Network Working Group                                            E. Lear
Internet-Draft                                        Cisco Systems GmbH
Intended status: Standards Track                           H. Tschofenig
Expires: February 14, August 4, 2011                           Nokia Siemens Networks
                                                              H. Mauldin
                                                     Cisco Systems, Inc.
                                                            S. Josefsson
                                                                  SJD AB
                                                         August 13, 2010
                                                        January 31, 2011

                 A SASL & GSS-API Mechanism for OpenID


   OpenID 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 and GSS-API mechanism for OpenID that allows the
   integration of existing OpenID Identity Providers with applications
   using SASL and GSS-API.

Status of this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Applicability for non-HTTP Use Cases . . . . . . . . . . . . .  5
     2.1.  Binding SASL to OpenID in the Relying Party  . . . . . . .  8
     2.2.  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  OpenID SASL Mechanism Specification  . . . . . . . . . . . . . 10
     3.1.  Advertisement  . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Initiation . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.3.  Authentication Request . . . . . . . . . . . . . . . . . . 10
     3.4.  Server Response  . . . . . . . . . . . . . . . . . . . . . 11
   4.  OpenID GSS-API Mechanism Specification . . . . . . . . . . . . 12
     4.1.  GSS-API Principal Name Types for OpenID  . . . . . . . . . 12
   5.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
     6.1.  Binding OpenIDs to Authorization Identities  . . . . . . . 15
     6.2.  RP redirected by malicious URL to take an improper
           action . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  Session Swapping (Cross-Site Request Forgery)  . . . . . . 15
     6.4.  User Privacy . . . . . . . . . . . . . . . . . . . . . . . 16
     6.5.  Collusion between RPs  . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  Room for Improvement . . . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   9. 19
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 19 20
   Appendix A.  Changes . . . . . . . . . . . . . . . . . . . . . . . 20 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 22

1.  Introduction

   OpenID [OpenID] is a three-party protocol that provides a means for a
   user to offer identity assertions and other attributes to a web
   server (Relying Party) via the help of an identity provider.  The
   purpose of this system is to provide a way to verify that an end user
   controls an identifier.

   Simple Authentication and Security Layer (SASL) [RFC4422] (SASL) is
   used by application protocols such IMAP, POP and XMPP, with the goal
   of modularizing authentication and security layers, so that newer
   mechanisms can be added as needed.  This memo specifies just such a

   The Generic Security Service Application Program Interface (GSS-API)
   [RFC2743] 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].
   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 OpenID in order to assert identity and other
   attributes to relying parties.  As such, while servers (as relying
   parties) will advertise SASL mechanisms, clients will select the
   OpenID mechanism.

   The OpenID mechanism described in this memo aims to re-use the
   available OpenID specification 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

   Figure 1 describes the interworking between OpenID 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
   OpenID Provider (OP) are necessary.  To accomplish this goal indirect
   messaging required by the OpenID specification is tunneled within

                                    |           |
                                   >|  Relying  |
                                  / |  Party    |
                                //  |           |
                              //    +-----------+
                            //            ^
                   OpenID //           +--|--+
                        //             | O|  |
                       /             S | p|  |
                     //              A | e|  |
                   //                S | n|  |
                 //                  L | I|  |
               //                      | D|  |
             </                        +--|--+
      +------------+                      v
      |            |                 +----------+
      |  OpenID    |   OpenID        |          |
      |  Provider  |<--------------->|  Client  |
      |            |                 |          |
      +------------+                 +----------+

                    Figure 1: Interworking Architecture

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

1.2.  Applicability

   Because this mechanism transports information that should not be
   controlled by an attacker, the OpenID mechanism MUST only be used
   over channels protected by TLS [RFC5246], and the client MUST
   successfully validate the server certificate, or similar integrity
   protected and authenticated channels.

