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Versions: 00 01 02 03 04 05 06 07 08 RFC 6616

Network Working Group                                            E. Lear
Internet-Draft                                        Cisco Systems GmbH
Intended status: Standards Track                           H. Tschofenig
Expires: January 12, 2012                         Nokia Siemens Networks
                                                              H. Mauldin
                                                     Cisco Systems, Inc.
                                                            S. Josefsson
                                                                  SJD AB
                                                           July 11, 2011


                 A SASL & GSS-API Mechanism for OpenID
                    draft-ietf-kitten-sasl-openid-04

Abstract

   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

   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 January 12, 2012.

Copyright Notice

   Copyright (c) 2011 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



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   (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 . . . . . . . . . . . . . . . . . . . . . . . . .  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.  Initiation . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Authentication Request . . . . . . . . . . . . . . . . . . 11
     3.3.  Server Response  . . . . . . . . . . . . . . . . . . . . . 11
     3.4.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 12
   4.  OpenID GSS-API Mechanism Specification . . . . . . . . . . . . 13
     4.1.  GSS-API Principal Name Types for OpenID  . . . . . . . . . 13
   5.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     6.1.  Binding OpenIDs to Authorization Identities  . . . . . . . 18
     6.2.  RP redirected by malicious URL to take an improper
           action . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.3.  Session Swapping (Cross-Site Request Forgery)  . . . . . . 18
     6.4.  User Privacy . . . . . . . . . . . . . . . . . . . . . . . 19
     6.5.  Collusion between RPs  . . . . . . . . . . . . . . . . . . 19
   7.  Room for Improvement . . . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  Changes . . . . . . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26











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1.  Introduction

   OpenID [OpenID] is a web-based 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 [RFC3501], POP [RFC1939] and
   XMPP [RFC3920], with the goal of modularizing authentication and
   security layers, so that newer mechanisms can be added as needed.
   This memo specifies just such a mechanism.

   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 [RFC5801].  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 OpenID
   mechanism to the 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) [RFC5246], will continued to be used.  This
   specification is appropriate for use when a browser is available.

   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 through
   the SASL/GSS-API mechanism.








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

                    Figure 1: Interworking Architecture

1.1.  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].

   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, and the client MUST successfully
   validate the server certificate, or similar integrity protected and
   authenticated channels.  [RFC5280][RFC6125]











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2.  Applicability for non-HTTP Use Cases

   OpenID was originally envisioned for HTTP [RFC2616] and HTML
   [W3C.REC-html401-19991224] 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
       (e.g., http://user.example.com).

   2.  After normalizing the User-Supplied Identifier as described in
       Section 7.2 of [OpenID], 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.  This process is desccribed in
       Section 8 of [OpenID].

   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.




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   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 authentication and transmits the
        User-Supplied Identifier as its first response.  The SASL
        mechanism is client-first, and as explained in [RFC4422] the
        server will send an empty challenge if needed.

   3.   After normalizing the User-Supplied Identifier as discussed in
        [OpenID], 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



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        requires the OpenID message content to be encoded using a
        Universal Resource Idenitifier (URI).  [RFC3986]

   6.   The SASL client now sends an response consisting of "=", to
        indicate that 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.



















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         SASL Serv.       Client          OP
            |>-----(1)----->|              | Advertisement
            |               |              |
            |<-----(2)-----<|              | Initiation
            |               |              |
            |> - - (3) - - - - - - - - - ->| Discovery
            |                              |
            |>- - -(4)- - - - - - - - - - >| Association
            |<- - -(4)- - - - - - - - - - <|
            |               |              |
            |>-----(5)----->|              | Indirect Auth Request
            |               |              |
            |<-----(6)-----<|              | Client "=" 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 HTTPS


   Note the directionality in SASL is such that the client MUST send the
   "=" 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



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   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 "=" client SASL
   response.  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 "=" client
   response.

   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.
































