TOC 
OAuth Working GroupB. Campbell
Internet-DraftPing Identity
Intended status: Standards TrackC. Mortimore
Expires: March 19, 2013Salesforce
 September 15, 2012


SAML 2.0 Bearer Assertion Profiles for OAuth 2.0
draft-ietf-oauth-saml2-bearer-14

Abstract

This specification defines the use of a SAML 2.0 Bearer Assertion as a means for requesting an OAuth 2.0 access token as well as for use as a means of client authentication.

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 http://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 March 19, 2013.

Copyright Notice

Copyright (c) 2012 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.



Table of Contents

1.  Introduction
    1.1.  Notational Conventions
    1.2.  Terminology
2.  HTTP Parameter Bindings for Transporting Assertions
    2.1.  Using SAML Assertions as Authorization Grants
    2.2.  Using SAML Assertions for Client Authentication
3.  Assertion Format and Processing Requirements
    3.1.  Authorization Grant Processing
    3.2.  Client Authentication Processing
4.  Authorization Grant Example
5.  Security Considerations
6.  IANA Considerations
    6.1.  Sub-Namespace Registration of urn:ietf:params:oauth:grant-type:saml2-bearer
    6.2.  Sub-Namespace Registration of urn:ietf:params:oauth:client-assertion-type:saml2-bearer
7.  References
    7.1.  Normative References
    7.2.  Informative References
Appendix A.  Acknowledgements
Appendix B.  Document History
§  Authors' Addresses




 TOC 

1.  Introduction

The Security Assertion Markup Language (SAML) 2.0 (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) [OASIS.saml‑core‑2.0‑os] is an XML-based framework that allows identity and security information to be shared 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 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 Assertion's subject for security related purposes.

The OAuth 2.0 Authorization Protocol (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [I‑D.ietf‑oauth‑v2] provides a method for making authenticated HTTP requests to a resource using an access token. Access tokens are issued to third-party clients by an authorization server (AS) with the (sometimes implicit) approval of the resource owner. In OAuth, an authorization grant is an abstract term used to describe intermediate credentials that represent the resource owner authorization. An authorization grant is used by the client to obtain an access token. Several authorization grant types are defined to support a wide range of client types and user experiences. OAuth also allows for the definition of new extension grant types to support additional clients or to provide a bridge between OAuth and other trust frameworks. Finally, OAuth allows the definition of additional authentication mechanisms to be used by clients when interacting with the authorization server.

The OAuth 2.0 Assertion Profile (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” September 2012.) [I‑D.ietf‑oauth‑assertions] is an abstract extension to OAuth 2.0 that provides a general framework for the use of Assertions as client credentials and/or authorization grants with OAuth 2.0. This specification profiles the OAuth 2.0 Assertion Profile (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” September 2012.) [I‑D.ietf‑oauth‑assertions] to define an extension grant type that uses a SAML 2.0 Bearer Assertion to request an OAuth 2.0 access token as well as for use as client credentials. The format and processing rules for the SAML Assertion defined in this specification are intentionally similar, though not identical, to those in the Web Browser SSO Profile defined in SAML Profiles (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,” March 2005.) [OASIS.saml‑profiles‑2.0‑os]. This specification is reusing, to the extent reasonable, concepts and patterns from that well-established Profile.

This document defines how a SAML Assertion can be used to request an access token when a client wishes to utilize an existing trust relationship, expressed through the semantics of (and digital signature calculated over) the SAML Assertion, without a direct user approval step at the authorization server. It also defines how a SAML Assertion can be used as a client authentication mechanism. The use of an Assertion for client authentication is orthogonal to and separable from using an Assertion as an authorization grant. They can be used either in combination or separately. Client assertion authentication is nothing more than an alternative way for a client to authenticate to the token endpoint and must be used in conjunction with some grant type to form a complete and meaningful protocol request. Assertion authorization grants may be used with or without client authentication or identification. Whether or not client authentication is needed in conjunction with an assertion authorization grant, as well as the supported types of client authentication, are policy decisions at the discretion of the authorization server.

The process by which the client obtains the SAML Assertion, prior to exchanging it with the authorization server or using it for client authentication, is out of scope.



