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Versions: (draft-jennings-dispatch-rfc4474bis) 00 01 draft-ietf-stir-rfc4474bis

Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                             C. Jennings
Expires: April 24, 2014                                            Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                        October 21, 2013


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

Abstract

   The baseline 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 requests.  It does so by defining new
   SIP header fields for conveying a signature used for validating the
   identity, and for conveying a reference to the credentials 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on April 24, 2014.

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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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Intermediary Authentication Services  . . . . . . . . . .   6
   4.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   6
   5.  Signature Generation and Validation . . . . . . . . . . . . .   7
     5.1.  Authentication Service Behavior . . . . . . . . . . . . .   7
       5.1.1.  Identity within a Dialog and Retargeting  . . . . . .  10
     5.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .  11
   6.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Credential Use by the Authentication Service  . . . . . .  13
     6.2.  Credential Use by the Verification Service  . . . . . . .  14
     6.3.  Handling Identity-Info URIs . . . . . . . . . . . . . . .  14
   7.  Identity and Telephone Numbers  . . . . . . . . . . . . . . .  16
   8.  Considerations for User Agents  . . . . . . . . . . . . . . .  17
   9.  Considerations for Proxy Servers  . . . . . . . . . . . . . .  18
   10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  18
   11. Compliance Tests and Examples . . . . . . . . . . . . . . . .  22
     11.1.  Identity-Info with a Singlepart MIME body  . . . . . . .  22
     11.2.  Identity for a Request with No MIME Body or Contact  . .  25
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  28
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  28
     13.1.  Handling of digest-string Elements . . . . . . . . . . .  29
     13.2.  Display-Names and Identity . . . . . . . . . . . . . . .  31
     13.3.  Securing the Connection to the Authentication Service  .  32
     13.4.  Domain Names, Certificates and Subordination . . . . . .  33



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     13.5.  Authorization and Transitional Strategies  . . . . . . .  35
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
     14.1.  Header Field Names . . . . . . . . . . . . . . . . . . .  36
     14.2.  428 'Use Identity Header' Response Code  . . . . . . . .  36
     14.3.  436 'Bad Identity-Info' Response Code  . . . . . . . . .  37
     14.4.  437 'Unsupported Certificate' Response Code  . . . . . .  37
     14.5.  438 'Invalid Identity Header' Response Code  . . . . . .  37
     14.6.  Identity-Info Parameters . . . . . . . . . . . . . . . .  37
     14.7.  Identity-Info Algorithm Parameter Values . . . . . . . .  38
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   16. Original RFC 4474 Requirements  . . . . . . . . . . . . . . .  38
   17. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  39
     17.1.  Motivation for Changes . . . . . . . . . . . . . . . . .  39
     17.2.  Changes to the Identity-Info Header  . . . . . . . . . .  41
     17.3.  Changes to the Identity Header . . . . . . . . . . . . .  42
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     18.2.  Informative References . . . . . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

1.  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 either a SIP URI, commonly a canonical
   address-of-record (AoR) employed to reach a user (such as
   'sip:alice@atlanta.example.com'), or a telephone number, which can be
   represented as either a TEL URI or as the user portion of a SIP URI.

   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
   they have no previous association -- just as in the telephone network
   today, one can receive a call from someone with whom one has no



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   previous association, and still have a reasonable assurance that the
   person's displayed calling party number (and/or Caller-ID) is
   accurate.  A cryptographic approach, like the one described in this
   document, can provide a much stronger and less spoofable assurance of
   identity than the telephone network provides today.

2.  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].

3.  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 calling party number
   (and/or Caller-ID) of a caller, which a human may review (making a
   policy decision) before answering a call.  An example of the former
   would be a voicemail service that compares the identity of the caller
   to a whitelist before determining whether it should allow the caller
   access to recorded messages.  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 provides a strong identity
   system for SIP requests in which authorization policies cannot be
   circumvented by impersonation.

