Network Working Group                                          S. Cantor
Internet-Draft                                     Shibboleth Consortium
Intended status: Standards Track                            S. Josefsson
Expires: April 27, October 26, 2018                                         SJD AB
                                                          April 24, 2017 2018

            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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   This Internet-Draft will expire on April 27, October 26, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Applicability for Non-HTTP Use Cases  . . . . . . . . . . . .   5
   4.  SAML Enhanced Client SASL Mechanism Specification . . . . . .   8
     4.1.  Advertisement . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Initiation  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Server Response . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  User Authentication with Identity Provider  . . . . . . .   9
     4.5.  Client Response . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Outcome . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.7.  Additional Notes  . . . . . . . . . . . . . . . . . . . .  10
   5.  SAML EC GSS-API Mechanism Specification . . . . . . . . . . .  10
     5.1.  GSS-API Credential Delegation . . . . . . . . . . . . . .  11
     5.2.  GSS-API Channel Binding . . . . . . . . . . . . . . . . .  12
     5.3.  Session Key Derivation  . . . . . . . . . . . . . . . . .  12
       5.3.1.  Generated by Identity Provider  . . . . . . . . . . .  13
       5.3.2.  Alternate Key Derivation Mechanisms . . . . . . . . .  14
     5.4.  Per-Message Tokens  . . . . . . . . . . . . . . . . . . .  14
     5.5.  Pseudo-Random Function (PRF)  . . . . . . . . . . . . . .  15
     5.6.  GSS-API Principal Name Types for SAML EC  . . . . . . . .  15
       5.6.1.  User Naming Considerations  . . . . . . . . . . . . .  16
       5.6.2.  Service Naming Considerations . . . . . . . . . . . .  17
   6.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     7.1.  Risks Left Unaddressed  . . . . . . . . . . . . . . . . .  26
     7.2.  User Privacy  . . . . . . . . . . . . . . . . . . . . . .  26
     7.3.  Collusion between RPs . . . . . . . . . . . . . . . . . .  27
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  GSS-API and SASL Mechanism Registration . . . . . . . . .  27
     8.2.  XML Namespace Name for SAML-EC  . . . . . . . . . . . . .  27
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     9.2.  Normative References for GSS-API Implementers . . . . . .  29
     9.3.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  XML Schema . . . . . . . . . . . . . . . . . . . . .  31
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  33
   Appendix C.  Changes  . . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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 use 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.  In a "dumb" client such as a web browser, various intrusive
   user interface techniques are used to determine the appropriate IdP
   to use because the request to the IdP is generated as an HTTP
   redirect by the RP, which does not generally have prior knowledge of
   the IdP to use.  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].

   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 one or more SAML <Assertion> elements
       containing authentication, and possibly attribute, statements
       about the subject.  Either the response or each assertion is
       signed, and the assertion(s) 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 unmodified.
   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.

   Note also that unlike an HTTP-based profile, the IdP cannot
   participate in the selection of, or evaluation of, the location to
   which the SASL Client Response will be delivered by the client.  The
   use of GSS-API Channel Binding is an important mitigation of the risk
   of a "Man in the Middle" attack between the client and RP, as is the
   use of a negotiated or derived session key in whatever protocol is
   secured by this mechanism.

   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 Enhanced Client 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.

   In accordance with [RFC5801] the "-PLUS" variant indicates that the
   server supports channel binding and would be selected by a client
   with that capability.

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 ("initresp") is as follows:

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

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

  del = "urn:oasis:names:tc:SAML:2.0:conditions:delegation"

  initresp = gs2-cb-flag "," [gs2-authzid] "," [hok] "," [mut] "," [del]

   The gs2-cb-flag 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 "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 "mut" 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

   The optional "del" field is a constant that signals the client's
   desire for the acceptor to request an assertion usable for delegation
   of the client's identity to the acceptor.

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,
   or must otherwise signal failure, it responds to the SASL server with
   a SOAP envelope containing a SOAP fault.