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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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

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



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1.2.  Terminology

All terms are as defined in The OAuth 2.0 Authorization Protocol (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [I‑D.ietf‑oauth‑v2], OAuth 2.0 Assertion Profile (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” September 2012.) [I‑D.ietf‑oauth‑assertions], and Security Assertion Markup Language (SAML) 2.0 (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) [OASIS.saml‑core‑2.0‑os].



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2.  HTTP Parameter Bindings for Transporting Assertions

The OAuth 2.0 Assertion Profile (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” September 2012.) [I‑D.ietf‑oauth‑assertions] defines generic HTTP parameters for transporting Assertions during interactions with a token endpoint. This section defines the values of those parameters for use with SAML 2.0 Bearer Assertions.



 TOC 

2.1.  Using SAML Assertions as Authorization Grants

To use a SAML Bearer Assertion as an authorization grant, use the following parameter values and encodings.

The value of the grant_type parameter MUST be urn:ietf:params:oauth:grant-type:saml2-bearer.

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 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648] and where the padding bits are set to zero. To avoid the need for subsequent encoding steps (by "application/x-www-form-urlencoded" (Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification,” December 1999.) [W3C.REC‑html401‑19991224], for example), the base64url encoded data SHOULD NOT be line wrapped and pad characters ("=") SHOULD NOT be included.

The following non-normative example demonstrates an Access Token Request with an assertion as an authorization grant (with extra line breaks for display purposes only):

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

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer&
  assertion=PHNhbWxwOl...[omitted for brevity]...ZT4


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2.2.  Using SAML Assertions for Client Authentication

To use a SAML Bearer Assertion for client authentication grant, use the following parameter values and encodings.

The value of the client_assertion_type parameter MUST be urn:ietf:params:oauth:client-assertion-type:saml2-bearer.

The value of the client_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 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648] and where the padding bits are set to zero. To avoid the need for subsequent encoding steps (by "application/x-www-form-urlencoded" (Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification,” December 1999.) [W3C.REC‑html401‑19991224], for example), the base64url encoded data SHOULD NOT be line wrapped and pad characters ("=") SHOULD NOT be included.

The following non-normative example demonstrates a client authenticating using an assertion during the presentation of an authorization code grant in an Access Token Request (with extra line breaks for display purposes only):

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

  grant_type=authorization_code&
  code=vAZEIHjQTHuGgaSvyW9hO0RpusLzkvTOww3trZBxZpo&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
  %3Aclient-assertion-type%3Asaml2-bearer&
  client_assertion=PHNhbW...[omitted for brevity]...ZT


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3.  Assertion Format and Processing Requirements

In order to issue an access token response as described in The OAuth 2.0 Authorization Protocol (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [I‑D.ietf‑oauth‑v2] or to rely on an Assertion for client authentication, the authorization server MUST validate the Assertion according to the criteria below. Application of additional restrictions and policy are at the discretion of the authorization server.



 TOC 

3.1.  Authorization Grant Processing

If present, the authorization server MUST also validate the client credentials.

If the Assertion is not valid, or its subject confirmation requirements cannot be met, the authorization server MUST construct an error response as defined in OAuth 2.0 (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [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.

For example:

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

  {
    "error":"invalid_grant",
    "error_description":"Audience validation failed"
  }


 TOC 

3.2.  Client Authentication Processing

If the client Assertion is not valid, or its subject confirmation requirements cannot be met, the authorization server MUST construct an error response as defined in OAuth 2.0 (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [I‑D.ietf‑oauth‑v2]. The value of the error parameter MUST be the invalid_client 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.



 TOC 

4.  Authorization Grant Example

Though non-normative, the following examples illustrate what a conforming 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="2010-10-01T20:07:34.619Z"
    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="2010-10-01T20:12:34.619Z"
       Recipient="https://authz.example.net/token.oauth2"/>
     </SubjectConfirmation>
    </Subject>
    <Conditions>
      <AudienceRestriction>
        <Audience>https://saml-sp.example.net</Audience>
      </AudienceRestriction>
    </Conditions>
    <AuthnStatement AuthnInstant="2010-10-01T20:07:34.371Z">
      <AuthnContext>
        <AuthnContextClassRef>
          urn:oasis:names:tc:SAML:2.0:ac:classes:X509
        </AuthnContextClassRef>
      </AuthnContext>
    </AuthnStatement>
  </Assertion>
 Figure 1: 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 might make the following HTTPS request (with extra line breaks for display purposes only):

