Internet DRAFT - draft-tschofenig-sip-saml

draft-tschofenig-sip-saml






SIP                                                        H. Tschofenig
Internet-Draft                                                   Siemens
Expires: September 6, 2006                                   J. Peterson
                                                           NeuStar, Inc.
                                                                 J. Polk
                                                                   Cisco
                                                               D. Sicker
                                                              CU Boulder
                                                               J. Hodges
                                                           NeuStar, Inc.
                                                           March 5, 2006


                      SIP SAML Profile and Binding
                    draft-tschofenig-sip-saml-05.txt

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   This Internet-Draft will expire on September 6, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies a Session Initiation Protocol (SIP) profile



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   of Security Assertion Markup Language (SAML) as well as a SAML SIP
   binding.  The defined SIP SAML Profile composes with the mechanisms
   defined in the SIP Identity specification and satisfy requirements
   presented in "Trait-based Authorization Requirements for the Session
   Initiation Protocol (SIP)".


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Specification Scope  . . . . . . . . . . . . . . . . . . . . .  5
   4.  SAML Introduction  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  SAML Assertions  . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Abstract Request/Response Protocol . . . . . . . . . . . .  9
   5.  Employing SAML in SIP  . . . . . . . . . . . . . . . . . . . . 10
   6.  Use-case Scenarios . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  PSTN-to-SIP Phone Call . . . . . . . . . . . . . . . . . . 12
     6.2.  SIP Conferencing . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Compensation using SIP and SAML  . . . . . . . . . . . . . 15
   7.  SIP SAML Profiles  . . . . . . . . . . . . . . . . . . . . . . 17
     7.1.  AS-driven SIP SAML URI-based Attribute  Assertion
           Fetch Profile  . . . . . . . . . . . . . . . . . . . . . . 17
       7.1.1.  Required Information . . . . . . . . . . . . . . . . . 17
       7.1.2.  Profile Overview . . . . . . . . . . . . . . . . . . . 17
       7.1.3.  Profile Description  . . . . . . . . . . . . . . . . . 21
       7.1.4.  Assertion Profile Description  . . . . . . . . . . . . 24
       7.1.5.  Assertion Verification . . . . . . . . . . . . . . . . 27
   8.  SAML SIP Binding . . . . . . . . . . . . . . . . . . . . . . . 29
     8.1.  SAML HTTP-URI-based SIP Binding  . . . . . . . . . . . . . 29
   9.  Example Signed SAML Assertion  . . . . . . . . . . . . . . . . 30
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
     10.1. Man-in-the-middle Attacks and Stolen Assertions  . . . . . 32
     10.2. Forged Assertion . . . . . . . . . . . . . . . . . . . . . 33
     10.3. Replay Attack  . . . . . . . . . . . . . . . . . . . . . . 33
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 34
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 36
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   15. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 38
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     16.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
   Intellectual Property and Copyright Statements . . . . . . . . . . 43






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1.  Introduction

   This document specifies composition of the Security Assertion Markup
   Language (SAML) V2.0 with SIP [RFC3261] in order to accommodate
   richer authorization mechanisms and enable "trait-based
   authorization."  Trait-based authorization is where one is authorized
   to make use of some resource based on roles or traits rather than
   ones identifier(s).  Motivations for trait-based authorization, along
   with use-case scenarios, are presented in [I-D.ietf-sipping-trait-
   authz].

   Security Assertion Markup Language (SAML) v2.0, "SAMLv2", is an XML-
   based framework for creating and exchanging security information.
   [OASIS.sstc-saml-exec-overview-2.0-cd-01] and [OASIS.sstc-saml-tech-
   overview-2.0-draft-08] provide non-normative overviews of SAMLv2.
   The SAMLv2 specification set is normatively defined by [OASIS.saml-
   conformance-2.0-os].

   Various means of providing trait-based authorization exist:
   authorization certificates, SPKI or extensions to the authenticated
   identity body [RFC3893].  The authors selected SAML due to its
   increasing use in environments such as the Liberty Alliance, and the
   Internet2 project, areas where the applicability to SIP is widely
   desired.



























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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The SIP network element "Authentication Service" is introduced in
   [I-D.ietf-sip-identity].  We reuse this term to refer to a network
   element that authenticates and authorizes a user and creates a "SIP
   identity assertion".  This system entity is the moral equivalent of a
   "SAML Authority" in the SAML terminology.

   For overall SIP terminology, see [RFC3261].

   In this specification, the term, or term component, "SAML" refers to
   SAML V2.0 in all cases.  For example, the term "SAML assertion"
   implicitly means "SAMLv2 assertion".  For overall SAML terminology,
   see [OASIS.saml-glossary-2.0-os].

   The below list maps other various SIP terms to their SAML
   (rough-)equivalents:



      Element, Network Element:               System Entity, Entity

      Authentication Service:                 SAML Authority

      Invitee, Invited User, Called Party, Callee - Relying Party

      Server, User Agent Server (UAS):        SAML Responder

      User Agent Client (UAC), client:        SAML Requester


   Additional terms concocted in the context of this specification:



      profile attribute(s):

         one or more attributes of a "user profile".

      user profile, subject profile:

         the set of various attributes accompanying (i.e., mapped to) a
         user account in many environments.




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3.  Specification Scope

   The scope of this specification is:

   o  Specify a SIP profile of SAML -- aka a "SIP SAML profile" -- such
      that a subject's profile attributes, and their domain's
      certificate, can be conveyed to a relying party using SAML.  In
      doing so, satisfy the requirements outlined in [I-D.ietf-sipping-
      trait-authz], and compose with [I-D.ietf-sip-identity].

   The following are outside the scope of this specification:

   o  Defining a means for configuring the runtime behavior, or
      deployment characteristics, of the Authentication Service.

      Discussion:

      For example, a SIP Authentication Service could be implemented
      such that its SAML-based features are employed, or not, on a
      subject-by-subject basis, and/or on a domain-by-domain basis.

   o  The definition of specific conveyed subject profile attributes
      (aka traits).

      Discussion:

      This specification defines a facility enabling "trait-based
      authorization" as discussed in [I-D.ietf-sipping-trait-authz].

      The attributes of interest in trait-based authorization will be
      ones akin to, for example: roles, organizational membership,
      access rights, or authentication event context.  Definition of
      such attributes is application- and/or deployment-context-
      dependent and are not defined in this specification.  However, The
      SAMLv2 specification defines several "SAML Attribute Profiles" for
      encoding attributes from various application domains, e.g., LDAP,
      UUID/GUID, DCE PAC, and XACML, in SAML assertions [OASIS.saml-
      profiles-2.0-os].

