Network Working Group                                          S. Cantor
Internet-Draft                                     Shibboleth Consortium
Intended status: Standards Track                            S. Josefsson
Expires: August 31, 2012 February 14, 2013                                        SJD AB
                                                       February 28,
                                                         August 13, 2012

            SAML Enhanced Client SASL and GSS-API Mechanisms


   Security Assertion Markup Language (SAML) 2.0 is a generalized
   framework for the exchange of security-related information between
   asserting and relying parties.  Simple Authentication and Security
   Layer (SASL) and the Generic Security Service Application Program
   Interface (GSS-API) are application frameworks to facilitate an
   extensible authentication model.  This document specifies a SASL and
   GSS-API mechanism for SAML 2.0 that leverages the capabilities of a
   SAML-aware "enhanced client" to address significant barriers to
   federated authentication in a manner that encourages reuse of
   existing SAML bindings and profiles designed for non-browser

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 31, 2012. February 14, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Applicability for Non-HTTP Use Cases . . . . . . . . . . . . .  6
   4.  SAML SASL Mechanism Specification  . . . . . . . . . . . . . .  9
     4.1.  Advertisement  . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Initiation . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Server Response  . . . . . . . . . . . . . . . . . . . . .  9 10
     4.4.  User Authentication with Identity Provider . . . . . . . . 10
     4.5.  Client Response  . . . . . . . . . . . . . . . . . . . . . 10
     4.6.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.7.  Additional Notes . . . . . . . . . . . . . . . . . . . . . 10
   5.  SAML EC GSS-API Mechanism Specification  . . . . . . . . . . . 11 12
     5.1.  Session Key Derivation . . . . . . . . . . . . . . . . . . 12
       5.1.1.  Bearer Assertion Session Keys  . . . . . . . . . . . . 13
       5.1.2.  Holder of Key Session Keys . . . . . . . . . . . . . . 14
     5.2.  Per-Message Tokens . . . . . . . . . . . . . . . . . . . . 14
     5.3.  Pseudo-Random Function (PRF) . . . . . . . . . . . . . . . 14
     5.4.  GSS-API Principal Name Types for SAML EC . . . . . . . . . 11 15
       5.4.1.  Support for User Name Form . . . . . . . . . . . . . . 15
       5.4.2.  Support for Host-Based Service Name Form . . . . . . . 16
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20 24
     7.1.  Risks Left Unaddressed . . . . . . . . . . . . . . . . . . 20 24
     7.2.  User Privacy . . . . . . . . . . . . . . . . . . . . . . . 20 24
     7.3.  Collusion between RPs  . . . . . . . . . . . . . . . . . . 21 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22 26
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 27
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 23 27
     9.2.  Normative References for GSS-API Implementers  . . . . . . 24 28
     9.3.  Informative References . . . . . . . . . . . . . . . . . . 24 29
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 25 30
   Appendix B.  Changes . . . . . . . . . . . . . . . . . . . . . . . 26 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 32

1.  Introduction

   Security Assertion Markup Language (SAML) 2.0
   [OASIS.saml-core-2.0-os] 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], and interoperability profiles
   [OASIS.saml-profiles-2.0-os] designed for different use cases.
   Additional profiles and extensions are also routinely developed and

   Simple Authentication and Security Layer (SASL) [RFC4422] 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 [RFC3920].  The effect is to make authentication modular, so
   that newer authentication mechanisms can be added as needed.

   The Generic Security Service Application Program Interface (GSS-API)
   [RFC2743] provides a framework for applications to support multiple
   authentication mechanisms through a unified programming interface.
   This document defines a pure SASL mechanism for SAML, but it conforms
   to the 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.  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.

   The mechanisms specified in this document allow a SASL- or GSS-API-
   enabled server to act as a SAML relying party, or service provider
   (SP), by advertising this mechanism as an option for SASL or GSS-API
   clients that support the use of SAML to communicate identity and
   attribute information.  Clients supporting this mechanism are termed
   "enhanced clients" in SAML terminology because they understand the
   federated authentication model and have specific knowledge of the
   IdP(s) associated with the user.  This knowledge, and the ability to
   act on it, addresses a significant problem with browser-based SAML
   profiles known as the "discovery", or "where are you from?"  (WAYF)
   problem.  Obviating the need for the RP to interact with the client
   to determine the right IdP (and its network location) is both a user
   interface and security improvement.