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

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-
  bearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ9IjIwMTEtMDU
  [...omitted for brevity...]aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-
 Figure 2: Example Request 



 TOC 

5.  Security Considerations

No additional security considerations apply beyond those described within The OAuth 2.0 Authorization Protocol (Hardt, D., “The OAuth 2.0 Authorization Framework,” August 2012.) [I‑D.ietf‑oauth‑v2], the OAuth 2.0 Assertion Profile (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” September 2012.) [I‑D.ietf‑oauth‑assertions], and in the Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0 (Hirsch, F., Philpott, R., and E. Maler, “Security and Privacy Considerations for the OASIS Security Markup Language (SAML) V2.0,” March 2005.) [OASIS.saml‑sec‑consider‑2.0‑os].



 TOC 

6.  IANA Considerations



 TOC 

6.1.  Sub-Namespace Registration of urn:ietf:params:oauth:grant-type:saml2-bearer

This is a request to IANA to please register the value grant-type:saml2-bearer in the registry urn:ietf:params:oauth established in An IETF URN Sub-Namespace for OAuth (Campbell, B. and H. Tschofenig, “An IETF URN Sub-Namespace for OAuth,” July 2012.) [I‑D.ietf‑oauth‑urn‑sub‑ns].



 TOC 

6.2.  Sub-Namespace Registration of urn:ietf:params:oauth:client-assertion-type:saml2-bearer

This is a request to IANA to please register the value client-assertion-type:saml2-bearer in the registry urn:ietf:params:oauth established in An IETF URN Sub-Namespace for OAuth (Campbell, B. and H. Tschofenig, “An IETF URN Sub-Namespace for OAuth,” July 2012.) [I‑D.ietf‑oauth‑urn‑sub‑ns].



 TOC 

7.  References



 TOC 

7.1. Normative References

[I-D.ietf-oauth-assertions] Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0,” draft-ietf-oauth-assertions-05 (work in progress), September 2012 (TXT, PDF).
[I-D.ietf-oauth-urn-sub-ns] Campbell, B. and H. Tschofenig, “An IETF URN Sub-Namespace for OAuth,” draft-ietf-oauth-urn-sub-ns-06 (work in progress), July 2012 (TXT).
[I-D.ietf-oauth-v2] Hardt, D., “The OAuth 2.0 Authorization Framework,” draft-ietf-oauth-v2-31 (work in progress), August 2012 (TXT, PDF).
[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.
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC4648] Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 4648, October 2006 (TXT).


 TOC 

7.2. Informative References

[OASIS.saml-deleg-cs] Cantor, S., Ed., “SAML V2.0 Condition for Delegation Restriction,” Nov 2009.
[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 (HTML).


 TOC 

Appendix A.  Acknowledgements

The following people contributed wording and concepts to this document: Paul Madsen, Patrick Harding, Peter Motykowski, Eran Hammer, Peter Saint-Andre, Ian Barnett, Eric Fazendin, Torsten Lodderstedt, Susan Harper, Scott Tomilson, Scott Cantor, Michael B. Jones, Hannes Tschofenig, David Waite, Phil Hunt, and Mukesh Bhatnagar.



 TOC 

Appendix B.  Document History

[[ to be removed by RFC editor before publication as an RFC ]]

draft-ietf-oauth-saml2-bearer-14

draft-ietf-oauth-saml2-bearer-13

draft-ietf-oauth-saml2-bearer-12

draft-ietf-oauth-saml2-bearer-11

draft-ietf-oauth-saml2-bearer-10

draft-ietf-oauth-saml2-bearer-09

draft-ietf-oauth-saml2-bearer-08

draft-ietf-oauth-saml2-bearer-07

draft-ietf-oauth-saml2-bearer-06

draft-ietf-oauth-saml2-bearer-05

draft-ietf-oauth-saml2-bearer-04

draft-ietf-oauth-saml2-bearer-03

draft-ietf-oauth-saml2-bearer-02

draft-ietf-oauth-saml2-bearer-01

draft-ietf-oauth-saml2-bearer-00

draft-campbell-oauth-saml-01

draft-campbell-oauth-saml-00



 TOC 

Authors' Addresses

  Brian Campbell
  Ping Identity Corp.
Email:  brian.d.campbell@gmail.com
  
  Chuck Mortimore
  Salesforce.com
Email:  cmortimore@salesforce.com