Authenticated Identity Management in the Session Initiation Protocol (SIP)
draft-ietf-stir-rfc4474bis-10

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Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                             C. Jennings
Expires: January 8, 2017                                           Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                                C. Wendt
                                                                 Comcast
                                                            July 7, 2016

  Authenticated Identity Management in the Session Initiation Protocol
                                 (SIP)
                   draft-ietf-stir-rfc4474bis-10.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 a
   SIP header field 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.

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   This Internet-Draft will expire on January 8, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   6
   5.  Signature Generation and Validation . . . . . . . . . . . . .   7
     5.1.  Authentication Service Behavior . . . . . . . . . . . . .   7
     5.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .  10
       5.2.1.  Handling 'canon' parameters . . . . . . . . . . . . .  12
   6.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Credential Use by the Authentication Service  . . . . . .  13
     6.2.  Credential Use by the Verification Service  . . . . . . .  14
     6.3.  Handling 'info' parameter URIs  . . . . . . . . . . . . .  15
     6.4.  Credential System Requirements  . . . . . . . . . . . . .  15
   7.  Identity Types  . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Authority for Telephone Numbers . . . . . . . . . . . . .  18
     7.2.  Telephone Number Canonicalization Procedures  . . . . . .  18
     7.3.  Authority for Domain Names  . . . . . . . . . . . . . . .  19
     7.4.  URI Normalization . . . . . . . . . . . . . . . . . . . .  20
   8.  Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  24
   10. Backwards Compatibililty with RFC4474 . . . . . . . . . . . .  25
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  25
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
     12.1.  Protected Request Fields . . . . . . . . . . . . . . . .  27
       12.1.1.  Protection of the To Header and Retargeting  . . . .  29
     12.2.  Unprotected Request Fields . . . . . . . . . . . . . . .  30
     12.3.  Malicious Removal of Identity Headers  . . . . . . . . .  30
     12.4.  Securing the Connection to the Authentication Service  .  31
     12.5.  Authorization and Transitional Strategies  . . . . . . .  32
     12.6.  Display-Names and Identity . . . . . . . . . . . . . . .  33
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
     13.1.  Identity-Info Parameters . . . . . . . . . . . . . . . .  33
     13.2.  Identity-Info Algorithm Parameter Values . . . . . . . .  34
     13.3.  Response Codes defined in RFC4474  . . . . . . . . . . .  34
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  35
   15. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  35

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   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     16.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

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
   'sip:alice@atlanta.example.com'), or a telephone number, which can be
   represented as either a TEL URI [RFC3966] or as the user portion of a
   SIP URI.

   [RFC3261] specifies several places within a SIP request where users
   can express an identity for themselves, most prominently 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].  Ideally, a
   cryptographic approach to identity can provide a much stronger and
   less spoofable assurance of identity than the Caller ID services that
   the telephone network provides today.

   [RFC3261] encourages user agents (UAs) to implement a number of
   potential authentication mechanisms, including Digest authentication,
   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 for its part
   Digest authentication is limited by the fact that the originator and
   destination must share a prearranged secret.  Practically speaking,
   originating user agents need to be able to securely communicate their
   users' identity to destinations with which they have no previous
   association.

   As an initial attempt to address this gap, [RFC4474] specified a
   means of signing portions of SIP requests in order to provide an
   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
   existing deployments commonly altered.  This specification therefore

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   revises RFC 4474 in light of recent reconsideration of the problem
   space to align with the threat model in [RFC7375], and aligns the
   signature format with PASSporT [I-D.ietf-stir-passport].

2.  Terminology

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

3.  Background

   Per [RFC7340], problems such as robocalling, voicemail hacking, and
   swatting are enabled by an attacker's ability to impersonate someone
   else.  The 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 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.  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.  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 in many environments.  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 validates outgoing requests.  An
   authentication service may be implemented either as part of a user
   agent or as a proxy server; typically, it is a component of a network
   intermediary like a proxy to which originating user agents send
   unsigned requests.  Once the sender of the message has been
   authenticated, the authentication service then computes and adds
   cryptographic information (including a digital signature over some
   components of messages) 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.

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   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 can 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 for authorizing the use of names
   in the example.com 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
   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 constructing tokens based on
   the PASSporT [I-D.ietf-stir-passport] format, a JSON [RFC7159] object
   comprising values copied from certain header field values in the SIP
   request.  The authentication service then computes a signature over
   those JSON object in a manner following PASSporT.  That signature is
   then placed in a SIP Identity header.  In order to assist in the
   validation of the Identity header, this specification also describes
   some metadata fields associated with the 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]).  Future
   work might specify ways that a SIP implementation could gateway
   PASSporT objects to other protocols.

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   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.

4.  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 example.com and
   the address-of-record sip:alice@example.com, wants to communicate
   with Bob at sip:bob@example.org.  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
   example.com domain.