   This document proposes an authentication architecture for SIP in
   which requests are processed by a logical authentication service that
   may be implemented as part of a user agent or as a proxy server.
   Once a message has been authenticated, the service then adds new
   cryptographic information to requests to communicate to other SIP
   entities that the sending user has been authenticated and its use of
   the From header field has been authorized.

   But authorized by whom?  Identities are issued to users by
   authorities.  When a new user becomes associated with example.com,
   the administrator of the SIP service for that domain will issue them
   an identity in that namespace, such as alice@example.com.  Alice may
   then send REGISTER requests to example.com that make her user agents
   eligible to receive requests for sip:alice@example.com.  In some
   cases, Alice may be the owner of the domain herself, and may issue
   herself identities as she chooses.  But ultimately, it is the



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   controller of the SIP service at example.com that must be responsible
   authorizing the use of names in the example.com domain.  Therefore,
   the credentials needed to prove this authorization must ultimately
   derive from the domain owner: either a user agent gives requests to
   the domain name owner in order for them to be signed by the domain
   owner's credentials, or the user agent must possess credentials that
   prove in some fashion that the domain owner has given the user agent
   the right to a name.

   The situation is however more complicated for telephone numbers.
   Authority over telephone numbers does not correspond directly to
   Internet domains.  While a user could register at a SIP domain with a
   username that corresponds to a telephone number, any connection
   between the administrator of that domain and the assignment of
   telephone numbers is not reflected on the Internet.  Telephone
   numbers do not share the domain-scope property described above, as
   they are dialed without any domain component.  This document thus
   assumes the existence of a separate means of establishing authority
   over telephone numbers, for cases where the telephone number is the
   identity of the user.  As with SIP URIs, the necessary credentials to
   prove authority for a name might reside either in the endpoint or at
   some intermediary.

   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 authentication service adding a SIP header, the
   Identity header.  In order to assist in the validation of this
   assurance, this specification also describes an Identity-Info header
   that can be used by the recipient of a request to recover the
   credentials of the signer.  Note that the scope of this document is
   limited to providing this identity assurance for SIP requests;
   solving this problem for SIP responses is outside the scope of this
   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 credentials; if they do, they can act as an authentication
   service.  However, end-user credentials may be neither practical nor
   affordable, 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 credentials
   for the various devices could also be burdensome.  This trade-off
   needs to be understood by implementers of this specification.





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3.1.  Intermediary Authentication Services

   In cases where a user agent does not possess its own credentials to
   sign an Identity header, the user agent must send its request through
   an intermediary that will provide a signed Identity header based on
   the contents of the request.  This requires, among other things, that
   intermediaries have some means of authenticating the user agents
   sending requests.

   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').  For such SIP URIs, by the
   definition of identity used in this document, registration proves the
   identity of the user to a registrar.  Similar checks might be
   performed for telephone numbers as identities.  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.

   RFC 3261 [RFC3261] already describes an intermediary architecture
   very similar to the one proposed in this document 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
   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.

4.  Overview of Operations





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   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.
   This value may be Alice's AoR, or it may be some other value that the
   proxy server has authority over, such as a telephone number.  It then
   computes a hash over some particular headers, including the From
   header field (and, optionally the body) in the message.  This hash is
   signed with the appropriate credential (example.com, in the
   sip:alice@example.com case) and inserted in a new header field in the
   SIP message, the 'Identity' header.

   The proxy, as the holder of the private key for 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 keying material necessary to validate its credentials, 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
   authority over the identity 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).

5.  Signature Generation and Validation

5.1.  Authentication Service Behavior

   This document defines a 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 one or more credentials that can be used to sign for a domain
   or a telephone number (see Section 6.1).  Intermediaries that
   instantiate this role MUST be capable of authenticating one or more



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   SIP users who can register for that identity.  Commonly, this role
   will be instantiated by a proxy server, since these entities are more
   likely to have a static hostname, hold corresponding credentials, and
   have access to SIP registrar capabilities that allow them to
   authenticate users.  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 10 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.

