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Versions: (RFC 4474) 00 01 draft-jennings-stir-rfc4474bis

Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                             C. Jennings
Expires: November 30, 2013                                         Cisco
                                                           E.K. Rescorla
                                                              RTFM, Inc.
                                                            May 29, 2013


  Authenticated Identity Management in the Session Initiation Protocol
                                 (SIP)
                 draft-jennings-dispatch-rfc4474bis-00

Abstract

   The existing security mechanisms in the Session Initiation Protocol
   (SIP) are inadequate for cryptographically assuring the identity of
   the end users that originate SIP requests, especially in an
   interdomain context.  This document defines a mechanism for securely
   identifying originators of SIP messages.  It does so by defining two
   new SIP header fields, Identity, for conveying a signature used for
   validating the identity, and Identity-Info, for conveying a reference
   to the certificate of the signer.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 30, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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Table of Contents

   1.  Preamble  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Scope of Proposed Changes . . . . . . . . . . . . . . . . . .   5
     2.1.  Changes to the Identity-Info Header . . . . . . . . . . .   5
     2.2.  Changes to the Identity Header  . . . . . . . . . . . . .   6
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Overview of Operations  . . . . . . . . . . . . . . . . . . .  10
   7.  Authentication Service Behavior . . . . . . . . . . . . . . .  11
     7.1.  Identity within a Dialog and Retargeting  . . . . . . . .  14
   8.  Verifier Behavior . . . . . . . . . . . . . . . . . . . . . .  15
   9.  Considerations for User Agent . . . . . . . . . . . . . . . .  16
   10. Considerations for Proxy Servers  . . . . . . . . . . . . . .  17
   11. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  18
   12. Compliance Tests and Examples . . . . . . . . . . . . . . . .  21
     12.1.  Identity-Info with a Singlepart MIME body  . . . . . . .  21
     12.2.  Identity for a Request with No MIME Body or Contact  . .  24
   13. Identity and the TEL URI Scheme . . . . . . . . . . . . . . .  27
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  28
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  29
     15.1.  Handling of digest-string Elements . . . . . . . . . . .  29
     15.2.  Display-Names and Identity . . . . . . . . . . . . . . .  32
     15.3.  Securing the Connection to the Authentication Service  .  33
     15.4.  Domain Names and Subordination . . . . . . . . . . . . .  34
     15.5.  Authorization and Transitional Strategies  . . . . . . .  35
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36



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     16.1.  Header Field Names . . . . . . . . . . . . . . . . . . .  37
     16.2.  428 'Use Identity Header' Response Code  . . . . . . . .  37
     16.3.  436 'Bad Identity-Info' Response Code  . . . . . . . . .  37
     16.4.  437 'Unsupported Certificate' Response Code  . . . . . .  38
     16.5.  438 'Invalid Identity Header' Response Code  . . . . . .  38
     16.6.  Identity-Info Parameters . . . . . . . . . . . . . . . .  38
     16.7.  Identity-Info Algorithm Parameter Values . . . . . . . .  38
     16.8.  Acknowledgements . . . . . . . . . . . . . . . . . . . .  39
     16.9.  Original RFC 4474 Requirements . . . . . . . . . . . . .  39
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     17.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Preamble

   The sip-identity drafts that lead to RFC 4474 [RFC4474] were first
   published in 2002.  Since that point many things have changed that
   impact that design.

   o  The DNS root has been signed.

   o  SPAM continues to be a problem.

   o  A clearer understanding has evolved of the use of B2BUAs including
      standardization such as the STRAW WG.

   o  Multipart MIME has failed as a SIP extension mechanism.

   o  Widespread identity providers such as Facebook have emerged.

   o  Techniques for non-carrier entities to verify phone numbers and
      then use them for addressing (such as Apple's iMessage) have been
      shown to be commercially feasible.

   o  Substantial portions of commercial, government, and personal voice
      communications rely on SIP at some stage in the communications.

   o  The cost of operating large databases has fallen and outsourced
      versions of these databases have become cheaply available.

   o  Extensive experience and user research has improved our
      understanding of how to present security information to users.

   o  The world is in the a huge transition to mobile devices.  Even the
      most limited modern mobile devices have user interface and
      computational capabilities that greatly exceed a 2002-era SIP
      phone.



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   The authors believe that the confluence of changing technology, the
   evolution of mobile devices and internet, and a political will to
   change make this the right time to consider an expansion of the scope
   of 4474 to solve the following problems:

   o  Assert strong identity for domain scoped names such as
      alice@example.com

   o  Assert strong identity for E.164 numbers such as +1 408 555-1212

   o  Provide organization attributes such as "this is a Bank".

   o  Work for calls crossing even the most adverse networks such as the
      PSTN.

   o  Provide reliable information about who is calling before the call
      is answered to help stop SPAM.

   o  Provide reliable information about who you are talking to.

   o  Work with evolving non SIP based communications systems such as
      WebRTC.

   We believe it is possible to solve all of these in a way that is
   commercially viable, deployable, and provides a delightful user
   experience.

   The core problem in a global identity system with delegated names is
   understanding who is authorized to make assertions about a given
   name.  The proposal is to solve that problem with a two pronged
   approach.

   First have responsibility for a number delegated down from the root
   in a series of delegation sub delegation towards the user.  For
   example, the North American number operator may assign a portion of
   the +1 space to a service provider.  That service provider may assign
   a sub space to a company and that company may assign a number to a
   user.  At each level of delegation, cryptographic credentials could
   be provided that allow the user to prove the space was delegated to
   them given some common trust root.  This approach is referred to as
   "delegation" and effectively works from the top down.

   The other prong to solving the problem is called "claims" and works
   via a bottom up approach.  The end user of a number basically claims
   it and some trusted system validates this claim.  The validation may
   be as simple as sending a SMS to the number or more complicated such
   as the VIPR system.




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   The delegation approach creates an easier user experience but is
   harder to deploy from a business incentive point of view so our
   approach is to do both and work down from the top and up from the
   bottom with a meet in the middle approach to coverage of the full
   name space.

   User agents that have a credential (whether of the delegation or
   claim variety) for a name can create two types of assertions: replay
   assertions and reliance assertions.  These two assertion are signed
   over a different set of the call information and protect against
   different types of attacks.  Some networks might modify the signaling
   in ways that impact one of these assertions but not the other.

   The assertions are passed from the caller to the callee for
   verification.  This can be done by either passing the assertion along
   with the signaling, or alternatively passing it through a web based
   Call Detail Service (CDS) where the caller saves the assertion on the
   Call Detail Service and the callee retrieves it from the Call Detail
   Service service.  There are some call signaling environments, such as
   when a call passes through the PSTN, where it is not possible to
   transfer the assertion in the call signaling path.  The Call Detail
   Service is in place to make things work in this environment thought
   some privacy information around who is calling who is reveled to the
   Call Detail Service service.  An outline for this design is described
   in [I-D.rescorla-callerid-fallback].

2.  Scope of Proposed Changes

2.1.  Changes to the Identity-Info Header

   Currently, RFC4474 restricts the subject of the certificate to a
   domain name, and accordingly the RFC4474 Identity-Info header
   contains a URI which designates a certificate whose subject (more
   precisely, subjectAltName) must correspond to the domain of the URI
   in the From header field value of the SIP request.  Per the analysis
   in [I-D.peterson-secure-origin-ps], we propose to relax this to allow
   designating an alternative authority for telephone numbers, when
   telephone numbers appear in the From header field value.

   These changes will allow the Identity-Info URI to point to the
   certificate with authority over the calling telephone number.  A
   verification service will therefore authorize a SIP request when the
   telephone number in the From header field value agrees with the
   subject of the certificate.  Verification services must of course
   trust the certificate authority that issued the certificate in
   question.  To implement this change to the Identity-Info header, we
   must allow for two possibilities for the conveyance of a telephone
   number in a request: appearing within a tel URI or appearing as the



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   user portion of a SIP URI.  Therefore, we must prescribe verification
   service behind in the case where the From header field value URI
   contains a telephone user part followed by a domain -- which should
   the verification service expect to find in a certificate?

   There are also a few other potential changes within the scope of a
   revision to the Identity-Info header.  We might consider implementing
   enough flexibility in the URI to allow a model more like the IdP
   model described in [I-D.rescorla-rtcweb-generic-idp]; this could be
   useful as RTCWeb sees increasing deployment.  We also should consider
   any implications of the signing of the DNSSEC root and the DANE
   specifications to the existing Identity-Info uses with domain name.
   At a high level, it is not expected that the proposed changes will
   radically alter the semantics of Identity-Info.

   Although deployment of RFC4474 to date has been essentially non-
   existent, we will during this revision process consider any realistic
   backwards compatibility concerns.

2.2.  Changes to the Identity Header

   Per the analysis in [I-D.peterson-secure-origin-ps], we propose to
   change the signature mechanism that RFC44474 specified for the
   Identity header: in particular, to replace this signature mechanism
   with one that is more likely to survive end-to-end in SIP networks
   where intermediaries act as back-to-back user agents rather than
   proxy servers.

