wdiff draft-ietf-sip-saml-06.txt draft-ietf-sip-saml-07.txt

SIP H. Tschofenig
Internet-Draft Nokia Siemens Networks
Intended status: Experimental J. Hodges
Expires: September 9, 2009 Unaffiliated 2010
J. Peterson
NeuStar, Inc.
J. Polk
Cisco
D. Sicker
CU Boulder
March 8, 2009 2010

SIP SAML Profile and Binding
draft-ietf-sip-saml-06.txt
draft-ietf-sip-saml-07.txt

Abstract

This document specifies a Session Initiation Protocol (SIP) profile
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)".

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on September 9, 2009. 2010.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.

Abstract

This Code Components extracted from this document specifies a Session Initiation Protocol (SIP) profile
of Security Assertion Markup Language (SAML) as well must
include Simplified BSD License text as a SAML SIP
binding. The defined SIP SAML Profile composes with the mechanisms
defined described in Section 4.e of
the SIP Identity specification Trust Legal Provisions and satisfy requirements
presented are provided without warranty as
described in "Trait-based Authorization Requirements for the Session
Initiation Protocol (SIP)". BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 5
3. SAML Introduction . . . . . . . . . . . . . . . . . . . . . . 8 7
3.1. SAML Assertions . . . . . . . . . . . . . . . . . . . . . 9 8
3.2. Abstract Request/Response Protocol . . . . . . . . . . . . 10 9
4. Specification Scope . . . . . . . . . . . . . . . . . . . . . 11 10
5. Employing SAML in SIP . . . . . . . . . . . . . . . . . . . . 12
6. URI Parameter Definition . . . . . . . . . . . . . . . . . . . 14
7. SIP SAML Profiles . . . . . . . . . . . . . . . . . . . . . . 14
6.1. 15
7.1. AS-driven SIP SAML URI-based Attribute Assertion
Fetch Profile . . . . . . . . . . . . . . . . . . . . . . 14
6.1.1. 15
7.1.1. Required Information . . . . . . . . . . . . . . . . . 14
6.1.2. 15
7.1.2. Profile Overview . . . . . . . . . . . . . . . . . . . 14
6.1.3. 16
7.1.3. Profile Description . . . . . . . . . . . . . . . . . 18
6.1.4. 19
7.2. Caller-driven SIP SAML Conveyed Assertion Profile Description . . . . . . 22
8. Assertion Profile . . . . . . 21
6.1.5. Assertion Verification . . . . . . . . . . . . . . . . 23
6.2. Caller-driven SIP SAML Conveyed
8.1. Assertion Profile Description . . . . 24
7. SAML SIP Binding . . . . . . . . . . 23
8.1.1. Element: <Assertion> . . . . . . . . . . . . . 26
7.1. SAML HTTP-URI-based SIP Binding . . . . 23
8.2. Assertion Verification . . . . . . . . . 26
8. Example SAML Assertions . . . . . . . . . 25
9. SAML SIP Binding . . . . . . . . . . 27
9. Authentication Service Behavior . . . . . . . . . . . . . 27
9.1. SAML HTTP-URI-based SIP Binding . . 32
10. Verifier Behavior . . . . . . . . . . . 27
10. Example SAML Assertions . . . . . . . . . . . 34
11. SAML-Info Header Field . . . . . . . . 28
11. Security Considerations . . . . . . . . . . . . 36
12. Extended RFC 4474 SIP Identity Signature Mechanism . . . . . . 38
13. Security Considerations . 33
11.1. Man-in-the-Middle Attacks and Stolen Assertions . . . . . 33
11.2. Privacy . . . . . . . . . . . . . . . 41
13.1. Man-in-the-Middle Attacks and Stolen Assertions . . . . . 41
13.2. . . . . . 33
11.3. Forged Assertion . . . . . . . . . . . . . . . . . . . . . 41
13.3. 34
11.4. Replay Attack . . . . . . . . . . . . . . . . . . . . . . 42
14. 35
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 43
15. 36
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
16. 37
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
16.1. Header Field Names 38
14.1. URI Parameter . . . . . . . . . . . . . . . . . . . . 45
16.2. . . 38
14.2. 477 'Binding to SIP Message failed' Response Code . . . . 45
16.3. 38
14.3. 478 'Unknown SAML Assertion Content' Response Code . . . . 45
16.4. 38
14.4. 479 'Invalid SAML Assertion' Response Code . . . . . . . . 46
16.5. 480 'Use SAML Header' Response Code 39
15. Change Log . . . . . . . . . . . 46
17. Change Log . . . . . . . . . . . . . . . 40
15.1. -06 to -07 . . . . . . . . . . . 47
17.1. . . . . . . . . . . . . . 40
15.2. -05 to -06 . . . . . . . . . . . . . . . . . . . . . . . . 47
17.2. 40
15.3. -04 to -05 . . . . . . . . . . . . . . . . . . . . . . . . 47
17.3. 40
15.4. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 47
17.4. 40
15.5. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 47
17.5. 40
15.6. -00 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 48
18. 41
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
18.1. 42
16.1. Normative References . . . . . . . . . . . . . . . . . . . 49
18.2. 42
16.2. Informative References . . . . . . . . . . . . . . . . . . 50 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52 45

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."
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). identity. Motivations for trait-based authorization, along with
use-case scenarios, are presented in [RFC4484].