   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, and the user portion is not a telephone number, 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, or the identity field is a SIP or SIPS URI with a telephone
   number in the user portion, the authentication service determines
   whether or not it is responsible for this telephone number; see
   Section 7 for more information.  If the authentication service is not
   authoritative 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:








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   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 credential, or preferably by providing a password that unlocks
      said private key.

   Authorization of the use of a particular username or telephone number
   in the user part of the From header field is a matter of local policy
   for the authentication service, see Section 6.1 for more information.

   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 13.2.

   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



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   to upset any verifiers.  If the Date header contains a time different
   by more than ten minutes from the current time noted by the
   authentication service, the authentication service SHOULD reject the
   request.  This behavior is not mandatory because a user agent client
   (UAC) could only exploit the Date header in order to cause a request
   to fail verification; the 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 credential.  For
   more information on the security properties associated with the Date
   header field value, see Section 10.

   [TBD: Should consider a lower threshold than ten minutes?  With the
   removal of other elements from the sig, that's a lot of leeway.]

   Step 4:

   The authentication service MAY form an identity-reliance signature
   and add an Identity-Reliance header to the request containing this
   signature.  The Identity-Reliance header provides body security
   properties that are useful for non-INVITE transactions, and in
   environments where body security of INVITE transactions is necessary.
   Details on the generation of this header is provided in Section 10.

   Step 5:

   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 credential can be acquired; see
   Section 6.3 for more on credential acquisition.  Details on the
   syntax of both of these headers are provided in Section 10.

   Finally, the authentication service MUST forward the message
   normally.

5.1.1.  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.





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   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
   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].

5.2.  Verifier Behavior

   This document introduces a new logical role for SIP entities called a
   verification service or verifier.  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.





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   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:

   In order to determine whether the signature for the URI in the From
   header field value should be over the entire URI or just a
   canonicalized telephone number, the verification service must follow
   the process described in Section 7.  That section also describes the
   procedures the verification service must follow to determine if the
   signer is authoritative for a telephone number.  For domains, the
   verifier MUST follow the process described in Section 13.4 to
   determine if the signer is authoritative for the URI in the From
   header field.

   Step 2:

   The verifier must first ensure that it possesses the proper keying
   material to validate the signature in the Identity header field.  See
   Section 6.2 for more information on these procedures.

   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 10.  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:

   If the request contains an Identity-Reliance header, the verifier
   SHOULD verify the signature in the Identity-Reliance header field,
   following the procedures for generating the hashed reliance-digest-
   string described in Section 10.  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 SHOULD
   be returned.

   Step 5:









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   The verifier MUST validate the Date header in the manner described in
   Section 13.1; 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.

6.  Credentials

   SIP entities cannot reliably predict where SIP requests will
   terminate.  When choosing a credential scheme for deployments of this
   specification, it is therefore essential that the trust anchor(s) for
   credentials be widely trusted, or that deployments restrict the use
   of this mechanism to environments where the reliance on particular
   trust anchors is assured by business arrangements or similar
   constraints.

   For more on the use of certificates for domain names as a credential
   system, see Section 13.4.

6.1.  Credential Use by the Authentication Service

   In order to act as an authentication service, a SIP entity must have
   access to the private keying material of one or more credentials that
   cover URIs, domain names or telephone numbers.  These credentials may
   represent authority over only a single name (such as
   alice@example.com), an entire domain (such as example.com), or
   potentially a set of domains.  Similarly, a credential may represent
   authority over a single telephone number or a range of telephone
   numbers.  The way that the scope of a credential is expressed is
   specific to the credential mechanism.

   Authorization of the use of a particular username or telephone number
   in the user part of 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 non-telephone
   number user parts, 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



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   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.  This is a suitable approach for
   telephone numbers in particular.  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.

