Network Working GroupK. Wierenga
Internet-DraftCisco Systems, Inc.
Intended status: Standards TrackE. Lear
Expires: January 13, 2011Cisco Systems GmbH
 July 12, 2010

A SASL Mechanism for SAML


Security Assertion Markup Language (SAML) has found its usage on the Internet for Web Single Sign-On. Simple Authentication and Security Layer (SASL) and the Generic Security Service Application Program Interface (GSS-API) are application frameworks to generalize authentication. This memo specifies a SASL mechanism and GSS-API mechanism for SAML 2.0 that allows the integration of existing SAML Identity Providers with applications using SASL and GSS-API.

Status of this Memo

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Table of Contents

1.  Introduction
2.  Terminology
3.  Applicability for non-HTTP Use Cases
4.  SAML SASL Mechanism Specification
    4.1.  Advertisement
    4.2.  Initiation
    4.3.  Server Redirect
    4.4.  Client Empty Response and other
    4.5.  Outcome and parameters
5.  SAML GSS-API Mechanism Specification
    5.1.  GSS-API Principal Name Types for SAML
6.  Channel Binding
7.  Example
8.  Security Considerations
    8.1.  Binding SAML subject identifiers to Authorization Identities
    8.2.  User Privacy
    8.3.  Collusion between RPs
9.  IANA Considerations
10.  Normative References
Appendix A.  Acknowledgments
Appendix B.  Changes
§  Authors' Addresses


1.  Introduction

Security Assertion Markup Language (SAML) 2.0 [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,” March 2005.) is a modular specification that provides various means for a user to be identified to a relying party (RP) through the exchange of (typically signed) assertions issued by an identity provider (IdP). It includes a number of protocols, protocol bindings [OASIS.saml‑bindings‑2.0‑os] (Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E. Maler, “Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.), and interoperability profiles [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,” March 2005.) designed for different use cases.

Simple Authentication and Security Layer (SASL) [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) is a generalized mechanism for identifying and authenticating a user and for optionally negotiating a security layer for subsequent protocol interactions. SASL is used by application protocols like IMAP, POP and XMPP. The effect is to make modular authentication, so that newer authentication mechanisms can be added as needed. This memo specifies just such a mechanism.

The Generic Security Service Application Program Interface (GSS-API) [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) provides a framework for applications to support multiple authentication mechanisms through a unified interface. This document defines a pure SASL mechanism for OpenID, but it conforms to the new bridge between SASL and the GSS-API called GS2 [I‑D.ietf‑sasl‑gs2] (Josefsson, S. and N. Williams, “Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family,” January 2010.). This means that this document defines both a SASL mechanism and a GSS-API mechanism. We want to point out that the GSS-API interface is optional for SASL implementers, and the GSS-API considerations can be avoided in environments that uses SASL directly without GSS-API.

As currently envisioned, this mechanism is to allow the interworking between SASL and SAML in order to assert identity and other attributes to relying parties. As such, while servers (as relying parties) will advertise SASL mechanisms (including SAML), clients will select the SAML SASL mechanism as their SASL mechanism of choice.

The SAML mechanism described in this memo aims to re-use the available SAML deployment to a maximum extent and therefore does not establish a separate authentication, integrity and confidentiality mechanism. It is anticipated that existing security layers, such as Transport Layer Security (TLS), will continued to be used.

Figure 1 (Interworking Architecture) describes the interworking between SAML and SASL: this document requires enhancements to the Relying Party and to the Client (as the two SASL communication end points) but no changes to the SAML Identity Provider are necessary. To accomplish this goal some indirect messaging is tunneled within SASL, and some use of external methods is made.