   The proxy 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 could be Alice's AoR, but in other cases it could be some
   different value that the authentication service has authority over,
   such as a telephone number.  The proxy authentication service then
   constructs a PASSporT object which contains a JSON representations of
   headers and claims which mirror certain parts of the SIP request,
   including the identity in the From header field.  As a part of
   generating the PASSporT object, the authentication service signs a
   hash of those headers and claims with the appropriate credential for
   the identity (in this case, the certificate for example.com, which
   covers the identity sip:alice@example.com), and the signature is
   inserted by the proxy server into the Identity header field value of
   the request.  Optionally, the JSON headers and claims themselves may
   also be included in the object, encoded in the "canon" parameter of
   the Identity header.

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   The proxy, as the holder of the private key for the example.com
   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 inserts an "info"
   parameter into the Identity header 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.

5.  Signature Generation and Validation

5.1.  Authentication Service Behavior

   This document specifies a role for SIP entities called an
   authentication service.  The authentication service role can be
   instantiated, for example, 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 6.1).  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.

   An authentication service adds the Identity header to SIP requests.
   The procedures below define the steps that must be taken when each an
   header is added.  More than one may appear in a single request, and
   an authentication service may add an Identity header to a request
   that already contains one or more Identity headers.  If the Identity
   header added follows extended signing procedures beyond the baseline
   given in Section 8, then it differentiates the header with a "ppt"
   parameter per the fourth step below.

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

   Step 1:

   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 7.1 for more information.  An
   authentication service proceeding with a signature over a telephone
   number MUST then follow the canonicalization procedures described in
   Section 7.2.  If the authentication service is not authoritative for
   the identity in question, it SHOULD process and forward the request
   normally unless the local policy is to block such requests.  The
   authentication service MUST NOT add an Identity header if the
   authentication service does not have the authority to make the claim
   it asserts.

   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.

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

   Note that this check is performed only on the addr-spec in the
   identity 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').  For
   more information, see Section 12.6.

   Step 3:

   An authentication service MUST add a Date header field to SIP
   requests that do not have one.  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 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 both the Date header and the current time fall
   within the validity period of its credential.

   See Section 12 for information on how the Date header field assists
   verifiers.

   Step 4:

   Subsequently, the authentication service MUST form a PASSporT object
   and add a corresponding an Identity header to the request containing
   this signature.  For baseline PASSporT objects headers (without an
   Identity header "ppt" parameter), this follows the procedures in
   Section 8; if the authentication service is using an alternative
   "ppt" format, it MUST add an appropriate "ppt" parameter and follow
   the procedures associated with that extension (see Section 9).  After
   the Identity header has been added to the request, the authentication
   service MUST also add a "info" parameter to the Identity header.  The
   "info" parameter contains a URI from which the authentication
   service's credential can be acquired; see Section 6.3 for more on
   credential acquisition.

   Step 5:

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   In the circumstances described below, an authentication service will
   add a "canon" parameter to the Identity header.  The syntax of
   "canon" is given in Section 8; essentially, it contains a base64
   encoding of the JSON header and claims in the PASSporT object.  The
   presence of "canon" is OPTIONAL baseline PASSporT objects in SIP as a
   because the information carried in the baseline PASSporT object's
   headers and claims is usually redundant with information already
   carried elsewhere in the SIP request.  Omitting "canon" can
   significantly reduce SIP message size, especially when the PASSporT
   object contains media keys.

   When however an authentication service creates a PASSporT that uses
   extension claims beyond the baseline PASSporT object, including
   "canon" is REQUIRED in order for the verification service to be
   capable of validating the signature.  See Section 9.

   Also, in some cases, a request signed by an authentication service
   will be rejected by the verification service on the receiving side,
   and the authentication service will receive a SIP 4xx status code in
   the backwards direction, such as a 438 indicating a verification
   failure.  If the authentication service did not originally send the
   Identity header with the "canon" parameter, it SHOULD retry a request
   once after receiving a 438 response, this time including the "canon".
   The information in "canon" is useful on the verification side for
   debugging errors, and there are some known causes of verification
   failures (such as the Date header changing in transit, see
   Section 12.1 for more information) that can be resolved by the
   inclusion of "canon".

   Finally, the authentication service MUST forward the message
   normally.

5.2.  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 one or more Identity headers, it inspects the
   signature(s) to verify the identity of the sender of the message.
   The results of a verification are provided as input to an
   authorization process that is outside the scope of this document.

   A SIP request may contain zero, one, or more Identity headers.  A
   verification service performs the procedures above on each Identity
   header that appears in a request.  If the verifier does not support
   an Identity header present in a request due to the presence of an
   unsupported "ppt" parameter, or if no Identity header is present, and
   the presence of an Identity header is required by local policy (for
   example, based on a per-sending-domain policy, or a per-sending-user

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   policy), then a 428 'Use Identity Header' response MUST be sent in
   the backwards direction.  For more on this and other failure
   responses, see Section 13.3.