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-16] 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 for encoding authorization exist: information exists, such as
authorization certificates [RFC3281], SPKI [RFC2693], or extensions
to the authenticated identity body [RFC3893]. The authors selected This document focuses
on an encoding of the authorization information using SAML due assertions
but does not exclude other formats to its increasing use be used utilized in environments, such as the Liberty
Alliance, and the Internet2 project, areas where the applicability to
SIP is widely desired. future.

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
[RFC4474]. 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 logical 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 defined 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.

3. SAML Introduction

SAML [OASIS.sstc-saml-exec-overview-2.0-cd-01]
[OASIS.sstc-saml-tech-overview-2.0-draft-16] 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
an assertion itself, or the subject of an assertion.

Thus,

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:

o Assertions

o Abstract Request/Response protocol

We describe each in turn below:

3.1. 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.

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.

3.2. 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.

4. Specification Scope

The scope of this specification is:

o Specify a SIP profile of SAML -- also known as a "SIP SAML
profile" -- such that a subject's profile attributes. attributes, and their
domain's certificate, can be conveyed to a relying party using
SAML. In doing so, satisfy the requirements outlined in [RFC4484].
[RFC4484], and compose with [RFC4474].

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 [RFC4484].

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
useful in addressing this need.

5. Employing SAML in SIP

Employing SAML in SIP necessitates devising a new SAML profile(s) and
binding(s) because 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 requiring specification of
SIP-specific SAML profile(s) and binding(s). This is technically
straightforward as both SAML and SIP are explicitly extensible.

The SIP SAML Profiles defined in this document make use of concepts
defined by [RFC4474] "Enhancements for Authenticated Identity
Management in the Session Initiation Protocol (SIP)" -- also known as
"SIP Identity". In particular, they leverage SIP Identity allows the "mediated
authentication architecture" utilizing SIP UA client and an entity
on behalf of the Authentication Service
(AS). UA client to attach a SAML assertion (or a reference
to it). Since intermediaries, like an outbound SIP proxy, are not
allowed to modify the body of a SIP message such an intermediary
would attach a pointer to the assertion instead.

The overall semantic being that specific details on how the AS SAML assertion is vouching requested are
outside the scope of this document. Possible mechanisms are to use a
software library that it did
indeed authenticate can be accessed via an API, a separate
authorization server that can be queried via HTTP (as envisioned the calling party.
'OAuth Web Resource Authorization Profiles' specification
[I-D.hardt-oauth]), or any other mechanism. As such, this document
does not further describe the functional split between the party that
attaches the SAML assertion to the SIP message and the party that
creates the SAML assertion. The SIP Identity specification calls the
party that makes identity assertions about the caller "Authentication
Service (AS)". 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.

The approach used by this This document is similar uses the term SAML
Authority and Authentication Service interchangable particularly
because of the fact that the entity that attaches the SAML assertion
to the one used for SIP
Identity, i.e. message also uses the AS creates SIP Identity mechanism to bind it to
the message.

Note that technically there is a difference between attaching a
reference to a SAML assertion and makes it available
to the verifier via attaching a reference, in SAML assertion to the particular case
body of the AS-
driven a message. We define two different profiles to cover these
two cases:

AS-driven SIP SAML URI-based Attribute Assertion Fetch Profile.
Figure 1 illustrates Profile:

In case of this approach in profile the AS attaches a high-level summary fashion.
Figure 2, further below, illustrates additional details. reference to a SAML
assertion to the SIP message and makes it available to the
verifier. More details about this profile can be found in
Section 7.1.

Assertion-by-Value Profile:

In case of
the Assertion-by Value this profile the SAML assertion is made available to
the verifying party directly without the additional step of
utilizing a reference. This approach is described in Section 6.2.

+--------+ +--------------+ +--------+
|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/SAML-Info and |
| | w/SAML-Signature |
| |--------------------->|
| | |
| | |
| | HTTP GET SAML assn |
| |<==================== |
| | |
| | |
| | HTTP 200 OK + assn |
| |=====================>|
| | |
| 200 OK | |
|<----------------------+----------------------|
| | |

Figure 1: SIP-SAML-based Network Asserted Identity

Figure 1 shows an exchange based on the AS-driven SIP SAML URI-based
Attribute Assertion Fetch Profile where 7.2.

6. URI Parameter Definition

This document represents the AS creates a SAML
assertion, creates a reference URL pointing to it, and puts that reference into
the SAML-Info header before forwarding the SIP message. To tie authorization
information using a URI parameter. The grammar for this parameter is
(following the
SAML-Info field ABNF [RFC4234] in Section 25 of RFC 3261 [RFC3261]):

token-info =
"token-info" HCOLON ident-info *( SEMI ident-info-params )

ident-info = LAQUOT absoluteURI RAQUOT

ident-info-params = generic-param

Figure 1: 'token-info' ABNF Grammar

The "absoluteURI" MUST contain a URI which dereferences to the message a digial signature is computed resource
containing a SAML assertion. All implementations of this
specification MUST support the use of HTTP and
placed in HTTPS URIs. Such HTTP
and HTTPS URIs MUST follow the SAML-Signature header. Bob in our case acting as conventions of RFC 2585 [RFC2585], and
for those URIs the
verifier uses indicated resource MUST be of the reference to retrieve form
'application/samlassertion+xml' described in that specification.

An example of the SAML assertion, verifies
it and syntax of the SAML-Signature.

6. "token-info" parameter is given
below:

From: <tel:+17005554141;
token-info=https://example.com/assns/?ID=abcde>;
tag=1928301774

7. SIP SAML Profiles

This section defines two "SIP SAML profiles":

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

o The "Assertion-by-value" "Assertion-by-Value" Profile

6.1.

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

6.1.1.