2.  Applicability for non-HTTP Use Cases

   OpenID was originally envisioned for HTTP/HTML based communications,
   and with the associated semantic, the idea being that the user would
   be redirected by the Relying Party to an identity provider who
   authenticates the user, and then sends identity information and other
   attributes (either directly or indirectly) to the Relying Party.  The
   identity provider in the OpenID specifications is referred to as an
   OpenID Provider (OP).  The actual protocol flow, as copied from the
   OpenID 2.0 specification, is as follows:

   1.  The end user initiates authentication by presenting a User-
       Supplied Identifier to the Relying Party via their User-Agent

   2.  After normalizing the User-Supplied Identifier, the Relying Party
       performs discovery on it and establishes the OP Endpoint URL that
       the end user uses for authentication.  It should be noted that
       the User-Supplied Identifier may be an OP Identifier, which
       allows selection of a Claimed Identifier at the OP or for the
       protocol to proceed without a Claimed Identifier if something
       else useful is being done via an extension.

   3.  The Relying Party and the OP optionally establish an association
       -- a shared secret established using Diffie-Hellman Key Exchange.
       The OP uses an association to sign subsequent messages and the
       Relying Party to verify those messages; this removes the need for
       subsequent direct requests to verify the signature after each
       authentication request/response.

   4.  The Relying Party redirects the end user's User-Agent to the OP
       with an OpenID Authentication request.  This occurs as stated in
       Section 10.3 of [RFC2616].

   5.  The OP authenticates the end user and establishes whether the end
       user will authenticate to, and share specific attributes with,
       the Relying Party.  For instance, the OP often asks the user what
       to do.  The manner in which the end user authenticates to their
       OP and any policies surrounding such authentication is out of
       scope of OpenID.

   6.  The OP redirects the end user's User-Agent back to the Relying
       Party with either an assertion that authentication is approved or
       a message that authentication failed.

   7.  The Relying Party verifies the information received from the OP
       including checking the Return URL, verifying the discovered
       information, checking the nonce, and verifying the signature by
       using either the shared key established during the association or
       by sending a direct request to the OP.

   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, it is a design constraint that the OP behavior remain
   untouched, in order for implementations to interoperate with existing
   IdPs.  Hence, an analog flow that interfaces the three parties needs
   to be created.  In the analog, we note that unlike a web server, the
   SASL server 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.  This
   will be discussed below.  By doing so, we externalize much of the
   authentiction from SASL.

   The steps are shown from below:

   1.   The Relying Party or SASL server advertises support for the SASL
        OpenID mechanism to the client.

   2.   The client initiates a SASL authentiation and transmits the
        User-Supplied Identifier as well as an optional return_to

   3.   After normalizing the User-Supplied Identifier, the Relying
        Party performs discovery on it and establishes the OP Endpoint
        URL that the end user uses for authentication.

   4.   The Relying Party and the OP optionally establish an association
        -- a shared secret established using Diffie-Hellman Key
        Exchange.  The OP uses an association to sign subsequent
        messages and the Relying Party to verify those messages; this
        removes the need for subsequent direct requests to verify the
        signature after each authentication request/response.

   5.   The Relying Party transmits an authentication request to the OP
        to obtain an assertion in the form of an indirect request.
        These messages are passed through the client rather than
        directly between the RP and the OP.  OpenID defines two methods
        for indirect communication, namely HTTP redirects and HTML form
        submission.  Both mechanisms are not directly applicable for
        usage with SASL.  To ensure that a standard OpenID 2.0 capable
        OP can be used a new method is defined in this document that
        requires the OpenID message content to be encoded using a
        Universal Resource Idenitifier (URI).  [RFC3986]

   6.   The SASL client now sends an empty response, as authentication
        continues via the normal OpenID flow.

   7.   At this point the client application MUST construct a URL
        containing the content received in the previous message from the
        RP.  This URL is transmitted to the OP either by the SASL client
        application or an appropriate handler, such as a browser.

   8.   Next the client optionally authenticates to the OP and then
        approves or disapproves authentication to the Relying Party.
        The manner in which the end user is authenticated to their
        respective OP and any policies surrounding such authentication
        is out of scope of OpenID and and hence also out of scope for
        this specification.  This step happens out of band from SASL.

   9.   The OP will convey information about the success or failure of
        the authentication phase back to the RP, again using an indirect
        response via the client browser or handler.  The client
        transmits over HTTP the redirect of the OP result to the RP.
        This step happens out of band from SASL.

   10.  The RP MAY send an OpenID check_authentication request directly
        to the OP, if no association has been established, and the OP
        should be expected to respond.  Again this step happens out of
        band from SASL.

   11.  The SASL server sends an appropriate SASL response to the
        client, with optional Open Simple Registry (SREG) attributes.