   In order to verify an Identity header in a message, an entity acting
   as a verifier MUST perform the following steps, in the order here
   specified.  Note that when an Identity header contains the optional
   "canon" parameter, the verifier MUST follow the additional procedures
   in Section 5.2.1.

   Step 1:

   The verifier MUST inspect any optional "ppt" parameter appearing the
   Identity request.  If no "ppt" parameter is present, then the
   verifier proceeds normally below.  If a "ppt" parameter value is
   present, and the verifier does not support it, it MUST ignore the
   Identity header.  If a supported "ppt" parameter value is present,
   the verifier follows the procedures below, including the variations
   described in Step 5.

   Step 2:

   In order to determine whether the signature for the identity field
   should be over the entire identity field URI or just a canonicalized
   telephone number, the verification service MUST follow the
   canonicalization process described in Section 7.2.  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 7.3 to determine if the signer is authoritative for the
   identity field.

   Step 3:

   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 a URI in the "info" parameter of
   the Identity header.  See Section 6.2 for more information on these
   procedures.  If the verifier does not support the credential
   described in the "info" parameter, then it should consider the
   credential for this header unsupported.  If a SIP request contains no
   Identity headers with a supported credential, then the verifier MUST
   return a 437 "Unsupported Credential" response.

   Step 4:

   The verifier MUST furthermore ensure that the value of the Date
   header of the request meets local policy for freshness (usually,

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   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 12.1.  If the "canon" parameter is
   present, the verifier should follow the Date-related behavior in
   Section 5.2.1.

   Step 5:

   The verifier MUST validate the signature in the Identity header field
   over the PASSporT object.  For baseline PASSporT objects (with no
   Identity header "ppt" parameter) the verifier MUST follow the
   procedures for generating the signature over a PASSporT object
   described in Section 8.  If a "ppt" parameter is present (and per
   Step 1, is understood), the verifier follows the procedures for that
   "ppt" (see Section 9).  If a verifier determines that the that the
   signature in the Identity does not correspond to the reconstructed
   signed-identity-digest, then the Identity header should be considered
   invalid.

   The presence of multiple Identity headers within a message raises the
   prospect that a verification services could receive a message
   containing some valid and some invalid Identity headers.  If the
   verifier determines all Identity headers within a message are
   invalid, then a 438 'Invalid Identity Header' response MUST be
   returned.

   The verification of an Identity header does not entail any particular
   treatment of the request.  The handling of the message after the
   verification process depends on how the implementation service is
   implemented and on local policy.  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.

5.2.1.  Handling 'canon' parameters

   If the optional "canon" parameter of the Identity header is present,
   it contains a base64 encoding of the header and claim component of
   the PASSporT object constructed by the authentication service, and
   this it conveys any canonical telephone number formats created by the
   authentication service (see Section 7.2), as well as an "iat" claim
   corresponding to the Date header that the authentication service
   used.  The "canon" is provided purely as an optimization and
   debugging mechanism for the verification service.

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   When "canon" is present, the verification service MAY compute its own
   canonicalization of the numbers and compare them to the values 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.  Also, when "canon" is present, during Step 4 the
   verification service SHOULD compare the "iat" value in the "canon" to
   its Date header field value.  If the two are different, and the "iat"
   value is later but within verification service policy for freshness,
   the verification service SHOULD perform the computation required by
   Step 5 using the "iat" value instead of the Date value.  As some
   deployments in the field have been observed to change the Date header
   in transit, this procedure will prevent some unnecessary verification
   failures.

6.  Credentials

6.1.  Credential Use by the Authentication Service

   In order to act as an authentication service, a SIP entity must have
   access to the private keying material of one or more credentials that
   cover domain names or telephone numbers.  These credentials may
   represent authority over an entire domain (such as example.com) 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
   mechanism.

   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

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   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 alice@example.com or
   sip:alice@example.com.  This would require a modification to
   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 implementers 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 'sip:anonymous@example.com' (see [RFC3323]), then the
   'example.com' 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.

6.2.  Credential Use by the Verification Service

   In order to act as a verification service, a SIP entity must have a
   way to acquire and retain credentials for authorities over particular
   domain names and/or telephone numbers or number ranges.
   Dereferencing the URI found in the "info" parameter of the Identity
   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 "info" parameter,
   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 "info" parameter
   cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST
   be returned.

   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 6.4.

   Verification service implementations supporting this specification
   may wish to 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

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   local policy: for example, by their scope, or the URI given in the
   "info" parameter value.  Further consideration of how to cache
   credentials is deferred to the credential mechanism specifications.

6.3.  Handling 'info' parameter URIs

   An "info" parameter 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 "info parameter" 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 "info" parameter 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 "info"
   parameter performs that function.