   To accomplish this, we propose creating two distinct signatures
   within SIP requests: a replay assurance and a reliance assurance.
   The replay assurance prevents impersonation attacks by providing a
   signature over the From header field value and certain other headers
   which will allow a verification service to detect a cut-and-paste
   attack.  The reliance assurance protects against men-in-the-middle
   unilaterally changing other parameters of the call: these include the
   target of future requests (Contact header field) and the entirely of
   the SDP, including the target IP address and ports which, if
   unprotected, can allow a man-in-the-middle to impersonate an intended
   listener.  Verification services behavior would change to allow them
   to decide, based on their configuration in a deployment environment,
   whether the replay assurance alone can realistically survive network
   transit, or if the reliance assurance should be available.

   There are a number of ways to implement this change to the signature
   in the Identity header field.  One possibility is to design two new
   headers, which we might call "Identity-Reliance" and "Identity-
   Replay" with the reliance signature being over a canonical
   representation of the reliance field and then the Identity-Replay



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   header covering the From header field value, other headers needed for
   replay protection, and well as the contents of the Identity-Reliance
   header.  It might also be possible to preserve the existing Identity
   header as the reliance header.  There are however several similar
   alternatives we might consider, and some analysis will be required to
   identify the best option.

   In order to preserve critical security parameters even in adverse
   network conditions, the replay assurance integrity protection must
   always cover security parameters of the SDP required to negotiate
   media-level security.  There may be other exception cases, or
   extensibility mechanisms, worth considering here.  In cases where the
   From header field value of a SIP request contains a SIP URI with a
   telephone number user part, we will also consider replay assurance
   canonicalizations that do not cover the domain portion of the URI.

   We will furthermore give due consideration to changes in SIP
   architecture and deployment since the publication of RFC4474,
   including the ongoing work in the STRAW working group.

   As with Identity-Info, any necessary consideration will be given to
   backwards compatibility of the Identity header.

3.  Introduction

   This document provides enhancements to the existing mechanisms for
   authenticated identity management in the Session Initiation Protocol
   (SIP, RFC 3261 [RFC3261]).  An identity, for the purposes of this
   document, is defined as a SIP URI, commonly a canonical address-of-
   record (AoR) employed to reach a user (such as
   'sip:alice@atlanta.example.com').

   RFC 3261 [RFC3261] stipulates several places within a SIP request
   where a user can express an identity for themselves, notably the
   user-populated From header field.  However, the recipient of a SIP
   request has no way to verify that the From header field has been
   populated appropriately, in the absence of some sort of cryptographic
   authentication mechanism.

   RFC 3261 [RFC3261] specifies a number of security mechanisms that can
   be employed by SIP user agents (UAs), including Digest, Transport
   Layer Security (TLS), and S/MIME (implementations may support other
   security schemes as well).  However, few SIP user agents today
   support the end-user certificates necessary to authenticate
   themselves (via S/MIME, for example), and furthermore Digest
   authentication is limited by the fact that the originator and
   destination must share a prearranged secret.  It is desirable for SIP
   user agents to be able to send requests to destinations with which



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   they have no previous association -- just as in the telephone network
   today, one can receive a call from someone with whom one has no
   previous association, and still have a reasonable assurance that the
   person's displayed Caller-ID is accurate.  A cryptographic approach,
   like the one described in this document, can probably provide a much
   stronger and less spoofable assurance of identity than the telephone
   network provides today.

4.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919].

5.  Background

   The usage of many SIP applications and services is governed by
   authorization policies.  These policies may be automated, or they may
   be applied manually by humans.  An example of the latter would be an
   Internet telephone application that displays the Caller-ID of a
   caller, which a human may review before answering a call.  An example
   of the former would be a presence service that compares the identity

   of potential subscribers to a whitelist before determining whether it
   should accept or reject the subscription.  In both of these cases,
   attackers might attempt to circumvent these authorization policies
   through impersonation.  Since the primary identifier of the sender of
   a SIP request, the From header field, can be populated arbitrarily by
   the controller of a user agent, impersonation is very simple today.
   The mechanism described in this document aspires to provide a strong
   identity system for SIP in which authorization policies cannot be
   circumvented by impersonation.

   All RFC 3261 [RFC3261] compliant user agents support Digest
   authentication, which utilizes a shared secret, as a means for
   authenticating themselves to a SIP registrar.  Registration allows a
   user agent to express that it is an appropriate entity to which
   requests should be sent for a particular SIP AoR URI (e.g.,
   'sip:alice@atlanta.example.com').

   By the definition of identity used in this document, registration is
   a proof of the identity of the user to a registrar.  However, the
   credentials with which a user agent proves its identity to a
   registrar cannot be validated by just any user agent or proxy server
   -- these credentials are only shared between the user agent and their
   domain administrator.  So this shared secret does not immediately
   help a user to authenticate to a wide range of recipients.



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   Recipients require a means of determining whether or not the 'return
   address' identity of a non-REGISTER request (i.e., the From header
   field value) has legitimately been asserted.

   The AoR URI used for registration is also the URI with which a UA
   commonly populates the From header field of requests in order to
   provide a 'return address' identity to recipients.  From an
   authorization perspective, if you can prove you are eligible to
   register in a domain under a particular AoR, you can prove you can
   legitimately receive requests for that AoR, and accordingly, when you
   place that AoR in the From header field of a SIP request other than a
   registration (like an INVITE), you are providing a 'return address'
   where you can legitimately be reached.  In other words, if you are
   authorized to receive requests for that 'return address', logically,
   it follows that you are also authorized to assert that 'return
   address' in your From header field.  This is of course only one
   manner in which a domain might determine how a particular user is
   authorized to populate the From header field; as an aside, for other
   sorts of URIs in the From (like anonymous URIs), other authorization
   policies would apply.

   Ideally, then, SIP user agents should have some way of proving to
   recipients of SIP requests that their local domain has authenticated
   them and authorized the population of the From header field.  This
   document proposes a mediated authentication architecture for SIP in
   which requests are sent to a server in the user's local domain, which
   authenticates such requests (using the same practices by which the
   domain would authenticate REGISTER requests).  Once a message has
   been authenticated, the local domain then needs some way to
   communicate to other SIP entities that the sending user has been
   authenticated and its use of the From header field has been
   authorized.  This document addresses how that imprimatur of
   authentication can be shared.

   RFC 3261 [RFC3261] already describes an architecture very similar to
   this in Section 26.3.2.2, in which a user agent authenticates itself
   to a local proxy server, which in turn authenticates itself to a
   remote proxy server via mutual TLS, creating a two-link chain of
   transitive authentication between the originator and the remote
   domain.  While this works well in some architectures, there are a few
   respects in which this is impractical.  For one, transitive trust is
   inherently weaker than an assertion that can be validated end-to-end.
   It is possible for SIP requests to cross multiple intermediaries in
   separate administrative domains, in which case transitive trust
   becomes even less compelling.

   One solution to this problem is to use 'trusted' SIP intermediaries
   that assert an identity for users in the form of a privileged SIP



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   header.  A mechanism for doing so (with the P-Asserted-Identity
   header) is given in [12].  However, this solution allows only hop-
   by-hop trust between intermediaries, not end-to-end cryptographic
   authentication, and it assumes a managed network of nodes with strict
   mutual trust relationships, an assumption that is incompatible with
   widespread Internet deployment.

   Accordingly, this document specifies a means of sharing a
   cryptographic assurance of end-user SIP identity in an interdomain or
   intradomain context that is based on the concept of an
   'authentication service' and a new SIP header, the Identity header.
   Note that the scope of this document is limited to providing this
   identity assurance for SIP requests; solving this problem for SIP
   responses is more complicated and is a subject for future work.

   This specification allows either a user agent or a proxy server to
   provide identity services and to verify identities.  To maximize end-
   to-end security, it is obviously preferable for end-users to acquire
   their own certificates and corresponding private keys; if they do,
   they can act as an authentication service.  However, end- user
   certificates may be neither practical nor affordable, given the
   difficulties of establishing a Public Key Infrastructure (PKI) that
   extends to end-users, and moreover, given the potentially large
   number of SIP user agents (phones, PCs, laptops, PDAs, gaming
   devices) that may be employed by a single user.  In such
   environments, synchronizing keying material across multiple devices
   may be very complex and requires quite a good deal of additional
   endpoint behavior.  Managing several certificates for the various
   devices is also quite problematic and unpopular with users.
   Accordingly, in the initial use of this mechanism, it is likely that
   intermediaries will instantiate the authentication service role.

6.  Overview of Operations

   This section provides an informative (non-normative) high-level
   overview of the mechanisms described in this document.

   Imagine the case where Alice, who has the home proxy of example.com
   and the address-of-record sip:alice@example.com, wants to communicate
   with sip:bob@example.org.

   Alice generates an INVITE and places her identity in the From header
   field of the request.  She then sends an INVITE over TLS to an
   authentication service proxy for her domain.