   The SAML mechanism described in this document is an adaptation of an
   existing SAML profile, the Enhanced Client or Proxy (ECP) Profile
   (V2.0) [SAMLECP20], 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) or Secure Shell (SSH), will continued to be used.

   Figure 1 describes the interworking between SAML and SASL: this
   document requires enhancements to the RP and to the client (as the
   two SASL communication endpoints) but no changes to the SAML IdP are
   assumed apart from its support for the applicable SAML profile.  To
   accomplish this, a SAML protocol exchange between the RP and the IdP,
   brokered by the client, is tunneled within SASL.  There is no assumed
   communication between the RP and the IdP, but such communication may
   occur in conjunction with additional SAML-related profiles not in
   scope for this document.

                                       |  SAML     |
                                       |  Relying  |
                                       |  Party    |
                                       |           |
                                          | S|  |
                                        S | A|  |
                                        A | M|  |
                                        S | L|  |
                                        L |  |  |
                                          |  |  |
         +------------+                      v
         |            |                 +----------+
         |  SAML      |     SAML SOAP   |          |
         |  Identity  |<--------------->|  Client  |
         |  Provider  |      Binding    |          |
         +------------+                 +----------+

                    Figure 1: Interworking Architecture

2.  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 also assumed to be familiar with the terms used in the
   SAML 2.0 specification, and an understanding of the Enhanced Client
   or Proxy (ECP) Profile (V2.0) [SAMLECP20] is necessary, as part of
   this mechanism explicitly reuses and references it.

   This document can be implemented without knowledge of GSS-API since
   the normative aspects of the GS2 protocol syntax have been duplicated
   in this document.  The document may also be implemented to provide a
   GSS-API mechanism, and then knowledge of GSS-API is essential.  To
   faciliate these two variants, the references has been split into two
   parts, one part that provides normative references for all readers,
   and one part that adds additional normative references required for
   implementers that wish to implement the GSS-API portion.

3.  Applicability for Non-HTTP Use Cases

   While SAML is designed to support a variety of application scenarios,
   the profiles for authentication defined in the original standard are
   designed around HTTP [RFC2616] applications.  They are not, however,
   limited to browsers, because it was recognized that browsers suffer
   from a variety of functional and security deficiencies that would be
   useful to avoid where possible.  Specifically, the notion of an
   "Enhanced Client" (or a proxy acting as one on behalf of a browser,
   thus the term "ECP") was specified for a software component that acts
   somewhat like a browser from an application perspective, but includes
   limited, but sufficient, awareness of SAML to play a more conscious
   role in the authentication exchange between the RP and the IdP.  What
   follows is an outline of the Enhanced Client or Proxy (ECP) Profile
   (V2.0) [SAMLECP20], as applied to the web/HTTP service use case:

   1.  The Enhanced Client requests a resource of a Relying Party (RP)
       (via an HTTP request).  In doing so, it advertises its "enhanced"
       capability using HTTP headers.

   2.  The RP, desiring SAML authentication and noting the client's
       capabilities, responds not with an HTTP redirect or form, but
       with a SOAP [W3C.soap11] envelope containing a SAML
       <AuthnRequest> along with some supporting headers.  This request
       identifies the RP (and may be signed), and may provide hints to
       the client as to what IdPs the RP finds acceptable, but the
       choice of IdP is generally left to the client.

   3.  The client is then responsible for delivering the body of the
       SOAP message to the IdP it is instructed to use (often via
       configuration ahead of time).  The user authenticates to the IdP
       ahead of, during, or after the delivery of this message, and
       perhaps explicitly authorizes the response to the RP.