         SASL Serv.       Client          OP
            |>-----(1)----->|              | Advertisement
            |               |              |
            |<-----(2)-----<|              | Initiation
            |               |              |
            |> - - (3) - - - - - - - - - ->| Discovery
            |                              |
            |>- - -(4)- - - - - - - - - - >| Association
            |<- - -(4)- - - - - - - - - - <|
            |               |              |
            |>-----(5)----->|              | Indirect Auth Request
            |               |              |
            |<-----(6)-----<|              | Client Empty Response
            |               |               |
            |               |>- - (7)- - ->| Client GET to the OP (ext)
            |               |              |
            |               |<- - (8)- - ->| Client / OP Auth. (ext.)
            |               |              |
            |<- - -(9)- - - + - - - - - - <| HTTP(s) Indirect id_res
            |               |              |
            |<- - -(10)- - - - - - - - - ->| Optional check_authenticate
            |               |              |
            |>-----(11)---->|              | SASL completion with status

        ----- = SASL
        - - - = HTTP or SSL

   Note the directionality in SASL is such that the client MUST send an
   empty response.  Specifically, it processes the redirect and then
   awaits a final SASL decision, while the rest of the OpenID
   authentication process continues.

2.1.  Binding SASL to OpenID in the Relying Party

   To ensure that a specific request is bound, and in particular to ease
   interprocess communication, it may be necessary for the relying party
   to encode some sort of nonce in the URIs it transmits through the
   client for success or failure.  This can be done in any number of
   ways.  Examples would include making changes to the base URI or
   otherwise including an additional fragment.

2.2.  Discussion

   As mentioned above OpenID is primarily designed to interact with web-
   based applications.  Portions of the authentication stream are only
   defined in the crudest sense.  That is, when one is prompted to
   approve or disapprove an authentication, anything that one might find
   on a browser is allowed, including JavaScript, fancy style-sheets,
   etc.  Because of this lack of structure, implementations will need to
   invoke a fairly rich browser in order to insure that the
   authentication can be completed.

   Once there is an outcome, the SASL server needs to know about it.
   The astute will hopefully by now have noticed an empty client SASL
   challenge.  This is not to say that nothing is happening, but rather
   that authentication flow has shifted from SASL to OpenID, and will
   return when the server has an outcome to hand to the client.  The
   alternative to this flow is some signal from the HTML browser to the
   SASL client of the results that is in turn passed to the SASL server.
   The IPC issue this raises is substantial.  Better, we conclude, to
   externalize the authentication to the browser, and have an empty
   client challenge.

   OpenID is also meant to be used in serial within the web.  As such,
   there are no transaction-ids within the protocol.  A transaction id,
   can be included by the RP by appending it to the return_to URL.

3.  OpenID SASL Mechanism Specification

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

3.1.  Advertisement

   To advertise that a server supports OpenID, during application
   session initiation, it displays the name "OPENID20" in the list of
   supported SASL mechanisms.

3.2.  Initiation

   A client initiates an OpenID authentication with SASL by sending the
   GS2 header followed by the XRI or URI, as specified in the OpenID
   specification.  The GS2 header carries the optional authorization

       initial-response = gs2-header Auth-Identifier
       Auth-Identifier = Identifier ; authentication identifier
       Identifier = URI | XRI      ;  Identifer is specified in
                                   ;  Sec. 7.2 of the OpenID 2.0 spec.

   The "gs2-header" is specified in [I-D.ietf-sasl-gs2], and it is used
   as follows.  The "gs2-nonstd-flag" MUST NOT be present.  The "gs2-cb-
   flag" MUST be "n" because channel binding is not supported by this
   mechanism.  The "gs2-authzid" carries the optional authorization

   The XRI syntax is defined in [XRI2.0].  URI is specified in

3.3.  Authentication Request

   The SASL Server sends an OpenID message that contains an openid.mode
   of either "checkid_immediate" or "checkid_setup", as specified in
   Section 9.1 of the OpenID 2.0 specification.

   As part of this request, the SASL server MUST append a unique
   transaction id to the &quote;return_to" portion of the request.  The
   form of this transaction is left to the RP to decide, but SHOULD be
   large enough to be resistant to being guessed or attacked.

   The client now sends that request via an HTTP GET to the OP, as if
   redirected to do so from an HTTP server.

