Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                             C. Jennings
Expires: March 17, 2016                                            Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                      September 14, 2015

  Authenticated Identity Management in the Session Initiation Protocol


   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

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

   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 March 17, 2016.

Copyright Notice

   Copyright (c) 2015 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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   ( 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  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   6
   4.  Signature Generation and Validation . . . . . . . . . . . . .   7
     4.1.  Authentication Service Behavior . . . . . . . . . . . . .   7
     4.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .   9
   5.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Credential Use by the Authentication Service  . . . . . .  11
     5.2.  Credential Use by the Verification Service  . . . . . . .  12
     5.3.  Handling Identity-Info URIs . . . . . . . . . . . . . . .  13
     5.4.  Credential Systems  . . . . . . . . . . . . . . . . . . .  13
   6.  Identity Types  . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Telephone Numbers . . . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Canonicalization Procedures . . . . . . . . . . . . .  15
     6.2.  Domain Names  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  23
     10.1.  Protected Request Fields . . . . . . . . . . . . . . . .  23
       10.1.1.  Protection of the To Header and Retargeting  . . . .  24
     10.2.  Unprotected Request Fields . . . . . . . . . . . . . . .  25
     10.3.  Malicious Removal of Identity Headers  . . . . . . . . .  26
     10.4.  Securing the Connection to the Authentication Service  .  26
     10.5.  Authorization and Transitional Strategies  . . . . . . .  27
     10.6.  Display-Names and Identity . . . . . . . . . . . . . . .  29
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Header Field Names . . . . . . . . . . . . . . . . . . .  29
     11.2.  Identity-Info Parameters . . . . . . . . . . . . . . . .  29
     11.3.  Identity-Info Algorithm Parameter Values . . . . . . . .  29
     11.4.  Identity-Extension Names . . . . . . . . . . . . . . . .  30
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  30
   13. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  30
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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1.  Introduction

   This document provides enhancements to the existing mechanisms for
   authenticated identity management in the Session Initiation Protocol
   (SIP, [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
   ''), or a telephone number, which can be
   represented as either a TEL URI [RFC3966] or as the user portion of a

   [RFC3261] stipulates several places within a SIP request where users
   can express an identity for themselves, primarily 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.  This leaves SIP vulnerable to a category
   of abuses, including impersonation attacks that enable robocalling
   and related problems as described in [RFC7340].

   [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.  A cryptographic approach, like
   the one described in this document, can provide a much stronger and
   less spoofable assurance of identity than the Caller ID services that
   the telephone network provides today.

   [RFC4474] previously specified a means of signing portions of SIP
   requests in order to provide that identity assurance.  However, RFC
   4474 was in several ways misaligned with deployment realities (see
   [I-D.rosenberg-sip-rfc4474-concerns]).  Most significantly, RFC 4474
   did not deal well with telephone numbers as identifiers, despite
   their enduring use in SIP deployments.  RFC 4474 also provided a
   signature over material that intermediaries in the field commonly
   altered.  This specification therefore revises RFC 4474 in light of
   recent reconsideration of the problem space to align with the threat
   model in [RFC7375].

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2.  Background

   Per [RFC7340], a key enabler of problems such as robocalling,
   voicemail hacking, and swatting lies in an attacker's ability to
   impersonate someone else.  This secure operation of most SIP
   applications and services depends on authorizing the source of
   communications as it is represented in a SIP request.  Such
   authorization policies can be automated or be a part of human
   operation of SIP devices.  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
   controller of a user agent, impersonation is very simple today.  The
   mechanism described in this document provides a strong identity
   system for detecting attempted impersonation in SIP requests.

   This identity architecture for SIP depends on a logical
   "authentication service" which processes and signs requests; it may
   be implemented either as part of a user agent or as a proxy server.
   Once the sender of the 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 claim of a particular identity has been authorized.  A
   "verification service" on the receiving end then validates this
   signature and enables policy decisions to be made based on the
   results of the verification.

   Identities are issued to users by authorities.  When a new user
   becomes associated with, the administrator of the SIP
   service for that domain can issue them an identity in that namespace,
   such as  Alice may then send REGISTER requests to that make her user agents eligible to receive requests
   for  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 that must be responsible for authorizing the use of names
   in the domain.  Therefore, for the purposes of baseline
   SIP, the credentials needed to prove a user is authorized to use a
   particular From header field 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

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   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,
   however.  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.
   It relies on the authentication service adding to requests a SIP
   header, the Identity header, which contains that cryptographic
   assurance.  In order to assist in the validation of the Identity
   header, 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 (see [RFC4916]).