   4.  Whether authentication succeeds or fails, the IdP responds with
       its own SOAP envelope, generally containing a SAML <Response>
       message for delivery to the RP.  In a successful case, the
       message will include a SAML <Assertion> containing
       authentication, and possibly attribute, information about the
       user.  Either the response or assertion alone is signed, and the
       assertion may be encrypted to a key negotiated with or known to
       belong to the RP.

   5.  The client then delivers the SOAP envelope containing the
       <Response> to the RP at a location the IdP directs (which acts as
       an additional, though limited, defense against MITM attacks).
       This completes the SAML exchange.

   6.  The RP now has sufficient identity information to approve the
       original HTTP request or not, and acts accordingly.  Everything
       between the original request and this response can be thought of
       as an "interruption" of the original HTTP exchange.

   When considering this flow in the context of an arbitrary application
   protocol and SASL, the RP and the client both must change their code
   to implement this SASL mechanism, but the IdP can remain untouched.
   The existing RP/client exchange that is tunneled through HTTP maps
   well to the tunneling of that same exchange in SASL.  In the parlance
   of SASL [RFC4422], this mechanism is "client-first" for consistency
   with GS2.  The steps are shown below:

   1.  The server MAY advertise the SAML20EC and/or SAML20EC-PLUS

   2.  The client initiates a SASL authentication with SAML20EC or

   3.  The server sends the client a challenge consisting of a SOAP
       envelope containing its SAML <AuthnRequest>.

   4.  The SASL client unpacks the SOAP message and communicates with
       its chosen IdP to relay the SAML <AuthnRequest> to it.  This
       communication, and the authentication with the IdP, proceeds
       separately from the SASL process.

   5.  Upon completion of the exchange with the IdP, the client responds
       to the SASL server with a SOAP envelope containing the SAML
       <Response> it obtained, or a SOAP fault, as warranted.

   6.  The SASL Server indicates success or failure.

   Note: The details of the SAML processing, which are consistent with
   the Enhanced Client or Proxy (ECP) Profile (V2.0) [SAMLECP20], are
   such that the client MUST interact with the IdP in order to complete
   any SASL exchange with the RP.  The assertions issued by the IdP for
   the purposes of the profile, and by extension this SASL mechanism,
   are short lived, and therefore cannot be cached by the client for
   later use.

   Encompassed in step four is the client-driven selection of the IdP,
   authentication to it, and the acquisition of a response to provide to
   the SASL server.  These processes are all external to SASL.

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

      SASL Serv.       Client          IdP
         |>-----(1)----->|              | Advertisement
         |               |              |
         |<-----(2)-----<|              | Initiation
         |               |              |
         |>-----(3)----->|              | SASL Server Response
         |               |              |
         |               |<- - -(4)- - >| SOAP AuthnRequest + user authn
         |               |              |
         |<-----(5)-----<|              | SASL Client Response
         |               |              |
         |>-----(6)----->|              | Server sends Outcome
         |               |              |

       ----- = SASL
       - - - = SOAP over HTTPS (external to SASL)

                       Figure 2: Authentication flow

4.  SAML SASL Mechanism Specification

   Based on the previous figures, the following operations are defined
   by the SAML SASL mechanism:

4.1.  Advertisement

   To advertise that a server supports this mechanism, during
   application session initiation, it displays the name "SAML20EC"
   and/or "SAML20EC-PLUS" in the list of supported SASL mechanisms
   (depending on its support for channel binding).

4.2.  Initiation

   A client initiates "SAML20EC" or "SAML20EC-PLUS" authentication.  If
   supported by the application protocol, the client MAY include an
   initial response, otherwise it waits until the server has issued an
   empty challenge (because the mechanism is client-first).

   The format of the initial client response is as follows:


     hok = "urn:oasis:names:tc:SAML:2.0:cm:holder-of-key"


     mutual = "urn:oasis:names:tc:SAML:2.0:profiles:SSO:ecp:2.0:" \

     initial-resp = gs2-cb-flag "," [gs2-authzid] "," [holder-of-key] [hok] "," [mutual]

   The gs2-cb-flag MUST be set as defined in [RFC5801] to indicate
   whether the client supports channel binding.  This takes the place of
   the PAOS HTTP header extension used in [SAMLECP20] to indicate
   channel binding support.