      In order for any trait-based system to be practical, participating
      entities must agree on attributes and traits that will be conveyed
      and subsequently relied upon.  Without such agreements, a trait-
      based system cannot be usefully deployed.  This specification does
      not discuss the manner in which participating entites might
      discover one another or agree on the syntax and semantics of
      attributes and traits.

      Note that SAMLv2 specifies a "metadata" facility that may be



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      useful in addressing this need.


















































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4.  SAML Introduction

   SAML [OASIS.sstc-saml-exec-overview-2.0-cd-01] [OASIS.sstc-saml-tech-
   overview-2.0-draft-08] defines an XML-based framework for exchanging
   "security assertions" between entities.  In the course of making, or
   relying upon such assertions, SAML system entities may use SAML
   protocols, or other protocols, to communicate regarding an assertion
   itself, or the subject of an assertion.

   Thus one can employ SAML to make and encode statements such as "Alice
   has these profile attributes and her domain's certificate is
   available over there, and I'm making this statement, and here's who I
   am."  Then one can cause such an assertion to be conveyed to some
   party who can then rely on it in some fashion for some purpose, for
   example input it into some local policy evaluation for access to some
   resource.  This is done in a particular "context of use".  Such a
   context of use could be, for example, deciding whether to accept and
   act upon a SIP-based invitation to initiate a communication session.

   The specification of how SAML is employed in a particular context of
   use is known as a "SAML profile".  The specification of how SAML
   assertions and/or protocol messages are conveyed in, or over, another
   protocol is known as a "SAML Binding".  Typically, a SAML profile
   specifies the SAML bindings that may be used in its context.  Both
   SAML profiles and SAML bindings reference other SAML specifications,
   especially the SAML Assertions and Protocols, aka "SAML Core",
   specification [OASIS.saml-core-2.0-os].

   There is an additional subtle aspect of SAML profiles that is worth
   highlighting -- the notion of a "SAML assertion profile".  A SAML
   assertion profile is the specification of the assertion contents in
   the context of a particular SAML profile.  It is possibly further
   qualified by a particular implementation and/or deployment context.
   Condensed examples of SAML assertion profiles are:

   o  The SAML assertion must contain at least one authentication
      statement and no other statements.  The relying party must be
      represented in the <AudienceRestriction> element.  The
      SubjectConfirmation Method must be Foo. etc.

   o  The SAML assertion must contain at least one attribute statement
      and may contain more than one.  The values for the subject's
      profile attributes named "Foo" and "Bar" must be present.  An
      authentication statement may be present. etc.

   The primary facets of SAML itself are:





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   o  Assertions

   o  Abstract Request/Response protocol

   We describe each in turn below:

4.1.  SAML Assertions

   A SAML assertion is a package of information including issuer and
   subject, conditions and advice, and/or attribute statements, and/or
   authentication statements and/or other statements.  Statements may or
   may not be present.  The SAML assertion "container" itself contains
   the following information:

   Issuing information:

      Who issued the assertion, when was it issued and the assertion
      identifier.


   Subject information:

      The name of the subject, the security domain and optional subject
      information, like public key.


   Conditions under which the  assertion is valid:

      Special kind of conditions like assertion validity period,
      audience restriction and target restriction.


   Additional advice:

      Explaining how the assertion was made, for example.

   In terms of SAML assertions containing SAML attribute statements or
   SAML authentication statements, here are explanatory examples:

      With a SAML assertion containing a SAML attribute statement, an
      issuing authority is asserting that the subject is associated with
      certain attributes with certain subject profile attribute values.
      For example, user jon@cs.example.com is associated with the
      attribute "Department", which has the value "Computer Science".

      With a SAML assertion containing a SAML authentication statement,
      an issuing authority is asserting that the subject was
      authenticated by certain means at a certain time.



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      With a SAML assertion containing both a SAML attribute statement
      and a SAML authentication statement, an issuing authority is
      asserting the union of the above.

4.2.  Abstract Request/Response Protocol

   SAML defines an abstract request/response protocol for obtaining
   assertions.  See Section 3 "SAML Protocols" of
   [OASIS.saml-core-2.0-os].  A request asks for an assertion.  A
   response returns the requested assertion or an error.  This abstract
   protocol may then be cast into particular contexts of use by binding
   it to specific underlying protocols, e.g., HTTP or SIP, and
   "profiling" it for the specific use case at hand.  The SAML HTTP-
   based web single sign-on profile is one such example (see Section 4.1
   Web Browser SSO Profile of [OASIS.saml-profiles-2.0-os]).  Trait-
   based SIP communication session establishment, the topic of this
   specification, is another.


































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5.  Employing SAML in SIP

   Employing SAML in SIP necessitates devising a new SAML profile or
   profiles because the those already specified in the SAMLv2
   specification set are specific to other use contexts, e.g., HTTP-
   based web browsing.  Although SIP bears some similarity to HTTP, it
   is a seperately distinct protocol, thus SAML profile specificity is
   warranted.  However, this should not present any untoward
   difficulties due to SAML's inherent and explicit extensibility, and
   also because SIP is similarly extensible.

   The "Authenticated Identity Management in SIP" specification
   [I-D.ietf-sip-identity] (aka "SIP Identity") facilitates the
   composition of SAML and SIP in that it defines a "mediated
   authentication architecture" where verifying endpoints verify SIP
   identity assertions -- i.e., the "Identity" header value -- signed by
   an Authentication Service (AS).  The semantic being that the AS is
   vouching that it did indeed authenticate the calling party.

   Such an Authentication Service, which likely has access to various
   pieces of information concerning the calling party, could also act as
   a SAML Authority, and make such information available to the callee
   via SAML.

   Since [I-D.ietf-sip-identity] stipulates that the AS must make its
   certificate available for retrieval and convey the availability and
   access mechanism via a URI, in the Identity-Info header, we have an
   opportunity to compose SIP Identity and SAML.

   Such composition can be accomplished by having the resource referred
   to by the URI in the Identity-Info be a SAML assertion conveying both
   the AS's certificate and user profile attributes.  This is the
   approach defined in this specification.  Figure 1 illustrates this
   approach in a high-level summary fashion.  Figure 5, further below,
   illustrates additional details.
