6.4.  Credential System Requirements

   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 "info" parameter could for example give
   an HTTP URL of the Content-Type 'application/pkix-cert' pointing to a
   certificate (following the conventions of [RFC2585]).  The "info"
   parameter 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
   system based on certificates is given in
   [I-D.ietf-stir-certificates], but this specification can work with
   other credential systems; for example, using the DNS was proposed in
   [I-D.kaplan-stir-cider].

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

      which URIs schemes the credential will use in the "info"
      parameter, and any special procedures required to dereference the
      URIs

      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 required to validate the credentials (e.g. for
      certificates, any algorithms used by certificate authorities to
      sign certificates themselves)

   It is furthermore required that all credential specifications
   describe how the associated credentials will support the mandatory
   signing algorithm(s) required by PASSporT [I-D.ietf-stir-passport].

   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 transaction (a minute would most
   likely suffice).

7.  Identity Types

   This specification focuses primarily on cases where the called and
   calling parties identified in the To and From header field values use
   telephone numbers, as this remains the dominant use case in the
   deployment of SIP.  However, this specification also works with
   "greenfield" identifiers (of the form "sip:user@host"), and
   potentially other identifiers when SIP interworks with another
   protocol.

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   The guidance in this section also applies to extracting the URI
   containing the originator's identity from the P-Asserted-Identity
   header field value instead of the From header field value.  In some
   environments, the P-Asserted-Identity header field is used in lieu of
   the From header field to convey the address-of-record or telephone
   number of the sender of a request; while it is not envisioned that
   many 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 identity derive
   from the P-Asserted-Identity header field rather than the From.

   Ultimately, in any case where local policy canonicalizes the idenity
   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 11.

   It may not be trivial to tell if a given URI contains a telephone
   number.  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 procedure on any SIP
   URI they inspect which contains a numeric user part.  Note that the
   same procedures are followed for creating the canonical form of URIs
   found in the From header field as they are in the To header field or
   the P-Asserted-Identity header field.

   First, implementations must look for obvious indications that the
   user-portion of the URI constitutes a telephone number.  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
   [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').  Thus, 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 number, a nationally-specific
   number, or even a private branch exchange number.  Once a telephone
   number has been detected, implementations should follow the
   procedures in Section 7.2.

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   If the URI field does not contain a telephone number, URI
   normalization procedures are invoked to canonicalize the URI before
   it is included in a PASSporT object in, for example, an "uri" claim.
   See Section 7.4 for that behavior.

7.1.  Authority for Telephone Numbers

   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 authoritative for telephone
   numbers or telephone number ranges, and verification services must
   trust the authority employed by the authentication service that signs
   a request.  Per Section 6.4, enrollment procedures and credential
   management are outside the scope of this document; approaches to
   credential management for telephone numbers are discussed in
   [I-D.ietf-stir-certificates].

7.2.  Telephone Number Canonicalization Procedures

   Once an implementation has identified a telephone number in the URI,
   it must construct a number string.  That requires performing the
   following steps:

      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 separators.

      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.  This will be the case, for
      example, for nationally-specific service numbers (e.g. 911, 112);
      however, the routing procedures associated with those numbers will
      likely make sure that the verification service understands the
      context of their use.

      Other transformations during canonicalization MAY be made in
      accordance with specific policies used within a 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

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      country-code and region code digits; another domain might prefix
      usernames with trunk-routing codes and need to remove the prefix.
      This specification cannot anticipate all of the potential
      transformations that might be useful.

      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

   If the result of this procedure forms a complete telephone number,
   that number is used for the purpose of creating and signing the
   signed-identity-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 result of the
   canonicalization process of the From header field value may also be
   recorded through the use of the "canon" parameter of the Identity(see
   Section 8).

   If the result of the canonicalization of the From header field value
   does not form a complete and valid telephone number, the
   authentication service and/or verification service SHOULD treat the
   entire URI as a SIP URI, and apply the procedures in Section 7.4.

7.3.  Authority for 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 "info"
   parameter.  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, 'example.com' to be routed to
   'sip.example.com'.  As a result, a user with the AoR
   'sip:jon@example.com' may process requests through a host like
   'sip.example.com', and it may be that latter host that acts as an
   authentication service.

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   To meet the second of these problems, a domain that deploys an
   authentication service on a subordinate host MUST be willing to
   supply that host with the private keying material associated with a
   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 [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.

7.4.  URI Normalization

   Just as telephone numbers may undergo a number of syntactic
   transformation during transit, the same can happen to SIP and SIPS
   URIs without telephone numbers as they traverse certain
   intermediaries.  Therefore, when generating a PASSporT object based
   on a SIP request, any SIP and SIPS URIs must be transformed into a
   canonical form which captures the address-of-record represented by
   the URI before they are provisioned in PASSporT claims such as "uri".
   The URI normalization procedures required are as follows.