   The client MUST handle both user authentication to the OP and
   confirmation or rejection of the authentiation of the RP.

   After all authentication has been completed by the OP, and after the
   response has been sent to the client, the client will relay the
   response to the Relying Party via HTTP or SSL.

3.4.  Server Response

   The Relying Party now validates the response it received from the
   client via HTTP or SSL, as specified in the OpenID specification.

   The response by the Relying Party consists of an application specific
   response code indicating success or failure of authentication.  In
   the additional data, the server MAY include OpenID Simple Registry
   (SREG) attributes that are listed in Section 4 of [SREG1.0].  They
   are encoded as follows:

   1.  Strip "openid.sreg." from each attribute name.

   2.  Treat the concatentation of results as URI parameters that are
       separated by an ambersand (&) and encode as one would a URI,
       absent the scheme, authority, and the question mark.

   For example:

   More formally:

         outcome_data = [ sreg_avp *( "," sreg_avp ) ]
         sreg_avp     = sreg_attr "=" sreg_val
         sreg_attr    = sreg_word
         sreg_val     = sreg_word
         sreg_word    = 1* ( unreserved / pct-encoded )
                        ; pct-encoded from Section 2.1 of RFC 3896
                        ; unreserved from Section 2.3 of RFC 3896

   If the application protocol allows, openid.error and
   openid.error_code and any other useful diagnostic information SHOULD
   be included in authentication failures.

4.  OpenID 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 OpenID 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 OpenID 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 OpenID is

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

   The OpenID mechanism does not support per-message tokens or

4.1.  GSS-API Principal Name Types for OpenID

   OpenID supports standard generic name syntaxes for acceptors such as
   GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4.1).

   OpenID supports only a single name type for initiators:
   GSS_C_NT_USER_NAME.  GSS_C_NT_USER_NAME is the default name type for

   OpenID name normalization is covered by the OpenID specification, see
   [OpenID] section 7.2.

   The query, display, and exported name syntaxes for OpenID principal
   names are all the same.  There are no OpenID-specific name syntaxes
   -- applications should use generic GSS-API name types such as
   Section 4).  The exported name token does, of course, conform to
   [RFC2743], Section 3.2, but the "NAME" part of the token should be
   treated as a potential input string to the OpenID name normalization

   GSS-API name attributes may be defined in the future to hold the
   normalized OpenID Identifier.

5.  Example

   Suppose one has an OpenID of http://openid.example, and wishes to
   authenticate his IMAP connection to mail.example (where .example is
   the top level domain specified in [RFC2606]).  The user would input
   his Openid into his mail user agent, when he configures the account.
   In this case, no association is attempted between the OpenID Consumer
   and the OP.  The client will make use of the return_to attribute to
   capture results of the authentication to be redirected to the server.
   The authentication on the wire would then look something like the

       (S = IMAP server; C = IMAP client)

       C: < connects to IMAP port>
       S: * OK
       S: C1 OK Capability Completed
       C: C2 AUTHENTICATE OPENID biwsaHR0cDovL29wZW5pZC5leGFtcGxlLw==
       [  This is the base64 encoding of "n,,http://openid.example/".
          Server performs discovery on http://openid.example/ ]
       S: + aHR0cDovL29wZW5pZC5leGFtcGxlL29wZW5pZC8/b3BlbmlkLm5z
       [ This is the base64 encoding of "http://openid.example/openid/
          with line breaks and spaces added here for readibility.
       [ The client now sends the URL it received to a browser for
         processing. The user logs into http://openid.example, and
         agrees to authenticate imap://mail.example.  A redirect is
         passed back to the client browser who then connects to
         https://imap.example/consumer via SSL with the results.
         From an IMAP perspective, however, the client sends an empty
         response, and awaits mail.example.
         Server mail.example would now contact openid.example with an
         openid.check_authenticate message.  After that...
       S: + ZW1haWw9bGVhckBtYWlsLmV4YW1wbGUsZnVsbG5hbWU9RWxp
         [ Here the IMAP server has returned an SREG attribute of
           Line break added in this example for clarity. ]
         [ In IMAP client must send a blank response to receive data
           that is included in a success response. ]
       S: C2 OK

6.  Security Considerations

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

6.1.  Binding OpenIDs to Authorization Identities

   As specified in [RFC4422], the server is responsible for binding
   credentials to a specific authorization identity.  It is therefore
   necessary that either some sort of registration process takes place
   to register specific OpenIDs, or that only specific trusted OpenID
   Providers be allowed.  Some out of band knowledge may help this
   process along.  For instance, users of a particular domain may
   utilize a particular OP that enforces a mapping.