   The authentication service authenticates Alice (possibly by sending a
   Digest authentication challenge) and validates that she is authorized
   to assert the identity that is populated in the From header field.



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   This value may be Alice's AoR, or it may be some other value that the
   policy of the proxy server permits her to use.  It then computes a
   hash over some particular headers, including the From header field
   and the bodies in the message.  This hash is signed with the
   certificate for the domain (example.com, in Alice's case) and
   inserted in a new header field in the SIP message, the 'Identity'
   header.

   The proxy, as the holder of the private key of its domain, is
   asserting that the originator of this request has been authenticated
   and that she is authorized to claim the identity (the SIP address-
   of-record) that appears in the From header field.  The proxy also
   inserts a companion header field, Identity-Info, that tells Bob how
   to acquire its certificate, if he doesn't already have it.

   When Bob's domain receives the request, it verifies the signature
   provided in the Identity header, and thus can validate that the
   domain indicated by the host portion of the AoR in the From header
   field authenticated the user, and permitted the user to assert that
   From header field value.  This same validation operation may be
   performed by Bob's user agent server (UAS).

7.  Authentication Service Behavior

   This document defines a new role for SIP entities called an
   authentication service.  The authentication service role can be
   instantiated by a proxy server or a user agent.  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 (see
   Section 11 for information on how the Date header field assists
   verifiers).  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.





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   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
   [RFC3261], 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; see
   Section 13 for more information.  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 an Identity header;
   see below for more information on authentication service handling of
   an existing Identity 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.  Some possible ways in which
   this authentication might be performed include:

      If the authentication service is instantiated by a SIP
      intermediary (proxy server), it may challenge the request with a
      407 response code using the Digest authentication scheme (or
      viewing a Proxy-Authentication header sent in the request, which
      was sent in anticipation of a challenge using cached credentials,
      as described in RFC 3261 [RFC3261], Section 22.3).  Note that if
      that proxy server is maintaining a TLS connection with the client
      over which the client had previously authenticated itself using
      Digest authentication, the identity value obtained from that
      previous authentication step can be reused without an additional
      Digest challenge.

      If the authentication service is instantiated by a SIP user agent,
      a user agent can be said to authenticate its user on the grounds
      that the user can provision the user agent with the private key of
      the domain, or preferably by providing a password that unlocks
      said private key.



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   Authorization of the use of a particular username in the From header
   field is a matter of local policy for the authentication service, one
   that depends greatly on the manner in which authentication is
   performed.  For example, one policy might be as follows: the username
   given in the 'username' parameter of the Proxy-Authorization header
   MUST correspond exactly to the username in the From header field of
   the SIP message.  However, there are many cases in which this is too
   limiting or inappropriate; a realm might use 'username' parameters in
   Proxy-Authorization that do not correspond to the user-portion of SIP
   From headers, or a user might manage multiple accounts in the same
   administrative domain.  In this latter case, a domain might maintain
   a mapping between the values in the 'username' parameter of Proxy-
   Authorization and a set of one or more SIP URIs that might
   legitimately be asserted for that 'username'.  For example, the
   username can correspond to the 'private identity' as defined in Third
   Generation Partnership Project (3GPP), in which case the From header
   field can contain any one of the public identities associated with
   this private identity.  In this instance, another policy might be as
   follows: the URI in the From header field MUST correspond exactly to
   one of the mapped URIs associated with the 'username' given in the
   Proxy-Authorization header.  Various exceptions to such policies
   might arise for cases like anonymity; if the AoR asserted in the From
   header field uses a form like 'sip:anonymous@example.com', then the
   'example.com' proxy should authenticate that the user is a valid user
   in the domain and insert the signature over the From header field as
   usual.

   Note that this check is performed on the addr-spec in the From header
   field (e.g., the URI of the sender, like
   'sip:alice@atlanta.example.com'); it does not convert the display-
   name portion of the From header field (e.g., 'Alice Atlanta').
   Authentication services MAY check and validate the display-name as
   well, and compare it to a list of acceptable display-names that may
   be used by the sender; if the display-name does not meet policy
   constraints, the authentication service MUST return a 403 response
   code.  The reason phrase should indicate the nature of the problem;
   for example, "Inappropriate Display Name".  However, the display-name
   is not always present, and in many environments the requisite
   operational procedures for display-name validation may not exist.
   For more information, see Section 15.2.

   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



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   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 Identity 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.  For
   more information on the security properties associated with the Date
   header field value, see Section 11.

   Step 4:

   The authentication service MUST form the identity signature and add
   an Identity header to the request containing this signature.  After
   the Identity header has been added to the request, the authentication
   service MUST also add an Identity-Info header.  The Identity-Info
   header contains a URI from which its certificate can be acquired.
   Details on the generation of both of these headers are provided in
   Section 11.

   Finally, the authentication service MUST forward the message
   normally.

7.1.  Identity within a Dialog and Retargeting

   Retargeting is broadly defined as the alteration of the Request-URI
   by intermediaries.  More specifically, retargeting supplants the
   original target URI with one that corresponds to a different user, a
   user that is not authorized to register under the original target
   URI.  By this definition, retargeting does not include translation of
   the Request-URI to a contact address of an endpoint that has
   registered under the original target URI, for example.

   When a dialog-forming request is retargeted, this can cause a few
   wrinkles for the Identity mechanism when it is applied to requests
   sent in the backwards direction within a dialog.  This section
   provides some non-normative considerations related to this case.

   When a request is retargeted, it may reach a SIP endpoint whose user
   is not identified by the URI designated in the To header field value.
   The value in the To header field of a dialog-forming request is used
   as the From header field of requests sent in the backwards direction
   during the dialog, and is accordingly the header that would be signed
   by an authentication service for requests sent in the backwards
   direction.  In retargeting cases, if the URI in the From header does
   not identify the sender of the request in the backwards direction,
   then clearly it would be inappropriate to provide an Identity



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   signature over that From header.  As specified above, if the
   authentication service is not responsible for the domain in the From
   header field of the request, it MUST NOT add an Identity header to
   the request, and it should process/forward the request normally.

   Any means of anticipating retargeting, and so on, is outside the
   scope of this document, and likely to have equal applicability to
   response identity as it does to requests in the backwards direction
   within a dialog.  Consequently, no special guidance is given for
   implementers here regarding the 'connected party' problem;
   authentication service behavior is unchanged if retargeting has
   occurred for a dialog-forming request.  Ultimately, the
   authentication service provides an Identity header for requests in
   the backwards dialog when the user is authorized to assert the
   identity given in the From header field, and if they are not, an
   Identity header is not provided.

   For further information on the problems of response identity and the
   potential solution spaces, see [15].

8.  Verifier Behavior

   This document introduces a new logical role for SIP entities called a
   server.  When a verifier receives a SIP message containing an
   Identity header, it may inspect the signature to verify the identity
   of the sender of the message.  Typically, the results of a
   verification are provided as input to an authorization process that
   is outside the scope of this document.  If an Identity 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 Identity 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 MUST 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 Identity-
   Info header field value.





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   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 Identity-Info header, unless they
   have some more efficient implementation-specific way of acquiring
   certificates for that domain.  If the URI scheme in the Identity-Info
   header cannot be dereferenced, then a 436 'Bad Identity-Info'
   response MUST be returned.  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 15.4 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 Identity header field,
   following the procedures for generating the hashed digest-string
   described in Section 11.  If a verifier determines that the signature
   on the message does not correspond to the reconstructed digest-
   string, then a 438 'Invalid Identity Header' response MUST be
   returned.

   Step 4:

   The verifier MUST validate the Date, Contact, and Call-ID headers in
   the manner described in Section 15.1; recipients that wish to verify
   Identity signatures MUST support all of the operations described
   there.  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.

9.  Considerations for User Agent

   This mechanism can be applied opportunistically to existing SIP
   deployments; accordingly, it requires no change to SIP user agent
   behavior in order for it to be effective.  However, because this
   mechanism does not provide integrity protection between the UAC and
   the authentication service, a UAC SHOULD implement some means of
   providing this integrity.  TLS would be one such mechanism, which is



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   attractive because it MUST be supported by SIP proxy servers, but is
   potentially problematic because it is a hop-by-hop mechanism.  See
   Section 15.3 for more information about securing the channel between
   the UAC and the authentication service.

   When a UAC sends a request, it MUST accurately populate the From
   header field with a value corresponding to an identity that it
   believes it is authorized to claim.  In a request, it MUST set the
   URI portion of its From header to match a SIP, SIPS, or TEL URI AoR
   that it is authorized to use in the domain (including anonymous URIs,
   as described in RFC 3323 [RFC3323]).  In general, UACs SHOULD NOT use
   the TEL URI form in the From header field (see Section 13).

   Note that this document defines a number of new 4xx response codes.
   If user agents support these response codes, they will be able to
   respond intelligently to Identity-based error conditions.

   The UAC MUST also be capable of sending requests, including mid-call
   requests, through an 'outbound' proxy (the authentication service).
   The best way to accomplish this is using pre-loaded Route headers and
   loose routing.  For a given domain, if an entity that can instantiate
   the authentication service role is not in the path of dialog-forming
   requests, identity for mid-dialog requests in the backwards direction
   cannot be provided.