   This specification allows either a user agent or a proxy server to
   provide the authentication service function and/or the verification
   service function.  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 authentication services.  However,
   for some deployments 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 prohibitively complex and require
   quite a good deal of additional endpoint behavior.  Managing several
   credentials for the various devices could also be burdensome.  In
   these cases, implementation the authentication service at an
   intermediary may be more practical.  This trade-off needs to be
   understood by implementers of this specification.

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3.  Overview of Operations

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

   Imagine a case where Alice, who has the home proxy of and
   the address-of-record, wants to communicate
   with Bob at  They have no prior relationship,
   and Bob implements best practices to prevent impersonation attacks.

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

   The authentication service authenticates Alice (possibly by sending a
   Digest authentication challenge) and validates that she is authorized
   to assert the identity that she populated in the From header field.
   This value is Alice's AoR, but in other cases it could be some
   different value that the proxy server has authority over, such as a
   telephone number.  The proxy then computes a hash over some
   particular headers and fields, including part of the From header
   field of the message.  This hash is signed with the appropriate
   credential for the identity (, in the 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 the
   domain, is asserting that the originator of this request has been
   authenticated and that she is authorized to claim the identity 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 (a public key), in
   case 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).  As the request has been validated, it is
   rendered to Bob. If the validation was unsuccessful, some other
   treatment would be applied by the receiving domain.

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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 by 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.

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

   Step 1:

   First, the authentication service must determine whether it is
   authoritative for the identity of the sender of the request.  In
   ordinary operations, the authentication service decides this by
   inspecting the URI value from the addr-spec component of From header
   field; this URI 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).  If the identity
   field uses the TEL URI scheme [RFC3966], 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.  An
   authentication service proceeding with a signature over a telephone
   number MUST then follow the canonicaliaation procedures described in
   Section 6.1.1.  If the authentication service is not authoritative
   for the identity in question, it SHOULD process and forward the
   request normally, but it MUST NOT follow the steps below to add an
   Identity header.  An authentication service MUST NOT add an Identity
   header to a request that already has one.

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   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 authenticate the request with
      the authentication scheme used for registration in its domain
      (e.g., Digest authentication).

      If the authentication service is instantiated by a SIP user agent,
      a user agent may authenticate its own user through any system-
      specific means, perhaps simply by virtue of having physical access
      to the user agent.

   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
   identity field (e.g., the URI of the sender, like
   ''); 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 could return a 403 response
   code.  In this case, 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, so no normative guidance is given here.  For more information,
   see Section 10.6.

   Step 3:

   An authentication service MUST add a Date header field to SIP
   requests if one is not already present.  The authentication service
   MUST 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 sixty seconds will
   ensure that the request is unlikely to upset any verifiers.  If the
   Date header contains a time different by more than one minute from
   the current time noted by the authentication service, the
   authentication service SHOULD reject the request.  This behavior is

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   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.

   See Section 7 for information on how the Date header field assists

   Step 4:

   The authentication service MAY add extend the identity mechanism by
   adding one or more Identity-Extension headers to the request.  Only
   implementations that extend this base mechanism MAY populate this
   header field and add this signature.  See Section 8.

   Step 5:

   Subsequently, 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 the authentication
   service's 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

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.

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   Step 1:

   In order to determine whether the signature for the identity field
   should be over the entire URI or just a canonicalized telephone
   number, the verification service MUST follow the canonicalization
   process described in Section 6.1.1.  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 identity field.

   Step 2:

   The verifier must first ensure that it possesses the proper keying
   material to validate the signature in the Identity header field,
   which usually involves dereferencing the Identity-Info header.  See
   Section 5.2 for more information on these procedures.

   Step 3:

   The verifier MUST must furthermore ensure that the value of the Date
   header meets local policy for freshness (usually, within sixty
   seconds) and that it falls within the validity period of the
   credential used to sign the Identity header.  For more on the attacks
   this prevents, see Section 10.1.