                                    |           |
                                   >|  Relying  |
                                  / |  Party    |
                                //  |           |
                              //    +-----------+
                   SAML/    //            ^
                   HTTPs  //           +--|--+
                        //             | S|  |
                       /             S | A|  |
                     //              A | M|  |
                   //                S | L|  |
                 //                  L |  |  |
               //                      |  |  |
             </                        +--|--+
      +------------+                      v
      |            |                 +----------+
      |  SAML      |     HTTPs       |          |
      |  Identity  |<--------------->|  Client  |
      |  Provider  |                 |          |
      +------------+                 +----------+

 Figure 1: Interworking Architecture 


2.  Terminology

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

The reader is assumed to be familiar with the terms used in the SAML 2.0 specification.


3.  Applicability for non-HTTP Use Cases

While SAML itself is merely a markup language, its common use case these days is with HTTP. What follows is a typical flow:

  1. The browser requests a resource of a Relying Party (RP) (via an HTTP request).
  2. The RP sends an HTTP redirect as described in Section 10.3 of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) to the browser to the Identity Provider (IdP) or an IdP discovery service with an authentication request that contains the name of resource being requested, some sort of a cookie and a return URL,
  3. The user authenticates to the IdP and perhaps authorizes the authentication to the service provider.
  4. In its authentication response, the IdP redirects the browser back to the RP with an authentication assertion (stating that the IdP vouches that the subject has successfully authenticated), optionally along with some additional attributes.
  5. RP now has sufficient identity information to approve access to the resource or not, and acts accordingly. The authentication is concluded.

When considering this flow in the context of SASL, we note that while the RP and the client both must change their code to implement this SASL mechanism, the IdP must remain untouched. The RP already has some sort of session (probably a TCP connection) established with the client. However, it may be necessary to redirect a SASL client to another application or handler. This will be discussed below. The steps are shown from below:

  1. The Relying Party or SASL server advertises support for the SASL SAML20 mechanism to the client
  2. The client initiates a SASL authentication with SAML20 and sends an IdP identity
  3. The Relying Party transmits an authentication request encoded using a Universal Resource Identifier (URI) as described in RFC 3986 [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) and a redirect to the IdP
  4. The SASL client now sends an empty response, as authentication continues via the normal SAML flow.
  5. At this point the SASL client MUST construct a URL containing the content received in the previous message from the RP. This URL is transmitted to the IdP either by the SASL client application or an appropriate handler, such as a browser.
  6. Next the client authenticates to the IdP. The manner in which the end user is authenticated to the IdP and any policies surrounding such authentication is out of scope for SAML and hence for this draft. This step happens out of band from SASL.
  7. The IdP will convey information about the success or failure of the authentication back to the the RP in the form of an Authentication Statement or failure, using a indirect response via the client browser or the handler. This step happens out of band from SASL.
  8. The SASL Server sends an appropriate SASL response to the client, along with an optional list of attributes

Please note: What is described here is the case in which the client has not previously authenticated. If the client can handle SAML internally it is possible that the client already holds a valid SAML authentication token so that the user does not need to be involved in the process anymore, but that would still be external to SASL.

With all of this in mind, the flow appears as follows:

         SASL Serv.       Client          IdP
            |>-----(1)----->|              | Advertisement
            |               |              |
            |<-----(2)-----<|              | Initiation
            |               |              |
            |>-----(3)----->|              | Authentication Request
            |               |              |
            |<-----(4)-----<|              | Empty Response
            |               |              |
            |               |< - - - - - ->| Client<>IDP
            |               |              | Authentication
            |               |              |
            |<- - - - - - - - - - - - - - -| Authentication Statement
            |               |              |
            |>-----(6)----->|              | SASL completion with
            |               |              | status
            |               |              |

       ----- = SASL
       - - - = HTTP or HTTPs (external to SASL)

 Figure 2: Authentication flow 


4.  SAML SASL Mechanism Specification

Based on the previous figure, the following operations are performed with the SAML SASL mechanism:


4.1.  Advertisement

To advertise that a server supports SAML 2.0, during application session initiation, it displays the name "SAML20" in the list of supported SASL mechanisms.