   The optional "gs2-authzid" field holds the authorization identity, as
   requested by the client.

   The optional "holder-of-key" "hok" field is a constant that signals the client's
   support for stronger security by means of a locally held key.  This
   takes the place of the PAOS HTTP header extension used in [SAMLECP20]
   to indicate "holder of key" support.

   The optional "mutual" field is a constant that signals the client's
   desire for mutual authentication.  If set, the SASL server MUST
   digitally sign its SAML <AuthnRequest> message.  The URN constant
   above is a single string; the linefeed is shown for RFC formatting

4.3.  Server Response

   The SASL server responds with a SOAP envelope constructed in
   accordance with section 2.3.2 of [SAMLECP20].  This includes adhering
   to the SOAP header requirements of the SAML PAOS Binding
   [OASIS.saml-bindings-2.0-os], for compatibility with the existing
   profile.  Various SOAP headers are also consumed by the client in
   exactly the same manner prescribed by that section.

4.4.  User Authentication with Identity Provider

   Upon receipt of the Server Response (Section 4.3), the steps
   described in sections 2.3.3 through 2.3.6 of [SAMLECP20] are
   performed between the client and the chosen IdP.  The means by which
   the client determines the IdP to use, and where it is located, are
   out of scope of this mechanism.

   The exact means of authentication to the IdP are also out of scope,
   but clients supporting this mechanism MUST support HTTP Basic
   Authentication as defined in [RFC2617] and TLS client authentication
   as defined in [RFC5246].

4.5.  Client Response

   Assuming a response is obtained from the IdP, the client responds to
   the SASL server with a SOAP envelope constructed in accordance with
   section 2.3.7 of [SAMLECP20].  This includes adhering to the SOAP
   header requirements of the SAML PAOS Binding
   [OASIS.saml-bindings-2.0-os], for compatibility with the existing
   profile.  If the client is unable to obtain a response from the IdP,
   it responds to the SASL server with a SOAP envelope containing a SOAP

4.6.  Outcome

   The SAML protocol exchange having completed, the SASL server will
   transmit the outcome to the client depending on local validation of
   the client responses.  This outcome is transmitted in accordance with
   the application protocol in use.

4.7.  Additional Notes

   Because this mechanism is an adaptation of an HTTP-based profile,
   there are a few requirements outlined in [SAMLECP20] that make
   reference to a response URL that is normally used to regulate where
   the client returns information to the RP.  There are also security-
   related checks built into the profile that involve this location.

   For compatibility with existing IdP and profile behavior, and to
   provide for secure identification of the RP to the client, mutual authentication, the SASL server MUST populate the
   responseConsumerURL and AssertionConsumerServiceURL attributes with
   its service name,
   expressed as an absolute URI. name.  The parties then perform the steps described in
   [SAMLECP20] as usual.

   Similarly, the use of HTTP status signaling between the RP and client
   mandated by [SAMLECP20] may not be applicable.

5.  SAML EC 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 Enhanced Clients mechanism is also a GSS-API mechanism.
   The messages are the same, but a) the GS2 header on the client's
   first message is excluded when SAML EC 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 EC is

   SAML EC security contexts always have OID-TBD (IANA to assign: see
   IANA considerations).  The DER encoding of the OID is TBD.

   The mutual_state request flag (GSS_C_MUTUAL_FLAG) MAY be set to TRUE.  SAML EC does not support credential
   delegation, therefore SAML EC security contexts alway have TRUE,
   resulting in the
   deleg_state "mutual-auth" option set in the initial client
   response.  The security context mutual_state flag (GSS_C_DELEG_FLAG) is set to FALSE.

   The mutual authentication property of this mechanism relies on
   successfully comparing TRUE only
   if the server identity from the existing security
   layer (TLS, SSH, etc.) with digitally signs its SAML <AuthnRequest> message, and
   the negotiated target name.  Since identity provider signals this to the client in an <ecp:
   RequestAuthenticated> SOAP header block.