   Step 4:

   The verifier MUST validate 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

   Step 5:

   If the request contains one or more Identity-Extension headers, then
   if the verifier supports the included extension(s), it SHOULD verify
   any associated fields following the procedures specified in that
   extension (see Section 8).  If a verifier determines that such a
   signature in the message does not correspond to the reconstructed
   digest-string, then a 438 'Invalid Identity Header' response SHOULD
   be returned.  If the verifier does not support the extension(s), then
   the verifier takes no further action.

   The handling of the message after the verification process depends on
   how the implementation service is implemented, and on local policy.

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   This specification does not propose any authorization policy for user
   agents or proxy servers to follow based on the presence of a valid
   Identity header, the presence of an invalid Identity header, or the
   absence of an Identity header, but it is anticipated that local
   policies could involve making different forwarding decisions in
   intermediary implementations, or changing how the user is alerted, or
   how identity is rendered, in user agent implementations.

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 domain names or telephone numbers.  These credentials may
   represent authority over an entire domain (such as or
   potentially a set of domains enumerated by the credential.
   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

   Authorization of the use of a particular username or telephone number
   in the identity 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
   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.

   This specification could also be used with credentials that cover a
   single name or URI, such as or

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   authentication service behavior to operate on a whole URI rather than
   a domain name.  Because this is not believed to be a pressing use
   case, this is deferred to future work, but implementors should note
   this as a possible future direction.

   Exceptions to such authentication service policies arise for cases
   like anonymity; if the AoR asserted in the From header field uses a
   form like '' (see [RFC3323]), then the
   '' proxy might authenticate only 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
   domain names and/or telephone numbers or number ranges.
   Dereferencing the Identity-Info header (as described in the next
   section) MUST be 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
   other implementation-specific way of acquiring the needed keying
   material, such as an offline store of periodically-updated
   credentials.  If the URI in the Identity-Info header cannot be
   dereferenced, then a 436 'Bad Identity-Info' response MUST be

   This specification does not propose any particular policy for a
   verification service to determine whether or not the holder of a
   credential is the appropriate party to sign for a given SIP identity.
   Guidance on this is deferred to the credential mechanism
   specifications, which must meet the requirements in Section 5.4.

   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 may be indexed in accordance with local policy:
   for example, by their scope, or the URI given in the Identity-Info
   header field value.  Further consideration of how to cache
   credentials is deferred to the credential mechanism specifications.

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5.3.  Handling Identity-Info URIs

   An Identity-Info header MUST contain a URI which dereferences to a
   resource that contains the public key components of the credential
   used by the authentication service to sign a request.  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 serves as 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 a range of telephone and a user agent belonging to Alice has
   acquired a credential for a single telephone number within that
   range.  Either would be eligible to sign a SIP request for the number
   in question.  Verification services however need a means to
   differentiate which one performed the signature.  The Identity-Info
   header performs that function.

   If the optional "canon" parameter is present, it contains the result
   of the number canonicalization process performed by the
   authentication service (see Section 6.1.1) on the identity in the
   From.  This value is provided purely informationally as an
   optimization for the verification service.  The verification service
   MAY compute its own canonicalization of the number and compare this
   to the value in the "canon" parameter before performing any
   cryptographic functions in order to ascertain whether or not the two
   ends agree on the canonical number form.

5.4.  Credential Systems

   This document makes no recommendation for the use of any specific
   credential system.  Today, there are two primary credential systems
   in place for proving ownership of domain names: certificates (e.g.,
   X.509 v3, see [RFC5280]) and the domain name system itself (e.g.,
   DANE, see [RFC6698]).  It is envisioned that either could be used in
   the SIP identity 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 [RFC2585]).  The Identity-
   Info headers may use the DNS URL scheme (see [RFC4501]) to designate
   keys in the DNS.

   While no comparable public credentials exist for telephone numbers,
   either approach could be applied to telephone numbers.  A credential

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   system based on certificates is given in
   [I-D.ietf-stir-certificates].  One based on the domain name system is
   given in [I-D.kaplan-stir-cider].

   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

      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 RFC 4474, 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

   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 transaction (a minute would most
   likely suffice).

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

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   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.

   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.

   In order to work with any such authorities, authentication and
   verification services must be able to 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 header field values in the username portion of
   a SIP URI (e.g., ';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
   ''.  It may be non-trivial for a
   service to ascertain in this case whether the URI contains a
   telephone number or not.

