method is
   the use of an assertion which supports the case when a client wishes
   to exchange an existing security token for an access token.  However
   the Oauth 2.0 leaves the specific format and validation of the
   assertion out of scope.

   This specification profiles the specific use of a SAML 2.0 bearer
   assertion in requesting an access token using the assertion
   grant_type from OAuth 2.0.  The format and processing rules for the
   SAML assertion defined in this specification are intentionally
   similar to those in the Web Browser SSO Profile defined in
   [OASIS.saml-profiles-2.0-os] with the goal of reusing, to the extent
   reasonable, concepts and patterns from that well established profile.

1.1.  Notational 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].

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.


2.  SAML Assertion Access Token Request

   A SAML assertion is used to request an access token when a client
   wishes to utilize an existing trust relationship, expressed though
   the semantics of the SAML assertion, to establish authorization



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   without directly involving the resource owner's approval at the
   resource server.

   The process by which the client obtains the assertion is out of
   scope.



        +--------+                                  +---------------+
        |        |                                  |               |
        |        |>--(A)-- SAML 2.0 Assertion ----->| Authorization |
        | Client |                                  |     Server    |
        |        |<--(B)---- Access Token ---------<|               |
        |        |                                  |               |
        +--------+                                  +---------------+


                 Figure 1: Assertion Access Token Request

   The request/response flow illustrated in Figure 1 includes the
   following steps:

   (A)  The client sends an access token request to the authorization
        server and includes a SAML 2.0 assertion.

   (B)  The authorization server validates the assertion per the
        processing rules defined in this specification and issues an
        access token.

2.1.  Client Requests Access Token

   The client requests an access token by making an HTTP "POST" request
   to the token endpoint using an assertion as an access grant.  The
   client makes an access token request, as defined in the OAuth, with
   the following parameter definitions taking precedence in the
   constructed URI:

   assertion_type
         REQUIRED.  The value of the assertion_format parameter MUST be
         "http://oauth.net/assertion_type/saml/2.0/bearer"

   assertion
         REQUIRED.  The value of the assertion parameter MUST contain a
         single SAML 2.0 Assertion.  The SAML assertion XML data MUST be
         encoded using base64url, where the encoding adheres to the
         definition in Section 5 of RFC4648 [RFC4648] and where the
         padding bits set to zero.  To to avoid the need for subsequent
         encoding steps (by "application/x-www-form-urlencoded"



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         [W3C.REC-html401-19991224], for example), the base64url encoded
         data SHOULD NOT be line wrapped and pad characters ("=") SHOULD
         NOT be included.

2.2.  Assertion Format and Processing Requirements

   The authorization server MUST validate the assertion according to the
   criteria below and, if valid, issues an access token response as
   described in [I-D.ietf.oauth-v2].  The access token SHOULD be issued
   only for the subject of the assertion

   o  The Assertion's <Issuer> element MUST contain a unique identifier
      for the entity that issued the assertion; the Format attribute
      MUST be omitted or have a value of
      "urn:oasis:names:tc:SAML:2.0:nameid-format:entity".

   o  The assertion MUST contain a <Subject> element that identifies the
      resource owner for whom the access token is being requested.

   o  The <Subject> element MUST contain a single <SubjectConfirmation>
      element and it MUST have a Method attribute with a value of
      "urn:oasis:names:tc:SAML:2.0:cm:bearer".

   o  The <SubjectConfirmation> element MUST contain a single
      <SubjectConfirmationData> element.

   o  The <SubjectConfirmationData> element MUST have a Recipient
      attribute with a value indicating the token endpoint URL of the
      authorization server.  The authorization server MUST verify that
      the value of the Recipient attribute matches the token endpoint
      URL (or an acceptable alias) to which the assertion was delivered.

   o  The <SubjectConfirmationData> element MUST have a NotOnOrAfter
      attribute that limits the window during which the assertion can be
      confirmed.  The authorization server MUST verify that the
      NotOnOrAfter instant has not passed, subject to allowable clock
      skew between systems.

   o  The <SubjectConfirmationData> element MUST NOT contain a NotBefore
      attribute.

   o  The <SubjectConfirmationData> element MAY also contain an Address
      attribute limiting the client address from which the assertion can
      be delivered.  Verification of the Address is at the discretion of
      the authorization server.

   o  If the assertion issuer authenticated the subject, the assertion
      SHOULD contain a single <AuthnStatement> representing that



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      authentication event.

   o  If the assertion was issued with the intention that the client act
      autonomously on behalf of the subject, an <AuthnStatement> SHOULD
      NOT be included.