   Following the ABNF of RFC3261, the SIP or SIPS URI in question MUST
   discard all elements after the "hostport" of the URI, including all
   uri-parameters and headers, from its ayntax.  Of the userinfo
   component of the SIP URI, only the user element will be retained: any
   password (and any leading ":" before the password) MUST be removed,
   and since this userinfo necessarily does not contain a telephone-

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   subscriber component, no further parameters can appear in the user
   portion.

   The hostport portion of the SIP or SIPS URI MUST similarly be
   stripped of any trailing port along with the ":" that proceeds the
   port, leaving only the host.

   The ABNF of this canonical URI form (following the syntax defined in
   RFC3261) is:

             canon-uri =  ( "sip" / "sips" ) ":" user "@" host

   Finally, the URI will be subject to syntax-based URI normalization
   procedures of [RFC3986] Section 6.2.2, especially to perform case
   normalization and percent-encoding normalization.  However, note that
   normalization procedures face known challenges in some
   internationalized environments (see [I-D.ietf-iri-comparison]) and
   that perfect normalization of URIs may not be possible in those
   environments.

   For future PASSporT applications, it may be desirable to provide an
   identifier without an attached protocol scheme.  Future
   specifications that define PASSporT claims for SIP as a using
   protocol could use these basic procedures, but eliminate the scheme
   component.  A more exact definition is left to future specifications.

8.  Header Syntax

   The Identity and Identity-Info headers that were previously defined
   in RFC4474 are deprecated.  This revised specification collapses the
   grammar of Identity-Info into the Identity header via the "info"
   parameter.  Note that unlike the prior specification in RFC4474, the
   Identity header is now allowed to appear more than one time in a SIP
   request.  The revised grammar for the Identity header is (following
   the ABNF [RFC4234] in RFC 3261 [RFC3261]):

   Identity = "Identity" HCOLON signed-identity-digest SEMI ident-info *( SEMI ident-info-params )
   signed-identity-digest = LDQUOT *base64-char RDQUOT
   ident-info = "info" EQUAL ident-info-uri
   ident-info-uri = LAQUOT absoluteURI RAQUOT
   ident-info-params = ident-info-alg / ident-type / canonical-str / ident-info-extension
   ident-info-alg = "alg" EQUAL token
   ident-type = "ppt" EQUAL token
   canonical-str = "canon" EQUAL *base64-char
   ident-info-extension = generic-param

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

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   In addition to "info" parameter, and the "alg" parameter previously
   defined in RFC4474, this specification includes the optional "canon"
   and "ppt" parameters.  Note that in RFC4474, the signed-identity-
   digest (see ABNF above) was given as quoted 32LHEX, whereas here it
   is given as a quoted sequence of base64-char.

   The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see
   Section 6.3 for more on choosing how to advertise credentials through
   this parameter.

   The signed-identity-digest is the signed hash component of a PASSporT
   object [I-D.ietf-stir-passport], a signature which PASSporT generates
   over a pair of JSON objects.  The first PASSporT object contains
   header information, and the second contains claims, following the
   conventions of JWT [RFC7519]; some header and claim values will
   mirror elements of the SIP request.  Once these two JSON objects have
   been generated, they will be encoded, then hashed with a SHA-256
   hash.  Those two hashes are then concatenated (header then claims)
   into a string separated by a single "." per baseline PASSporT.
   Finally, that string is signed to generate the signed-identity-digest
   value of the Identity header.

   For SIP implementations to populate the PASSporT header object from a
   SIP request, the following elements message MUST be placed as the
   values corresponding to the designated JSON keys:

      First, per baseline [I-D.ietf-stir-passport], the JSON key "typ"
      key MUST have the value "passport".

      Second, the JSON key "alg" MUST mirror the value of the optional
      "alg" parameter in the SIP Identity header.  Note if the "alg"
      parameter is absent, the default value is "ES256".

      Third, the JSON key "x5u" MUST have a value equivalent to the
      quoted URI in the "info" parameter.

      Fourth, the optional JSON key "ppt", if present, MUST have a value
      equivalent to the quoted value of the "ppt" parameter of the
      Identity header.  If the "ppt" parameter is absent from the
      header, the "ppt" key MUST NOT not appear in the JSON heaer
      object.

   For example:

   { "typ":"passport",
     "alg":"ES256",
     "x5u":"https://www.example.com/cert.pkx" }

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   To populate the PASSporT claims JSON object from a SIP request, the
   following elements MUST be placed as values corresponding to the
   designated JSON keys:

      First, the JSON "orig" array MUST be populated.  If the
      originating identity is a telephone number, then the array MUST be
      populated with a "tn" claim with a value set to the value of the
      quoted originating identity, a canonicalized telephone number (see
      Section 7.2).  Otherwise, the array MUST be populated with a "uri"
      claim, set to the value of the AoR of the UA sending the message
      as taken from addr-spec of the From header field, per the
      procedures in Section 7.4.