6.2.  Credential Use by the Verification Service

   In order to act as a verification service, a SIP entity must have a
   way to acquire and retain credentials for authorities over particular
   URIs, domain names and/or telephone numbers.  The Identity-Info
   header (as described in the next section) is supported by all
   verification service implementations to create a baseline means of
   credential acquisition.  Provided that the credential used to sign a
   message is not previously known to the verifier, SIP entities SHOULD
   discover this credential by dereferencing the Identity-Info header,
   unless they have some more efficient implementation-specific way of
   acquiring certificates.  If the URI scheme in the Identity-Info
   header cannot be dereferenced, then a 436 'Bad Identity-Info'
   response MUST be returned.

   In the case the credential is a certificate, 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 certificate
   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.

   Verification service implementations supporting this specification
   SHOULD have some means of retaining credentials (in accordance with
   normal practices for credential lifetimes and revocation) in order to
   prevent themselves from needlessly downloading the same credential
   every time a request from the same identity is received.  Credentials
   cached in this manner SHOULD be indexed by their scope, or the URI
   given in the Identity-Info header field value.

6.3.  Handling Identity-Info URIs




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   A URI in an Identity-Info header MUST contain a URI which
   dereferences to a resource containing the credential used by the
   authentication service to sign a request.  Much as is the case with
   the trust anchor(s) required for deployments of this specification,
   it is essential that a URI in the Identity-Info header be
   dereferencable by any entity that can receive the request.  For
   common cases, this means that the URI must be dereferencable by any
   entity on the public Internet.  In constrained deployment
   environments, a service private to the environment might be used
   instead.

   Beyond providing a means of accessing credentials for an identity,
   the Identity-Info header further services a means of differentiating
   which particular credential was used to sign a request, when there
   are potentially multiple authorities eligible to sign.  For example,
   imagine a case where a domain implements the authentication service
   role for example.com, and a user agent belonging to Alice has
   acquired a credential for alice@example.com.  Either would be
   eligible to sign a SIP request from alice@example.com.  Verification
   services however need a means to differentiate which one performed
   the signature.  The Identity-Info header performs that function.

   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).

   Beyond HTTP, implementations may support any of several alternative
   mechanism for acquiring credentials.  When implemented as part of a
   user agent, for example, an authentication service might include its
   credential as an additional MIME body in the SIP request, and refer
   to the certificate with a CID URI (per [RFC2392]).  Uses of SIP
   outside of the request transaction may be suitable for transmitting
   certificates in some environments, such as through a SUBSCRIBE/NOTIFY
   exchange.  As DANE deployment increases with the widespread adoption
   of DNSSEC, implementations may want to rely on keying material stored
   in the DNS.  The Identity-Info headers may use the DNS URL scheme to
   indicate keys in the DNS.




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   [TBD: Should we add some kind of hash or similar indication to the
   Identity-Info header to make it easier for verifiers to ascertain
   that they already possess a credential without dereferencing the
   URI?]

7.  Identity and Telephone Numbers

   Since many SIP applications provide a Voice over IP (VoIP) service,
   telephone numbers are commonly used as identities in SIP deployments.
   In order for telephone numbers to be used with the mechanism
   described in this document, authentication services must enroll with
   an authority that issues credentials for telephone numbers or
   telephone number ranges, and verification services must trust the
   authority employed by the authentication service that signs a
   request.

   Given the existence of such authorities, authentication and
   verification services must furthermore identify when a request should
   be signed by an authority for a telephone number, and when it should
   be signed by an authority for a domain.  Telephone numbers most
   commonly appear in SIP requests in the username portion of a SIP URI
   (e.g., 'sip:+17005551008@chicago.example.com;user=phone').  The user
   part of that URI conforms to the syntax of the TEL URI scheme (RFC
   3966 [RFC3966]).  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 both of these cases, it's clear that
   the signer must have authority over the telephone number, not the
   domain name of the SIP URI.  It is also possible, however, for
   requests to contain a URI like 'sip:7005551000@chicago.example.com'.
   It may be non-trivial for a service to ascertain in this case whether
   the URI contains a telephone number or not.