6.1.1.  Canonicalization Procedures

   In order to determine whether or not the user portion of a SIP URI is
   a telephone number, authentication services and verification services
   must perform the following canonicalization procedure on any SIP URI
   they inspect which contains a wholly numeric user part.  Note that
   the same procedures are followed for creating the canonical form of
   URIs found in both the From and To header field values.

      First, implementations must assess if the user-portion of the URI
      constitutes a telephone number.  In some environments, numbers
      will be explicitly labeled by the use of TEL URIs or the
      'user=phone' parameter, or implicitly by the presence of the '+'
      indicator at the start of the user-portion.  Absent these
      indications, if there are numbers present in the user-portion,
      implementations may also detect that the user-portion of the URI
      contains a telephone number by determining whether or not those
      numbers would be dialable or routable in the local environment --
      bearing in mind that the telephone number may be a valid E.164

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      number, a nationally-specific number, or even a private branch
      exchange number.

      Once an implementation has identified a telephone number, it must
      construct a number string.  Implementations MUST drop any leading
      +'s, any internal dashes, parentheses or other non-numeric
      characters, excepting only the leading "#" or "*" keys used in
      some special service numbers (typically, these will appear only in
      the To header field value).  This MUST result in an ASCII string
      limited to "#", "*" and digits without whitespace or visual

      Next, an implementation must assess if the number string is a
      valid, globally-routable number with a leading country code.  If
      not, implementations SHOULD convert the number into E.164 format,
      adding a country code if necessary; this may involve transforming
      the number from a dial string (see [RFC3966]), removing any
      national or international dialing prefixes or performing similar
      procedures.  It is only in the case that an implementation cannot
      determine how to convert the number to a globally-routable format
      that this step may be skipped.

      In some cases, further transformations MAY be made in accordance
      with specific policies used within the local domain.  For example,
      one domain may only use local number formatting and need to
      convert all To/From user portions to E.164 by prepending country-
      code and region code digits; another domain might prefix usernames
      with trunk-routing codes and need to remove the prefix.  Also, in
      some networks, the P-Asserted-Identity header field value is used
      in lieu of the From header field to convey the telephone number of
      the sender of a request; while it is not envisioned that most of
      those networks would or should make use of the Identity mechanism
      described in this specification, where they do, local policy might
      therefore dictate that the canonical string derive from the P-
      Asserted-Identity header field rather than the From.  In any case
      where local policy canonicalizes the number into a form different
      from how it appears in the From header field, the use of the
      "canon" parameter by authentication services is RECOMMENDED, but
      because "canon" itself could then divulge information about users
      or networks, implementers should be mindful of the guidelines in
      Section 9.

      The resulting canonical number string will be used as input to the
      hash calculation during signing and verifying processes.

   The ABNF of this number string is:

             tn-spec = [ "#" / "*" ] 1*DIGIT

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   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.  Practically, entities that perform the authentication
   service role will sometimes alter the telephone numbers that appear
   in the To and From header field values, converting them to this
   format (though note this is not a function that [RFC3261] permits
   proxy servers to perform).  The authentication service MAY also add
   the result of the canonicalization process of the From header field
   value to the "canon" parameter of the Identity-Info header.  If the
   result of the canonicalization of the From header field value 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.

6.2.  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 it 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
   to a subordinate domain and utilize the procedures in RFC 3263
   [RFC3263] that allow requests for, say, '' to be routed to
   ''.  As a result, a user with the AoR
   '' may process requests through a host like
   '', 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 '',
   the corresponding SRV records point to the service
   ''), 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

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   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 [RFC2818], Section 3.1.  While RFC 2818 [RFC2818]
   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

   Baseline RFC4474 defined the Identity and Identity-Info headers.
   This document updates that specification and adds the optional
   Identity-Extension header (the grammar for which appears in
   Section 8).  Identity and Identity-Info are REQUIRED for securing
   requests with this specification, and may appear only once in a
   request, while Identity-Extension can be present multiple times.  The
   grammar for the first two headers is (following the ABNF [RFC4234] in
   RFC 3261 [RFC3261]):