      Second, the JSON "dest" array MUST be populated.  If the
      destination identity is a telephone number, then the array MUST be
      populated with a "tn" claim with a value set to the value of the
      quoted destination identity, a canonicalized telephone number (see
      Section 7.2).  Otherwise, the array MUST be populated with a "uri"
      claim, set to the value of the addr-spec component of the To
      header field, which is the AoR to which the request is being sent,
      per the procedures in Section 7.4.

      Third, the JSON key "iat" MUST appear, set to the value of a
      quoted encoding of the value of the SIP Date header field as a
      JSON NumericDate (as UNIX time, per [RFC7519] Section 2).

      Fourth, if the request contains an SDP message body, and if that
      SDP contains one or more "a=fingerprint" attributes, then the JSON
      key "mky" MUST appear with the algorithm(s) and value(s) of the
      fingerprint attributes (if they differ), following the format
      given in [I-D.ietf-stir-passport] Section 3.2.2.2.

   For example:

      { "orig":{"tn":"12155551212"},
        "dest":{"tn":"12155551213"},
        "iat":"1443208345" }

   For more information on the security properties of these SIP message
   elements, and why their inclusion mitigates replay attacks, see
   Section 12 and [RFC3893].  Note that future extensions to the
   PASSporT object could introduce new claims, and that further SIP
   procedures could be required to extract further information from the
   SIP request to populate the values of those claims; see Section 9.

   The "orig" and "dest" arrays may contain identifiers of heterogeneous
   type; for example, the "orig" array might contain a "tn" claim, while
   the "dest" contains a "uri" claim.  Also note that in some cases, the

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   "orig" and "dest" arrays might be populated with more than one value.
   This could for example occur when multiple "dest" identities are
   specified in a meshed conference.  Defining how a SIP implementation
   would provision multiple originating or destination identities is
   left as a subject for future specification.

   After these two JSON objects, the header and the claims, have been
   constructed, they must each be hashed per [I-D.ietf-stir-passport]
   Section 3.3.  The signed value of those concatenated hashes then
   becomes the signed-identity-string of the Identity header.  The
   hashing and signing algorithm is specified by the 'alg' parameter of
   the Identity header and the mirrored "alg" parameter of PASSporT.
   This specification inherits from the PASSporT specification one value
   for the 'alg' parameter: 'ES256', as defined in [RFC7519], which
   connotes an ECDSA P-256 digital signature.  All implementations of
   this specification MUST support the required signing algorithms of
   PASSporT.

   The complete form of the Identity header will therefore look like the
   following example:

  Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
      eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
      pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \
          info=<https://biloxi.example.org/biloxi.cer>;alg=ES256

   In a departure from JWT practice, the SIP usage of PASSporT MAY NOT
   include the base64 encoded version of the JSON objects in the
   Identity header: only the signature component of the PASSporT is
   REQUIRED.  Optionally, as a debugging measure or optimization, the
   base64 encoded concatenation of the JSON header and claims may be
   included as the value of a "canon" parameter of the Identity header.
   Note that this may be lengthy string.

9.  Extensibility

   For the extensibility of baseline PASSporT with now claims, see
   [I-D.ietf-stir-passport] Section 4.

   As future requirements may warrant increasing the scope of the
   Identity mechanism, this specification defines an optional "ppt"
   parameter of the Identity header, which mirrors the "ppt" header key
   in PASSporT.  The "ppt" parameter value MUST consist of a token
   containing an extension specification, which denotes an extended set
   of one or more signed claims per the type extensibility mechanism
   specified in [I-D.ietf-stir-passport].

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   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 an extension; authors of extensions should provide
   appropriate extension-specific guidance to application developers on
   this point.

   If any claim in an extension contains a JSON value that does not
   correspond to any field of the SIP request, but then the optional
   "canon" parameter MUST be used for the Identity header containing
   that extension.

10.  Backwards Compatibililty with RFC4474

   This specification introduces several significant changes from the
   RFC4474 version of the Identity header.  However, due to the problems
   enumerated in [I-D.rosenberg-sip-rfc4474-concerns], it is not
   believed that the original Identity header has seen any deployment,
   or even implementation in deployed products.

   As such, this mechanism contains no provisions for signatures
   generated with this specification to work with RFC4474-compliant
   implementations, nor any related backwards-compatibility provisions.
   Hypothetically, were an RFC4474-compliant implementation to receive
   messages containing this revised version of the Identity header, it
   would likely fail the request due to the absence of an Identity-Info
   header with a 436 response code.  Implementations of this
   specification, for debugging purposes, might interpret a 436 with a
   reason phrase of "Bad Identity-Info" as an indication that the
   request has failed because it reached a (hypothetical)
   RFC4474-compliant verification service.

