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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: August 18, 2014                                           Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                       February 14, 2014


  Authenticated Identity Management in the Session Initiation Protocol
                                 (SIP)
                 draft-jennings-stir-rfc4474bis-01.txt

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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   5
   4.  Signature Generation and Validation . . . . . . . . . . . . .   6
     4.1.  Authentication Service Behavior . . . . . . . . . . . . .   6
       4.1.1.  Intermediary Authentication Services  . . . . . . . .   9
     4.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Identity within a Dialog and Retargeting  . . . . . . . .  11
   5.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Credential Use by the Authentication Service  . . . . . .  12
     5.2.  Credential Use by the Verification Service  . . . . . . .  13
     5.3.  Handling Identity-Info URIs . . . . . . . . . . . . . . .  14
     5.4.  Credential Systems  . . . . . . . . . . . . . . . . . . .  14
   6.  Identity Types  . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Telephone Numbers . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Usernames with Domain Names . . . . . . . . . . . . . . .  16
   7.  Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  22
     10.1.  Handling of digest-string Elements . . . . . . . . . . .  22
     10.2.  Securing the Connection to the Authentication Service  .  24
     10.3.  Authorization and Transitional Strategies  . . . . . . .  25
     10.4.  Display-Names and Identity . . . . . . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  Header Field Names . . . . . . . . . . . . . . . . . . .  27
     11.2.  428 'Use Identity Header' Response Code  . . . . . . . .  27
     11.3.  436 'Bad Identity-Info' Response Code  . . . . . . . . .  27
     11.4.  437 'Unsupported Credential' Response Code . . . . . . .  28
     11.5.  438 'Invalid Identity Header' Response Code  . . . . . .  28
     11.6.  Identity-Info Parameters . . . . . . . . . . . . . . . .  28
     11.7.  Identity-Info Algorithm Parameter Values . . . . . . . .  28
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   13. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  29
   14. Informative References  . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31





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1.  Introduction

   This document provides enhancements to the existing mechanisms for
   authenticated identity management in the Session Initiation Protocol
   (SIP, RFC 3261 [1]).  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 [1] 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 [1] 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
   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.  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 to make 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



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   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
   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 currently 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



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   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, their user agents 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.

3.  Overview of Operations

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

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

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

   The authentication service authenticates Alice (possibly by sending a
   Digest authentication challenge) and validates that she is authorized
   to assert the identity that is populated in the From header field.
   This value may be Alice's AoR, or in other cases it may be some
   different 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) of
   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-



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   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).

4.  Signature Generation and Validation

4.1.  Authentication Service Behavior

   This document specifies a role for SIP entities called an
   authentication service.  The authentication service role can be
   instantiated by an intermediary such as 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 5.1).
   Intermediaries that instantiate this role MUST be capable of
   authenticating one or more 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 7 for information on how the Date header field assists
   verifiers).

   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



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   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 [1], 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 6.1 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 following the steps below to add an
   Identity header; see below for more information on authentication
   service handling of an existing Identity header. [where?]

   Step 2:

   The authentication service MUST then 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 [1], 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 5.1 for more information.

   Note that this check is performed only 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').



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   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 10.4.

   Step 3:

   The authentication service SHOULD ensure that any preexisting Date
   header in the request is accurate.  Local policy can dictate
   precisely how accurate the Date must be; a RECOMMENDED maximum
   discrepancy of ten minutes will ensure that the request is unlikely
   to upset any verifiers.  If the Date header contains a time different
   by more than ten minutes from the current time noted by the
   authentication service, the authentication service SHOULD reject the
   request.  This behavior is not mandatory because a user agent client
   (UAC) could only exploit the Date header in order to cause a request
   to fail verification; the 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 7.

   [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 7.
   If the authentication service is adding an Identity-Reliance header,
   it MUST also 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.

   Step 5:





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   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 5.3 for more on credential acquisition.  Details on the
   syntax of both of these headers are provided in Section 7.

   Finally, the authentication service MUST forward the message
   normally.

4.1.1.  Intermediary Authentication Services

   In cases where a user agent does not possess its own credentials to
   sign an Identity header, the user agent can 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 [1] 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 [1] 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.

   This specification assumes that UACs will have an appropriate means
   to discover an authentication service that can sign with a credential



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   corresponding to the UAC's identity.  Most likely, this information
   will simply be provisioned in UACs.

   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 RFC 3325 [9].  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.

4.2.  Verifier Behavior

   This document specifies a logical role for SIP entities called a
   verification service, or verifier.  When a verifier receives a SIP
   message containing an Identity header, it inspects the signature to
   verify the identity of the sender of the message.  Typically, the
   results of a verification are provided as input to an authorization
   process that is outside the scope of this document.  If an Identity
   header is not present in a request, and one is required by local
   policy (for example, based on a per-sending-domain policy, or a per-
   sending-user policy), then a 428 'Use Identity Header' response MUST
   be sent.

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

   Step 1:

   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 6.1.  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 6.2 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 5.2 for more information on these procedures.