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3.  OpenID SASL Mechanism Specification

   This section specifies the details of the OpenID SASL mechanism.
   Recall section 5 of [RFC4422] for what needs to be described here.

   The name of this mechanism "OPENID20".  The mechanism is capable of
   transferring an authorization identity (via "gs2-header").  The
   mechanism does not offer a security layer.

   The mechanism is client-first.  The first mechanism message from the
   client to the server is the "initial-response" described below.  As
   described in [RFC4422], if the application protocol does not support
   sending a client-response together with the authentication request,
   the server will send an empty server-challenge to let the client
   begin.

   The second mechanism message is from the server to the client, the
   "authentication_request" described below.

   The third mechanism message is from client to the server, and is the
   fixed message consisting of "=".

   The fourth mechanism message is from the server to the client,
   described below as "outcome_data" (with SREG attributes), sent as
   additional data when indicating a successful outcome.

3.1.  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
   identity.

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


   The "gs2-header" is specified in [RFC5801], 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
   identity.

   The XRI syntax is defined in [XRI2.0].  URI is specified in
   [RFC3986].




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3.2.  Authentication Request

   The SASL Server sends the URL resulting from the OpenID
   authentication request, containing an "openid.mode" of either
   "checkid_immediate" or "checkid_setup", as specified in Section 9.1
   of the OpenID 2.0 specification.

          authentication-request = URI

   As part of this request, the SASL server MUST append a unique
   transaction id to the "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(S), as specified previously in
   the transaction ("return_to").

3.3.  Server Response

   The Relying Party now validates the response it received from the
   client via HTTP or HTTPS, as specified in the OpenID specification,
   using the "return_to" URI given previsiously in the transaction.

   The response by the Relying Party constitutes a SASL mechanism
   outcome, and SHALL be used to set state in the server accordingly,
   and it shall be used by the server to report that state to the SASL
   client as described in [RFC4422] Section 3.6.  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: email=lear@example.com&fullname=Eliot%20Lear

   More formally:



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         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 3986
                        ; unreserved from Section 2.3 of RFC 3986


   In the case of failures, the response MUST follow this syntax:

        outcome_data = "openid.error" "=" sreg_val *( "," sregp_avp )

3.4.  Error Handling

   [RFC4422] Section 3.6 explicitly prohibits additional information in
   an unsuccessful authentication outcome.  Therefore, the openid.error
   and openid.error_code are to be sent as an additional challenge in
   the event of an unsuccessful outcome.  In this case, as the protocol
   is lock step, the client will follow with an additional exchange
   containing "=", after which the server will respond with an
   application-level outcome.





























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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
   OpenID user takes the role of the GSS-API Initiator and the OpenID
   Relying Party takes the role of the GSS-API Acceptor.  The OpenId
   Provider does not have a role in GSS-API, and is considered an
   internal matter for the OpenID 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 OID-TBD (IANA to assign: see
   IANA considerations).

   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 mutual authentication property of this mechanism relies on
   successfully comparing the TLS server identity with the negotiated
   target name.  Since the TLS channel is managed by the application
   outside of the GSS-API mechanism, the mechanism itself is unable to
   confirm the name while the application is able to perform this
   comparison for the mechanism.  For this reason, applications MUST
   match the TLS server identity with the target name, as discussed in
   [RFC6125].

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

   The [RFC5587] mechanism attributes for this mechanism are
   GSS_C_MA_MECH_CONCRETE, GSS_C_MA_ITOK_FRAMED, and GSS_C_MA_AUTH_INIT.

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.



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   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
   GSS_C_NT_USER_NAME and GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743],
   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
   rules.