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     +--------+           +--------------+          +--------+
     |Alice@  |           |Authentication|          | Bob@   |
     |example |           |Service       |          |example2|
     |.com    |           |@example.com  |          |com     |
     |        |           |              |          |        |
     +---+----+           +------+-------+          +---+----+
         |                       |                      |
         |      INVITE           |                      |
         |---------------------->|                      |
         | From:alice@foo.com    |                      |
         |                       |                      |
         |  407 Proxy auth. req. |                      |
         |<----------------------|                      |
         |     Challenge         |                      |
         |                       |                      |
         |       ACK             |                      |
         |---------------------->|                      |
         |                       |                      |
         | INVITE w/authn creds  |                      |
         |---------------------->|                      |
         |                       | INVITE               |
         |                       | w/Identity header    |
         |                       |--------------------->|
         |                       | and Identity-Info    |
         |                       |                      |
         |                       | HTTP GET SAML assn   |
         |                       |<==================== |
         |                       | and domain cert      |
         |                       |                      |
         |                       | HTTP 200 OK + assn   |
         |                       |=====================>|
         |                       | and domain cert      |
         |            200 OK     |                      |
         |<----------------------+----------------------|
         |                       |                      |


   Figure 1: SIP-SAML-based Network Asserted Identity

   Since the AS already being trusted to create and add the Identity
   header containing the SIP Identity Assertion, and to supply a pointer
   to its domain certificate, having it point instead to a SAML
   assertion conveying the domain certificate and possibly some user
   profile attributes, does not significantly alter the first-order
   security considerations examined in [I-D.ietf-sip-identity].  This
   specification provides some additional security considerations
   analysis below in Section 10.




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6.  Use-case Scenarios

   This section shows message flows based on scenarios in [I-D.ietf-
   sipping-trait-authz] enriched with a SAML based solution.
   Section 6.2 shows a SIP conferencing scenario with role-based access
   control using SAML.  A future version of this document will cover
   more scenarios from [I-D.ietf-sipping-trait-authz].

6.1.  PSTN-to-SIP Phone Call

   Alice, using a phone connected to the PSTN, wants to make a call to
   Bob, which resides in a SIP network.  Her call is switched through
   the PSTN by means of PSTN signaling (outside the scope of this
   document) to the PSTN/SIP gateway.  At the gateway, the call is
   converted from SS7 signaling to SIP signaling.  Since Alice's PSTN
   phone was previously "authenticated" via PSTN signaling mechanisms,
   the gateway is able to assert her phone's identity (e.g., her
   telephone number) via SIP Identity and SAML-based mechanisms (e.g.,
   in order to convey profile attributes) to Bob's SIP proxy, which also
   dereferences the URI in the Identity-Info header in order to obtain
   the SAML assertion and the PSTN/SIP Gateway's domain certificate.
   Alice's INVITE is then forwarded from the SIP/PSTN gateway to Bob's
   phone, and is secured via whatever means is locally established in
   Bob's administrative domain.



                                                 +-----------+
   +----------------------+                      |           |
   |                      |       SS7            |  PSTN/SIP |
   |  Public Switched     |--------------------->|  Gateway  |
   |                      |                      |           |
   |                      |                      |           |
   | Telephone Network    |                   +--+-----------+------+
   |         ^            |                   |     | ^ V           |
   +---------+------------+                   |     | ^ V SIP-Ident |
             | SS7                            |     v ^ V +SAML     |
             |                                |    +--------+       |
                                      O       |    | Bob's  |       |
             O                       /|\ <----+----| SIP    |       |
            /|\                      / \  SIP |    | Proxy  |       |
            / \                      Bob      |    |        |       |
           Alice                              |    +--------+       |
                                              |     SIP based       |
                                              |     Network         |
                                              +---------------------+





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   Figure 2: PSTN to SIP call

   Note that the INVITE emitted by the PSTN/SIP gateway could
   alternatively be simply forwarded by Bob's SIP Proxy to Bob's phone,
   and Bob's phone could take on the SIP Identity "verifier" role, which
   is being played by Bob's SIP proxy in the figure.

   Whichever approach is employed is a decision local to Bob's
   administrative domain and can be made independently.

6.2.  SIP Conferencing

   This section is meant to foster discussion about the usage of SAML in
   the domain of conferencing.  A user who routes its SIP message
   through the Authentication Service (Asserting Party) towards a
   conferencing server may want or need various of her profile
   attributes included and may also need to be authenticated by the
   conference server.  The following properties could be provided by
   this procedure:

   o  The user identity can be replaced to allow the user to be
      anonymous with regard to the Focus.  This can be accomplished via
      [RFC3323] in combination with [I-D.ietf-sip-identity], per the
      latter, or,

   o  The user identity could be asserted to the Focus, via [I-D.ietf-
      sip-identity] mechanisms, and/or,

   o  the SAML assertion could provide additional user profile
      information such as group membership (belongs to the students,
      staff, faculty group of university X).  This could, for non-
      identity-based authorization systems, imply certain rights.

   The corresponding SIP message flow (in high level detail) could have
   the following shape:
















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       +-----+          +-----------+       +-----------+
       |     |          | SIP Proxy |       | Focus     |
       |User |          |(Asserting |       | (Relying  |
       |     |          | party)    |       | party)    |
       +--+--+          +-----+-----+       +-----+-----+
          |     INVITE        |                   |
          |sip:conf-factory   |    INVITE         |
          |------------------>|    w/Identity hdr |
          |                   |------------------>|
          |                   |                   |
          |                   | HTTP GET SAML assn|
          |                   |<==================|
          |                   | and domain cert   |
          |                   |                   |
          |                   | HTTP 200 OK + assn|
          |                   |==================>|
          |                   | and domain cert   |
          |                   |                   |
          |                   |                   |
          |                   | Ringing           |
          | Ringing           |<------------------|
          |<------------------|                   |
          |                   |                   |
          |                   | OK                |
          | OK                |<------------------|
          |<------------------|                   |
          |                   |                   |
          |    ACK            |                   |
          |------------------>|    ACK            |
          |                   |------------------>|
          |                   |                   |
          |                   |                   |
                     ... many more msgs...


   Figure 3: SIP Conferencing and SAML

   However, there are obvious scaling issues with the conference server
   having to do the outbound requests in order to obtain SAML assertions
   and certificates for conference participants.

   This could be addressed by creating another SIP SAML Profile where
   the caller obtains the necessary information, e.g., SAML assertions,
   and places them into its SIP request message prior to sending it.
   This would obviate the need for the callee relying party to make
   requests in order to obtain said information.  This is a topic for
   future work, and possibly future revisions of this specification.




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6.3.  Compensation using SIP and SAML

   This section describes a scenario where SAML is used in SIP to
   realize compensation functionality as described in [I-D.jennings-
   sipping-pay].

   Note that this scenario is not directly addressed by the SIP SAML
   Profile and SAML SIP Binding presently defined in this specification.
   Rather, this use case calls for additional such profiles and bindings
   to be developed.