   If the mutual_state flag is not requested, or is not set, then the
   security layer is managed by the application outside of the GSS-API mechanism, the
   mechanism itself is unable to confirm responsible for authenticating the
   name.  For acceptor.  In this reason,
   case, applications MUST match the server identity from the existing
   security layer with the target name.  For TLS, this matching MUST be perfored
   performed as discussed in [RFC6125].  For SSH, this matching MUST be
   performed as discussed in [RFC4462].  Applications
   may rely on the GSS-API mechanism to perform this matching by passing
   the server identity as the targ_name parameter to

   The SAML EC mechanism does not support per-message tokens or

   The lifetime of a security context established with this mechanism
   SHOULD be limited by the value of a SessionNotOnOrAfter attribute, if
   any, in the <AuthnStatement> of the SAML assertion received by the

5.1.  GSS-API Principal Name Types for SAML EC

   SAML EC supports standard generic name syntaxes for acceptors such as
   GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4.1).  These
   service names

   SAML EC supports credential delegation through the issuance of SAML
   assertions that the issuing identity provider will accept as proof of
   authentication by a service on behalf of a user.  Such assertions
   MUST contain an <AudienceRestriction> condition element identifying
   the identity provider, and a <SubjectConfirmation> element that the
   acceptor can satisy.  In such a case, the security context will have
   its deleg_state flag (GSS_C_DELEG_FLAG) set to TRUE.

5.1.  Session Key Derivation

   Some GSS-API features (discussed in the following sections) require a
   session key be established as a result security context
   establishment.  In the common case of a "bearer" assertion in SAML,
   there is no secure mechanism by which such a key can be established.
   In other cases such as assertions based on "holder of key"
   confirmation, there may be.

   Information defining or describing the session key, or a process for
   deriving one, is communicated by the client to the server using an
   <ecp:SessionKey> SOAP header block, as defined in [SAMLECP20].  This
   header contains a <ds:KeyInfo> element as a generic container, and is
   designed to support reuse of mechanisms defined by [XMLENC11] or
   other specifications.

5.1.1.  Bearer Assertion Session Keys

   In the event that a client is not capable of supporting the "holder
   of key" option (or if other infrastructure components do not do so),
   but still wishes to make use of a session key, the client MAY
   generate a random value of 128 bits.  The octets are then base64-
   encoded and placed in a <CipherData> element inside a <ecp:
   SessionKey> SOAP header block in the following manner:

<ecp:SessionKey S:mustUnderstand="1"
  <ds:KeyInfo xmlns:ds="">
    <xenc11:DerivedKey xmlns:xenc11="">
      <xenc11:KeyDerivationMethod Algorithm="TBD">
        <xenc:CipherData xmlns:xenc="">

   The derivation algorithm of TBD (IANA to assign: see IANA
   considerations)\ signifies the client-generated approach described

   Applications that rely on this mechanism do so with the understanding
   that it is not a secure approach, but merely for compatibility when
   the risk is accepted.

5.1.2.  Holder of Key Session Keys

   In the event that a client is proving possession of a secret or
   private key, the session key can be communicated in a manner that
   proves its authenticity, or a formal key agreement algorithm may be
   supported.  For example, if the server has an elliptic curve public
   key, the ECDH-ES key agreement algorithm, as defined in [XMLENC11]
   may be used.

   If a key agreement or derivation process is not possible, then the
   mechanism defined in the previous section can be used, with the added
   benefit that the client's request will be strongly authenticated by a
   key.  However, if channel binding is not used, the generated key
   SHOULD be wrapped or transported under the protection of a key
   belonging to the server, such as an RSA public key.  The <xenc:
   EncryptedKey> element and associated key wrap and transport
   algorithms (see [XMLENC11]) can be used for this purpose.

   Note that this server no purpose in the bearer case, since if channel
   binding is not used, an attacker can generate its own key, and if it
   is used, there is no MITM to see the key.

5.2.  Per-Message Tokens

   The per-message tokens SHALL be the same as those for the Kerberos V
   GSS-API mechanism [RFC4121] (see Section 4.2 and sub-sections), using
   the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962].