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 mutual authentication, the SASL server MUST populate the
   responseConsumerURL and AssertionConsumerServiceURL attributes with
   its service name.  As discussed in Section 5.6.2, most SASL profiles
   rely on a service name format of "service@host", but regardless of
   the form, the service name is used directly rather than transformed
   into an absolute URI if it is not already one, and MUST be percent-
   encoded per [RFC3986].

   The IdP MUST securely associate the service name with the SAML
   entityID claimed by the SASL server, such as through the use of SAML
   metadata [OASIS.saml-metadata-2.0-os].  If metadata is used, a SASL
   service's <SPSSODescriptor> role MUST contain a corresponding
   <AssertionConsumerService> whose Location attribute contains the
   appropriate service name, as described above.  The Binding attribute
   MUST be one of "urn:ietf:params:xml:ns:samlec" (RECOMMENDED) or
   "urn:oasis:names:tc:SAML:2.0:bindings:PAOS" (for compatibility with
   older implementations of the ECP profile in existing identity
   provider software).

   Finally, note that 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 Enhanced Client SASL mechanism is also a GSS-API mechanism.
   The messages are the same, but a) the GS2 [RFC5801] 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 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,
   resulting in the "mut" option set in the initial client response.
   The security context mutual_state flag is set to TRUE only if the
   server digitally signs its SAML <AuthnRequest> message and the
   signature and signing credential are appropriately verified by the
   identity provider.  The identity provider signals this to the client
   in an <ecp:RequestAuthenticated> SOAP header block.

   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> element(s) of the SAML assertion(s)
   received by the RP.  By convention, in the rare case that multiple
   valid/confirmed assertions containing <AuthnStatement> elements are
   received, the most restrictive SessionNotOnOrAfter is generally

5.1.  GSS-API Credential Delegation

   This mechanism can support credential delegation through the issuance
   of SAML assertions that an identity provider will accept as proof of
   authentication by a service on behalf of a subject.  An initiator may
   request delegation of its credentials by setting the "del" option
   field in the initial client response to

   An acceptor, upon receipt of this constant, requests a delegated
   assertion by including in its <AuthnRequest> message a <Conditions>
   element containing an <AudienceRestriction> identifying the IdP as a
   desired audience for the assertion(s) to be issued.  In the event
   that the specific identity provider to be used is unknown, the
   constant "urn:oasis:names:tc:SAML:2.0:conditions:delegation" may be
   used as a stand-in, per Section 2.3.2 of [SAMLECP20].

   Upon receipt of an assertion satisfying this property, and containing
   a <SubjectConfirmation> element that the acceptor can satisfy, the
   security context may have its deleg_state flag (GSS_C_DELEG_FLAG) set
   to TRUE.

   The identity provider, if it issues a delegated assertion to the
   acceptor, MUST include in the SOAP response to the initiator a
   <samlec:Delegated> SOAP header block, indicating that delegation was
   enabled.  It has no content, other than mandatory SOAP attributes (an
   example follows):

   <samlec:Delegated xmlns:samlec="urn:ietf:params:xml:ns:samlec"
       S:actor="" />

   Upon receipt of such a header block, the initiator MUST fail the
   establishment of the security context if it did not request
   delegation in its initial client response to the acceptor.  It SHOULD
   signal this failure to the acceptor with a SOAP fault message in its
   final client response.

   As noted previously, the exact means of client authentication to the
   IdP is formally out of scope of this mechanism.  This extends to the
   use of a delegation assertion as a means of authentication by an
   acceptor acting as an initiator.  In practice, some profile of
   [WSS-SAML] is used to attach the assertion and a confirmation proof
   to the SOAP message from the client to the IdP.

5.2.  GSS-API Channel Binding

   GSS-API channel binding [RFC5554] is a protected facility for
   exchanging a cryptographic name for an enclosing channel between the
   initiator and acceptor.  The initiator sends channel binding data and
   the acceptor confirms that channel binding data has been checked.

   The acceptor SHOULD accept any channel binding provided by the
   initiator if null channel bindings are passed into
   gss_accept_sec_context.  Protocols such as HTTP Negotiate [RFC4559]
   depend on this behavior of some Kerberos implementations.

   The exchange and verification of channel binding information is
   described by [SAMLECP20].