4.2.  Initiation

A client initiates a "SAML20" authentication with SASL by sending the GS2 header followed by the authentication identifier. The GS2 header carries the optional authorization identity.

     initial-response = gs2-header Idp-Identifier
     IdP-Identifier = Identifier ; IdP identifier
     Identifier = URI            ; IdP URI

The "gs2-header" is specified in [I‑D.ietf‑sasl‑gs2] (Josefsson, S. and N. Williams, “Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family,” January 2010.), and it is used as follows. The "gs2-nonstd-flag" MUST NOT be present. Regarding the channel binding "gs2-cb-flag" field, see Section 5. The "gs2- authzid" carries the optional authorization identity. URI is specified in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).


4.3.  Server Redirect

The SASL Server transmits a redirect to the IdP that the user provided, with a SAML authentication request in the form of a SAML assertion as one of the parameters.


4.4.  Client Empty Response and other

The SASL client hands the URI it received from the server in the previous step to either a browser or other appropriate handler to continue authentication externally while sending an empty response to the SASL server. The URI is encoded according to Section 3.4 of the SAML bindings 2.0 specification (Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E. Maler, “Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) [OASIS.saml‑bindings‑2.0‑os].


4.5.  Outcome and parameters

The SAML authentication having completed externally, the SASL server will transmit the outcome


5.  SAML GSS-API Mechanism Specification

This section and its sub-sections and all normative references of it not referenced elsewhere in this document are INFORMATIONAL for SASL implementors, but they are NORMATIVE for GSS-API implementors.

The SAML SASL mechanism is actually also a GSS-API mechanism. The messages are the same, but

a) the GS2 header on the client's first message and channel binding data is excluded when SAML is used as a GSS-API mechanism, and

b) the RFC2743 section 3.1 initial context token header is prefixed to the client's first authentication message (context token).

The GSS-API mechanism OID for SAML is XXXXX

SAML security contexts always have the mutual_state flag (GSS_C_MUTUAL_FLAG) set to TRUE. SAML does not support credential delegation, therefore SCRAM security contexts alway have the deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.

The SAML mechanism does not support per-message tokens or GSS_Pseudo_random.


5.1.  GSS-API Principal Name Types for SAML

SAML supports standard generic name syntaxes for acceptors such as GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.), Section 4.1). SAML supports only a single name type for initiators: GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for SAML. The query, display, and exported name syntaxes for SAML principal names are all the same. There are no SAML-specific name syntaxes -- applications should use generic GSS-API name types such as GSS_C_NT_USER_NAME and GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.), Section 4). The exported name token does, of course, conform to [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.), Section 3.2. GSS-API name attributes may be defined in the future to hold the SAML Subject Identifier.


6.  Channel Binding

The "gs2-cb-flag" MUST use "n" because channel binding data cannot be integrity protected by the SAML negotiation.


7.  Example

Suppose the user has an identity at the SAML IdP and a Jabber Identifier (JID) "", and wishes to authenticate his XMPP connection to The authentication on the wire would then look something like the following:

Step 1: Client initiates stream to server:

<stream:stream xmlns='jabber:client'
to='' version='1.0'>

Step 2: Server responds with a stream tag sent to client:

xmlns='jabber:client' xmlns:stream=''
id='some_id' from='' version='1.0'>

Step 3: Server informs client of available authentication mechanisms:

 <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>

Step 4: Client selects an authentication mechanism:

<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='SAML20'></auth>

Step 5: Server sends a BASE64 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648] encoded challenge to client in the form of an HTTP Redirect to the SAML IdP with the SAML Authentication Request as specified in the redirection url:


The decoded challenge is:

HTTP/1.1 302 Object Moved Date: 22 Oct 2009 07:00:49 GMT Location:

Where the decoded SAMLRequest looks like:

<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
    ID="_bec424fa5103428909a30ff1e31168327f79474984" Version="2.0"
    IssueInstant="2007-12-10T11:39:34Z" ForceAuthn="false"
 <saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
 <samlp:NameIDPolicy xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
     SPNameQualifier="" AllowCreate="true" />
 Ê    urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport

Step 5 (alt): Server returns error to client:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>