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

Contact Information:

Hannes Tschofenig (Hannes.Tschofenig@nsn.com)

SAML Confirmation Method Identifiers:

The SAML V2.0 confirmation method identifier is used in this
profile.

Description:

Given below.

Updates:

None.

6.1.2.

7.1.2. Profile Overview

Figure 2 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.

Although this profile is overview is cast in terms of a SIP INVITE
transaction, the reader should note that the mechanism specified
herein, may be applied to any SIP request message.

Figure 2 begins on the next page.

+------------------+ +------------------+ +-----------------+
| 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
| | |
G = | | SAML-Info: token-info: |
| | https://example.com|
| N | | /assns/?ID=abcde |
| | | | SAML-Signature |
| | | |
| + | |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 |
| | | 200 OK | |
| V |<----------------------+----------------------| (6)
. - | | |
V

Figure 2: 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 2. 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. 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 [RFC4474] and [RFC3261].
Depending on the chosen SIP security mechanism for client
authentication either digest authentication, client side
authentication of Transport Layer Security, or a combination
of both is used to provide the AS with a strong assurance
about the identity of Alice.

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

First, the AS authorizes the received INVITE message, message as
specified in [RFC4474] and [RFC3261]. If the authorization
procedure
is successful, the AS creates constructs and caches a SAML assertion
asserting Alice's profile attributes required by Bob's
domain (example2.com)., (example2.com), and also containing a the domain's
(example.com) public key certificate, or a reference to it.
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
and puts the value for the SAML-Info header it adds to into the INVITE
message. token-info parameter.

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
[RFC4474]. In order to accomplish this task, it needs to
obtain Alice's domain certificate that is contained in the
SAML assertion. certificate. It obtains the HTTP-based HTTP-
based SAML URI
Reference reference from the message's SAML-Info header token-info
parameter and dereferences it per Section 7.1. 9.1. 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 binding between
the SAML assertion and the SIP message is verified. A
detailed description can be found in Section 10. Various
elements contained in the SAML assertion are inspected and
the processing AS continues its
verification of the INVITE message. If successful, it
returns a 200 OK message is continued. 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.

6.1.3.

7.1.3. Profile Description

The following sections provide detailed definitions of the individual
profile steps. The relevant illustration is Figure 3, 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.

+------------------+ +------------------+ +------------------+
| 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 + SAML-Info |
| | + SIP-Signature token-info |
| |--------------------->| (3)
| | |
| | URI resolution |
| |<=====================| (4)
| | (via HTTP) |
| | |
| | HTTP/1.1 200 OK |
| |=====================>| (5)
| | |
| | |
| | ? | (6)
| | |

Figure 3: AS-driven SIP SAML Attribute Fetch Profile: Message Flow

6.1.3.1.

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 [RFC4474]. If the SIP request sent by the
caller in substep 1a is deemed insufficiently authenticated by the AS
per the rules stipulated by [RFC4474] 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 [RFC4474]. Substeps 1b and 1c as shown
in Figure 3 are non-normative and illustrative only.

6.1.3.2.

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 [RFC4474]. 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 [RFC4474] Steps 1 and 2, and if successful, this
procedure procedes to the next step below.

6.1.3.3.

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 9, 5
"Authentication Service Behavior" of [RFC4474], 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 6.1.4 8.1 of this
specification.

The AS MUST SHOULD construct an HTTP HTTPS, and MAY construct an HTTP, URI
per Section "3.7.5.1 URI Syntax" of [OASIS.saml-bindings-2.0-os]. To enable proper caching, the
HTTP URI pointing to the SAML assertion MUST be unique, i.e., if
the content of the SAML assertion changes then the HTTP URI
reference MUST be different than any previously used HTTP URI
references used before.

The AS MUST use the URI constructed in the immediately preceding
substep as the value of the SAML-Info header token-info parameter that is added to
the SIP request message.

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 10.

6.1.3.4. 6 "Verifier Behavior" of [RFC4474]. The
verifier MUST perform the steps enumerated in the aforementioned
section, with the following modifications:

Step 1 of [RFC4474] Section 6 maps to and is updated by, the
following two steps in this procedure.

Steps 2, 3, and 4 of [RFC4474] 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 SAML-Info header. token-info parameter. To do so, the verifier MUST
employ the "SAML HTTP-URI-based SIP Binding" specified in
Section 7.1.

6.1.3.5. 9.1.

7.1.3.5. AS Returns SAML Assertion

This step also employs Section 7.1 9.1 "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 SAML-Info' token-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 6.1.5 8.2
"Assertion Verification", below. If successful, then this procedure
continues with the next step.

6.1.3.6.

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.

6.1.4.

7.2. Caller-driven SIP SAML Conveyed Assertion Profile Description

This section defines

For the particulars "Assertion-by-value" profile we assume that a SAML assertion
is obtained out-of-band and attached to the body of how the sender, i.e., SIP message.
Note that any SIP message may be used to convey the SAML Authority, MUST construct certain portions assertion
even though SIP INVITE may be the most appropriate candiate. The
verification step described in Section 8.2 is applicable to this
profile as well as the description on the content of the assertion
illustrated in Section 8.1.

8. Assertion Profile

This section provides some guidance on what information should be put
into a SAML assertion by the SAML
assertions it issues. Authority and how that information
is then used by the Verifier.

8.1. Assertion Profile Description

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 8. 10.

Overall SAML assertion profile requirements:

If a SAML assertion is signed then it MUST be signed by the same
key that is used in the Transport Layer Security mechanism
utilized with HTTPS. 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].

6.1.4.1.