   +--------+              +--------+                +--------+
   |Payment |              | User   |                |Merchant|
   |Provider|              | Alice  |                |Bob     |
   |        |              |        |                |        |
   |        |              |        |                |        |
   +---+----+              +---+----+                +---+----+
       |                       |                         |
       |                       |  1) Call                |
       |                       |------------------------>|
       |                       |                         |
       |                       |  2) 402 + Payment Offer |
       |  3) Request for       |<------------------------|
       |     a Payment         |                         |
       |<----------------------|                         |
       |                       |                         |
       |  4) SAML Assertion    |                         |
       |     (=Receipt)        |                         |
       |---------------------->|                         |
       |                       |  5) Call + Receipt      |
       |                       |------------------------>|
       |                       |                         |
       |                       |  6) 200 OK              |
       |                       |<------------------------|
       |                       |                         |


   Figure 4: Message flow for SIP payment

   User Alice and the Merchant Bob interacts with each other using SIP
   and the Alice uses HTTP to exchange messages with a Payment Provider.
   Initially, Alice makes a call to Bob (1).  Bob determines that a
   payment is required and includes information about the payment in an
   Offer body of a 402 (Payment Required) response to Alice (2).  Alice
   looks at this Offer and decides to make a payment.  Alice therefore
   instructs her Payment Provider to make a transfer from the Alice's



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   account to the Merchants's account (3) using a request for a SAML
   assertion with the extensions defined in this document.  The Payment
   Provider returns a receipt for this transfer (4).  This receipt is a
   SAML Assertion (or a SAML Artifact, if the exchange is triggered by a
   proxy or if desired by the Customer).  Alice resubmits the call to
   Bob but this time provides the Receipt for the transaction (5).  Bob
   determines whether the Receipt is valid (by checking the digital
   signature and the content of the assertion) and continues with the
   call processing, if the authorization was succesful.

   The Offer contains information about the three parties, the Payment
   Provider, that are acceptable to the Merchant Bob, the amount of
   transaction, the account identifier for Bob at the Payment Provider,
   and a replay protection indicator to make it easier for the Merchant
   Bob to avoid replay attacks.  User Alice includes this information
   when making the Request for Payment to the Payment Provider; adds its
   own account information and authorization password; and sends this to
   the Payment Provider, which produces a Receipt for the transaction if
   it is successful.  This transfer from Alice to the Payment Provider
   is made across an encrypted, integrity protected channel.  The
   Receipt includes a timestamp when the Payment Provider made the
   transaction and protects the Receipt with a digital signature.  Alice
   resubmits the call to the Merchant Bob with the Receipt from the
   Payment Provier.  Merchant Bob can check for replay attacks using the
   timestamp and a replay protection indiciator initially provided with
   the Offer.  Bob can then check the signature is valid using the
   Payment Provider's public key.
























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7.  SIP SAML Profiles

   This section defines one SIP SAML profile:

      The "AS-driven SIP SAML URI-based Attribute Assertion Fetch
      Profile"

7.1.  AS-driven SIP SAML URI-based Attribute  Assertion Fetch Profile

7.1.1.  Required Information

   The information given in this section is similar to the info provided
   when registering something, a MIME Media Type, say, with IANA.  In
   this case, it is for registering this profile with the OASIS SSTC.
   See Section 2 "Specification of Additional Profiles" in [OASIS.saml-
   profiles-2.0-os].

   Identification:

      urn:ietf:params:sip:sip-saml-profile:as:uri:attr:1.0

      @@ NOTE: This URN must be agreed upon, and then registered with
         IANA per [RFC3553].

   Contact Information:

      @@ someone's or something's contact info goes here

   SAML Confirmation Method Identifiers:

      The SAML V2.0 "{bearer,hok,?}" confirmation method identifier is
      used in this profile.

   Description:

      Given below.

   Updates:

      None.

7.1.2.  Profile Overview

   Figure 5 illustrates this profile's overall protocol flow.  The
   following steps correspond to the labeled interactions in the figure.
   Within an individual step, there may be one or more actual message
   exchanges depending upon the protocol binding employed for that
   particular step and other implementation-dependent behavior.



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   Although this profile is overview is cast in terms of a SIP INVITE
   transaction, the reader should note that the mechanism specified
   herein, and in [I-D.ietf-sip-identity], may be applied to any SIP
   request message.

   Figure 5 begins on the next page.













































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     +------------------+    +------------------+   +-----------------+
     |     Caller       |    |Authn Service (AS)|   |     Callee      |
     |Alice@example.com |    |  @example.com    |   | Bob@example2.com|
     +--------+---------+    +--------+---------+   +--------+--------+
   -    -     |                       |                      | (steps)
   ^    ^     |      INVITE           |                      |
   |    |     |---------------------->|                      |   (1a)
   |          | From:alice@foo.com    |                      |
   |    C     | To:sip:bob@example.com|                      |
   |    S     |                       |                      |
   |    e     |  407 Proxy auth. req. |                      |
   |    q     |<----------------------|                      |   (1b)
   |    =     |  Challenge            |                      |
   |    N     |                       |                      |
   |          |      ACK              |                      |
   |    |     |---------------------->|                      |   (1c)
   |    V     |                       |                      |
   |    -     |                       |                      |
        ^     | INVITE + authorization|                      |
   D    |     | header w/ creds       |                      |
        |     |---------------------->|                      |   (2)
   I    |     | From:alice@foo.com    |                      |
        |     | To:sip:bob@example.com|                      |
   A          | Proxy-Authorization:..|                      |
        C     |                       | INVITE               |
   L    S     |                       |--------------------->|   (3)
        e     |                       | From:alice@foo.com   |
   O    q     |                       | To:sip:bob@example2.com
              |                       | Identity: .....      |
   G    =     |                       | Identity-Info:       |
              |                       |   https://example.com|
   |    N     |                       |     /assns/?ID=abcde |
   |          |                       |                      |
   |    +     |                       |URI resolution (eg. HTTP)
   |          |                       |<=====================|   (4)
   |    1     |                       | GET /assns/?ID=abcde |
   |          |                       |                      |
   |    |     |                       | HTTP/1.1 200 OK      |
   |    |     |                       |=====================>|   (5)
   |    |     |                       |  <saml:Assertion>    |
   |    |     |                       |   <saml:Subject>     |
   |    |     |                       |    <saml:NameID>     |
   |    |     |                       |      Alice@example.com
   |    |     |                       |     <saml:SubjConf>
   |    |     |                       |      <saml:SubjConfData>
   |    |     |                       |       <ds:KeyInfo>...
   |    |     |                       |   <saml:AttrStatement>
   |    |     |                       |     foo=bar          |



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   |    |     |            200 OK     |                      |
   |    V     |<----------------------+----------------------|   (6)
   .    -     |                       |                      |
   V


   Figure 5: AS-driven SIP SAML Attribute Fetch Profile: Example INVITE
   Transaction

   Step 1. Initial SIP Transaction between Caller and AS

           This optional initial step is comprised of substeps 1a, 1b,
           and 1c in Figure 5.  In this step, the caller, Alice, sends a
           SIP request message, illustrated as an INVITE, indicating Bob
           as the callee (1a), is subsequently challenged by the AS
           (1b), and sends an ACK in response to the challenge (1c).
           The latter message signals the completion of this SIP
           transaction (which is an optional substep of this profile).