Step 6: Client sends a BASE64 encoded empty response to the challenge:

<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>

[ The client now sends the URL to a browser for processing. The browser engages in a normal SAML authentication flow (external to SASL), like redirection to the Identity Provider (, the user logs into, and agrees to authenticate to A redirect is passed back to the client browser who sends the AuthN response to the server, containing the subject-identifier as an attribute. If the AuthN response doesn't contain the JID, the server maps the subject-identifier received from the IdP to a JID]

Step 7: Server informs client of successful authentication:

<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

Step 7 (alt): Server informs client of failed authentication:

<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>

Step 8: Client initiates a new stream to server:

<stream:stream xmlns='jabber:client'
to='' version='1.0'>

Step 9: Server responds by sending a stream header to client along with any additional features (or an empty features element):

<stream:stream xmlns='jabber:client'
id='c2s_345' from='' version='1.0'>
 <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
 <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>

Step 10: Client binds a resource:

   <iq type='set' id='bind_1'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>

Step 11: Server informs client of successful resource binding:

   <iq type='result' id='bind_1'>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>

Please note: line breaks were added to the base64 for clarity.


8.  Security Considerations

This section will address only security considerations associated with the use of SAML with SASL applications. For considerations relating to SAML in general, the reader is referred to the SAML specification and to other literature. Similarly, for general SASL Security Considerations, the reader is referred to that specification.


8.1.  Binding SAML subject identifiers to Authorization Identities

As specified in [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), the server is responsible for binding credentials to a specific authorization identity. It is therefore necessary that only specific trusted IdPs be allowed. This is typical part of SAML trust establishment between RP's and IdP.


8.2.  User Privacy

The IdP is aware of each RP that a user logs into. There is nothing in the protocol to hide this information from the IdP. It is not a requirement to track the visits, but there is nothing that prohibits the collection of information. SASL servers should be aware that SAML IdPs will track - to some extent - user access to their services.


8.3.  Collusion between RPs

It is possible for RPs to link data that they have collected on you. By using the same identifier to log into every RP, collusion between RPs is possible. In SAML, targeted identity was introduced. Targeted identity allows the IdP to transform the identifier the user typed in to an opaque identifier. This way the RP would never see the actual user identifier, but a randomly generated identifier. This is an option the user has to understand and decide to use if the IdP is supporting it.


9.  IANA Considerations

The IANA is requested to register the following SASL profile:

SASL mechanism profile: SAML20

Security Considerations: See this document

Published Specification: See this document

For further information: Contact the authors of this document.

Owner/Change controller: the IETF

Note: None


10. Normative References

[I-D.ietf-sasl-gs2] Josefsson, S. and N. Williams, “Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family,” draft-ietf-sasl-gs2-20 (work in progress), January 2010 (TXT).
[OASIS.saml-bindings-2.0-os] Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E. Maler, “Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0,” OASIS Standard saml-bindings-2.0-os, March 2005.
[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.
[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.
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” RFC 2616, June 1999 (TXT, PS, PDF, HTML, XML).
[RFC2743] Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” RFC 2743, January 2000 (TXT).
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[RFC4422] Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” RFC 4422, June 2006 (TXT).
[RFC4648] Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 4648, October 2006 (TXT).


Appendix A.  Acknowledgments

The authors would like to thank Scott Cantor, Joe Hildebrand, Josh Howlett, Leif Johansson, Simon Josefsson, Diego Lopez, Hank Mauldin, RL 'Bob' Morgan and Hannes Tschofenig for their review and contributions.


Appendix B.  Changes

This section to be removed prior to publication.


Authors' Addresses

  Klaas Wierenga
  Cisco Systems, Inc.
  Haarlerbergweg 13-19
  Amsterdam, Noord-Holland 1101 CH
Phone:  +31 20 357 1752
  Eliot Lear
  Cisco Systems GmbH
  Richtistrasse 7
  Wallisellen, ZH CH-8304
Phone:  +41 44 878 9200