5.3.  Session Key Derivation

   Some GSS-API features (discussed in the following sections) require a
   session key be established as a result of security context
   establishment.  In the common case of a "bearer" assertion in SAML, a
   mechanism is defined to communicate a key to both parties via the
   identity provider.  In other cases such as assertions based on
   "holder of key" confirmation bound to a client-controlled key, there
   may be additional methods defined in the future, and extension points
   are provided for this purpose.

   Information defining or describing the session key, or a process for
   deriving one, is communicated between the initiator and acceptor
   using a <samlec:SessionKey> element, defined by the XML schema in
   Appendix A.  This element is a SOAP header block.  The content of the
   element further depends on the specific use in the mechanism.  The
   Algorithm XML attribute identifies a mechanism for key derivation.
   It is omitted to identify the use of an Identity Provider-generated
   key (see following section) or will contain a URI value identifying a
   derivation mechanism defined outside this specification.  Each header
   block's mustUnderstand and actor attributes MUST be set to "1" and
   "" respectively.

   In the acceptor's first response message containing its SAML request,
   one or more <samlec:SessionKey> SOAP header blocks MUST be included.
   The element MUST contain one or more <EncType> elements containing
   the number of a supported encryption type defined in accordance with
   [RFC3961].  Encryption types should be provided in order of
   preference by the acceptor.

   In the final client response message, a single <samlec:SessionKey>
   SOAP header block MUST be included.  A single <EncType> element MUST
   be included to identify the chosen encryption type used by the

   All parties MUST support the "aes128-cts-hmac-sha1-96" encryption
   type, number 17, defined by [RFC3962].

   Further details depend on the mechanism used, one of which is
   described in the following section.

5.3.1.  Generated by Identity Provider

   The identity provider, if issuing a bearer assertion for use with
   this mechanism, SHOULD provide a generated key for use by the
   initiator and acceptor.  This key is used as pseudorandom input to
   the "random-to-key" function for a specific encryption type defined
   in accordance with [RFC3961].  The key is base64-encoded and placed
   inside a <samlec:GeneratedKey> element.  The identity provider does
   not participate in the selection of the encryption type and simply
   generates enough pseudorandom bits to supply key material to the
   other parties.

   The resulting <samlec:GeneratedKey> element is placed within the
   <saml:Advice> element of the assertion issued.  The identity provider
   MUST encrypt the assertion (implying that it MUST have the means to
   do so, typically knowledge of a key associated with the RP).  If
   multiple assertions are issued (allowed, but not typical), the
   element need only be included in one of the assertions issued for use
   by the relying party.

   A copy of the element is also added as a SOAP header block in the
   response from the identity provider to the client (and then removed
   when constructing the response to the acceptor).

   If this mechanism is used by the initiator, then the
   <samlec:SessionKey> SOAP header block attached to the final client
   response message will identify this via the omission of the Algorithm
   attribute and will identify the chosen encryption type using the
   <samlec:EncType> element:

   <samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"

   Both the initiator and acceptor MUST execute the chosen encryption
   type's random-to-key function over the pseudorandom value provided by
   the <samlec:GeneratedKey> element.  The result of that function is
   used as the protocol and session key.  Support for subkeys from the
   initiator or acceptor is not specified.

5.3.2.  Alternate Key Derivation Mechanisms

   In the event that a client is proving possession of a secret or
   private key, a formal key agreement algorithm might be supported.
   This specification does not define such a mechanism, but the
   <samlec:SessionKey> element is extensible to allow for future work in
   this space by means of the Algorithm attribute and an optional
   <ds:KeyInfo> child element to carry extensible content related to key

   However a key is derived, the <samlec:EncType> element will identify
   the chosen encrytion type, and both the initiator and acceptor MUST
   execute the encryption type's random-to-key function over the result
   of the key agreement or derivation process.  The result of that
   function is used as the protocol key.

5.4.  Per-Message Tokens

   The per-message tokens SHALL be the same as those for the Kerberos V5
   GSS-API mechanism [RFC4121] (see Section 4.2 and sub-sections).

   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_INTEG_FLAG) security context flags are always set to TRUE.