8.1.1. 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.

6.1.4.1.1.

8.1.1.1. 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

This field contains the domain certificate of the AS.

6.1.4.1.2.

8.1.1.2. Element: <Subject>

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

The value of the <NameID> element MUST be the Address of Record
(AoR).

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.

6.1.4.1.3.

8.1.1.3. Element: <Conditions>

The <Conditions> element SHOULD contain an <AudienceRestriction>
element, which itself SHOULD contain an <Audience> element. When
included the value of the <Audience> element MUST 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.

6.1.4.1.4.

8.1.1.4. 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.

6.1.5.

8.2. Assertion Verification

This section specifies the steps that a verifier participating in
this profile MUST has to perform in addition to
verify a SAML assertion created according to the "Verifier Behavior"
specified in profile from
Section 6 of [RFC4474]. 8.1.1.

The steps are:

1. Before Step 1 in Section 6 of [RFC4474], the verifier MUST
extract the AS's domain certificate from the <ds:X509Certificate> <ds:
X509Certificate> XML element at the end of the element path
given in Section 6.1.4.1.1. 8.1.1.1.

2. Perform Step 1 in Section 6 of [RFC4474].

3. After Step 1 in Section 6 of [RFC4474], 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]. The 479
'Invalid SAML Assertion' response code is used when the verifier
is unable to process the SAML assertion.

4. Perform Step 2 in Section 6 of [RFC4474].

5. Verify that the signer of the SIP message's Identity header
field is the same as the signer of the SAML assertion. assertion, if SIP
Identity is used to bind the token-info parameter to the SIP
signaling message. Note that without such protection certain
attacks are feasible as described in Section 11.

6. Verify that the content of the SAML assertion, if present, assertion matches with the
information carried in the SIP message. This may include the
following checks:

* 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.

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.

11. Verify that the validity period denoted by the NotBefore and
NotOnOrAfter attributes of the <Conditions> element meets the
requirements given in Section 6.1.4.1.3.

6.2. Caller-driven SIP SAML Conveyed Assertion Profile

For the "Assertion-by-value" profile we assume that a SAML assertion
is obtained out-of-band and attached to the body of the SIP message.
Note that any SIP message may be used to convey the SAML assertion
even though SIP INVITE may be the most appropriate candiate. The
verification step described in Section 6.1.5 is applicable to this
profile as well as the description on the content of the assertion
illustrated in Section 6.1.4.

7. 8.1.1.3.

9. SAML SIP Binding

This section specifies one SAML SIP Binding at this time. Additional
bindings may be specified in future revisions of this specification.
The description in Section 6.1.4 8.1 is applicable to this profile.

7.1.

9.1. 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.

10. Example SAML Assertions

This section presents two examples of a SAML assertion, one unsigned
(for clarity), the other signed (for accuracy).

In the first example, Figure 4, the assertion is attesting with
respect to the subject (lines 7-15) "Alice@example.com" (line 11).
The validity conditions are expressed in lines 16-23, via both a
validity period expressed as temporal endpoints, and an "audience
restriction" stating that this assertion's semantics are valid for
only the relying party named "example2.com". Also, the assertion's
issuer is noted in lines 4-5.

The above items correspond to some aspects of this specification's
SAML assertion profile, as noted below in Security Considerations
dicussions, see: Section 13.1 11.1 and Section 13.2. 11.3.

In lines 24-36, Alice's telephone number is conveyed, in a "typed"
fashion, using LDAP/X.500 schema as the typing means.

1 <Assertion ID="_a75adf55-01d7-40cc-929f-dbd8372ebdfc"
2 IssueInstant="2003-04-17T00:46:02Z" Version="2.0"
3 xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
4 <Issuer>
5 example.com
6 </Issuer>
7 <Subject>
8 <NameID
9 Format=
10 "urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
11 Alice@example.com
12 </NameID>
13 <SubjectConfirmation
14 Method="urn:oasis:names:tc:SAML:2.0:cm:sender-vouches"/>
15 </Subject>
16 <Conditions NotBefore="2003-04-17T00:46:02Z"
17 NotOnOrAfter="2003-04-17T00:51:02Z">
18 <AudienceRestriction>
19 <Audience>
20 example2.com
21 </Audience>
22 </AudienceRestriction>
23 </Conditions>
24 <AttributeStatement>
25 <saml:Attribute
26 xmlns:x500=
27 "urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
28 NameFormat=
29 "urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
30 Name="urn:oid:2.5.4.20"
31 FriendlyName="telephoneNumber">
32 <saml:AttributeValue xsi:type="xs:string">
33 +1-888-555-1212
34 </saml:AttributeValue>
35 </saml:Attribute>
36 </AttributeStatement>
37 </Assertion>

Figure 4: Unsigned SAML Assertion Illustrating Conveyance of
Subject Attribute

In the second example, Figure 5, the information described above is
the same, the addition is that this version of the assertion is
signed. All the signature information is conveyed in the <ds:
signature> element, lines 7-47. Thus this assertion's origin and its
integrity are assured. Since this assertion is the same as the one
in the first example above, other than having a signature added, the
second example below addresses the same Security Considerations
aspects, plus those requiring a Signature.