   Step 2. Caller sends SIP Request Message with Authorization
           Credentials to the AS

           Alice then sends an INVITE message in response to the
           challenge, or uses cached credentials for the domain if step
           1 was skipped, as specified in [I-D.ietf-sip-identity] and
           [RFC3261].  Depending on the chosen SIP security mechanism
           either digest authentication, S/MIME or Transport Layer
           Security is used to provide the AS with a strong assurance
           about the identity of Alice.

   Step 3. AS Authorizes the SIP Request and Forwards it to Callee

           First, the AS authorizes the received INVITE message as
           specified in [I-D.ietf-sip-identity] and [RFC3261].  If the
           authorization is successful, the AS will form the "identity
           signature" for the message and add Identity and Identity-Info
           headers to the message.  The AS also at this time constructs
           and caches a SAML assertion asserting Alice's profile
           attributes required by Bob's domain (example2.com), and also
           containing a the domain's (example.com) public key
           certificate, or a reference to it.  This certificate MUST
           contain the public key corresponding to the private key used
           to construct the signature whose value was placed in the
           Identity header.  The AS constructs a HTTP-based SAML URI
           Reference incorporating the assertion's Assertion ID (see
           section 2.3.3 of [OASIS.saml-core-2.0-os]).  The AS uses this
           URI as the value for the Identity-Info header it adds to the
           INVITE message.



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           The AS determines which profile attributes (if any) to assert
           in the <AttributeStatement> via local configuration and/or
           obtaining example2.com's metadata
           [OASIS.saml-metadata-2.0-os].  The AS then sends the updated
           INVITE message to Bob.

   Step 4. Callee Dereferences HTTP-based SAML URI Reference

           Bob's UAC or SIP Proxy receives the message and begins
           verifying it per the "Verifier Behavior" specified in
           [I-D.ietf-sip-identity].  In order to accomplish this task,
           it needs to obtain Alice's domain certificate.  It obtains
           the HTTP-based SAML URI Reference from the message's
           Identity-Info header and dereferences it per Section 8.1.
           Note that this is not a SIP message, but an HTTP message
           [RFC2616].

   Step 5. AS Returns SAML Assertion

           Upon receipt of the above HTTP request, which contains an
           embedded reference to Alice's SAML Assertion, Alice's AS
           returns her assertion in an HTTP response message.

           Upon receipt of Alice's SAML Assertion, the AS continues its
           verification of the INVITE message.  If successful, it
           returns a 200 OK message directly to Alice.  Otherwise it
           returns an appropriate SIP error response.

   Step 6. Callee Returns SIP 200 OK to Caller

           If Bob determines, based upon Alice's identity as asserted by
           the AS, and as further substantiated by the information in
           the SAML assertion, to accept the INVITE, he returns a SIP
           200 OK message directly to Alice.

7.1.3.  Profile Description

   The following sections provide detailed definitions of the individual
   profile steps.  The relevant illustration is Figure 6, below.  Note
   that this profile is agnostic to the specific SIP request, and also
   that the Sender and Authentication Service (AS) may be seperate or
   co-located in actuality.









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     +------------------+    +------------------+   +------------------+
     |     Sender       |    |Authn Service (AS)|   |    Verifier      |
     |      (UAC)       |    |    (Sender's)    |   |(UAS or Proxy Svr)|
     +--------+---------+    +--------+---------+   +--------+---------+
              |                       |                      | (steps)
              |    SIP Request        |                      |
              |---------------------->|                      |   (1a)
              |                       |                      |
              |  407 Proxy auth. req. |                      |
              |<----------------------|                      |   (1b)
              |  Challenge            |                      |
              |                       |                      |
              |      ACK              |                      |
              |---------------------->|                      |   (1c)
              |                       |                      |
              |                       |                      |
              |SIP Req + authorization|                      |
              | header w/ creds       |                      |
              |---------------------->|                      |   (2)
              |                       |                      |
              |                       |                      |
              |                       | SIP Req + Ident &    |
              |                       | authz headers        |
              |                       |--------------------->|   (3)
              |                       |                      |
              |                       | URI resolution       |
              |                       |<=====================|   (4)
              |                       | (via HTTP)           |
              |                       |                      |
              |                       | HTTP/1.1 200 OK      |
              |                       |=====================>|   (5)
              |                       |                      |
              |                       |                      |
              |                       |                  ?   |   (6)
              |                       |                      |


   Figure 6: AS-driven SIP SAML Attribute Fetch Profile: Message Flow

7.1.3.1.  Initial SIP Transaction between  Sender and AS

   This OPTIONAL step maps to Steps 1 and 2 of Section 5 "Authentication
   Service Behavior" of [I-D.ietf-sip-identity].  If the SIP request
   sent by the caller in substep 1a is deemed insufficiently
   authenticated by the AS per the rules stipulated by [I-D.ietf-sip-
   identity] Steps 1 and 2, then the AS MUST authenticate the sender of
   the message.  The particulars of how this is accomplished depend upon
   implementation and/or deployment instantiation as discussed in



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   [I-D.ietf-sip-identity].  Substeps 1b and 1c as shown in Figure 6 are
   non-normative and illustrative only.

7.1.3.2.  Sender sends SIP Request Message with  Authorization
          Credentials to the AS

   This step maps to Steps 1 and 2 of Section 5 "Authentication Service
   Behavior" of [I-D.ietf-sip-identity].  This request is presumed to be
   made in a context such that the AS will not challenge it -- i.e., the
   AS will consider the sender of the message to be authenticated.  If
   this is not true, then this procedure reverts back to Step 1, above.

   Otherwise, the AS carries out all other processing of the message as
   stipulated in [I-D.ietf-sip-identity] Steps 1 and 2, and if
   successful, this procedure procedes to the next step below.

7.1.3.3.  AS Authorizes the SIP Request and Forwards it to Verifier

   This first portion of this step maps to Steps 3 and 4 of Section 5
   "Authentication Service Behavior" of [I-D.ietf-sip-identity], which
   the AS MUST perform, although with the following additional substeps:

      The AS MUST construct a SAML assertion according to the "Assertion
      Profile Description" specified in Section 7.1.4 of this
      specification.