   o  Other statements, in particular, <AttributeStatement> elements MAY
      be included in the assertion.

   o  The assertion MUST contain an <AudienceRestriction> element with
      an <Audience> element containing a URI reference that identifies
      the authorization server, or the service provider SAML entity of
      its controlling domain, as an intended audience.  The
      authorization server MUST verify that it is an intended audience
      for the assertion.

   o  The authorization server MAY ensure that bearer assertions are not
      replayed, by maintaining the set of used ID values for the length
      of time for which the assertion would be considered valid based on
      the NotOnOrAfter attribute in the <SubjectConfirmationData>.

   o  The assertion MUST be digitally signed by the issuer and the
      authorization server MUST verify the signature.

   o  Encrypted elements MAY appear in place of their plain text
      counterparts as defined in [OASIS.saml-core-2.0-os].

   o  The authorization server MUST verify that the assertion is valid
      in all other respects per [OASIS.saml-core-2.0-os].

2.3.  Error Response

   If the assertion is not valid, or its subject confirmation
   requirements cannot be met, the the authorization server MUST
   construct an error response as defined in [I-D.ietf.oauth-v2].  The
   value of the error parameter MUST be the "invalid_grant" error code.
   The authorization server MAY include additional information regarding
   the reasons the assertion was considered invalid using the
   error_description or error_uri parameters.












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   For example:


   HTTP/1.1 400 Bad Request
   Content-Type: application/json
   Cache-Control: no-store

   {
     "error":"invalid_grant",
     "error_description":"invalid signature"
   }


2.4.  Example (non-normative)

   Though non-normative, the following examples illustrate what a
   conformant assertion and access token request would look like.

   Below is an example SAML 2.0 Assertion (whitespace formatting is for
   display purposes only):


   <Assertion IssueInstant="1970-01-01T00:00:00.000Z"
     ID="ef1xsbZxPV2oqjd7HTLRLIBlBb7"
     Version="2.0"
     xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
    <Issuer>https://saml-idp.example.com</Issuer>
    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
     [...omitted for brevity...]
    </ds:Signature>
    <Subject>
     <NameID
       Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
      brian@example.com
     </NameID>
     <SubjectConfirmation
       Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
      <SubjectConfirmationData
       NotOnOrAfter="1970-01-01T00:05:00.000Z"
       Recipient="https://authz.example.net/token.oauth2"/>
      </SubjectConfirmation>
    </Subject>
    <Conditions>
     <AudienceRestriction>
      <Audience>https://saml-sp.example.net</Audience>
     </AudienceRestriction>
    </Conditions>
   </Assertion>



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                   Figure 2: Example SAML 2.0 Assertion

   To present the assertion shown in the previous example as part of an
   access token request, for example, the client makes the following
   HTTPS request (line breaks are for display purposes only):


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

   grant_type=assertion&assertion_type=http%3A%2F%2Foauth.net%2Fasse
   rtion_type%2Fsaml%2F2.0%2Fbearer&assertion=PEFzc2VydGlvbiBJc3N1ZU
   [...omitted for brevity...]b24-PC9Db25kaXRpb25zPjwvQXNzZXJ0aW9uPg

                         Figure 3: Example Request


3.  Security Considerations

   No additional considerations beyond those described within the OAuth
   2.0 Protocol [I-D.ietf.oauth-v2] and in the Security and Privacy
   Considerations for the OASIS Security Assertion Markup Language
   (SAML) V2.0 [OASIS.saml-sec-consider-2.0-os].


4.  IANA Considerations

   This document has no actions for IANA.


Appendix A.  Contributors

   The following people contributed wording and concepts to this
   document: Patrick Harding, Peter Motyka, Peter Saint-Andre and David
   Waite


5.  References

5.1.  Normative References

   [I-D.ietf.oauth-v2]
              Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
              OAuth 2.0 Protocol", Jun 2010.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,



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              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

5.2.  Informative References

   [OASIS.saml-profiles-2.0-os]
              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Maler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard OASIS.saml-profiles-2.0-os, March 2005.

   [OASIS.saml-sec-consider-2.0-os]
              Hirsch, F., Philpott, R., and E. Maler, "Security and
              Privacy Considerations for the OASIS Security Markup
              Language (SAML) V2.0", OASIS Standard saml-sec-consider-
              2.0-os, March 2005.

   [W3C.REC-html401-19991224]
              Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.


Authors' Addresses

   Brian Campbell (editor)
   Ping Identity Corp.

   Email: brian.d.campbell@gmail.com


   Chuck Mortimore
   Salesforce.com

   Email: cmortimore@salesforce.com








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