1 <Assertion ID="_a75adf55-01d7-40cc-929f-dbd8372ebdfc"
2 IssueInstant="2003-04-17T00:46:02Z" Version="2.0"
3 xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
4 <Issuer>
5 example.com
6 </Issuer>
7 <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
8 <ds:SignedInfo>
9 <ds:CanonicalizationMethod
10 Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
11 <ds:SignatureMethod
12 Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
13 <ds:Reference
14 URI="#_a75adf55-01d7-40cc-929f-dbd8372ebdfc">
15 <ds:Transforms>
16 <ds:Transform
17 Algorithm=
18 "http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
19 <ds:Transform
20 Algorithm=
21 "http://www.w3.org/2001/10/xml-exc-c14n#">
22 <InclusiveNamespaces
23 PrefixList="#default saml ds xs xsi"
24 xmlns=
25 "http://www.w3.org/2001/10/xml-exc-c14n#"/>
26 </ds:Transform>
27 </ds:Transforms>
28 <ds:DigestMethod
29 Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
30 <ds:DigestValue>
31 Kclet6XcaOgOWXM4gty6/UNdviI=
32 </ds:DigestValue>
33 </ds:Reference>
34 </ds:SignedInfo>
35 <ds:SignatureValue>
36 hq4zk+ZknjggCQgZm7ea8fI7...Hr7wHxvCCRwubfZ6RqVL+wNmeWI4=
37 </ds:SignatureValue>
38 <ds:KeyInfo>
39 <ds:X509Data>
40 <ds:X509Certificate>
41 MIICyjCCAjOgAwIBAgICAnUwDQYJKoZIhvcNAQEEBQAwgakxNVBAYTAlVT
42 MRIwEAYDVQQIEwlXaXNjb ..... dnP6Hr7wHxvCCRwubnZAv2FU78pLX
43 8I3bsbmRAUg4UP9hH6ABVq4KQKMknxu1xQxLhpR1ylGPdioG8cCx3w/w==
44 </ds:X509Certificate>
45 </ds:X509Data>
46 </ds:KeyInfo>
47 </ds:Signature>
48 <Subject>
49 <NameID
50 Format=
51 "urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
52 Alice@example.com
53 </NameID>
54 <SubjectConfirmation
55 Method="urn:oasis:names:tc:SAML:2.0:cm:sender-vouches"/>
56 </Subject>
57 <Conditions NotBefore="2003-04-17T00:46:02Z"
58 NotOnOrAfter="2003-04-17T00:51:02Z">
59 <AudienceRestriction>
60 <Audience>
61 example2.com
62 </Audience>
63 </AudienceRestriction>
64 </Conditions>
65 <AttributeStatement>
66 <saml:Attribute
67 xmlns:x500=
68 "urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
69 NameFormat=
70 "urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
71 Name="urn:oid:2.5.4.20"
72 FriendlyName="telephoneNumber">
73 <saml:AttributeValue xsi:type="xs:string">
74 +1-888-555-1212
75 </saml:AttributeValue>
76 </saml:Attribute>
77 </AttributeStatement>
78 </Assertion>

Figure 5: Signed SAML Assertion Illustrating Conveyance of Subject
Attribute

9. Authentication Service Behavior

[RFC4474] defined a new role for SIP entities called an
authentication service. This document re-uses the concept and hence
the same constraints and properties apply to this document. The
subsequent text is copied from [RFC4474] and modified to fit to the
usage of SAML.

Any entity that instantiates the authentication service role MUST
possess the private key of a domain certificate. Intermediaries that
instantiate this role MUST be capable of authenticating one or more
SIP users that can register in that domain. Commonly, this role will
be instantiated by a proxy server, since these entities are more
likely to have a static hostname, hold a corresponding certificate,
and have access to SIP registrar capabilities that allow them to
authenticate users in their domain. It is also possible that the
authentication service role might be instantiated by an entity that
acts as a redirect server, but that is left as a topic for future
work.

SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present.
Similarly, authentication services MUST add a Content- Length header
field to SIP requests if one is not already present; this can help
verifiers to double-check that they are hashing exactly as many bytes
of message-body as the authentication service when they verify the
message.

Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP
request:

Step 1:

The authentication service MUST extract the identity of the sender
from the request. The authentication service takes this value
from the From header field; this AoR will be referred to here as
the 'identity field'. If the identity field contains a SIP or SIP
Secure (SIPS) URI, the authentication service MUST extract the
hostname portion of the identity field and compare it to the
domain(s) for which it is responsible (following the procedures in
RFC 3261, Section 16.4, used by a proxy server to determine the
domain(s) for which it is responsible). If the identity field
uses the TEL URI scheme, the policy of the authentication service
determines whether or not it is responsible for this identity. If
the authentication service is not responsible for the identity in
question, it SHOULD process and forward the request normally, but
it MUST NOT add a SAML-Info and a SAML-Signature header.

Step 2:

The authentication service MUST determine whether or not the
sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message.

Step 3:

The authentication service SHOULD ensure that any preexisting Date
header in the request is accurate. Local policy can dictate
precisely how accurate the Date must be; a RECOMMENDED maximum
discrepancy of ten minutes will ensure that the request is
unlikely to upset any verifiers. If the Date header contains a
time different by more than ten minutes from the current time
noted by the authentication service, the authentication service
SHOULD reject the request. This behavior is not mandatory because
a user agent client (UAC) could only exploit the Date header in
order to cause a request to fail verification; the SAML-Signature
header is not intended to provide a source of non-repudiation or a
perfect record of when messages are processed. Finally, the
authentication service MUST verify that the Date header falls
within the validity period of its certificate.

Step 4:

The authentication service MUST form the signature and add the
SAML-Signature header to the request containing this signature.
After the SAML-Signature header has been added to the request, the
authentication service MUST also add an SAML-Info header. The
SAML-Info header contains a URI from which the SAML assertion and
a domain certificate can be acquired.