      The AS SHOULD construct an HTTPS, and MAY construct an HTTP, URI
      per Section "3.7.5.1 URI Syntax" of [OASIS.saml-bindings-2.0-os].

      The AS MUST use the URI constructed in the immediately preceding
      substep as the value of the Identity-Info header that is added to
      the SIP request message per Step 4 of Section 5 of [I-D.ietf-sip-
      identity].

   Upon successful completion of all of the above, the AS forwards the
   request message.

   At this point in this step, after perhaps traversing some number of
   intermediaries, the SIP request message arrives at a SIP network
   entity performing the "verifier" role.  This role and its behavior
   are specified in Section 6 "Verifier Behavior" of [I-D.ietf-sip-
   identity].  The verifier MUST perform the steps enumerated in the
   aforementioned section, with the following modifications:

      Step 1 of [I-D.ietf-sip-identity] Section 6 maps to and is updated
      by, the following two steps in this procedure.





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      Steps 2, 3, and 4 of [I-D.ietf-sip-identity] Section 6 may be
      mapped across this latter portion of this step, and/or the
      following two steps, as appropriate.

7.1.3.4.  Verifier Dereferences HTTP-based SAML  URI Reference

   The verifier SHOULD ascertain whether it has a current cached copy of
   the SIP message sender's SAML assertion and domain certificate.  If
   not, or if the verifier chooses to (e.g., due to local policy), it
   MUST dereference the the HTTP-based SAML URI Reference found in the
   SIP message's Identity-Info header.  To do so, the verifier MUST
   employ the "SAML HTTP-URI-based SIP Binding" specified in
   Section 8.1.

7.1.3.5.  AS Returns SAML Assertion

   This step also employs Section 8.1 "SAML HTTP-URI-based SIP Binding".

   If the prior step returns an HTTP error (e.g., 4xx series), then this
   procedure terminates and the verifier returns (upstream) a SIP 436
   'Bad Identity-Info' Response code.

   Otherwise, the HTTP response message will contain a SAML assertion
   and be denoted as such via the MIME media type of "application/
   samlassertion+xml" [IANA.application.samlassertion-xml].  The
   verifier MUST perform the verification steps specified in
   Section 7.1.5 "Assertion Verification", below.  If successful, then
   this procedure continues with the next step.

7.1.3.6.  Verifier performs Next Step

   The SIP request was successfully processed.  The verifier now
   performs its next step, which depends at least in part on the type of
   SIP request it received.

7.1.4.  Assertion Profile Description

   This section defines the particulars of how the sender, i.e., the
   SAML Authority, MUST construct certain portions of the SAML
   assertions it issues.  The schema for SAML assertions themselves is
   defined in Section 2.3 of [OASIS.saml-core-2.0-os].

   An example SAML assertion, formulated according to this profile is
   given in Section 9.

   Overall SAML assertion profile requirements:





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      The SAML assertion MUST be signed by the same key as used to sign
      the contents of the Identity header field.  Signing of SAML
      assertions is defined in Section 5.4 of [OASIS.saml-core-2.0-os].

   In the following subsections, the SAML assertion profile is specified
   element-by-element, in a top-down, depth-first manner, beginning with
   the outermost element, "<Assertion>".  Where applicable, the
   requirements for an element's XML attributes are also stated, as a
   part of the element's description.  Requirements for any given
   element or XML attribute are only stated when, in the context of use
   of this profile, they are not already sufficiently defined by
   [OASIS.saml-core-2.0-os].

7.1.4.1.  Element: <Assertion>

   Attribute: ID

      The value for the ID XML attribute SHOULD be allocated randomly
      such that the value meets the randomness requirments specified in
      Section 1.3.4 of [OASIS.saml-core-2.0-os].

   Attribute: IssueInstant

      The value for the IssueInstant XML attribute SHOULD be set at the
      time the SAML assertion is created (and cached for subsequent
      retrieval).  This time instant value MAY be temporally the same as
      that encoded in the SIP message's Date header, and MUST be at
      least temporally later, although it is RECOMMENDED that it not be
      10 minutes or more later.

7.1.4.1.1.  Element: <Issuer>

   The value for the Issuer XML element MUST be a value that matches
   either the Issuer or the Issuer Alternative Name fields [RFC3280] in
   the certificate conveyed by the SAML assertion in the ds:
   X509Certificate element located on this path within the SAML
   assertion:

                <Assertion
                  <ds:Signature
                    <ds:KeyInfo
                      <ds:X509Data
                        <ds:X509Certificate

7.1.4.1.2.  Element: <Subject>

   The <Subject> element SHOULD contain both a <NameID> element and a
   <SubjectConfirmation> element.



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   The value of the <NameID> element MUST be the same as the Address of
   Record (AoR) value used in computing the "signed-identity-digest"
   which forms the value of the Identity header.  See Section 9 of
   [I-D.ietf-sip-identity].

   The <SubjectConfirmation> element attribute Method SHOULD be set to
   the value:

      urn:oasis:names:tc:SAML:2.0:cm:sender-vouches

   Although it MAY be set to some other implementation- and/or
   deployment-specific value.  The <SubjectConfirmation> element itself
   SHOULD be empty.

7.1.4.1.3.  Element: <Conditions>

   The <Conditions> element SHOULD contain an <AudienceRestriction>
   element, which itself SHOULD contain an <Audience> element.  The
   value of the <Audience> element SHOULD be the same as the addr-spec
   of the SIP request's To header field.

   The following XML attributes of the <Conditions> element MUST be set
   as follows:

   Attribute: NotBefore

      The value of the NotBefore XML attribute MUST be set to a time
      instant the same as the value for the IssueInstant XML attribute
      discussed above, or to a later time.

   Attribute: NotOnOrAfter

      The value of the NotOnOrAfter XML attribute MUST be set to a time
      instant later than the value for NotBefore.

7.1.4.1.4.  Element: <AttributeStatement>

   The SAML assertion MAY contain an <AttributeStatement> element.  If
   so, the <AttributeStatement> element will contain attribute-value
   pairs, e.g., of a user profile nature, encoded according to either
   one of the "SAML Attribute Profiles" as specified in [OASIS.saml-
   profiles-2.0-os], or encoded in some implementation- and/or
   deployment-specific attribute profile.

   The attribute-value pairs SHOULD in fact pertain to the entity
   identified in the SIP From header field, since a SAML assertion
   formulated per this overall section is stating that they do.




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7.1.5.  Assertion Verification

   This section specifies the steps that a verifier participating in
   this profile MUST perform in addition to the "Verifier Behavior"
   specified in Section 6 of [I-D.ietf-sip-identity].