11.  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 an authority asserts the originator can claim the URI
   given in the From header field.  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], implementers 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

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   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.  Similar practices could be used to support opportunistic
   signing of SIP requests for UA-integrated authentications services
   with self-signed certificates, though that is outside the scope of
   this specification and is left as a matter for future investigation.

   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".

   It is worth noting two features of this more anonymous form of
   identity.  One can eliminate any identifying information in a domain
   through the use of the domain 'anonymous.invalid," but we must then
   acknowledge that it is difficult for a domain to be both anonymous
   and authenticated.  The use of the "anonymous.invalid" domain entails
   that no corresponding authority for the domain can exist, and as a
   consequence, authentication service functions for that domain are
   meaningless.  The second feature is more germane to the threats this
   document mitigates [RFC7375].  None of the relevant attacks, all of
   which rely on the attacker taking on the identity of a victim or
   hiding their identity using someone else's identity, are enabled by
   an anonymous identity.  As such, the inability to assert an authority
   over an anonymous domain is irrelevant to our threat model.

   [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.
   Presumably, that 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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   The optional "canon" parameter of the Identity header specified in
   this document provides the complete JSON objects used to generate the
   signed-identity-digest of the Identity header, including the
   canonicalized form of the telephone number of the originator of a
   call, if the signature is over a telephone number.  In some contexts,
   local policy may require a canonicalization which differs
   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.  As a side note, history shows that closed
   networks never stay closed and one should design their implementation
   assuming connectivity to the broader Internet.

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

12.  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, 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 of RFC3261 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 addresses the solution-specific attacks against
   in-band solutions enumerated in [RFC7375] Section 4.1.

12.1.  Protected Request Fields

   The From header field value (in ordinary operations) indicates the
   identity of the sender of the message.  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.  Note that
   in some deployments the identity of the sender may reside in P-

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   Asserted-Id instead.  The sender's identity 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.

   It has been observed in the wild that some networks change the Date
   header field value of SIP requests in transit, and that alternative
   behavior might be necessary to accommodate that use case.
   Verification services that observe a signature validation failure MAY
   therefore reconstruct the Date header field component of the
   signature from the "iat" carried in PASSporT via the "canon"
   parameter: provided that time recorded by "iat" falls within the
   local policy for freshness that would ordinarily apply to the Date
   header, the verification service MAY treat the signature as valid,
   provided it keeps adequate state to detect recent replays.  Note that
   this will require the inclusion of the "canon" parameter by
   authentication services in networks where such failures are observed.

   The To header field value provides the identity of the SIP user that
   this request originally targeted.  Covering the identity in the To
   header field with 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 identity offers additional protection against cut-and-
   paste attacks beyond the Date header field.  For example, without a
   signature over the To identity, 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 12.1.1.

   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

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   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 media 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.

12.1.1.  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
   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 direction 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
   [I-D.peterson-sipping-retarget].

   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

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   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.

12.2.  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
   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.

12.3.  Malicious Removal of Identity Headers

   In the end analysis, the Identity header cannot protect itself.  Any
   attacker could remove the header 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

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   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.

12.4.  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
   intermediary-based authentication service.  The reasons for this are
   twofold:

      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
      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 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

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   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.

12.5.  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.
   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
   practices.

   One cannot expect the Identity header 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.

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   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.

12.6.  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 through the
   "ppt" name mechanism.

   In the absence of signing the display-name, authentication services
   might check and validate it, 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.

13.  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
   document.

13.1.  Identity-Info Parameters

   The IANA has already created a registry for Identity-Info parameters.
   This specification defines a new value called "canon" as defined in
   Section 6.3.  Note however that unlike in RFC4474, Identity-Info
   parameters now appear in the Identity header.

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13.2.  Identity-Info Algorithm Parameter Values

   The IANA has already created a registry for Identity-Info "alg"
   parameter values.  Note that now, the "alg" parameter appears in the
   Identity header rather than the deprecated Identity-Info header.
   Since the algorithms for signing PASSporT objects are defined in
   PASSporT rather than in this specification, there is no longer a need
   for an algorithm parameter registry for the Identity header.  This
   registry is therefore deprecated.

13.3.  Response Codes defined in RFC4474

   RFC4474 defined four response codes for failure conditions specific
   to the Identity header and its original mechanism.  These status
   codes are retained in this specification, with some modifications.

   The semantics of the 428 'Use Identity Header' response code are
   slightly altered by the potential presence of the "ppt" parameter.
   Now, a 428 response MUST be sent when an Identity header is required,
   but no Identity header without a "ppt" parameter, or with a supported
   "ppt" value, has been received.  In the case where one or more
   Identity headers with unsupported "ppt" values have been received,
   then a verification service SHOULD send a 428 with the reason phrase
   "Use Supported PASSporT Format".  Note however that this
   specification gives no guidance on how a verification service might
   decide to require an Identity header for a particular SIP request.
   Such authorization policies are outside the scope of this
   specification.