Finally, the authentication service MUST forward the message
normally.

10. Verifier Behavior

When a verifier receives a SIP message containing an SAML-Signature
and SAML-Info header, it may inspect these two header fields.
Typically, the results of a verification are provided as input to an
authorization process that is outside the scope of this document. If
an SAML-Info and SAML-Signature header is not present in a request,
and one is required by local policy (for example, based on a per-
sending-domain policy, or a per-sending-user policy), then a 428 'Use
SAML Header' response MUST be sent.

In order to verify the identity of the sender of a message, an entity
acting as a verifier MUST perform the following steps, in the order
here specified.

Step 1:

The verifier has to acquire the certificate for the signing
domain. Implementations supporting this specification SHOULD have
some means of retaining domain certificates (in accordance with
normal practices for certificate lifetimes and revocation) in
order to prevent themselves from needlessly downloading the same
certificate every time a request from the same domain is received.
Certificates cached in this manner should be indexed by the URI
given in the SAML-Info header field value.

Provided that the domain certificate used to sign this message is
not previously known to the verifier, SIP entities SHOULD discover
this certificate by dereferencing the SAML-Info header, unless
they have some more efficient implementation-specific way of
acquiring certificates for that domain. The domain certificate
can be found in the SAML assertion, either by reference or by
value. The verifier processes this certificate in the usual ways,
including checking that it has not expired, that the chain is
valid back to a trusted certification authority (CA), and that it
does not appear on revocation lists. Once the certificate is
acquired, it MUST be validated following the procedures in RFC
3280 [RFC3280]. If the certificate cannot be validated (it is
self-signed and untrusted, or signed by an untrusted or unknown
certificate authority, expired, or revoked), the verifier MUST
send a 437 'Unsupported Certificate' response.

Step 2:

The verifier MUST follow the process described in Section 13.4 of
[RFC4474] to determine if the signer is authoritative for the URI
in the From header field.

Step 3:

The verifier MUST verify the signature in the SAML-Signature
header field, following the procedures for generating the hashed
digest-string described in Section 12. If a verifier determines
that the signature on the message does not correspond to the
reconstructed digest- string, then a 479 'Invalid SAML Assertion'
response MUST be returned.

Step 4:

The verifier MUST validate the Date, Contact, and Call-ID headers.
It must furthermore ensure that the value of the Date header falls
within the validity period of the certificate whose corresponding
private key was used to sign the Identity header.

11. SAML-Info Header Field

This document introduces the SIP header field "SAML-Info" to carry a
reference to a SAML assertion. This header MAY appear in any SIP
header and MAY also appear more than once.

The grammar for this header is (following the ABNF [RFC4234] in
Section 25 of RFC 3261 [RFC3261]):

SAML-Info =
"SAML-Info" HCOLON ident-info *( SEMI ident-info-params )

ident-info = LAQUOT absoluteURI RAQUOT

ident-info-params = generic-param

Figure 6: SAML-Info ABNF grammar

The "absoluteURI" portion of the SAML-Info header MUST contain a URI
which dereferences to a resource containing a SAML assertion. All
implementations of this specification MUST support the use of HTTP
and HTTPS URIs in the SAML-Info header. Such HTTP and HTTPS URIs
MUST follow the conventions of RFC 2585 [RFC2585], and for those URIs
the indicated resource MUST be of the form 'application/
samlassertion+xml' described in that specification.

No parameters are defined for the SAML-Info header in this document.
Future experimental RFCS may define additional SAML-Info header
parameters.

This document adds the following entries to Table 2 of RFC 3261
[RFC3261]:

Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
SAML-Info R a o o - o o o

SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o

Figure 7: New SAML-Info Row for RFC3261 Table 2

The SAML-Info header MUST NOT appear in CANCEL.

12. Extended RFC 4474 SIP Identity Signature Mechanism

To allow the SIP Identity mechanism [RFC4474] to protect also the
reference to the SAML assertion additional SIP header fields need to
be protected by the signature calculation mechanisms. The extended
signature computation is included in the SAML-Signature header field.

The grammar for this new header is (following the ABNF [RFC4234] in
RFC 3261 [RFC3261]):

SAML-Signature = "SAML-Signature" HCOLON ( signed-identity-digest
sig-info-fields sig-info-alg ) / sig-info-extension
signed-identity-digest = LDQUOT 32LHEX RDQUOT
sig-info-alg = "alg" EQUAL token
sig-info-fields = "fields" EQUAL token
sig-info-extension = generic-param

The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character:

o The AoR of the UA sending the message, or addr-spec of the From
header field (referred to occasionally here as the 'identity
field').

o The addr-spec component of the To header field, which is the AoR
to which the request is being sent.

o The callid from Call-Id header field.

o The digit (1*DIGIT) and method (method) portions from CSeq header
field, separated by a single space (ABNF SP, or %x20). Note that
th CSeq header field allows linear whitespace (LWS) rather than SP
to separate the digit and method portions, and thus the CSeq
header field may need to be transformed in order to be
canonicalized. The authentication service MUST strip leading
zeros from the 'digit' portion of the Cseq before generating the
digest-string.

o The Date header field, with exactly one space each for each SP and
the weekday and month items case set as shown in BNF in RFC 3261.
RFC 3261 specifies that the BNF for weekday and month is a choice
amongst a set of tokens. The RFC 2234 rules for the BNF specify
that tokens are case sensitive. However, when used to construct
the canonical string defined here, the first letter of each week
and month MUST be capitalized, and the remaining two letters must
be lowercase. This matches the capitalization provided in the
definition of each token. All requests that use the Identity
mechanism MUST contain a Date header.

o The addr-spec component of the Contact header field value. If the
request does not contain a Contact header, this field MUST be
empty (i.e., there will be no whitespace between the fourth and
fifth "|" characters in the canonical string).

o The sig-info-params parameter contains a list of SIP header fields
whose values have to be included into the signature calculation.
The individual field names in small letters are encoded in the
token parameter of the sig-info-fields, each name separated by a
"|" character.

o The body content of the message with the bits exactly as they are
in the Message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that
the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that
follows that CRLF. If the message has no body, then message-body
will be empty, and the final "|" will not be followed by any
additional characters.