   Identity = "Identity" HCOLON signed-identity-digest
   signed-identity-digest = LDQUOT *base64-char RDQUOT

   Identity-Info = "Identity-Info" HCOLON ident-info
                    *( SEMI ident-info-params )
   ident-info = LAQUOT absoluteURI RAQUOT
   ident-info-params = ident-info-alg / canonical-str / ident-info-extension
   ident-info-alg = "alg" EQUAL token
   canonical-str = "canon" EQUAL tn-spec
   ident-info-extension = generic-param

   base64-char = ALPHA / DIGIT / "/" / "+"

   This follows the original specification of Identity and Identity-Info
   in RFC4474, except for the addition of the "canon" parameter.  Note
   that in RFC4474, the signed-identity-digest was given as quoted
   32LHEX, whereas here it is given as a quoted sequence of base64-char.

   The signed-identity-digest is a signed hash of a canonical string
   generated from certain components of a SIP request.  To create the

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   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 6.1.1).  Otherwise, the first slot contains the AoR of the
      UA sending the message as taken from addr-spec of the From header

      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 (again,
      see Section 6.1.1).  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 [RFC3261].  RFC 3261 specifies that the BNF for
      weekday and month is a choice amongst a set of tokens.  The RFC
      4234 [RFC4234] 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, if the request contains an SDP message body, and if that
      SDP contains one or more "a=fingerprint" attributes, the value(s)
      of the attributes if they differ.  Each attribute value consists
      of all characters following the colon after "a=fingerprint"
      including the algorithm description and hexadecimal key
      representation, any whitespace, carriage returns, and "/" line
      break indicators.  If multiple non-identical "a=fingerprint"
      attributes appear in an SDP body, then all non-identical
      attributes values MUST be concatenated, with no separating
      character, after sorting the values in alphanumeric order.  If the
      SDP body contains no "a=fingerprint" attribute, the fifth element
      MUST be empty, containing no whitespace, resulting in a "||" in
      the signed-identity-digest.

      Sixth, the Identity-Extension header field value(s), if there is
      at least one Identity-Extension header field in the request.  If

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      multiple Identity-Extension header fields are in the request, they
      MUST be concatenated after sorting the header field values in
      alphanumeric order, with each entry separated by a vertical line,
      "|" or %x7C.  If the message contains no Identity-Extension
      header, then the sixth slot MUST be empty, containing no
      whitespace, resulting in a "||" in the signed-identity-digest.

   For more information on the security properties of these headers, and
   why their inclusion mitigates replay attacks, see Section 10 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 "|" [ sdp-fingerprint ] "|"
                                   [ signed-identity-extension-digest ]

   sdp-fingerprint = byte-string

   For the definition of 'tn-spec' see Section 6.1.1.

   After the digest-string is formed, it 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-sha256'; further values MUST be defined in a
   Standards Track RFC, see Section 11.3 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 [RFC3370] and base64
   encode the results as specified in RFC 3548 [RFC3548].  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 '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.

8.  Extensibility

   As future requirements may warrant increasing the scope of the
   Identity mechanism, this specification defines an optional Identity-
   Extension header.  Each extension header field value MUST consist of
   a right hand side identifying the extension, an equals sign, and then

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   a left hand side consisting of a signature over an element in a SIP

   Future specifications that define extensions to the Identity
   mechanism must explicitly designate which elements of a SIP request
   are to be signed, how a canonical string of those elements is
   generated by both the authentication service and the verifier, and
   the mechanism and algorithms used to generate the signature (it is
   RECOMMENDED that these follow the algorithm choice of this
   specification).  Note that per verifier behavior in Section 4.2,
   verifying an extension is always optional.  An authentication service
   cannot assume that verifiers will understand any given extension.
   Verifiers that do support an extension may then trigger appropriate
   application-level behavior in the presence of a signature over
   additional part of the SIP request; authors of Identity extensions
   should provide appropriate extension-specific guidance to application
   developers on this point.

    Identity-Extension = "Identity-Extension" HCOLON identity-extension-string
    identity-extension-string = identity-extension-name EQUAL *base64-char
    identity-extension-name = token
        signed-identity-extension-digest = LDQUOT *base64-char RDQUOT

   Defining a new Identity-Extension requires a Standards Action; see
   Section 11.4.