   For 436 'Bad Identity-Info' response, the default reason phrase is
   now renamed 'Bad Identity info', as the deprecation of the Identity-
   Info header has made 'info' a parameter of the Identity header.
   Again, given the potential presence of multiple Identity headers,
   this response code is sent when the verification service is unable to
   deference the URIs and/or acquire the credentials associated with all
   Identity headers in the request.  This failure code could be
   repairable if the authentication service resends the request with an
   'info' parameter pointing to a credential that the verification
   service can access.

   The 437 'Unsupported Certificate' default reason phrase is now
   changed to 'Unsupported Credential'.  This response is sent when a
   verification service can acquire, or already holds, the credential
   represented by the 'info' parameter of at least one Identity header
   in the request, but does not support said credential(s), for reasons
   such as failing to trust the issuing CA, or failing to support the
   algorithm with which the credential was signed.

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   Finally, the 438 'Invalid Identity Header' response now indicates
   that of the set of Identity headers in a request, no header with a
   valid and supported PASSporT object has been received.  Like the 428
   response, this is sent by a verification service when its local
   policy dictates that a broken signature in an Identity header is
   grounds for rejecting a request.  Note that in some cases, an
   Identity header may be broken for other reasons than that an
   originator is attempting to spoof an identity: for example, when a
   transit network alters the Date header of the request.  Relying on
   the full PASSporT object presented through the "canon" parameter can
   repair some of these conditions (see Section 5.2.1), so the
   recommended way to attempt to repair this failure is to retry the
   request with "canon".

14.  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, Eric Burger, Alan Ford, Philippe
   Fouquart, Michael Hamer, Henning Schulzrinne, and Richard Barnes for
   their comments.

15.  Changes from RFC4474

   The following are salient changes from the original RFC 4474:

      Generalized the credential mechanism; credential enrollment,
      acquisition and trust 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

      Removed the Identity-Info header and relocated its components into
      parameters of the Identity header

      The Identity header can now appear multiple times in one request

      Replaced previous signed-identity-digest format with PASSporT
      (signing algorithms now defined there)

      Revised status code descriptions

16.  References

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16.1.  Normative References

   [I-D.ietf-stir-passport]
              Wendt, C. and J. Peterson, "Persona Assertion Token",
              draft-ietf-stir-passport-03 (work in progress), June 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [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,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, June 2002,
              <http://www.rfc-editor.org/info/rfc3263>.

   [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,
              <http://www.rfc-editor.org/info/rfc3280>.

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
              <http://www.rfc-editor.org/info/rfc3370>.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,
              <http://www.rfc-editor.org/info/rfc3966>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

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   [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,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC6919]  Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
              for Use in RFCs to Indicate Requirement Levels", RFC 6919,
              DOI 10.17487/RFC6919, April 2013,
              <http://www.rfc-editor.org/info/rfc6919>.

16.2.  Informative References

   [I-D.ietf-iri-comparison]
              Masinter, L. and M. D&#258;&#378;rst, "Comparison,
              Equivalence and Canonicalization of Internationalized
              Resource Identifiers", draft-ietf-iri-comparison-02 (work
              in progress), October 2012.

   [I-D.ietf-stir-certificates]
              Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", draft-ietf-stir-
              certificates-06 (work in progress), July 2016.

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

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

   [I-D.rosenberg-sip-rfc4474-concerns]
              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,
              <http://www.rfc-editor.org/info/rfc2585>.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 2002,
              <http://www.rfc-editor.org/info/rfc3323>.

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   [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,
              <http://www.rfc-editor.org/info/rfc3325>.

   [RFC3548]  Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, DOI 10.17487/RFC3548, July 2003,
              <http://www.rfc-editor.org/info/rfc3548>.

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              DOI 10.17487/RFC3893, September 2004,
              <http://www.rfc-editor.org/info/rfc3893>.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
              October 2005, <http://www.rfc-editor.org/info/rfc4234>.

   [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,
              <http://www.rfc-editor.org/info/rfc4474>.

   [RFC4501]  Josefsson, S., "Domain Name System Uniform Resource
              Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
              <http://www.rfc-editor.org/info/rfc4501>.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <http://www.rfc-editor.org/info/rfc4916>.

   [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, <http://www.rfc-editor.org/info/rfc5763>.

   [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, <http://www.rfc-editor.org/info/rfc6698>.

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   [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,
              <http://www.rfc-editor.org/info/rfc6973>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <http://www.rfc-editor.org/info/rfc7340>.

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,
              <http://www.rfc-editor.org/info/rfc7375>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

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

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

   Email: ekr@rtfm.com

   Chris Wendt
   Comcast
   One Comcast Center
   Philadelphia, PA  19103
   USA

   Email: chris-ietf@chriswendt.net

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