The precise formulation of this digest-string is, therefore
(following the ABNF [RFC4234] in RFC 3261 [RFC3261]):

The signfields parameter represent the concatination of the values of
the SIP header fields that are included in the signature calculation.

Note again that the first addr-spec MUST be taken from the From
header field value, the second addr-spec MUST be taken from the To
header field value, and the third addr-spec MUST be taken from the
Contact header field value, provided the Contact header is present in
the request.

After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain. The hashing and signing algorithm is
specified by the 'alg' parameter. This document defines only one
value for the 'alg' parameter: 'rsa-sha1'. All implementations of
this specification MUST support 'rsa-sha1'. When the 'rsa-sha1'
algorithm is specified in the 'alg' parameter of Identity-Info, the
hash and signature MUST be generated as follows: compute the results
of signing this string with sha1WithRSAEncryption as described in RFC
3370 [RFC3370] and base64 encode the results as specified in RFC 3548
[RFC3548]. A 1024-bit or longer RSA key MUST be used. The result is
placed in the SAML-Signature header field.

This document adds the following entries to Table 2 of RFC 3261
[RFC3261]:

Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
SAML-Signature R a o o - o o o

SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o

Note, in the table above, that this mechanism does not protect the
CANCEL method. The CANCEL method cannot be challenged, because it is
hop-by-hop, and accordingly authentication service behavior for
CANCEL would be significantly limited. Note as well that the
REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to this mechanism, and the use of
Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility
reasons. The SAML-Signature header MUST NOT appear in CANCEL.

13. Security Considerations

This section discusses security considerations when using SAML with
SIP.

13.1.

11.1. 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. By 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 utilize the SAML assertion in
another exchange in order 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 because the values in the SAML assertion, which
are tied to the SIP message, will not verify.

Also,

Furthermore, the SIP SAML assertion profile specified herein that may contain restrictions
regarding the
subject's SAML assertion must adhere to causes parties it to can be not useful
to arbitrary parties. The subject's assertion:

* used by. Finally, the assertion
should be signed, signed and thus causing any alterations to break its
integrity and make such alterations detectable.

* relying party is represented

11.2. Privacy

Threat:

The ability for other entities to obtain additional information
about an individual, such as role in an organization or other
authorization relevant information raises privacy concerns.

Since the SAML assertion's Audience
Restriction.

* Issuer assertion itself is represented not confidentiality protected
nor the exchange of the reference to the SAML assertion an
intermediary or a third party adversary would be allowed to gain
additional information about an individual

Countermeasures:

To address the threats three cases need to be differentiated.

First, a third party that did not participate in any of the
exchange is prevented from eavesdropping on the content of the
SAML assertion.

* validity period for assertion by employing confidentiality protection of the SIP
signaling exchange as well as the HTTP exchange. This ensures
that an eavesdropper on the wire is unable to obtain information.
However, this does not prevent intermediaries, such as SIP proxies
from observing a URL to a SAML assertion (in the token-info
parameter). To deal with this second type of attacker depending
on the environment where such a threat must be addressed it is
necessary to authenticate the entity that tries to resolve the
reference to a SAML assertion and to only provide a positive
response (with the SAML assertion) if the requestor is authorized
to obtain the desired information. When a SAML assertion is
carried inband then such a protection is restricted.

13.2. more difficult to
accomplish as the SAML assertion would have to be confidentiality
protected with the key of the intented recipient, for example
using S/MIME. Finally, the last type of threat concerns the
intented recipient of the SAML assertion itself. Proper
permissions for the distribution of information about the caller
via the content of the SAML assertion to certain recipients need
to be available. This permission must be provided by the caller
itself or, in certain circumstances, by someone on behalf of the
caller. From a technical point of view, some form of
authorization policies will be required.

11.3. 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. itself or protecting the transport of
the SAML assertion from the AS to the verifying party using TLS.
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.

13.3.

11.4. Replay Attack

Threat:

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

Countermeasures:

The SAML assertion may contain several elements to prevent replay
attacks. There is, however, a clear tradeoff between the
replaying an assertion and re-using it over multiple SIP
exchanges/sessions.

14.

Additionally, the SAML assertion can be tied to the SIP exchange
with the help of the SIP Identity mechanism. RFC 4474 [RFC4474]
signs certain header fields and the SIP message body and thereby
helps to protect message modifications. If a recipient knows that
all messages from a certain originator arrive with SIP Identity
protection applies then downgrading attacks are not possible.

12. Contributors

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

15.

13. Acknowledgments

We would like to thank RL 'Bob' Morgan, Stefan Goeman, Shida
Schubert, Jason Fischl, Sebastian Felis, Nie Pin, Marcos Dytz, Erkki
Koivusalo, Richard Barnes, Marc Willekens, Marc Willekens, Steffen
Fries and Vijay Gurbani for their comments to this draft.

The "AS-
driven "AS-driven SIP SAML URI-based Attribute Assertion Fetch Profile"
is based on an idea by Jon Peterson.