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





































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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.
   Note the use of [RFC4959] for initial response.  The authentication
   on the wire would then look something like the following:








































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       (S = IMAP server; C = IMAP client)

       C: < connects to IMAP port>
       S: * OK
       C: C1 CAPABILITY
       S: * CAPABILITY IMAP4rev1 SASL-IR SORT [...] AUTH=OPENID20
       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
            PWh0dHA6Ly9zcGVjcy5vcGVuaWQubmV0L2F1dGgvMi4wJm9wZW5p
            ZC5yZXR1cm5fdG89aHR0cHM6Ly9tYWlsLmV4YW1wbGUvY29uc3Vt
            ZXIvMWVmODg4YyZvcGVuaWQuY2xhaW1lZF9pZD1odHRwczovL29w
            ZW5pZC5leGFtcGxlLyZvcGVuaWQuaWRlbnRpdHk9aHR0cHM6Ly9v
            cGVuaWQuZXhhbXBsZS8mb3BlbmlkLnJlYWxtPWltYXA6Ly9tYWls
            LmV4YW1wbGUmb3BlbmlkLm1vZGU9Y2hlY2tpZF9zZXR1cA==
       [ This is the base64 encoding of "http://openid.example/openid/
             ?openid.ns=http://specs.openid.net/auth/2.0
             &openid.return_to=https://mail.example/consumer/1ef888c
             &openid.claimed_id=https://openid.example/
             &openid.identity=https://openid.example/
             &openid.realm=imap://mail.example
             &openid.mode=checkid_setup"
          with line breaks and spaces added here for readibility.
       ]
       C: PQ==
       [ 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 the "="
         response, and awaits mail.example.
         Server mail.example would now contact openid.example with an
         openid.check_authenticate message.  After that...
       ]
       S: + ZW1haWw9bGVhckBtYWlsLmV4YW1wbGUsZnVsbG5hbWU9RWxp
            b3QlMjBMZWFy
         [ Here the IMAP server has returned an SREG attribute of
           email=lear@mail.example,fullname=Eliot%20Lear.
           Line break added in this example for clarity. ]
       C:
         [ In IMAP client must send a blank response to receive data
           that is included in a success response. ]
       S: C2 OK

   In this example, the SASL server / RP has made use of a transaction



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   id 1ef888c.


















































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6.  Security Considerations

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

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.  It is RECOMMENDED that only http or https
   schemas be accepted.

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.



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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.
































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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 the given
   URL is the beginning of a sensitive transaction, and that increased
   scrutiny should be given.  A signal of some form would need to come
   from an appropriately authorized agent that the sensitive transaction
   is complete.  An example behavior during this sensitive period might
   be increased scrutiny of broken trust chains in certificates, or
   perhaps disallowing such trust chains altogether.







































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8.  IANA Considerations

   The IANA is requested to update the SASL Mechanism Registry using the
   following template, as described in [RFC4422].

   SASL mechanism name: OPENID20

   Security Considerations: See this document

   Published specification: See this document

   Person & email address to contact for further information: Authors of
   this document

   Intended usage: COMMON

   Owner/Change controller: IETF

   Note: None

   The IANA is further requested to assign an OID for this GSS mechanism
   in the SMI numbers registry, with the prefix of
   iso.org.dod.internet.security.mechanisms (1.3.6.1.5.5) and to
   reference this specification in the registry.



























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9.  Acknowledgments

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














































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10.  References

10.1.  Normative References

   [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.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5587]  Williams, N., "Extended Generic Security Service Mechanism
              Inquiry APIs", RFC 5587, July 2009.

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, July 2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.



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   [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.

10.2.  Informative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC4959]  Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
              Simple Authentication and Security Layer (SASL) Initial
              Client Response", RFC 4959, September 2007.

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
























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Appendix A.  Changes

   This section to be removed prior to publication.

   o  03 Clarifies messages and ordering, and replace the empty message
      with a "=" message.

   o  02 Address all WGLC comments.

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

   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.





























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Authors' Addresses

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

   Phone: +41 44 878 9200
   Email: lear@cisco.com


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


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

   Phone: +1 (800) 553-6387
   Email: hmauldin@cisco.com


   Simon Josefsson
   SJD AB
   Hagagatan 24
   Stockholm  113 47
   SE

   Email: simon@josefsson.org
   URI:   http://josefsson.org/










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