   The steps are:

   1.   Before Step 1 in Section 6 of [I-D.ietf-sip-identity], the
        verifier MUST extract the AS's domain certificate from the <ds:
        X509Certificate> XML element at the end of the element path
        given in Section 7.1.4.1.1.

   2.   Perform Step 1 in Section 6 of [I-D.ietf-sip-identity].

   3.   After Step 1 in Section 6 of [I-D.ietf-sip-identity], but before
        Step 2 of that section, the verifier MUST verify the SAML
        assertion's signature via the procedures specified in Section
        5.4 of [OASIS.saml-core-2.0-os] as well as [W3C.xmldsig-core].

        @@ TODO: do we need to define a new SIP error response code for
        when a SAML assn signature is bad? e.g., '4xx Invalid SAML
        asssertion'.

   4.   Perform Step 2 in Section 6 of [I-D.ietf-sip-identity].

   5.   Verify that the signer of the SIP message's Identity header
        field is the same as the signer of the SAML assertion.

   6.   Perform Steps 3 and 4 in Section 6 of [I-D.ietf-sip-identity].

   7.   Verify that the SAML assertion's <Issuer> element value matches
        the Issuer or the Issuer Alternative Name fields [RFC3280] in
        the AS's domain certificate.

   8.   Verify that the SAML assertion's <NameID> element value is the
        same as the Address of Record (AoR) value in the "signed-
        identity-digest".  See Section 9 of [I-D.ietf-sip-identity].

   9.   Verify that the SAML assertion's <SubjectConfirmation> element
        value is set to whichever value was configured at
        implementation- or deployment-time.  The default value is:

           urn:oasis:names:tc:SAML:2.0:cm:sender-vouches

   10.  Verify that the SAML assertion contains an <Audience> element,
        and that its value matches the value of the addr-spec of the SIP
        To header field.



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   11.  Verify that the validity period denoted by the NotBefore and
        NotOnOrAfter attributes of the <Conditions> element meets the
        requirements given in Section 7.1.4.1.3.
















































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8.  SAML SIP Binding

   This section specifies one SAML SIP Binding at this time.  Additional
   bindings may be specified in future revisions of this specification.

8.1.  SAML HTTP-URI-based SIP Binding

   This section specifies the "SAML HTTP-URI-based SIP Binding",
   (SHUSB).

   The SHUSB is a profile of the "SAML URI Binding" specified in Section
   3.7 of [OASIS.saml-bindings-2.0-os].  The SAML URI Binding specifies
   a means by which SAML assertions can be referenced by URIs and thus
   be obtained through resolution of such URIs.

   This profile of the SAML URI Binding is congruent with the SAML URI
   Binding -- including support for HTTP-based URIs being mandatory to
   implement -- except for the following further restrictions which are
   specified in the interest of interoperability (section numbers refer
   to [OASIS.saml-bindings-2.0-os]):

   Section 3.7.5.3 Security Considerations:

      Support for TLS 1.0 or SSL 3.0 is mandatory to implement.

   Section 3.7.5.4 Error Reporting:

      All SHOULDs in this section are to be interpreted as MUSTs.























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9.  Example Signed SAML Assertion

   Below is an example of a signed SAML assertion:


   <Assertion ID="_a75adf55-01d7-40cc-929f-dbd8372ebdfc"
      IssueInstant="2003-04-17T00:46:02Z" Version="2.0"
      xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
      <Issuer>
         example.com
      </Issuer>
      <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
         <ds:SignedInfo>
            <ds:CanonicalizationMethod
               Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
            <ds:SignatureMethod
               Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
            <ds:Reference URI="#_a75adf55-01d7-40cc-929f-dbd8372ebdfc">
               <ds:Transforms>
                  <ds:Transform
                      Algorithm=
              "http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
                  <ds:Transform
                     Algorithm=
                        "http://www.w3.org/2001/10/xml-exc-c14n#">
                     <InclusiveNamespaces
                        PrefixList="#default saml ds xs xsi"
                        xmlns=
                           "http://www.w3.org/2001/10/xml-exc-c14n#"/>
                  </ds:Transform>
               </ds:Transforms>
               <ds:DigestMethod
                   Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
               <ds:DigestValue>
                  Kclet6XcaOgOWXM4gty6/UNdviI=
               </ds:DigestValue>
            </ds:Reference>
         </ds:SignedInfo>
         <ds:SignatureValue>
           hq4zk+ZknjggCQgZm7ea8fI7...Hr7wHxvCCRwubnmIfZ6RqVL+wNmeWI4=
         </ds:SignatureValue>
         <ds:KeyInfo>
            <ds:X509Data>
                <ds:X509Certificate>
       MIICyjCCAjOgAwIBAgICAnUwDQYJKoZIhvcNAQEEBQAwgakxCzAJBgNVBAYTAlVT
       MRIwEAYDVQQIEwlXaXNjb .....  dnP6Hr7wHxvCCRwubnmIfZ6QZAv2FU78pLX
       8I3bsbmRAUg4UP9hH6ABVq4KQKMknxu1xQxLhpR1ylGPdiowMNTrEG8cCx3w/w==
                </ds:X509Certificate>



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            </ds:X509Data>
         </ds:KeyInfo>
      </ds:Signature>
      <Subject>
         <NameID
           Format=
              "urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
           Alice@example.com
         </NameID>
         <SubjectConfirmation
             Method="urn:oasis:names:tc:SAML:2.0:cm:sender-vouches"/>
      </Subject>
      <Conditions NotBefore="2003-04-17T00:46:02Z"
                  NotOnOrAfter="2003-04-17T00:51:02Z">
         <AudienceRestriction>
         </AudienceRestriction>
      </Conditions>
      <AttributeStatement>
          ...
      </AttributeStatement>
   </Assertion>






























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10.  Security Considerations

   This section discusses security considerations when using SAML with
   SIP.

10.1.  Man-in-the-middle Attacks and Stolen Assertions

   Threat:

      By making SAML assertions available via HTTP-based requests by a
      potentially unbounded set of requesters, it is conceivably
      possible that anyone would be able to simply request one and
      obtain it.  SIP intermediaries on the signaling path for example.
      Or, an HTTP intermediary/proxy could intercept the assertion as it
      is being returned to a requester.

      The attacker could then conceivably attempt to impersonate the
      subject (the putative caller) to some SIP-based target entity.

   Countermeasures:

      Such an attack is implausible for several reasons.  The primary
      reason is that a message constructed by an imposter using a stolen
      assertion that conveys the public key certificate of some domain
      will not verify per [I-D.ietf-sip-identity] because the imposter
      will not have the corresponding private key with which to generate
      the signed Identity header value.