9.  Privacy Considerations

   The purpose of this mechanism is to provide a strong identification
   of the originator of a SIP request, specifically a cryptographic
   assurance that the URI given in the From header field value can
   legitimately be claimed by the originator.  This URI may contain a
   variety of personally identifying information, including the name of
   a human being, their place of work or service provider, and possibly
   further details.  The intrinsic privacy risks associated with that
   URI are, however, no different from those of baseline SIP.  Per the
   guidance in [RFC6973], implementors should make users aware of the
   privacy trade-off of providing secure identity.

   The identity mechanism presented in this document is compatible with
   the standard SIP practices for privacy described in [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., cannot be signed
   by an authentication service.  The construction of the Identity

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   header is the same for private URIs as it is for any other sort of

   Note, however, that even when 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 for that domain are meaningless.

   [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.

   The optional "canon" parameter of the Identity-Info header specified
   in this document provides a canonicalized form of the telephone
   number of the originator of a call.  In some contexts, local policy
   may be used to populate a "canon" that may differ substantially from
   the original From header field.  Depending on those policies,
   potentially the "canon" parameter might divulge information about the
   originating network or user that might not appear elsewhere in the
   SIP request.  Were it to be used to reflect the contents of the P-
   Asserted-Identity header field, for example, then "canon" would need
   to be removed when the P-Asserted-Identity header is removed to avoid
   any such leakage outside of a trust domain.  Since, in those
   contexts, the canonical form of the sender's identity could not be
   reassembled by a verifier, and thus the Identity signature validation
   process would fail, using P-Asserted-Identity with the Identity
   "canon" parameter in this fashion is NOT RECOMMENDED outside of
   environments where SIP requests will never leave the trust domain.

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

10.  Security Considerations

   This document describes a mechanism that provides a signature over
   the Date header field of SIP requests, parts of the To and From
   header fields, the request method, and when present any media keying
   material in the message body.  In general, the considerations related
   to the security of these headers are the same as those given in
   [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.
   It adddresses the solution-specific attacks again in-band solutions
   enumerated in [RFC7375] Section 4.1.

10.1.  Protected Request Fields

   The From header field value 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.  This is the key piece of
   information that this mechanism secures; the remainder of the signed
   parts of a SIP request are present to provide reference integrity and
   to prevent certain types of cut-and-paste attacks.

   The Date header field value protects against cut-and-paste attacks,
   as described in [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 60 seconds of the receipt of a message).  Note
   that per baseline [RFC3261] behavior, servers keep state of recently
   received requests, and thus if an Identity header is replayed by an
   attacker within the Date interval, verifiers can detect that it is
   spoofed; because a message with an identical Date from the same
   source had recently been received.

   The To header field value provides the identity of the SIP user that
   this request originally targeted.  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 To
   offers additional protection against cut-and-paste attacks beyond the

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   Date header field: for example, without a signature over the To, an
   attacker who receives a call from a target could immediately forward
   the INVITE to the target's voicemail service within the Date
   interval, and the voicemail service would have no way knowing that
   the Identity header it received had been originally signed for a call
   intended for a different number.  However, note the caveats below in
   Section 10.1.1.

   Without the request 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 transaction (for example, a re-INVITE) could be
   replayed in potentially confusing or malicious ways to enable

   When signing a request that contains a fingerprint of keying material
   in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
   signature over that fingerprint.  This signature prevents certain
   classes of impersonation attacks in which an attacker forwards or
   cut-and-pastes a legitimate request: although the target of the
   attack may accept the request, the attacker will be unable to
   exchange media with the target as they will not possess a key
   corresponding to the fingerprint.  For example there are some baiting
   attacks, launched with the REFER method or through social
   engineering, where the attacker receives a request from the target
   and reoriginates it to a third party: these might not be prevented by
   only a signature over the From, To and Date, but could be prevented
   by securing a fingerprint for DTLS-SRTP.  While this is a different
   form of impersonation than is commonly used for robocalling,
   ultimately 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
   subsequent communication.  This signature also, per [RFC7258],
   reduces the potential for passive monitoring attacks against the SIP
   media.  In environments where DTLS-SRTP is unsupported, however, no
   field is signed and no protections are provided.