   Step 3:



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   The verifier MUST verify the signature in the Identity header field,
   following the procedures for generating the hashed digest-string
   described in Section 7.  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 7.  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:

   The verifier MUST validate the Date header in the manner described in
   Section 10.1; 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 credential used to sign the Identity header.

4.3.  Identity within a Dialog and Retargeting

   The mechanism is this document provides a signature over the URI in
   the To header field value.  The recipient of a request must compare
   that value to their own identity in order to determine whether or not
   the identity information in this call might have been replayed.
   Retargeting, however, complicates this evaluation.

   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,
   potentially 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.

   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.
   Moreover, 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.  But in retargeting cases, if the URI in



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   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 see
   [17].

5.  Credentials

5.1.  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



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   administrative domain.  In this latter case, a domain might maintain
   a mapping between the values in the 'username' parameter of Proxy-
   Authorization and a set of one or more SIP URIs that might
   legitimately be asserted for that 'username'.  For example, the
   username can correspond to the 'private identity' as defined in Third
   Generation Partnership Project (3GPP), in which case the From header
   field can contain any one of the public identities associated with
   this private identity.  In this instance, another policy might be as
   follows: the URI in the From header field MUST correspond exactly to
   one of the mapped URIs associated with the 'username' given in the
   Proxy-Authorization header.  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.

5.2.  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.

   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 max be indexed in accordance with local policy:
   for example, by their scope, or the URI given in the Identity-Info
   header field value.

   [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?]





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5.3.  Handling Identity-Info URIs

   An Identity-Info header MUST contain a URI which dereferences to a
   resource which contains the public key components of 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 could plausibly 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.

5.4.  Credential Systems

   This document makes no specific recommendation for the use of any
   credential system.  Today, there are two primary credential systems
   in place for proving ownership of domain names: certificates (e.g.,
   X.509 v3, see [8]) and the domain name system itself (e.g., DANE, see
   [10]).  It is envisioned that either could be used in the SIP
   context: an Identity-Info header could for example give an HTTP URL
   of the form 'application/pkix-cert' pointing to a certificate
   (following the conventions of [3]).  The Identity-Info headers may
   use the DNS URL scheme (see [11]( to indicate keys in the DNS.

   While no comparable public credentials exist for telephone numbers,
   either approach could be applied to telephone numbers.  A credential
   system based on certificates is given in draft-peterson-stir-
   certificates [TBD - fix after submitting].  One based on the domain
   name system is given in [18].

   In order for a credential system to work with this mechanism, its
   specification must detail:

      which URIs schemes the credential will use in the Identity-Info
      header, and any special procedures required to dereference the
      URIs



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      how the verifier can learn the scope of the credential.

      any special procedures required to extract keying material from
      the resources designated by the URI

      any algorithms that would appear in the Identity-Info "alg"
      parameter other than 'rsa-sha256.'  Note that per the IANA
      Considerations of this document (Section 11.7), new algorithms can
      only be specified by Standards Action.

   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.

   Note that credential systems must address key lifecycle management
   concerns: were a domain to change the credential 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 credential, 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).

   [TBD: What will the normative language here be?  Support for which
   mechanisms?]

6.  Identity Types

6.1.  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.  Enrollment procedures and credential management are outside
   the scope of this document.

   Given the existence of such authorities, authentication and
   verification services must identify when a request should be signed
   by an authority for a telephone number, and when it should be signed



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   by an authority for a domain.  Telephone numbers most commonly appear
   in SIP header field values 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 [5]).  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 canonicalization algorithm - drop the characters, +'s, assess if
   its a valid local number (if so, append country code), etc]

   [TBD define tn-spec here for ABNF purposes]

   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 by both 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 6.2.

   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.

6.2.  Usernames with Domain Names

   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 credential acquired from the Identity-Info
   header.  While this might seem that this should be a straightforward
   process, it is complicated by two deployment realities.  In the first
   place, credentials 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



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   to a subordinate domain and utilize the procedures in RFC 3263 [2]
   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.

   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
   credential 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 credential by following the procedures
   defined in RFC 2818 [7], Section 3.1.  While RFC 2818 [7] deals with
   the use of HTTP in TLS and is specific to certificates, 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.

7.  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 [12] in RFC 3261 [1]):






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   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.]

   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 local policy for
   authentication services.  To create the contents of the signed-
   identity-reliance-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; should there be a special case for security parameters that
   would appear in SDP?]

   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:





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      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 6.1).  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 6.1).  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 the
      BNF of RFC 3261 [1].  RFC 3261 specifies that the BNF for weekday
      and month is a choice amongst a set of tokens.  The RFC 4234 [12]
      rules for the BNF specify that tokens are case sensitive.
      However, when used to construct the canonical string defined here,
      the first letter of each week and month MUST be capitalized, and
      the remaining two letters must be lowercase.  This matches the
      capitalization provided in the definition of each token.  All
      requests that use the Identity mechanism MUST contain a Date
      header.

      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.