We would also like to thank Eric Rescorla for his expert review.

16.

14. IANA Considerations

16.1. Header Field Names

When a SAML assertion is attached to the body of the message then the
"application/samlassertion+xml" MIME media type is used. This document specifies two new SIP header fields: 'SAML-Info' (see
Section 11 and 'SAML-Signature' (see Section 12). MIME
type is already registered with IANA and no further action is requested
to add these two headers to
required from IANA.

14.1. URI Parameter

This document extends the header sub-registry under
http://www.iana.org/assignments/sip-parameters.

Header Name: SAML-Info

Compact Form: y

Header registry of URI parameters, as defined RFC
3969 [RFC3969] with the following value:

Parameter Name: SAML-Signature

Compact Form: y

16.2. token-info

Predefined Values: No

Reference: This document

14.2. 477 'Binding to SIP Message failed' Response Code

This document registers a new SIP response code. It is sent when a
verifier receives a SAML assertion but the Subject and Condition
elements cannot be matched to the content in the SIP message, i.e.,
the binding between the SIP message and the SAML assertion cannot be
accomplished. This response code is defined by the following
information, which has been added to the method and response-code
sub-registry under http://www.iana.org/assignments/sip-parameters.

Response Code Number: 477

Default Reason Phrase: Binding to SIP Message failed

16.3.

14.3. 478 'Unknown SAML Assertion Content' Response Code

This document registers a new SIP response code. It is used when the
verifier is unable to parse the content of the SAML assertion,
because, for example, the assertion contains only unknown elements in
in the SAML assertion, or the SAML assertion XML document is garbled.
This response code is defined by the following information, which has
been added to the method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.

Response Code Number: 478

Default Reason Phrase: Unknown SAML Assertion Content

16.4.

14.4. 479 'Invalid SAML Assertion' Response Code

This document registers a new SIP response code. It is used when the
verifier is unable to process the SAML assertion. A verifier may be
unable to process the SAML assertion in case the assertion is self-
signed, or signed by a root certificate authority for whom the
verifier does not possess a root certificate. This response code is
defined by the following information, which has been added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.

Response Code Number: 479

Default Reason Phrase: Invalid SAML Assertion

16.5. 480 'Use SAML Header' Response Code

This document registers a new SIP response code. It is used when a
SAML-Info and SAML-Signature header is not present in a request, and
one is required by local policy (for example, based on a per-sending-
domain policy, or a per-sending-user policy). This response code is
defined by the following information, which has been added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.

Response Code Number: 480

Default Reason Phrase: Use SAML Header

17.

15. Change Log

RFC Editor - Please remove this section before publication.

17.1. -05 to

15.1. -06

In response to -07

Undo changes made in version 6.

Removed the review comments by Eric Rescorla header fields and switched to a URI parameter

Editorial changes

15.2. -05 to -06

Defined a new signature SIP header field was added.

17.2. Identity signature mechanism.

15.3. -04 to -05

Changed the document type to experimental

Removed option tag

Added the Caller-driven SIP SAML Conveyed Assertion Profile

Defined a new header (SAML-Info)

Changed the description for usage with this new header

Updated security considerations

Minor editorial cleanups

17.3.

15.4. -03 to -04

Updated IANA consideration section.

Added option tag

Updated acknowledgments section

Minor editorial changes to the security considerations section

17.4.

15.5. -02 to -03

Denoted that this I-D is intended to update RFC4474 per SIP working
group consensus at IETF-69. This is the tact adopted in order to
address the impedance mismatch between the nature of the URIs
specified as to be placed in the Identity-Info header field, and what
is specified in RFC4474 as the allowable value of that header field.

Added placeholder "TBD" section for a to-be-determined "call-by-
value" profile, per SIP working group consensus at IETF-69.

Removed use-case appendicies (per recollection of JHodges during
IETF-69 discussion as being WG consensus, but such is not noted in
the minutes).

17.5.

15.6. -00 to -02

Will detail in -04 rev.

18.

Initial specifications to kickstart the work.

16. References

18.1.

16.1. Normative References

[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.

[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, May 1999.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[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.

[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.

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

[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 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.

[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC 3969, December 2004.

[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.

[RFC4484] Peterson, J., Polk, J., Sicker, D., and H. Tschofenig,
"Trait-Based Authorization Requirements for the Session
Initiation Protocol (SIP)", RFC 4484, August 2006.

[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/>.

18.2.

16.2. Informative References

[I-D.hardt-oauth]
Hardt, D., Tom, A., Eaton, B., and Y. Goland, "OAuth Web
Resource Authorization Profiles", draft-hardt-oauth-01
(work in progress), January 2010.

[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.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-protocol-ext-thirdparty-cd-01]
Cantor, S., "SAML Protocol Extension for Third-Party
Requests", OASIS SSTC Committee Draft sstc-saml-protocol-
ext-thirdparty-cd-01, March 2006.

[OASIS.sstc-saml-tech-overview-2.0-draft-16]
Ragouzis, N., Hughes, J., Philpott, R., Maler, E., Madsen,
P., and T. Scavo, "Security Assertion Markup Language
(SAML) V2.0 Technical Overview", OASIS SSTC Working
Draft sstc-saml-tech-overview-2.0-draft-16, May 2008.

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

[RFC2693] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
B., and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
September 1999.

[RFC3281] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281,
April 2002.

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

[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.

Authors' Addresses

Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland

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

Jeff Hodges
Unaffiliated

Email: Jeff.Hodges@KingsMountain.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