6.2.  RP redirected by malicious URL to take an improper action

   In the initial SASL client response a user or host can transmit a
   malicious response to the RP for purposes of taking advantage of
   weaknesses in the RP's OpenID implementation.  It is possible to add
   port numbers to the URL so that the outcome is the RP does a port
   scan of the site.  The URL could send the connection to an internal
   host or even the local host, which the attacker would not normally
   have access to.  The URL could contain a protocol other than http or
   https, such as file or ftp.

   To mitigate this attack, implementations should carefully analyze
   URLs received, eliminating any that would in some way be privileged.
   A log of those sites that fail SHOULD be kept, and limitations on
   queries from clients should be imposed, just as with any other
   authentication attempt.

6.3.  Session Swapping (Cross-Site Request Forgery)

   There is no defined mechanism in the OpenID protocol to bind the
   OpenID session to the user's browser.  An attacker may forge a cross-
   site request in the log-in form, which has the user logging into a
   proper RP as the attacker.  The user would not recognize they are
   logged into the site as the attacker, and so may reveal information
   at the RP.  Cross-site request forgery is a widely exploited
   vulnerability at web sites.  This is only concern in the context SASL
   in as much as the client is not configured with the Relying Party
   (e.g., SASL server) in a safe manner.

6.4.  User Privacy

   The OP is aware of each RP that a user logs into.  There is nothing
   in the protocol to hide this information from the OP.  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
   OpenID Providers will be track - to some extent - user access to
   their services and any additional information that OP provides.

6.5.  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 OpenID 2.0, directed identity was introduced.
   Directed identity allows the OP to transform the identifier the user
   typed in to another 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 OP is
   supporting it.

7.  Room for Improvement

   We note one area where there is possible room for improvement over
   existing OpenID implementations.  Because SASL is often implemented
   atop protocols that have required some amount of provisioning, it may
   be possible for the SASL client to signal the browser that it should
   increase scrutiny of invalid credentials.  How this would be done is
   beyond the scope of this specification, but may be the subject of
   future updates.  For instance, the browser may wish to fail the
   request entirely if the certificate is invalid and has not been
   accessed prior to this point.  One thing that this would require
   would be the exposure of a "return_to" URL by the SASL server in case
   of such failures, so that the authentication is not left hanging.

8.  IANA Considerations

   The IANA is requested to register the following SASL profile:

   SASL mechanism profile: OPENID20

   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


9.  Acknowledgments

   The authors would like to thank Alexey Melenkov, Joe Hildebrand, Mark
   Crispin, Chris Newman, Leif Johansson, and Klaas Wierenga for their
   review and contributions.


10.  Normative References

              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.

   [OpenID]   OpenID Foundation, "OpenID Authentication 2.0 - Final",
              December 2007.

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

   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

   [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.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [SREG1.0]  OpenID Foundation, "OpenID Simple Registration Extension
              version 1.0", June 2006.

   [XRI2.0]   Reed, D. and D. McAlpin, "Extensible Resource Identifier
              (XRI) Syntax V2.0", OASIS Standard xri-syntax-V2.0-cs,
              September 2005.

Appendix A.  Changes

   This section to be removed prior to publication.

   o  01 Specific text around possible improvements for OOB browser
      control in security considerations.  Also talk about transaction

   o  00 WG -00 draft.  Slight wording modifications abou design
      constraints per Alexey.

   o  02 Correct single (significant) error on mechanism name.

   o  01 Add nonce discussion, add authorized identity, explain a
      definition.  Add gs2 support.

   o  00 Initial Revision.

Authors' Addresses

   Eliot Lear
   Cisco Systems GmbH
   Richtistrasse 7
   Wallisellen, ZH  CH-8304

   Phone: +41 44 878 9200

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445

   Henry Mauldin
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 (800) 553-6387

   Simon Josefsson
   Hagagatan 24
   Stockholm  113 47