   To address this problem, the authentication service and verification
   service both must perform the following canonicalization procedure on
   any SIP URI they inspect which contains a wholly numeric user part.
   [TBD: the algorithm] If the result of this procedure forms a complete
   telephone number, that number is used for the purpose of creating and
   signing the digest-string at the authentication service and
   verification service.  If the result does not form a complete
   telephone number, the authentication service and verification service
   should treat the entire URI as a SIP URI, and apply a domain
   signature per the procedures in Section 13.4.

   This specification assumes that UACs will have an appropriate means
   to discover an authentication service that can sign with a telephone
   number certificate corresponding to the UAC's telephone number.  Most
   likely, this information will simply be provisioned in UACs.




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   Certificates that prove authority over telephone numbers should
   contain the telephone number, or number range, in the [TBD] field of
   the certificate.  Verification services must compare the
   canonicalized telephone number to the contents of the [TBD] field in
   order to establish that the proper authority has signed the request.
   [TBD: This would refer to an external specification, most likely]

   In the longer term, it is possible that some directory or other
   discovery mechanism 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.

8.  Considerations for User Agents

   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
   attractive because it MUST be supported by SIP proxy servers, but is
   potentially problematic because it is a hop-by-hop mechanism.  See
   Section 13.3 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]).

   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



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   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.

9.  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 verification service, it MAY validate the Identity header
   before it makes a forwarding decision for a request.  Compliant proxy
   servers MUST NOT remove or modify an existing Identity or Identity-
   Info header in a request.

10.  Header Syntax

   This document specifies three SIP headers: Identity, Identity-
   Reliance and Identity- Info.  Each of these headers can appear only
   once in a SIP request; Identity-Reliance is OPTIONAL, while Identity
   and Identity-Info are REQUIRED for securing requests with this
   specification.  The grammar for these three headers is (following the
   ABNF [6] in RFC 3261 [1]):

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

      Identity-Reliance = "Identity-Reliance" HCOLON signed-identity-reliance-digest
      signed-identity-reliance-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


   [TBD: The version has the Identity-Reliance header covered under the
   Identity signature.  It is also possible to do this the other way
   around, where the base Identity signature is generated first, and
   Identity-Reliance would cover both the Identity header and the body.
   This is a trade-off of whether the authentication service should
   decide whether Identity-Reliance is needed or if the verification
   service should decide.  These have different properties, and some
   investigation would be needed to decide between them.]





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   The signed-identity-reliance-digest is a signed hash of a canonical
   string generated from certain components of a SIP request.  Creating
   this hash and the Identity-Reliance header field to contain it is
   OPTIONAL, and its usage is a matter of policy for authentication
   services.  To create the contents of the signed-identity-digest, the
   following element of a SIP message MUST be placed in a bit-exact
   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.

   [TBD: Explore alternatives to including the whole body for INVITE
   requests]

   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:

      First, the identity.  If the user part of the AoR in the From
      header field of the request contains a telephone number, then the
      canonicalization of that number goes into the first slot (see
      Section 7).  Otherwise, the first slot contains the AoR of the UA
      sending the message, or addr-spec of the From header field.

      Second, the target.  If the user part of the AoR in the To header
      field of the request contains a telephone number, then the
      canonicalization of that number goes into the second slot (see
      Section 7).  Otherwise, the second slot contains the addr-spec
      component of the To header field, which is the AoR to which the
      request is being sent.

      Third, the request method.

      Fourth, 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 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



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      token.  All requests that use the Identity mechanism MUST contain
      a Date header.

      Fifth, the Identity-Reliance header field value, if there is an
      Identity-Reliance field in the request.  If the message has no
      body, or no Identity-Reliance header, then the fifth slot will be
      empty, and the final "|" will not be followed by any additional
      characters.

      [TBD: Should there be a special case for security parameters that
      would appear in SDP?]

   For more information on the security properties of these headers, and
   why their inclusion mitigates replay attacks, see Section 13 and
   [RFC3893].  The precise formulation of this digest-string is,
   therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):

   digest-string = addr-spec / tn-spec "|" addr-spec / tn-spec "|"
                   Method "|" SIP-date "|" [ signed-identity-reliance-digest ]


   For the definition of 'tn-spec' see Section 7.