   The replay_det_state (GSS_C_REPLAY_FLAG), sequence_state
   (GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG) and integ_avail
   (GSS_C_CONF_FLAG) security context flags are always set to TRUE.

   The 128-bit session "protocol key" SHALL be a session key established
   in a manner described in the previous section.  "Specific keys" are
   then derived as usual as described in Section 2 of [RFC4121],
   [RFC3961], and [RFC3962].

   The terms "protocol key" and "specific key" are Kerberos V5 terms

   SAML20EC is PROT_READY as soon as the SAML response message has been

5.3.  Pseudo-Random Function (PRF)

   The GSS-API has been extended with a Pseudo-Random Function (PRF)
   interface in [RFC4401].  The purpose is to enable applications to
   derive a cryptographic key from an established GSS-API security
   context.  This section defines a GSS_Pseudo_random that is applicable
   for the SAML20EC GSS-API mechanism.

   The GSS_Pseudo_random() [RFC4401] SHALL be the same as for the
   Kerberos V GSS-API mechanism [RFC4402].  There is no acceptor-
   asserted sub-session key, thus GSS_C_PRF_KEY_FULL and
   GSS_C_PRF_KEY_PARTIAL are equivalent.  The protocol key to be used
   for the GSS_Pseudo_random() SHALL be the same as the key defined in
   the previous section.

5.4.  GSS-API Principal Name Types for SAML EC

   Services that act as SAML relying parties are typically identified by
   means of a URI called an "entityID".  Clients that are named in the
   <Subject> element of a SAML assertion are typically identified by
   means of a <NameID> element, which is an extensible XML structure
   containing, at minimum, an element value that names the subject and a
   Format attribute.

   In practice, a GSS-API client and server are unlikely to know the
   name of the initiator as it will be expressed by the SAML identity
   provider upon completion of authentication.  It is also generally
   incorrect to assume that a particular acceptor name will directly map
   into a particular RP entityID, because there is often a layer of
   naming indirection between particular services on hosts and the
   identity of a relying party in SAML terms.

   The SAML EC mechanism is compatible with the common/expected name
   types used for acceptors and initiators, GSS_C_NT_HOSTBASED_SERVICE
   and GSS_C_NT_USER_NAME.  The mechanism provides for validation of the
   host-based service name in conjunction with the SAML exchange.  It
   does not attempt to solve the problem of mapping between an initiator
   "username", the user's identity while authenticating to the identity
   provider, and the information supplied by the identity provider to
   the acceptor.  These relationships must be managed through local
   policy at the initiator and acceptor.

5.4.1.  Support for User Name Form

   The GSS_C_NT_USER_NAME form represents the name of an individual
   user.  The client relies on this value to determine the appropriate
   credentials to use in authenticating to the identity provider, and
   supplies it to the server for use by the acceptor.

   No SAML-specific mechanism name type is defined.  SAML-based
   information associated with the initiator SHOULD be expressed to the
   acceptor using GSS-API naming extensions
   [I-D.ietf-kitten-gssapi-naming-exts], in accordance with

   [I-D.ietf-abfab-gss-eap-naming].  This information MUST be evaluated
   by the mechanism at the server to determine whether to accept the
   initiator name as a valid, authenticated name for the client.
   Failure to establish this MUST result in failure of the mechanism.

5.4.2.  Support for Host-Based Service Name Form

   The GSS_C_NT_HOSTBASED_SERVICE name form represents a service running
   on a host; it is textually represented as "service@host".  This name
   form is required by most SASL profiles and is used by many existing
   applications that use the Kerberos GSS-API mechanism.  Such a name is
   used directly by this mechanism as the effective
   AssertionConsumerService of the server.

   This value is used in the construction of the responseConsumerURL and
   AssertionConsumerServiceURL attributes, and for eventual comparison
   and validation by the client before completing the exchange.  The
   value MUST be securely associated with the SAML "entityID" entityID claimed by
   the RP, server by the identity provider, such as through the use of SAML
   metadata [OASIS.saml-metadata-2.0-os].