      Also, the SIP SAML assertion profile specified herein that the
      subject's SAML assertion must adhere to causes it to be not useful
      to arbitrary parties.  The subject's assertion:

      *  should be signed, thus causing any alterations to break its
         integrity and make such alterations detectable.

      *  does not contain an authentication statement.  Thus no parties
         implementing this specification should be relying on SAML
         assertions specified herein as sufficient in and of themselves
         to allow access to resources.

      *  relying party is represented in the SAML assertion's Audience
         Restriction.

      *  Issuer is represented in the SAML assertion.

      *  validity period for assertion is restricted.





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      *  etc.

10.2.  Forged Assertion

   Threat:

      A malicious user could forge or alter a SAML assertion in order to
      communicate with the SIP entities.

   Countermeasures:

      To avoid this kind of attack, the entities must assure that proper
      mechanisms for protecting the SAML assertion are employed, e.g.,
      signing the SAML assertion itself.  Section 5.1 of [OASIS.saml-
      core-2.0-os] specifies the signing of SAML assertions.

      Additionally, the assertion content dictated by the SAML assertion
      profile herein ensures ample evidence for a relying party to
      verify the assertion and its relationship with the received SIP
      request.

10.3.  Replay Attack

   Threat:

      Theft of SIP message protected by the mechanisms described herein
      and replay of it at a later time.

   Countermeasures:

      There are various provisions within [I-D.ietf-sip-identity] that
      prevent a replay attack.



















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11.  Contributors

   The authors would like to thank Marcus Tegnander and Henning
   Schulzrinne for his contributions to earlier versions of this
   document.














































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12.  Acknowledgments

   We would like to thank RL 'Bob' Morgan and Stefan Goeman for their
   comments to this draft.















































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13.  Acknowledgments

   We thank RL 'Bob' Morgan for his inputs to this work.  The "AS-driven
   SIP SAML URI-based Attribute Assertion Fetch Profile" is based on an
   idea by Jon Peterson.

   Addtionally, the following persons provided input to this work:
   Stefan Goeman, Shida Schubert, Jason Fischl











































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14.  IANA Considerations

   This document contains a number of IANA considerations.  A future
   version of this document will list them in this section.















































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15.  Open Issues

   A list of open issues can be found at:
   http://www.tschofenig.com:8080/saml-sip/















































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16.  References

16.1.  Normative References

   [I-D.ietf-sip-identity]
              Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation  Protocol (SIP)", draft-ietf-sip-identity-06
              (work in progress), October 2005.

   [I-D.ietf-sipping-trait-authz]
              Peterson, J., "Trait-based Authorization Requirements for
              the Session Initiation Protocol  (SIP)",
              draft-ietf-sipping-trait-authz-02 (work in progress),
              January 2006.

   [OASIS.saml-bindings-2.0-os]
              Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E.
              Maler, "Bindings for the OASIS Security Assertion Markup
              Language (SAML) V2.0", OASIS
              Standard saml-bindings-2.0-os, March 2005.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [OASIS.saml-metadata-2.0-os]
              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.

   [OASIS.saml-profiles-2.0-os]
              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Maler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard OASIS.saml-profiles-2.0-os, March 2005.

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

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, August 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext



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              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, June 2003.

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              September 2004.

   [W3C.xmldsig-core]
              Eastlake, D., Reagle , J., and D. Solo, "XML-Signature
              Syntax and Processing", W3C Recommendation xmldsig-core,
              October 2000, <http://www.w3.org/TR/xmldsig-core/>.

16.2.  Informative References

   [I-D.ietf-sip-content-indirect-mech]
              Burger, E., "A Mechanism for Content Indirection in
              Session Initiation Protocol (SIP)  Messages",
              draft-ietf-sip-content-indirect-mech-05 (work in
              progress), October 2004.

   [I-D.ietf-sipping-certs]
              Jennings, C. and J. Peterson, "Certificate Management
              Service for The Session Initiation Protocol (SIP)",
              draft-ietf-sipping-certs-02 (work in progress), July 2005.

   [I-D.jennings-sipping-pay]
              Jennings, C., "Payment for Services in Session Initiation
              Protocol (SIP)", draft-jennings-sipping-pay-03 (work in
              progress), October 2005.

   [I-D.peterson-message-identity]
              Peterson, J., "Security Considerations for Impersonation



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              and Identity in Messaging  Systems",
              draft-peterson-message-identity-00 (work in progress),
              October 2004.

   [IANA.application.samlassertion-xml]
              OASIS Security Services Technical Committee (SSTC),
              "application/samlassertion+xml MIME Media Type
              Registration", IANA MIME Media Types Registry application/
              samlassertion+xml, December 2004.

   [OASIS.draft-saml-protocol-ext-02]
              Cantor, S., "SAML Protocol Extensions", OASIS SSTC Working
              Draft draft-saml-protocol-ext-02, Februrary 2006.

   [OASIS.saml-conformance-2.0-os]
              Mishra, P., Philpott, R., and E. Maler, "Conformance
              Requirements for the Security Assertion Markup Language
              (SAML) V2.0", OASIS Standard saml-conformance-2.0-os,
              March 2005.

   [OASIS.saml-glossary-2.0-os]
              Hodges, J., Philpott, R., and E. Maler, "Glossary for the
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard saml-glossary-2.0-os, March 2005.

   [OASIS.saml-sec-consider-2.0-os]
              Hirsch, F., Philpott, R., and E. Maler, "Security and
              Privacy Considerations for the OASIS Security Markup
              Language (SAML) V2.0", OASIS Standard saml-sec-consider-
              2.0-os, March 2005.

   [OASIS.sstc-saml-exec-overview-2.0-cd-01]
              Madsen, P. and E. Maler, "SAML V2.0 Executive Overview",
              OASIS SSTC Committee
              Draft sstc-saml-exec-overview-2.0-cd-01, April 2005.

   [OASIS.sstc-saml-tech-overview-2.0-draft-08]
              Hughes, J. and E. Maler, "Security Assertion Markup
              Language (SAML) V2.0 Technical Overview", OASIS SSTC
              Working Draft sstc-saml-tech-overview-2.0-draft-08,
              September 2005.

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.



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Authors' Addresses

   Hannes Tschofenig
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com


   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520
   US

   Email: jon.peterson@neustar.biz


   James Polk
   Cisco
   2200 East President George Bush Turnpike
   Richardson, Texas  75082
   US

   Email: jmpolk@cisco.com


   Douglas C. Sicker
   University of Colorado at Boulder
   ECOT 430
   Boulder, CO  80309
   US

   Email: douglas.sicker@colorado.edu


   Jeff Hodges
   NeuStar, Inc.
   2000 Broadway Street
   Redwood City, CA  94063
   US

   Email: Jeff.Hodges@neustar.biz






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