   After the digest-string or reliance-digest-string is formed, each
   MUST be hashed and signed with the certificate of authority over the
   identity.  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 [RFC3548].  A 1024-bit or
   longer RSA key MUST be used.  The result of the digest-string hash is
   placed in the Identity header field; the optional reliance-digest-
   string hash goes in the Identity-Reliance header.  For detailed
   examples of the usage of this algorithm, see Section 11.

   The 'absoluteURI' portion of the Identity-Info header MUST contain a
   URI; see Section 6.3 for more on choosing how to advertise
   credentials through Identity-Info.







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   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] (this repeats the registrations of RFC4474):

    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

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

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



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









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11.  Compliance Tests and Examples

   [TBD: Need to fix examples for RFC4474bis]

   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 13.4 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.

11.1.  Identity-Info with a Singlepart MIME body

   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



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        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>
      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



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      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|
   INVITE|Thu, 21 Feb 2002 13:02:03 GMT|


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

        ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
        ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
        FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=


   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="



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   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.

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

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

        -----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



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        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
   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



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   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


   [TBD: Fix example.]  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=

   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




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   biloxi.example.org then forwards the request normally.

12.  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 for using anonymous SIP URIs, an authentication
   service must possess a certificate corresponding to the host portion
   of the addr-spec of the From header field of the request;
   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 domain can exist, and as a consequence,
   authentication service functions are meaningless.

   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.

13.  Security Considerations




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13.1.  Handling of digest-string Elements

   This document describes a mechanism that provides a signature over
   the Date header field, and either the whole or part of the To and
   From header fields of SIP requests, as well as optional protections
   for the message body.  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.

   The From header field indicates the identity of the sender of the
   message, and the SIP address-of-record URI, or an embedded telephone
   number, 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 header field provides 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).
   The result of this is that if an Identity header is replayed within
   the Date interval, verifiers will recognize that it is invalid; if an
   Identity header is replayed after the Date interval, verifiers will
   recognize that it is invalid because the Date is stale.

   Without the method an INVITE request could be cut- and-pasted by an
   attacker and transformed into a MESSAGE 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.

   RFC4474 had protections for the Contact, Call-ID and CSeq.  These are
   removed from RFC4474bis.  The absence of these header values creates
   some opportunities for determined attackers to impersonate based on



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   cut-and-paste attacks; however, the absence of these headers does not
   seem impactful to preventing against the simple unauthorized claiming
   of a From header field value.

   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,
   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 an optional signature over the bodies of
   SIP requests.  This can help to protect non-INVITE transactions such
   as MESSAGE or NOTIFY, as well as INVITEs in those environments where
   intermediaries do not change SDP.  While this is not strictly
   necessary to prevent the impersonation attacks, 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 contents of the message.  There are furthermore
   some baiting attacks (where the attacker receives a request from the
   target and reoriginates it to a third party) that might not be
   prevented by only a signature over the From, To and Date, but could
   be prevented by securing SDP.  Note, however, that this is not
   perfect end-to-end security.  The authentication service itself, when
   instantiated at an intermediary, could conceivably change the body
   (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 bodies, 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.




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   In the end analysis, the Identity, Identity-Reliance 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 the originators of SIP
   requests can prove 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 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.

13.2.  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



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   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 13.1.

   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
   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.

13.3.  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.  A similar practice could be used for
      telephone numbers, though the application of certificates for
      telephone numbers to TLS is left as a matter for future study.





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      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.

   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.

13.4.  Domain Names, Certificates and Subordination

   When a verifier processes a request containing an Identity-Info
   header with a domain signature, 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 requests through a
   host like 'sip.example.com', and it may be that latter host that acts
   as an authentication service.





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   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
   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.

   [TBD: DANE?]



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

13.5.  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
   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



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   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.

14.  IANA Considerations

   [TBD: update for rfc4474bis or remove?]