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

   The query, display, and exported name syntaxes for SAML EC principal
   names are all the same.  There are no SAML EC-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 SAML EC name normalization

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

6.  Example

   Suppose the user has an identity at the SAML IdP and
   a Jabber Identifier (jid) "", and wishes to
   authenticate his XMPP connection to (and
   and have established a SAML-capable trust relationship).
   The authentication on the wire would then look something like the

   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 and sends the
   initial client response (it is base64 encoded as specified by the
   XMPP SASL protocol profile):

   <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='SAML20EC'>

   The initial response is "n,," which signals that channel binding is
   not used, there is no authorization identity, and the client does not
   support key-based confirmation.

   Step 5: Server sends a challenge to client in the form of a SOAP
   envelope containing its SAML <AuthnRequest>:

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

   The Base64 [RFC4648] decoded envelope:

    <paos:Request xmlns:paos="urn:liberty:paos:2003-08"
      messageID="c3a4f8b9c2d" S:mustUnderstand="1"
      S:mustUnderstand="1" ProviderName="Jabber at">
      ID="c3a4f8b9c2d" Version="2.0" IssueInstant="2007-12-10T11:39:34Z"
      <saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
      <samlp:NameIDPolicy AllowCreate="true"
      <samlp:RequestedAuthnContext Comparison="exact">

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

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

   Step 6: Client relays the request to IdP in a SOAP message
   transmitted over HTTP (over TLS).  HTTP portion not shown, use of
   Basic Authentication is assumed.  The body of the SOAP envelope is
   exactly the same as received in the previous step.

           <!-- same as above -->

   Step 7: IdP responds to client with a SOAP response containing a SAML
   <Response> containing a short-lived SSO assertion (shown as an
   encrypted variant in the example).

      <ecp:Response S:mustUnderstand="1"
      <samlp:Response ID="d43h94r389309r" Version="2.0"
          IssueInstant="2007-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
          <!-- contents elided -->

   Step 8: Client sends SOAP envelope containing the SAML <Response> as
   a response to the SASL server's challenge:

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

   The Base64 [RFC4648] decoded envelope:

      <paos:Response xmlns:paos="urn:liberty:paos:2003-08"
        S:mustUnderstand="1" refToMessageID="6c3a4f8b9c2d"/>
      <samlp:Response ID="d43h94r389309r" Version="2.0"
          IssueInstant="2007-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
          <!-- contents elided -->

   Step 9: Server informs client of successful authentication:

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

   Step 9 (alt): Server informs client of failed authentication:

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

   Step 10: Client initiates a new stream to server:

   <stream:stream xmlns='jabber:client'
   to='' version='1.0'>
   Step 11: 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 12: Client binds a resource:

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

   Step 13: 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.

7.  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

   Version 2.0 of the Enhanced Client or Proxy Profile [SAMLECP20] adds
   optional support for channel binding and use of "Holder of Key"
   subject confirmation.  The former is strongly recommended for use
   with this mechanism to detect "Man in the Middle" attacks between the
   client and the RP without relying on flawed commercial TLS
   infrastructure.  The latter may be impractical in many cases, but is
   a valuable way of strengthening client authentication, protecting
   against phishing, and improving the overall mechanism.

7.1.  Risks Left Unaddressed

   The adaptation of a web-based profile that is largely designed around
   security-oblivious clients and a bearer model for security token
   validation results in a number of basic security exposures that
   should be weighed against the compatibility and client simplification
   benefits of this mechanism.

   When channel binding is not used, protection against "Man in the
   Middle" attacks is left to lower layer protocols such as TLS, and the
   development of user interfaces able to implement that has not been
   effectively demonstrated.  Failure to detect a MITM can result in
   phishing of the user's credentials if the attacker is between the
   client and IdP, or the theft and misuse of a short-lived credential
   (the SAML assertion) if the attacker is able to impersonate a RP.
   SAML allows for source address checking as a minor mitigation to the
   latter threat, but this is often impractical.  IdPs can mitigate to
   some extent the exposure of personal information to RP attackers by
   encrypting assertions with authenticated keys.

7.2.  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 activity, but there is nothing technically
   that prohibits the collection of this information.  SASL servers
   should be aware that SAML IdPs will track - to some extent - user
   access to their services.