   As a recipient of a request, a user agent that can verify signed
   identities should also support an appropriate user interface to
   render the validity of identity to a user.  User agent
   implementations SHOULD differentiate signed From header field values
   from unsigned From header field values when rendering to an end-user
   the identity of the sender of a request.

10.  Considerations for Proxy Servers

   Domain policy may require proxy servers to inspect and verify the
   identity provided in SIP requests.  A proxy server may wish to
   ascertain the identity of the sender of the message to provide spam
   prevention or call control services.  Even if a proxy server does not
   act as an authentication service, it MAY validate the Identity header
   before it makes a forwarding decision for a request.  Proxy servers
   MUST NOT remove or modify an existing Identity or Identity-Info
   header in a request.









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11.  Header Syntax

   This document specifies two new SIP headers: Identity and Identity-
   Info.  Each of these headers can appear only once in a SIP message.
   The grammar for these two headers is (following the ABNF [6] in RFC
   3261 [1]):

      Identity = "Identity" HCOLON signed-identity-digest
      signed-identity-digest = LDQUOT 32LHEX RDQUOT

      Identity-Info = "Identity-Info" HCOLON ident-info
                       *( SEMI ident-info-params )
      ident-info = LAQUOT absoluteURI RAQUOT
      ident-info-params = ident-info-alg / ident-info-extension
      ident-info-alg = "alg" EQUAL token
      ident-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:

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

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

      The callid from Call-Id header field.

      The digit (1*DIGIT) and method (method) portions from CSeq header
      field, separated by a single space (ABNF SP, or %x20).  Note that
      the 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.

      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
      [RFC3261].  RFC 3261 specifies that the BNF for weekday and month
      is a choice amongst a set of tokens.  The RFC 2234 [RFC2234] rules
      for the BNF specify that tokens are case sensitive.  However, when
      used to construct the canonical string defined here, the first



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      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.

      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).

      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.

   For more information on the security properties of these headers, and
   why their inclusion mitigates replay attacks, see Section 15 and
   [RFC3893].  The precise formulation of this digest-string is,
   therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):

   digest-string = addr-spec "|" addr-spec "|" callid "|"
                   1*DIGIT SP Method "|" SIP-date "|" [ addr-spec ] "|"
                   message-body


   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 of the Identity-Info header (see
   below for more information on Identity-Info header parameters).  This
   document defines only one value for the 'alg' parameter: 'rsa-sha1';
   further values MUST be defined in a Standards Track RFC, see
   Section 14.7 for more information.  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



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   [RFC3548].  A 1024-bit or longer RSA key MUST be used.  The result is
   placed in the Identity header field.  For detailed examples of the
   usage of this algorithm, see Section 12.

   The 'absoluteURI' portion of the Identity-Info header MUST contain a
   URI which dereferences to a resource containing the certificate of
   the authentication service.  All implementations of this
   specification MUST support the use of HTTP and HTTPS URIs in the
   Identity-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/pkix-cert' described in
   that specification.  Note that this introduces key lifecycle
   management concerns; were a domain to change the key available at the
   Identity-Info URI before a verifier evaluates a request signed by an
   authentication service, this would cause obvious verifier failures.
   When a rollover occurs, authentication services SHOULD thus provide
   new Identity-Info URIs for each new certificate, and SHOULD continue
   to make older key acquisition URIs available for a duration longer
   than the plausible lifetime of a SIP message (an hour would most
   likely suffice).

   The Identity-Info header field MUST contain an 'alg' parameter.  No
   other parameters are defined for the Identity-Info header in this
   document.  Future Standards Track RFCs may define additional
   Identity-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
    ------------         -----   -----   ---  ---  ---  ---  ---  ---
    Identity               R       a      o    o    -    o    o    o

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

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

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







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   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 Identity and Identity-Info header MUST NOT appear in
   CANCEL.

12.  Compliance Tests and Examples

   The examples in this section illustrate the use of the Identity
   header in the context of a SIP transaction.  Implementers are advised
   to verify their compliance with the specification against the
   following criteria:

      Implementations of the authentication service role MUST generate
      identical base64 identity strings to the ones shown in the
      Identity headers in these examples when presented with the source
      message and utilizing the appropriate supplied private key for the
      domain in question.

      Implementations of the verifier role MUST correctly validate the
      given messages containing the Identity header when utilizing the
      supplied certificates (with the caveat about self-signed
      certificates below).

   Note that the following examples use self-signed certificates, rather
   than certificates issued by a recognized certificate authority.  The
   use of self-signed certificates for this mechanism is NOT
   RECOMMENDED, and it appears here only for illustrative purposes.
   Therefore, in compliance testing, implementations of verifiers SHOULD
   generate appropriate warnings about the use of self-signed
   certificates.  Also, the example certificates in this section have
   placed their domain name subject in the subjectAltName field; in
   practice, certificate authorities may place domain names in other
   locations in the certificate (see Section 15.4 for more information).

   Note that all examples in this section use the 'rsa-sha1' algorithm.

   Bit-exact reference files for these messages and their various
   transformations are supplied in Appendix B.

12.1.  Identity-Info with a Singlepart MIME body





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   Consider the following private key and certificate pair assigned to
   'atlanta.example.com' (rendered in OpenSSL format).

        -----BEGIN RSA PRIVATE KEY-----
        MIICXQIBAAKBgQDPPMBtHVoPkXV+Z6jq1LsgfTELVWpy2BVUffJMPH06LL0cJSQO
        aIeVzIojzWtpauB7IylZKlAjB5f429tRuoUiedCwMLKblWAqZt6eHWpCNZJ7lONc
        IEwnmh2nAccKk83Lp/VH3tgAS/43DQoX2sndnYh+g8522Pzwg7EGWspzzwIDAQAB
        AoGBAK0W3tnEFD7AjVQAnJNXDtx59Aa1Vu2JEXe6oi+OrkFysJjbZJwsLmKtrgtt
        PXOU8t2mZpi0wK4hX4tZhntiwGKkUPC3h9Bjp+GerifP341RMyMO+6fPgjqOzUDw
        +rPjjMpwD7AkcEcqDgbTrZnWv/QnCSaaF3xkUGfFkLx5OKcRAkEA7UxnsE8XaT30
        tP/UUc51gNk2KGKgxQQTHopBcew9yfeCRFhvdL7jpaGatEi5iZwGGQQDVOVHUN1H
        0YLpHQjRowJBAN+R2bvA/Nimq464ZgnelEDPqaEAZWaD3kOfhS9+vL7oqES+u5E0
        J7kXb7ZkiSVUg9XU/8PxMKx/DAz0dUmOL+UCQH8C9ETUMI2uEbqHbBdVUGNk364C
        DFcndSxVh+34KqJdjiYSx6VPPv26X9m7S0OydTkSgs3/4ooPxo8HaMqXm80CQB+r
        xbB3UlpOohcBwFK9mTrlMB6Cs9ql66KgwnlL9ukEhHHYozGatdXeoBCyhUsogdSU
        6/aSAFcvWEGtj7/vyJECQQCCS1lKgEXoNQPqONalvYhyyMZRXFLdD4gbwRPK1uXK
        Ypk3CkfFzOyfjeLcGPxXzq2qzuHzGTDxZ9PAepwX4RSk
        -----END RSA PRIVATE KEY-----


        -----BEGIN CERTIFICATE-----
        MIIC3TCCAkagAwIBAgIBADANBgkqhkiG9w0BAQUFADBZMQswCQYDVQQGEwJVUzEL
        MAkGA1UECAwCR0ExEDAOBgNVBAcMB0F0bGFudGExDTALBgNVBAoMBElFVEYxHDAa
        BgNVBAMME2F0bGFudGEuZXhhbXBsZS5jb20wHhcNMDUxMDI0MDYzNjA2WhcNMDYx
        MDI0MDYzNjA2WjBZMQswCQYDVQQGEwJVUzELMAkGA1UECAwCR0ExEDAOBgNVBAcM
        B0F0bGFudGExDTALBgNVBAoMBElFVEYxHDAaBgNVBAMME2F0bGFudGEuZXhhbXBs
        ZS5jb20wgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBAM88wG0dWg+RdX5nqOrU
        uyB9MQtVanLYFVR98kw8fTosvRwlJA5oh5XMiiPNa2lq4HsjKVkqUCMHl/jb21G6
        hSJ50LAwspuVYCpm3p4dakI1knuU41wgTCeaHacBxwqTzcun9Ufe2ABL/jcNChfa
        yd2diH6DznbY/PCDsQZaynPPAgMBAAGjgbQwgbEwHQYDVR0OBBYEFNmU/MrbVYcE
        KDr/20WISrG1j1rNMIGBBgNVHSMEejB4gBTZlPzK21WHBCg6/9tFiEqxtY9azaFd
        pFswWTELMAkGA1UEBhMCVVMxCzAJBgNVBAgMAkdBMRAwDgYDVQQHDAdBdGxhbnRh
        MQ0wCwYDVQQKDARJRVRGMRwwGgYDVQQDDBNhdGxhbnRhLmV4YW1wbGUuY29tggEA
        MAwGA1UdEwQFMAMBAf8wDQYJKoZIhvcNAQEFBQADgYEADdQYtswBDmTSTq0mt211
        7alm/XGFrb2zdbU0vorxRdOZ04qMyrIpXG1LEmnEOgcocyrXRBvq5p6WbZAcEQk0
        DsE3Ve0Nc8x9nmvljW7GsMGFCnCuo4ODTf/1lGdVr9DeCzcj10YUQ3MRemDMXhY2
        CtDisLWl7SXOORcZAi1oU9w=
         -----END CERTIFICATE-----