   For more information on the security properties of these headers, and
   why their inclusion mitigates replay attacks, see Section 10 and [4].
   The precise formulation of this digest-string is, therefore
   (following the ABNF[12] in RFC 3261 [1]):

   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 6.1.

   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



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   defines only one value for the 'alg' parameter: 'rsa-sha256'; 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-sha256'.  When the 'rsa-sha256' 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 [13] and base64
   encode the results as specified in RFC 3548 [14].  A 2048-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 8.

   The 'absoluteURI' portion of the Identity-Info header MUST contain a
   URI; see Section 5.3 for more on choosing how to advertise
   credentials through Identity-Info.

   This document adds (or amends) the following entries to Table 2 of
   RFC 3261 [1] (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



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   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.

8.  Examples

9.  Privacy Considerations

   The identity mechanism presented in this document is compatible with
   the standard SIP practices for privacy described in RFC 3323 [15].  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 [9] 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.




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   Finally, note that unlike RFC 3325 [9], the mechanism described in
   this specification adds no information to SIP requests that has
   privacy implications.

10.  Security Considerations

10.1.  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 replayed 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 [1] 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 [1], 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



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   certain transaction could be replayed in potentially confusing or
   malicious ways.

   RFC4474 originally 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 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, which is
   the primary scope of the current document.

   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.



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   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.

   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.  Once again, 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.

10.2.  Securing the Connection to the Authentication Service

   In the absence of user agent-based authentication services, 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



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      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.

      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.

10.3.  Authorization and Transitional Strategies

   Ultimately, the worth of an assurance provided by an Identity header
   is limited by the security practices of the authentication service
   that issues the assurance.  Relying on an Identity header generated
   by a remote administrative domain assumes that the issuing domain
   uses recommended administrative practices to authenticate its users.
   However, it is possible that some authentication services 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 authentication services is
   questionable.  While there is no magic way for a verifier to
   distinguish "good" from "bad" signers by inspecting a SIP request, it



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   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.

   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
      or telephone number that uses Identity, and in the future, view
      messages received from that sources without Identity headers with
      skepticism.

      A verifier could consult some sort of directory that indications
      whether a given caller should have a signed identity.  There are a
      number of potential ways in which this could be implemented.  This
      is left as a subject for future work.

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

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

10.4.  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.  Securing the display-name
   component of the From header field value is outside the scope of this
   document, but may be the subject of future work.





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11.  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.

11.1.  Header Field Names

   This document specifies three SIP headers: Identity, Identity-
   Reliance and Identity- Info.  Their syntax is given in Section 7.
   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
      Header Name: Identity-Reliance
      Compact Form:


11.2.  428 'Use Identity Header' Response Code

   This document registers a SIP response code, which is described in
   Section 4.2.  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

11.3.  436 'Bad Identity-Info' Response Code

   This document registers a SIP response code, which is described in
   Section 4.2.  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



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            Response Code Number: 436
            Default Reason Phrase: Bad Identity-Info

11.4.  437 'Unsupported Credential' Response Code

   This document registers a SIP response code, which is described in
   Section 4.2.  It is used when the verifier cannot validate the
   credential referenced by the URI of the Identity-Info header,
   because, for example, the credential is self-signed, or signed by an
   authority for whom the verifier does not trust.  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 Credential

11.5.  438 'Invalid Identity Header' Response Code

   This document registers a SIP response code, which is described in
   Section 4.2.  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

11.6.  Identity-Info Parameters

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

11.7.  Identity-Info Algorithm Parameter Values

   The IANA has created a registry for Identity-Info 'alg' parameter
   values.  This registry is to be prepopulated with a single entry for
   a value called 'rsa-sha256', 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




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   specification in which the value is described.  New values for the
   'alg' parameter may be defined only in Standards Track RFCs.

   A previous version of this specification defined the 'rsa-sha1' value
   for this registry.  That value is hereby deprecated, and should be
   removed.  It is not believed that any implementations are making use
   of this value.

   [TBD - consider EC for smaller credential sizes?]

12.  Acknowledgments

   Lots of people made significant contributions to this document.

13.  Changes from RFC4474

   Lots of people made significant contributions to this document.

      Generalized the credential mechanism; credential enrollment and
      acquisition is now outside the scope of this document

      Reduced the scope of the Identity signature to remove CSeq, Call-
      ID, Contact, and the message body.

      Added the Identity-Reliance header

      Deprecated 'rsa-sha1' in favor of new baseline signing algorithm

      [TBD - more]

14.  Informative References

   [1]        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.

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

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

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



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

   [6]        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.

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

   [8]        Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [9]        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.

   [10]       Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [11]       Josefsson, S., "Domain Name System Uniform Resource
              Identifiers", RFC 4501, May 2006.

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

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

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

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

   [16]       Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement", draft-ietf-stir-
              problem-statement-03 (work in progress), January 2014.

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





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   [18]       Kaplan, H., "A proposal for Caller Identity in a DNS-based
              Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
              (work in progress), July 2013.

Authors' Addresses

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

   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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