   It is also out of scope of the mechanism to determine under what
   conditions an IdP will release particular information to a relying
   party, and it is generally unclear in what fashion user consent could
   be established in real time for the release of particular
   information.  The SOAP exchange with the IdP does not preclude such
   interaction, but neither does it define that interoperably.

7.3.  Collusion between RPs

   Depending on the information supplied by the IdP, it may be possible
   for RPs to correlate data that they have collected.  By using the
   same identifier to log into every RP, collusion between RPs is
   possible.  SAML supports the notion of pairwise, or targeted/
   directed, identity.  This allows the IdP to manage opaque, pairwise
   identifiers for each user that are specific to each RP.  However,
   correlation is often possible based on other attributes supplied, and
   is generally a topic that is beyond the scope of this mechanism.  It
   is sufficient to say that this mechanism does not introduce new
   correlation opportunities over and above the use of SAML in web-based
   use cases.

8.  IANA Considerations

   The IANA is requested to assign a new entry for this GSS mechanism in
   the sub-registry for SMI Security for Mechanism Codes, whose prefix
   is ( and to
   reference this specification in the registry.

   The IANA is requested to register the following SASL profile:

   SASL mechanism profiles: SAML20EC and SAML20EC-PLUS

   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

   The IANA is requested to assign a URI to identify the key derivation
   algorithm described in this document for client-generated session

9.  References

9.1.  Normative References

              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.

              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.

              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.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

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

   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, May 2006.

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

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

   [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.

              Cantor, S., "SAML V2.0 Enhanced Client or Proxy Profile
              Version 2.0", OASIS Working Draft OASIS.sstc-saml-ecp-
              v2.0-wd04, August 2011.
              v2.0-wd05, July 2012.

              Box, D., Ehnebuske, D., Kakivaya, G., Layman, A.,
              Mendelsohn, N., Nielsen, H., Thatte, S., and D. Winer,
              "Simple Object Access Protocol (SOAP) 1.1", W3C
              Note soap11, May 2000, <>.

              Hirsch, F. and T. Roessler, "XML Encryption Syntax and
              Processing Version 1.1", W3C Editor's Draft W3C.xmlenc-
              core-11-ed, July 2012.

9.2.  Normative References for GSS-API Implementers

              Hartman, S. and J. Howlett, "Name Attributes for the GSS-
              API EAP mechanism", draft-ietf-abfab-gss-eap-naming-03
              (work in progress), July 2012.

              Williams, N., Johansson, L., Hartman, S., and S.
              Josefsson, "GSS-API Naming Extensions",
              draft-ietf-kitten-gssapi-naming-exts-15 (work in
              progress), May 2012.

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

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
              Encryption for Kerberos 5", RFC 3962, February 2005.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.

   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401, February 2006.

   [RFC4402]  Williams, N., "A Pseudo-Random Function (PRF) for the
              Kerberos V Generic Security Service Application Program
              Interface (GSS-API) Mechanism", RFC 4402, February 2006.

   [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.

9.3.  Informative References

              Cantor, S., Moreh, J., Philpott, R., and E. Maler,
              "Metadata for the Security Assertion Markup Language
              (SAML) V2.0", OASIS Standard saml-metadata-2.0-os,
              March 2005.

   [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.

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

Appendix A.  Acknowledgments

   The authors would like to thank Klaas Wierenga, Sam Hartman, and Nico
   Williams, and Jim Basney for their contributions.

Appendix B.  Changes

   This section to be removed prior to publication.

   o  02, major revision of GSS-API material and updated references

   o  01, SSH language added, noted non-assumption of HTTP error
      handling, added guidance on life of security context.

   o  00, Initial Revision, first WG-adopted draft.  Removed support for
      unsolicited SAML responses.

Authors' Addresses

   Scott Cantor
   Shibboleth Consortium
   2740 Airport Drive
   Columbus, Ohio  43219
   United States

   Phone: +1 614 247 6147

   Simon Josefsson
   Hagagatan 24
   Stockholm  113 47