   A user of atlanta.example.com, Alice, wants to send an INVITE to
   bob@biloxi.example.org.  She therefore creates the following INVITE
   request, which she forwards to the atlanta.example.org proxy server
   that instantiates the authentication service role:

      INVITE sip:bob@biloxi.example.org SIP/2.0
      Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
      To: Bob <sip:bob@biloxi.example.org>



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      From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 314159 INVITE
      Max-Forwards: 70
      Date: Thu, 21 Feb 2002 13:02:03 GMT
      Contact: <sip:alice@pc33.atlanta.example.com>
      Content-Type: application/sdp
      Content-Length: 147

      v=0
      o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
      s=Session SDP
      c=IN IP4 pc33.atlanta.example.com
      t=0 0
      m=audio 49172 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


   When the authentication service receives the INVITE, it authenticates
   Alice by sending a 407 response.  As a result, Alice adds an
   Authorization header to her request, and resends to the
   atlanta.example.com authentication service.  Now that the service is
   sure of Alice's identity, it calculates an Identity header for the
   request.  The canonical string over which the identity signature will
   be generated is the following (note that the first line wraps because
   of RFC editorial conventions):

      sip:alice@atlanta.example.com|sip:bob@biloxi.example.org|
      a84b4c76e66710|314159 INVITE|Thu, 21 Feb 2002 13:02:03 GMT|
      sip:alice@pc33.atlanta.example.com|v=0
      o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
      s=Session SDP
      c=IN IP4 pc33.atlanta.example.com
      t=0 0

      m=audio 49172 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


   The resulting signature (sha1WithRsaEncryption) using the private RSA
   key given above, with base64 encoding, is the following:

        ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
        ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
        FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=






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   Accordingly, the atlanta.example.com authentication service will
   create an Identity header containing that base64 signature string
   (175 bytes).  It will also add an HTTPS URL where its certificate is
   made available.  With those two headers added, the message looks like
   the following:

   INVITE sip:bob@biloxi.example.org SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
   To: Bob <sip:bob@biloxi.example.org>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Max-Forwards: 70
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Contact: <sip:alice@pc33.atlanta.example.com>
   Identity:
     "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
      ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
      FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
   Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
   Content-Type: application/sdp
   Content-Length: 147
    v=0
   o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
   s=Session SDP
   c=IN IP4 pc33.atlanta.example.com
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   atlanta.example.com then forwards the request normally.  When Bob
   receives the request, if he does not already know the certificate of
   atlanta.example.com, he dereferences the URL in the Identity-Info
   header to acquire the certificate.  Bob then generates the same
   canonical string given above, from the same headers of the SIP
   request.  Using this canonical string, the signed digest in the
   Identity header, and the certificate discovered by dereferencing the

   Identity-Info header, Bob can verify that the given set of headers
   and the message body have not been modified.

12.2.  Identity for a Request with No MIME Body or Contact

   Consider the following private key and certificate pair assigned to
   "biloxi.example.org".





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        -----BEGIN RSA PRIVATE KEY-----
        MIICXgIBAAKBgQC/obBYLRMPjskrAqWOiGPAUxI3/m2ti7ix4caqCTAuFX5cLegQ
        7nmquLOHfIhxVIqT2f06UA0lOo2NVofK9G7MTkVbVNiyAlLYUDEj7XWLDICf3ZHL
        6Fr/+CF7wrQ9r4kv7XiJKxodVCCd/DhCT9Gp+VDoe8HymqOW/KsneriyIwIDAQAB
        AoGBAJ7fsFIKXKkjWgj8ksGOthS3Sn19xPSCyEdBxfEm2Pj7/Nzzeli/PcOaic0k
        JALBcnqN2fHEeIGK/9xUBxTufgQYVJqvyHERs6rXX/iT4Ynm9t1905EiQ9ZpHsrI
        /AMMUYA1QrGgAIHvZLVLzq+9KLDEZ+HQbuCLJXF+6bl0Eb5BAkEA636oMANp0Qa3
        mYWEQ2utmGsYxkXSfyBb18TCOwCty0ndBR24zyOJF2NbZS98Lz+Ga25hfIGw/JHK
        nD9bOE88UwJBANBRSpd4bmS+m48R/13tRESAtHqydNinX0kS/RhwHr7mkHTU3k/M
        FxQtx34I3GKzaZxMn0A66KS9v/SHdnF+ePECQQCGe7QshyZ8uitLPtZDclCWhEKH
        qAQHmUEZvUF2VHLrbukLLOgHUrHNa24cILv4d3yaCVUetymNcuyTwhKj24wFAkAO
        z/jx1EplN3hwL+NsllZoWI58uvu7/Aq2c3czqaVGBbb317sHCYgKk0bAG3kwO3mi
        93/LXWT1cdiYVpmBcHDBAkEAmpgkFj+xZu5gWASY5ujv+FCMP0WwaH5hTnXu+tKe
        PJ3d2IJZKxGnl6itKRN7GeRh9PSK0kZSqGFeVrvsJ4Nopg==
        -----END RSA PRIVATE KEY-----


        -----BEGIN CERTIFICATE-----
        MIIC1jCCAj+gAwIBAgIBADANBgkqhkiG9w0BAQUFADBXMQswCQYDVQQGEwJVUzEL
        MAkGA1UECAwCTVMxDzANBgNVBAcMBkJpbG94aTENMAsGA1UECgwESUVURjEbMBkG
        A1UEAwwSYmlsb3hpLmV4YW1wbGUuY29tMB4XDTA1MTAyNDA2NDAyNloXDTA2MTAy
        NDA2NDAyNlowVzELMAkGA1UEBhMCVVMxCzAJBgNVBAgMAk1TMQ8wDQYDVQQHDAZC
        aWxveGkxDTALBgNVBAoMBElFVEYxGzAZBgNVBAMMEmJpbG94aS5leGFtcGxlLmNv
        bTCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAv6GwWC0TD47JKwKljohjwFMS
        N/5trYu4seHGqgkwLhV+XC3oEO55qrizh3yIcVSKk9n9OlANJTqNjVaHyvRuzE5F
        W1TYsgJS2FAxI+11iwyAn92Ry+ha//ghe8K0Pa+JL+14iSsaHVQgnfw4Qk/RqflQ
        6HvB8pqjlvyrJ3q4siMCAwEAAaOBsTCBrjAdBgNVHQ4EFgQU0Z+RL47W/APDtc5B
        fSoQXuEFE/wwfwYDVR0jBHgwdoAU0Z+RL47W/APDtc5BfSoQXuEFE/yhW6RZMFcx
        CzAJBgNVBAYTAlVTMQswCQYDVQQIDAJNUzEPMA0GA1UEBwwGQmlsb3hpMQ0wCwYD
        VQQKDARJRVRGMRswGQYDVQQDDBJiaWxveGkuZXhhbXBsZS5jb22CAQAwDAYDVR0T
        BAUwAwEB/zANBgkqhkiG9w0BAQUFAAOBgQBiyKHIt8TXfGNfpnJXi5jCizOxmY8Y
        gln8tyPFaeyq95TGcvTCWzdoBLVpBD+fpRWrX/II5sE6VHbbAPjjVmKbZwzQAtpp
        P2Fauj28t94ZeDHN2vqzjfnHjCO24kG3Juf2T80ilp9YHcDwxjUFrt86UnlC+yid
        yaTeusW5Gu7v1g==
        -----END CERTIFICATE-----


   Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
   at the end of the dialog initiated in the previous example.  He
   therefore creates the following BYE request, which he forwards to the
   'biloxi.example.org' proxy server that instantiates the
   authentication service role:

   BYE sip:alice@pc33.atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
   To: Alice <sip:alice@atlanta.example.com>;tag=1928301774



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   Call-ID: a84b4c76e66710
   CSeq: 231 BYE
   Content-Length: 0


   When the authentication service receives the BYE, it authenticates
   Bob by sending a 407 response.  As a result, Bob adds an
   Authorization header to his request, and resends to the
   biloxi.example.org authentication service.  Now that the service is
   sure of Bob's identity, it prepares to calculate an Identity header
   for the request.  Note that this request does not have a Date header
   field.  Accordingly, the biloxi.example.org will add a Date header to
   the request before calculating the identity signature.  If the
   Content-Length header were not present, the authentication service
   would add it as well.  The baseline message is thus:

   BYE sip:alice@pc33.atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
   To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Date: Thu, 21 Feb 2002 14:19:51 GMT
   Call-ID: a84b4c76e66710
   CSeq: 231 BYE
   Content-Length: 0


   Also note that this request contains no Contact header field.
   Accordingly, biloxi.example.org will place no value in the canonical
   string for the addr-spec of the Contact address.  Also note that
   there is no message body, and accordingly, the signature string will
   terminate, in this case, with two vertical bars.  The canonical
   string over which the identity signature will be generated is the
   following (note that the first line wraps because of RFC editorial
   conventions):

   sip:bob@biloxi.example.org|sip:alice@atlanta.example.com|
   a84b4c76e66710|231 BYE|Thu, 21 Feb 2002 14:19:51 GMT||

   The resulting signature (sha1WithRsaEncryption) using the private RSA
   key given above for biloxi.example.org, with base64 encoding, is the
   following:

   sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
   eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
   pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=





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   Accordingly, the biloxi.example.org authentication service will
   create an Identity header containing that base64 signature string.
   It will also add an HTTPS URL where its certificate is made
   available.  With those two headers added, the message looks like the
   following:

   BYE sip:alice@pc33.atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
   To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Date: Thu, 21 Feb 2002 14:19:51 GMT
   Call-ID: a84b4c76e66710
   CSeq: 231 BYE
   Identity:
     "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
      eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
      pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="
   Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
   Content-Length: 0


   biloxi.example.org then forwards the request normally.

13.  Identity and the TEL URI Scheme

   Since many SIP applications provide a Voice over IP (VoIP) service,
   telephone numbers are commonly used as identities in SIP deployments.
   In the majority of cases, this is not problematic for the identity
   mechanism described in this document.  Telephone numbers commonly
   appear in the username portion of a SIP URI (e.g.,
   'sip:+17005551008@chicago.example.com;user=phone').  That username
   conforms to the syntax of the TEL URI scheme (RFC 3966 [RFC3966]).
   For this sort of SIP address-of-record, chicago.example.com is the
   appropriate signatory.
















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   It is also possible for a TEL URI to appear in the SIP To or From
   header field outside the context of a SIP or SIPS URI (e.g.,
   'tel:+17005551008').  In this case, it is much less clear which
   signatory is appropriate for the identity.  Fortunately for the
   identity mechanism, this form of the TEL URI is more common for the
   To header field and Request-URI in SIP than in the From header field,
   since the UAC has no option but to provide a TEL URI alone when the
   remote domain to which a request is sent is unknown.  The local
   domain, however, is usually known by the UAC, and accordingly it can
   form a proper From header field containing a SIP URI with a username
   in TEL URI form.  Implementations that intend to send their requests
   through an authentication service SHOULD put telephone numbers in the
   From header field into SIP or SIPS URIs whenever possible.

   If the local domain is unknown to a UAC formulating a request, it
   most likely will not be able to locate an authentication service for
   its request, and therefore the question of providing identity in
   these cases is somewhat moot.  However, an authentication service MAY
   sign a request containing a TEL URI in the From header field.  This
   is permitted in this specification strictly for forward compatibility
   purposes.  In the longer-term, it is possible that ENUM [14] may
   provide a way to determine which administrative domain is responsible
   for a telephone number, and this may aid in the signing and
   verification of SIP identities that contain telephone numbers.  This
   is a subject for future work.

14.  Privacy Considerations

   The identity mechanism presented in this document is compatible with
   the standard SIP practices for privacy described in RFC 3323
   [RFC3323].  A SIP proxy server can act both as a privacy service and
   as an authentication service.  Since a user agent can provide any
   From header field value that the authentication service is willing to
   authorize, there is no reason why private SIP URIs that contain
   legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
   by an authentication service.  The construction of the Identity
   header is the same for private URIs as it is for any other sort of
   URIs.

   Note, however, that an authentication service must possess a
   certificate corresponding to the host portion of the addr-spec of the
   From header field of any request that it signs; accordingly, using
   domains like 'anonymous.invalid' will not be possible for privacy
   services that also act as authentication services.  The assurance
   offered by the usage of anonymous URIs with a valid domain portion is
   "this is a known user in my domain that I have authenticated, but I
   am keeping its identity private".  The use of the domain
   'anonymous.invalid' entails that no corresponding authority for the



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   domain can exist, and as a consequence, authentication service
   functions are meaningless.

   The "header" level of privacy described in RFC 3323 [RFC3323]
   requests that a privacy service alter the Contact header field value
   of a SIP message.  Since the Contact header field is protected by the
   signature in an Identity header, privacy services cannot be applied
   after authentication services without a resulting integrity
   violation.

   RFC 3325 [RFC3325] defines the "id" priv-value token, which is
   specific to the P-Asserted-Identity header.  The sort of assertion
   provided by the P-Asserted-Identity header is very different from the
   Identity header presented in this document.  It contains additional
   information about the sender of a message that may go beyond what
   appears in the From header field; P-Asserted-Identity holds a
   definitive identity for the sender that is somehow known to a closed
   network of intermediaries that presumably the network will use this
   identity for billing or security purposes.  The danger of this
   network-specific information leaking outside of the closed network
   motivated the "id" priv-value token.  The "id" priv-value token has
   no implications for the Identity header, and privacy services MUST
   NOT remove the Identity header when a priv-value of "id" appears in a
   Privacy header.

   Finally, note that unlike RFC 3325 [RFC3325], the mechanism described
   in this specification adds no information to SIP requests that has
   privacy implications.

15.  Security Considerations

15.1.  Handling of digest-string Elements

   This document describes a mechanism that provides a signature over
   the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
   requests.  While a signature over the From header field would be
   sufficient to secure a URI alone, the additional headers provide
   replay protection and reference integrity necessary to make sure that
   the Identity header will not be used in cut-and-paste attacks.  In
   general, the considerations related to the security of these headers
   are the same as those given in RFC 3261 [RFC3261] for including
   headers in tunneled 'message/sip' MIME bodies (see Section 23 in
   particular).  The following section details the individual security
   properties obtained by including each of these header fields within
   the signature; collectively, this set of header fields provides the
   necessary properties to prevent impersonation.





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   The From header field indicates the identity of the sender of the
   message, and the SIP address-of-record URI in the From header field
   is the identity of a SIP user, for the purposes of this document.
   The To header field provides the identity of the SIP user that this
   request targets.  Providing the To header field in the Identity
   signature serves two purposes: first, it prevents cut-and-paste
   attacks in which an Identity header from legitimate request for one
   user is cut-and-pasted into a request for a different user; second,
   it preserves the starting URI scheme of the request, which helps
   prevent downgrade attacks against the use of SIPS.

   The Date and Contact headers provide reference integrity and replay
   protection, as described in RFC 3261 [RFC3261], Section 23.4.2.
   Implementations of this specification MUST NOT deem valid a request
   with an outdated Date header field (the RECOMMENDED interval is that
   the Date header must indicate a time within 3600 seconds of the
   receipt of a message).  Implementations MUST also record Call-IDs
   received in valid requests containing an Identity header, and MUST
   remember those Call-IDs for at least the duration of a single Date
   interval (i.e., commonly 3600 seconds).  Because a SIP-compliant UA
   never generates the same Call-ID twice, verifiers can use the Call-ID
   to recognize cut-and-paste attacks; the Call-ID serves as a nonce.
   The result of this is that if an Identity header is replayed within
   the Date interval, verifiers will recognize that it is invalid
   because of a Call-ID duplication; if an Identity header is replayed
   after the Date interval, verifiers will recognize that it is invalid
   because the Date is stale.  The CSeq header field contains a numbered
   identifier for the transaction, and the name of the method of the
   request; without this information, an INVITE request could be cut-
   and-pasted by an attacker and transformed into a BYE request without
   changing any fields covered by the Identity header, and moreover
   requests within a certain transaction could be replayed in
   potentially confusing or malicious ways.

   The Contact header field is included to tie the Identity header to a
   particular user agent instance that generated the request.  Were an
   active attacker to intercept a request containing an Identity header,
   and cut-and-paste the Identity header field into its own request
   (reusing the From, To, Contact, Date, and Call-ID fields that appear
   in the original message), the attacker would not be eligible to
   receive SIP requests from the called user agent, since those requests
   are routed to the URI identified in the Contact header field.
   However, the Contact header is only included in dialog-forming
   requests, so it does not provide this protection in all cases.