   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.

14.1.  Header Field Names

   This document specifies two new SIP headers: Identity and Identity-
   Info.  Their syntax is given in Section 10.  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


14.2.  428 'Use Identity Header' Response Code

   This document registers a new SIP response code, which is described
   in Section 5.2.  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










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14.3.  436 'Bad Identity-Info' Response Code

   This document registers a new SIP response code, which is described
   in Section 5.2.  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


14.4.  437 'Unsupported Certificate' Response Code

   This document registers a new SIP response code, which is described
   in Section 5.2.  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


14.5.  438 'Invalid Identity Header' Response Code

   This document registers a new SIP response code, which is described
   in Section 5.2.  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


14.6.  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



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   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.

14.7.  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.

15.  Acknowledgements

   The authors would like to thank the many commentators on this
   document.

16.  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.







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      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.  Changes from RFC4474

17.1.  Motivation for Changes

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

   o  The DNS root has been signed.

   o  SPAM continues to be a problem.

   o  It has become clear that B2BUAs will continue to be a major factor
      in SIP deployments.

   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 middle of 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 change of the scope of
   4474 to solve the following problems:

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

   o  Continue to assert strong identity for domain scoped names such as
      alice@example.com

   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.

   o  Potentially, as future work explore organization attributes (e.g.,
      "this is a Bank").

   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.  The design of such a system is outside the scope of this
   draft, and perhaps of the IETF, but we believe it will have a twofold
   character:

   First, it will delegate responsibility for a number down from a root
   in a series of delegation sub delegation towards the user.  For
   example, the North American Numbering Plan Administrator assigns 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



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   be as simple as sending a SMS to the number or more complicated such
   as the VIPR system.

   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.  For the purposes of the current work, it is envisioned
   that a certificate authority could encompass both approaches.

   Authentication services that possess a credential (whether of the
   delegation or claim variety) for a telephone number or domain name
   can, in this mechanism, create one of two types of assertions: basic
   assertions and reliance assertions.  The basic assertion provides
   replay protection, whereas the reliance assertion provides a broader
   body protection.  Some networks might modify the signaling in ways
   that impact the reliance assertions but not the other, and thus the
   reliance assertion is optional.

   As in RFC4474, identity assertions are passed in-band in SIP from the
   caller to the callee for verification.  There are however some cases
   where in-band signaling cannot survive the call path, such as when
   the call passes through a gateway to the PSTN.  This specification
   assumes that other, out-of-band mechanisms may be used in cases where
   in-band identity is not carried end-to-end, but those mechanisms are
   outside the scope of this document.

17.2.  Changes to the Identity-Info Header

   RFC4474 restricted 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], this document relaxes that
   constraint 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 the
   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?

   Future version of this document may explore alternate ways of
   acquiring credentials, including the use of credentials other than
   certificates.  This might include 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.

17.3.  Changes to the Identity Header

   Per the analysis in [I-D.peterson-secure-origin-ps], this document
   changes 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 here create two distinct signatures within SIP
   requests: a basic assurance and a reliance assurance.  The basic
   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 some cut-and-paste attacks.
   The reliance assurance protects against attackers changing other
   parameters of the call: these include the entirely of the messaging
   body, including the target IP address and ports in SDP which, if
   unprotected, can allow an attacker to succeed with more sophisticated
   cut-and-paste attacks.  Authentication services behavior would change
   to allow them to decide, based on their policy in a deployment
   environment, whether only the basic assurance can realistically
   survive network transit, or if the reliance assurance should be
   available.  There are several similar design choices in this space to
   consider, and some analysis will be required to identify the best
   option.

   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.




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   [TBD: in order to preserve critical security parameters even in
   adverse network conditions, should the basic 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. ]

18.  References

18.1.  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.

   [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.

18.2.  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.




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   [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., "Secure Caller-ID Fallback Mode", draft-
              rescorla-callerid-fallback-00 (work in progress), May
              2013.

   [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.




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   [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


   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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