10.1.1.  Protection of the To Header and Retargeting

   The mechanism in this document provides a signature over the identity
   information in the To header field value of requests.  This provides
   a means for verifiers to detect replay attacks where a signed request
   originally sent to one target is modified and then forwarded by an
   attacker to another, unrelated target.  Armed with the original value
   of the To header field, the recipient of a request may compare it to
   their own identity in order to determine whether or not the identity
   information in this call might have been replayed.  However, any
   request may be legitimately retargeted as well, and as a result

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   legitimate requests may reach a SIP endpoint whose user is not
   identified by the URI designated in the To header field value.  It is
   therefore difficult for any verifier to decide whether or not some
   prior retargeting was "legitimate."  Retargeting can also cause
   confusion when identity information is provided for requests sent in
   the backwards in a dialog, as the dialog identifiers may not match
   credentials held by the ultimate target of the dialog.  For further
   information on the problems of response identity see

   Any means for authentication services or verifiers to anticipate
   retargeting 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 (see [RFC4916]); 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.  And per the threat model of
   [RFC7375], resolving problems with 'connected' identity has little
   bearing on detecting robocalling or related impersonation attacks.

10.2.  Unprotected Request Fields

   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 the simple
   unauthorized claiming of an identity for the purposes of robocalling,
   voicemail hacking, or swatting, 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

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   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.

10.3.  Malicious Removal of Identity Headers

   In the end analysis, the Identity, Identity-Info and Identity-
   Extension 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
   motivation 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.

10.4.  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

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   intermediary-based authentication service.  The reasons for this are

      If a user does not receive a certificate from the authentication
      service over the 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.

      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

   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

   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.

10.5.  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.

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   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
   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

   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

      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.

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10.6.  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, such as an extension.

11.  IANA Considerations

   This document relies on the headers and response codes defined in RFC
   4474.  It also retains the requirements for the specification of new
   algorithms or headers related to the mechanisms described in that

11.1.  Header Field Names

   This document also specifies a new SIP header called Identity-
   Extension.  Its syntax is given in Section 8.  A registry for
   Identity-Extension names is defined in Section 11.4.

      Header Name: Identity-Extension
      Compact Form: N/A

11.2.  Identity-Info Parameters

   The IANA has already created a registry for Identity-Info header
   parameters.  This specification defines a new value called "canon" as
   defined in Section 5.3.

11.3.  Identity-Info Algorithm Parameter Values

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

   RFC4474 defined the 'rsa-sha1' value for this registry.  That value
   is hereby deprecated, and should be treated as such.  It is not
   believed that any implementations are making use of this value.

   Future specifications may consider elliptical curves for smaller key

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11.4.  Identity-Extension Names

   This specification requests that the IANA create a new registry for
   Identity-Extension names.  The registry will consist solely of a list
   of names mapped to the Standards Track RFCs in which those extensions
   are defined.

   The syntax of Identity-Extension names is given in Section 8.
   Registering a new Identity-Extension name requires a Standards

   This specification does not provide any initial values for Identity-
   Extension names.

12.  Acknowledgments

   The authors would like to thank Stephen Kent, Brian Rosen, Alex
   Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
   Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
   Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer,
   Henning Schulzrinne, and Richard Barnes for their comments.

13.  Changes from RFC4474

   The following are salient changes from the original RFC 4474:

      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 any DTLS-SRTP fingerprint in SDP as a mandatory element of
      the digest-string

      Added the Identity-Extension header and extensibility mechanism

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

14.  References

14.1.  Normative References

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, 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,
              DOI 10.17487/RFC3280, April 2002,

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,

   [RFC5280]  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, DOI 10.17487/RFC5280, May 2008,

14.2.  Informative References

              Peterson, J., "Secure Telephone Identity Credentials:
              Certificates", draft-ietf-stir-certificates-02 (work in
              progress), July 2015.

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

              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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              Rosenberg, J., "Concerns around the Applicability of RFC
              4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
              progress), February 2008.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 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,
              DOI 10.17487/RFC3325, November 2002,

   [RFC3548]  Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, DOI 10.17487/RFC3548, July 2003,

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              DOI 10.17487/RFC3893, September 2004,

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
              October 2005, <>.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,

   [RFC4501]  Josefsson, S., "Domain Name System Uniform Resource
              Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <>.

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   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,

Authors' Addresses

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


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


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

   Phone: +1 650 678 2350

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