   It might seem attractive to provide a signature over some of the
   information present in the Via header field value(s).  For example,
   without a signature over the sent-by field of the topmost Via header,



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   an attacker could remove that Via header and insert its own in a cut-
   and-paste attack, which would cause all responses to the request to
   be routed to a host of the attacker's choosing.  However, a signature
   over the topmost Via header does not prevent attacks of this nature,
   since the attacker could leave the topmost Via intact and merely
   insert a new Via header field directly after it, which would cause
   responses to be routed to the attacker's host "on their way" to the
   valid host, which has exactly the same end result.  Although it is
   possible that an intermediary-based authentication service could
   guarantee that no Via hops are inserted between the sending user
   agent and the authentication service, it could not prevent an
   attacker from adding a Via hop after the authentication service, and
   thereby preempting responses.  It is necessary for the proper
   operation of SIP for subsequent intermediaries to be capable of
   inserting such Via header fields, and thus it cannot be prevented.
   As such, though it is desirable, securing Via is not possible through
   the sort of identity mechanism described in this document; the best
   known practice for securing Via is the use of SIPS.

   This mechanism also provides a signature over the bodies of SIP
   requests.  The most important reason for doing so is to protect
   Session Description Protocol (SDP) bodies carried in SIP requests.
   There is little purpose in establishing the identity of the user that
   originated a SIP request if this assurance is not coupled with a
   comparable assurance over the media descriptors.  Note, however, that
   this is not perfect end-to-end security.  The authentication service
   itself, when instantiated at a intermediary, could conceivably change
   the SDP (and SIP headers, for that matter) before providing a
   signature.  Thus, while this mechanism reduces the chance that a
   replayer or man-in-the-middle will modify SDP, it does not eliminate
   it entirely.  Since it is a foundational assumption of this mechanism
   that the users trust their local domain to vouch for their security,
   they must also trust the service not to violate the integrity of
   their message without good reason.  Note that RFC 3261 [RFC3261],
   Section 16.6 states that SIP proxy servers "MUST NOT add to, modify,
   or remove the message body."

   In the end analysis, the Identity and Identity-Info headers cannot
   protect themselves.  Any attacker could remove these headers from a
   SIP request, and modify the request arbitrarily afterwards.  However,
   this mechanism is not intended to protect requests from men-in-the-
   middle who interfere with SIP messages; it is intended only to
   provide a way that SIP users can prove definitively that they are who
   they claim to be.  At best, by stripping identity information from a
   request, a man-in-the-middle could make it impossible to distinguish
   any illegitimate messages he would like to send from those messages
   sent by an authorized user.  However, it requires a considerably
   greater amount of energy to mount such an attack than it does to



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   mount trivial impersonations by just copying someone else's From
   header field.  This mechanism provides a way that an authorized user
   can provide a definitive assurance of his identity that an
   unauthorized user, an impersonator, cannot.

   One additional respect in which the Identity-Info header cannot
   protect itself is the 'alg' parameter.  The 'alg' parameter is not
   included in the digest-string, and accordingly, a man-in-the-middle
   might attempt to modify the 'alg' parameter.  However, it is
   important to note that preventing men-in-the-middle is not the
   primary impetus for this mechanism.  Moreover, changing the 'alg'

   would at worst result in some sort of bid-down attack, and at best
   cause a failure in the verifier.  Note that only one valid 'alg'
   parameter is defined in this document and that thus there is
   currently no weaker algorithm to which the mechanism can be bid down.
   'alg' has been incorporated into this mechanism for forward-
   compatibility reasons in case the current algorithm exhibits
   weaknesses, and requires swift replacement, in the future.

15.2.  Display-Names and Identity

   As a matter of interface design, SIP user agents might render the
   display-name portion of the From header field of a caller as the
   identity of the caller; there is a significant precedent in email
   user interfaces for this practice.  As such, it might seem that the
   lack of a signature over the display-name is a significant omission.

   However, there are several important senses in which a signature over
   the display-name does not prevent impersonation.  In the first place,
   a particular display-name, like "Jon Peterson", is not unique in the
   world; many users in different administrative domains might
   legitimately claim that name.  Furthermore, enrollment practices for
   SIP-based services might have a difficult time discerning the
   legitimate display-name for a user; it is safe to assume that
   impersonators will be capable of creating SIP accounts with arbitrary
   display-names.  The same situation prevails in email today.  Note
   that an impersonator who attempted to replay a message with an
   Identity header, changing only the display-name in the From header
   field, would be detected by the other replay protection mechanisms
   described in Section 15.1.

   Of course, an authentication service can enforce policies about the
   display-name even if the display-name is not signed.  The exact
   mechanics for creating and operationalizing such policies is outside
   the scope of this document.  The effect of this policy would not be
   to prevent impersonation of a particular unique identifier like a SIP
   URI (since display-names are not unique identifiers), but to allow a



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   domain to manage the claims made by its users.  If such policies are
   enforced, users would not be free to claim any display-name of their
   choosing.  In the absence of a signature, man-in-the-middle attackers
   could conceivably alter the display-names in a request with impunity.
   Note that the scope of this specification is impersonation attacks,
   however, and that a man-in-the-middle might also strip the Identity
   and Identity-Info headers from a message.

   There are many environments in which policies regarding the display-
   name aren't feasible.  Distributing bit-exact and internationalizable
   display-names to end-users as part of the enrollment or registration
   process would require mechanisms that are not explored in this

   document.  In the absence of policy enforcement regarding domain
   names, there are conceivably attacks that an adversary could mount
   against SIP systems that rely too heavily on the display-name in
   their user interface, but this argues for intelligent interface
   design, not changes to the mechanisms.  Relying on a non-unique
   identifier for identity would ultimately result in a weak mechanism.

15.3.  Securing the Connection to the Authentication Service

   The assurance provided by this mechanism is strongest when a user
   agent forms a direct connection, preferably one secured by TLS, to an
   intermediary-based authentication service.  The reasons for this are
   twofold:

      If a user does not receive a certificate from the authentication
      service over this TLS connection that corresponds to the expected
      domain (especially when the user receives a challenge via a
      mechanism such as Digest), then it is possible that a rogue server
      is attempting to pose as an authentication service for a domain
      that it does not control, possibly in an attempt to collect shared
      secrets for that domain.

      Without TLS, the various header field values and the body of the
      request will not have integrity protection when the request
      arrives at an authentication service.  Accordingly, a prior
      legitimate or illegitimate intermediary could modify the message
      arbitrarily.











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   Of these two concerns, the first is most material to the intended
   scope of this mechanism.  This mechanism is intended to prevent
   impersonation attacks, not man-in-the-middle attacks; integrity over
   the header and bodies is provided by this mechanism only to prevent
   replay attacks.  However, it is possible that applications relying on
   the presence of the Identity header could leverage this integrity
   protection, especially body integrity, for services other than replay
   protection.

   Accordingly, direct TLS connections SHOULD be used between the UAC
   and the authentication service whenever possible.  The opportunistic
   nature of this mechanism, however, makes it very difficult to
   constrain UAC behavior, and moreover there will be some deployment
   architectures where a direct connection is simply infeasible and the
   UAC cannot act as an authentication service itself.  Accordingly,
   when a direct connection and TLS are not possible, a UAC should use
   the SIPS mechanism, Digest 'auth-int' for body integrity, or both
   when it can.  The ultimate decision to add an Identity header to a
   request lies with the authentication service, of course; domain
   policy must identify those cases where the UAC's security association
   with the authentication service is too weak.

15.4.  Domain Names and Subordination

   When a verifier processes a request containing an Identity-Info
   header, it must compare the domain portion of the URI in the From
   header field of the request with the domain name that is the subject
   of the certificate acquired from the Identity-Info header.  While it
   might seem that this should be a straightforward process, it is
   complicated by two deployment realities.  In the first place,
   certificates have varying ways of describing their subjects, and may
   indeed have multiple subjects, especially in 'virtual hosting' cases
   where multiple domains are managed by a single application.
   Secondly, some SIP services may delegate SIP functions to a
   subordinate domain and utilize the procedures in RFC 3263 [RFC3263]
   that allow requests for, say, 'example.com' to be routed to
   'sip.example.com'.  As a result, a user with the AoR
   'sip:jon@example.com' may process its requests through a host like
   'sip.example.com', and it may be that latter host that acts as an
   authentication service.

   To meet the second of these problems, a domain that deploys an
   authentication service on a subordinate host MUST be willing to
   supply that host with the private keying material associated with a
   certificate whose subject is a domain name that corresponds to the
   domain portion of the AoRs that the domain distributes to users.
   Note that this corresponds to the comparable case of routing inbound
   SIP requests to a domain.  When the NAPTR and SRV procedures of RFC



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   3263 are used to direct requests to a domain name other than the
   domain in the original Request-URI (e.g., for 'sip:jon@example.com',
   the corresponding SRV records point to the service
   'sip1.example.org'), the client expects that the certificate passed
   back in any TLS exchange with that host will correspond exactly with
   the domain of the original Request-URI, not the domain name of the
   host.  Consequently, in order to make inbound routing to such SIP
   services work, a domain administrator must similarly be willing to
   share the domain's private key with the service.  This design
   decision was made to compensate for the insecurity of the DNS, and it
   makes certain potential approaches to DNS-based 'virtual hosting'
   unsecurable for SIP in environments where domain administrators are
   unwilling to share keys with hosting services.