   The "protocol key" SHALL be a 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

5.5.  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 V5 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.

5.6.  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 in
   advance 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.

   To avoid complexity, and avoid unnecessary use of XML within the
   naming layer, the SAML EC mechanism relies on the common/expected
   name types used for acceptors and initiators,
   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.

   SAML-based information associated with the initiator SHOULD be
   expressed to the acceptor using GSS-API naming extensions [RFC6680],
   in accordance with [RFC7056].

5.6.1.  User Naming Considerations

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

   Upon successful completion of this mechanism, the server MUST
   construct the authenticated initiator name based on the <saml:NameID>
   element in the assertion it successfully validated.  The name is
   constructed as a UTF-8 string in the following form:

     name = element-value "!" Format  "!" NameQualifier
         "!" SPNameQualifier "!" SPProvidedID

   The "element-value" token refers to the content of the <saml:NameID>
   element.  The other tokens refer to the identically named XML
   attributes defined for use with the element.  If an attribute is not
   present, which is common, it is omitted (i.e., replaced with the
   empty string).  The Format value is never omitted; if not present,
   the SAML-equivalent value of "urn:oasis:names:tc:SAML:1.1:nameid-
   format:unspecified" is used.

   Not all SAML assertions contain a <saml:NameID> element.  In the
   event that no such element is present, including the exceptional
   cases of a <saml:BaseID> element or a <saml:EncryptedID> element that
   cannot be decrypted, the GSS_C_NT_ANONYMOUS name type MUST be used
   for the initiator name.

   As noted in the previous section, it is expected that most
   applications able to rely on SAML authentication would make use of
   naming extensions to obtain additional information about the user
   based on the assertion.  This is particularly true in the anonymous
   case, or in cases in which the SAML name is pseudonymous or transient
   in nature.  The ability to express the SAML name in
   GSS_C_NT_USER_NAME form is intended for compatibility with
   applications that cannot make use of additional information.

5.6.2.  Service Naming Considerations

   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.  As described
   in in the SASL mechanism's Section 4.7, such a name is used directly
   by this mechanism as the effective AssertionConsumerService
   "location" associated with the service and applied in IdP
   verification of the request against the claimed SAML entityID.

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, or want mutual authentication or

   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">
    <samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"
      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).  A generated key is included in
   the assertion and in a header for the client.

     <ecp:Response S:mustUnderstand="1"
     <samlec:GeneratedKey xmlns:samlec="urn:ietf:params:xml:ns:samlec">
     <samlp:Response ID="d43h94r389309r" Version="2.0"
         IssueInstant="2007-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
         <!-- contents elided, copy of samlec:GeneratedKey in Advice -->

   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"/>
     <samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"
     <samlp:Response ID="d43h94r389309r" Version="2.0"
         IssueInstant="2007-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
         <!-- contents elided, copy of samlec:GeneratedKey in Advice -->

   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.  Servers should be
   aware that SAML IdPs will track - to some extent - user access to
   their services.  This exposure extends to the use of session keys
   generated by the IdP to secure messages between the parties, but note
   that when bearer assertions are involved, the IdP can freely
   impersonate the user to any relying party in any case.

   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

8.1.  GSS-API and SASL Mechanism Registration

   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

8.2.  XML Namespace Name for SAML-EC

   A URN sub-namespace for XML constructs introduced by this mechanism
   is defined as follows:

   URI: urn:ietf:params:xml:ns:samlec

   Specification: See Appendix A of this document.

   Description: This is the XML namespace name for XML constructs
   introduced by the SAML Enhanced Client SASL and GSS-API Mechanisms.

   Registrant Contact: the IESG

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,
              DOI 10.17487/RFC2119, 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, DOI 10.17487/RFC2617, June 1999,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

              Cantor, S., "SAML V2.0 Enhanced Client or Proxy Profile
              Version 2.0", OASIS Committee Specification OASIS.sstc-
              saml-ecp-v2.0-cs01, August 2013.

              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, <>.

9.2.  Normative References for GSS-API Implementers

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743,
              DOI 10.17487/RFC2743, January 2000,

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <>.