   A verifier MUST evaluate the correspondence between the user's
   identity and the signing certificate by following the procedures
   defined in RFC 2818 [RFC2818], Section 3.1.  While RFC 2818 [RFC2818]
   deals with the use of HTTP in TLS, the procedures described are
   applicable to verifying identity if one substitutes the "hostname of
   the server" in HTTP for the domain portion of the user's identity in
   the From header field of a SIP request with an Identity header.

   Because the domain certificates that can be used by authentication
   services need to assert only the hostname of the authentication
   service, existing certificate authorities can provide adequate
   certificates for this mechanism.  However, not all proxy servers and
   user agents will be able to support the root certificates of all
   certificate authorities, and moreover there are some significant
   differences in the policies by which certificate authorities issue
   their certificates.  This document makes no recommendations for the
   usage of particular certificate authorities, nor does it describe any
   particular policies that certificate authorities should follow, but
   it is anticipated that operational experience will create de facto
   standards for authentication services.  Some federations of service
   providers, for example, might only trust certificates that have been
   provided by a certificate authority operated by the federation.  It
   is strongly RECOMMENDED that self-signed domain certificates should
   not be trusted by verifiers, unless some previous key exchange has
   justified such trust.

   For further information on certificate security and practices, see
   RFC 3280 [RFC3280].  The Security Considerations of RFC 3280
   [RFC3280] are applicable to this document.

15.5.  Authorization and Transitional Strategies

   Ultimately, the worth of an assurance provided by an Identity header
   is limited by the security practices of the domain that issues the



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   assurance.  Relying on an Identity header generated by a remote
   administrative domain assumes that the issuing domain used its
   administrative practices to authenticate its users.  However, it is
   possible that some domains will implement policies that effectively
   make users unaccountable (e.g., ones that accept unauthenticated
   registrations from arbitrary users).  The value of an Identity header
   from such domains is questionable.  While there is no magic way for a
   verifier to distinguish "good" from "bad" domains by inspecting a SIP
   request, it is expected that further work in authorization practices
   could be built on top of this identity solution; without such an
   identity solution, many promising approaches to authorization policy
   are impossible.  That much said, it is RECOMMENDED that
   authentication services based on proxy servers employ strong
   authentication practices such as token-based identifiers.

   One cannot expect the Identity and Identity-Info headers to be
   supported by every SIP entity overnight.  This leaves the verifier in
   a compromising position; when it receives a request from a given SIP
   user, how can it know whether or not the sender's domain supports
   Identity?  In the absence of ubiquitous support for identity, some
   transitional strategies are necessary.

      A verifier could remember when it receives a request from a domain
      that uses Identity, and in the future, view messages received from
      that domain without Identity headers with skepticism.

      A verifier could query the domain through some sort of callback
      system to determine whether or not it is running an authentication
      service.  There are a number of potential ways in which this could
      be implemented; use of the SIP OPTIONS method is one possibility.
      This is left as a subject for future work.

   In the long term, some sort of identity mechanism, either the one
   documented in this specification or a successor, must become
   mandatory-to-use for the SIP protocol; that is the only way to
   guarantee that this protection can always be expected by verifiers.

   Finally, it is worth noting that the presence or absence of the
   Identity headers cannot be the sole factor in making an authorization
   decision.  Permissions might be granted to a message on the basis of
   the specific verified Identity or really on any other aspect of a SIP
   request.  Authorization policies are outside the scope of this
   specification, but this specification advises any future
   authorization work not to assume that messages with valid Identity
   headers are always good.

16.  IANA Considerations




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   This document requests changes to the header and response-code sub-
   registries of the SIP parameters IANA registry, and requests the
   creation of two new registries for parameters for the Identity-Info
   header.

16.1.  Header Field Names

   This document specifies two new SIP headers: Identity and Identity-
   Info.  Their syntax is given in Section 11.  These headers are
   defined by the following information, which has been added to the
   header sub-registry under http://www.iana.org/assignments/sip-
   parameters

   Header Name: Identity
   Compact Form: y
   Header Name: Identity-Info
   Compact Form: n


16.2.  428 'Use Identity Header' Response Code

   This document registers a new SIP response code, which is described
   in Section 8.  It is sent when a verifier receives a SIP request that
   lacks an Identity header in order to indicate that the request should
   be re-sent with an Identity header.  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: 428
         Default Reason Phrase: Use Identity Header


16.3.  436 'Bad Identity-Info' Response Code

   This document registers a new SIP response code, which is described
   in Section 8.  It is used when the Identity-Info header contains a
   URI that cannot be dereferenced by the verifier (either the URI
   scheme is unsupported by the verifier, or the resource designated by
   the URI is otherwise unavailable).  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: 436
         Default Reason Phrase: Bad Identity-Info





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16.4.  437 'Unsupported Certificate' Response Code

   This document registers a new SIP response code, which is described
   in Section 8.  It is used when the verifier cannot validate the
   certificate referenced by the URI of the Identity-Info header,
   because, for example, the certificate 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: 437
         Default Reason Phrase: Unsupported Certificate


16.5.  438 'Invalid Identity Header' Response Code

   This document registers a new SIP response code, which is described
   in Section 8.  It is used when the verifier receives a message with
   an Identity signature that does not correspond to the digest-string
   calculated by the verifier.  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: 438
         Default Reason Phrase: Invalid Identity Header


16.6.  Identity-Info Parameters

   The IANA has created a new registry for Identity-Info headers.  This
   registry is to be prepopulated with a single entry for a parameter
   called 'alg', which describes the algorithm used to create the
   signature that appears in the Identity header.  Registry entries must
   contain the name of the parameter and the specification in which the
   parameter is defined.  New parameters for the Identity-Info header
   may be defined only in Standards Track RFCs.

16.7.  Identity-Info Algorithm Parameter Values

   The IANA has created a new registry for Identity-Info 'alg' parameter
   values.  This registry is to be prepopulated with a single entry for
   a value called 'rsa-sha1', which describes the algorithm used to
   create the signature that appears in the Identity header.  Registry
   entries must contain the name of the 'alg' parameter value and the
   specification in which the value is described.  New values for the
   'alg' parameter may be defined only in Standards Track RFCs.



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16.8.  Acknowledgements

   The authors would like to thank

16.9.  Original RFC 4474 Requirements

   The following requirements were crafted throughout the development of
   the mechanism described in this document.  They are preserved here
   for historical reasons.

      The mechanism must allow a UAC or a proxy server to provide a
      strong cryptographic identity assurance in a request that can be
      verified by a proxy server or UAS.

      User agents that receive identity assurances must be able to
      validate these assurances without performing any network lookup.

      User agents that hold certificates on behalf of their user must be
      capable of adding this identity assurance to requests.

      Proxy servers that hold certificates on behalf of their domain
      must be capable of adding this identity assurance to requests; a
      UAC is not required to support this mechanism in order for an
      identity assurance to be added to a request in this fashion.

      The mechanism must prevent replay of the identity assurance by an
      attacker.

      In order to provide full replay protection, the mechanism must be
      capable of protecting the integrity of SIP message bodies (to
      ensure that media offers and answers are linked to the signaling
      identity).

      It must be possible for a user to have multiple AoRs (i.e.,
      accounts or aliases) that it is authorized to use within a domain,
      and for the UAC to assert one identity while authenticating itself
      as another, related, identity, as permitted by the local policy of
      the domain.

17.  References

17.1.  Normative References

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

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.




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   [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.

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

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

   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

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

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

17.2.  Informative References

   [I-D.cooper-iab-secure-origin-00]
              Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba,
              "Secure Call Origin Identification", draft-cooper-iab-
              secure-origin-00 (work in progress), November 2012.

   [I-D.peterson-secure-origin-ps]
              Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Origin Identification: Problem Statement, Requirements,
              and Roadmap", draft-peterson-secure-origin-ps-00 (work in
              progress), May 2013.

   [I-D.peterson-sipping-retarget]
              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements", draft-peterson-sipping-
              retarget-00 (work in progress), February 2005.

   [I-D.rescorla-callerid-fallback]
              Rescorla, E.K., "Secure Caller-ID Fallback Mode", draft-
              rescorla-callerid-fallback-00 (work in progress), May
              2013.



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   [I-D.rescorla-rtcweb-generic-idp]
              Rescorla, E., "RTCWEB Generic Identity Provider
              Interface", draft-rescorla-rtcweb-generic-idp-01 (work in
              progress), March 2012.

   [RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

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

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263, June
              2002.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
              3966, December 2004.

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

   [RFC4475]  Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
              and H. Schulzrinne, "Session Initiation Protocol (SIP)
              Torture Test Messages", RFC 4475, May 2006.

   [RFC6919]  Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
              for Use in RFCs to Indicate Requirement Levels", RFC 6919,
              April 1 2013.

Authors' Addresses

   Jon Peterson
   NeuStar

   Email: jon.peterson@neustar.biz





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   Cullen Jennings
   Cisco
   400 3rd Avenue SW, Suite 350
   Calgary, AB  T2P 4H2
   Canada

   Email: fluffy@iii.ca


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Phone: +1 650 678 2350
   Email: ekr@rtfm.com

































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