   [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
              Encryption for Kerberos 5", RFC 3962,
              DOI 10.17487/RFC3962, 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,
              DOI 10.17487/RFC4121, July 2005,

   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401,
              DOI 10.17487/RFC4401, 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,
              DOI 10.17487/RFC4402, February 2006,

   [RFC5554]  Williams, N., "Clarifications and Extensions to the
              Generic Security Service Application Program Interface
              (GSS-API) for the Use of Channel Bindings", RFC 5554,
              DOI 10.17487/RFC5554, May 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, DOI 10.17487/RFC5801,
              July 2010, <>.

   [RFC6680]  Williams, N., Johansson, L., Hartman, S., and S.
              Josefsson, "Generic Security Service Application
              Programming Interface (GSS-API) Naming Extensions",
              RFC 6680, DOI 10.17487/RFC6680, August 2012,

   [RFC7056]  Hartman, S. and J. Howlett, "Name Attributes for the GSS-
              API Extensible Authentication Protocol (EAP) Mechanism",
              RFC 7056, DOI 10.17487/RFC7056, December 2013,

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

   [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,
              DOI 10.17487/RFC2616, June 1999,

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC3920,
              October 2004, <>.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,

              Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn,
              "XML Schema Part 1: Structures", W3C REC-xmlschema-1, May
              2001, <>.

              Monzillo, R., "Web Services Security SAML Token Profile
              Version 1.1.1", OASIS Standard OASIS.wss-SAMLTokenProfile,
              May 2012.

Appendix A.  XML Schema

   The following schema formally defines the
   "urn:ietf:params:xml:ns:samlec" namespace used in this document, in
   conformance with [W3C.REC-xmlschema-1] While XML validation is
   optional, the schema that follows is the normative definition of the
   constructs it defines.  Where the schema differs from any prose in
   this specification, the schema takes precedence.


     <import namespace=""/>
     <import namespace=""/>

     <element name="SessionKey" type="samlec:SessionKeyType"/>
     <complexType name="SessionKeyType">
         <element ref="samlec:EncType" maxOccurs="unbounded"/>
         <element ref="ds:KeyInfo" minOccurs="0"/>
       <attribute ref="S:mustUnderstand" use="required"/>
       <attribute ref="S:actor" use="required"/>
       <attribute name="Algorithm"/>

     <element name="EncType" type="integer"/>

     <element name="GeneratedKey" type="samlec:GeneratedKeyType"/>
     <complexType name="GeneratedKeyType">
         <extension base="base64Binary">
           <attribute ref="S:mustUnderstand"/>
           <attribute ref="S:actor"/>

     <element name="Delegated" type="samlec:DelegatedType"/>
     <complexType name="DelegatedType">
       <attribute ref="S:mustUnderstand" use="required"/>
       <attribute ref="S:actor" use="required"/>


Appendix B.  Acknowledgments

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

Appendix C.  Changes

   This section to be removed prior to publication.

   o  15,16,  15,16,17, avoid expiration

   o  14, address some minor comments

   o  13, clarify SAML metadata usage, adding a recommended Binding
      value alongside the backward-compatibility usage of PAOS

   o  12, clarifying comments based on WG feedback, with a normative
      change to use enctype numbers instead of names

   o  11, update EAP Naming reference to RFC

   o  10, update SAML ECP reference to final CS

   o  09, align delegation signaling to updated ECP draft

   o  08, more corrections, added a delegation signaling header

   o  07, corrections, revised section on delegation

   o  06, simplified session key schema, moved responsibility for
      random-to-key to the endpoints, and defined advertisement of
      session key algorithm and enctypes by acceptor

   o  05, revised session key material, added requirement for random-to-
      key, revised XML schema to capture enctype name, updated GSS
      naming reference

   o  04, stripped down the session key material to simplify it, and
      define an IdP-brokered keying approach, moved session key XML
      constructs from OASIS draft into this one

   o  03, added TLS key export as a session key option, revised GSS
      naming material based on list discussion

   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
   1050 Carmack Rd
   Columbus, Ohio  43212
   United States

   Phone: +1 614 247 6147

   Simon Josefsson
   Hagagatan 24
   Stockholm  113 47