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Versions: (draft-jennings-stir-rfc4474bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 Draft is active
In: Auth48
Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Obsoletes: 4474 (if approved)                                C. Jennings
Intended status: Standards Track                                   Cisco
Expires: August 13, 2017                                     E. Rescorla
                                                              RTFM, Inc.
                                                                C. Wendt
                                                                 Comcast
                                                        February 9, 2017


  Authenticated Identity Management in the Session Initiation Protocol
                                 (SIP)
                   draft-ietf-stir-rfc4474bis-16.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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 13, 2017.

Copyright Notice

   Copyright (c) 2017 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   4
   4.  Identity Header Field Syntax  . . . . . . . . . . . . . . . .   6
     4.1.  PASSporT Construction . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Example Full and Compact Forms of PASSporT in
               Identity  . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Example of Operations . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Example Identity Header Construction  . . . . . . . . . .  11
   6.  Signature Generation and Validation . . . . . . . . . . . . .  13
     6.1.  Authentication Service Behavior . . . . . . . . . . . . .  13
       6.1.1.  Handling Repairable Errors  . . . . . . . . . . . . .  15
     6.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .  16
       6.2.1.  Authorization of Requests . . . . . . . . . . . . . .  18
       6.2.2.  Failure Response Codes Sent by a Verification Service  18
       6.2.3.  Handling Retried Requests . . . . . . . . . . . . . .  20
       6.2.4.  Handling the full form of PASSporT  . . . . . . . . .  20
   7.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  Credential Use by the Authentication Service  . . . . . .  21
     7.2.  Credential Use by the Verification Service  . . . . . . .  22
     7.3.  'info' parameter URIs . . . . . . . . . . . . . . . . . .  23
     7.4.  Credential System Requirements  . . . . . . . . . . . . .  23
   8.  Identity Types  . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  Differentiating Telephone Numbers from URIs . . . . . . .  25
     8.2.  Authority for Telephone Numbers . . . . . . . . . . . . .  26
     8.3.  Telephone Number Canonicalization Procedures  . . . . . .  26
     8.4.  Authority for Domain Names  . . . . . . . . . . . . . . .  27
     8.5.  URI Normalization . . . . . . . . . . . . . . . . . . . .  29
   9.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  30
   10. Backwards Compatibility with RFC4474  . . . . . . . . . . . .  30
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  31
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  33
     12.1.  Protected Request Fields . . . . . . . . . . . . . . . .  33
       12.1.1.  Protection of the To Header and Retargeting  . . . .  35
     12.2.  Unprotected Request Fields . . . . . . . . . . . . . . .  35
     12.3.  Malicious Removal of Identity Headers  . . . . . . . . .  36



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     12.4.  Securing the Connection to the Authentication Service  .  36
     12.5.  Authorization and Transitional Strategies  . . . . . . .  37
     12.6.  Display-Names and Identity . . . . . . . . . . . . . . .  38
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
     13.1.  SIP Header Fields  . . . . . . . . . . . . . . . . . . .  39
     13.2.  SIP Response Codes . . . . . . . . . . . . . . . . . . .  39
     13.3.  Identity-Info Parameters . . . . . . . . . . . . . . . .  39
     13.4.  Identity-Info Algorithm Parameter Values . . . . . . . .  40
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  40
   15. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  40
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  41
     16.2.  Informative References . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

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 canonical address-of-record (AoR) SIP URI
   employed to reach a user (such as 'sip:alice@atlanta.example.com'),
   or a telephone number, which commonly appears in 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, in the absence of some sort of
   cryptographic authentication mechanism, the recipient of a SIP
   request has no way to verify that the From header field has been
   populated appropriately.  This leaves SIP vulnerable to a category of
   abuses, including impersonation attacks that facilitate or enable
   robocalling, voicemail hacking, swatting, and related problems as
   described in [RFC7340].  Ideally, a cryptographic approach to
   identity can provide a much stronger and assurance of identity than
   the Caller ID services that the telephone network provides today, and
   one less vulnerable to spoofing.

   [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




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   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, RFC4474 was in several ways misaligned
   with deployment realities (see [I-D.rosenberg-sip-rfc4474-concerns]).
   Most significantly, RFC4474 did not deal well with telephone numbers
   as identifiers, despite their enduring use in SIP deployments.
   RFC4474 also provided a signature over material that intermediaries
   in existing deployments commonly altered.  This specification
   therefore deprecates the RFC4474 syntax and behavior, reconsidering
   the problem space in light of the threat model in [RFC7375] and
   aligning the signature format with PASSporT [I-D.ietf-stir-passport].
   Backwards compatibility considerations are given in Section 10.

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].

   In addition, this document uses three terms specific to the
   mechanism:

      Identity: An identifier for the user of a communications service;
      for the purposes of SIP, either a SIP URI or a telephone number.
      Identities are derived from an "identity field" in a SIP request
      such as the From header field.

      Authentication Service: A logical role played by a SIP entity that
      adds Identity headers to SIP requests.

      Verification Service (or "Verifier"): A logical role played by a
      SIP entity that validates Identity headers in a SIP request.

3.  Architectural Overview

   The identity architecture for SIP defined in this specification
   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 originator
   of the message has been authenticated, through pre-arranged means
   with the authentication service, the authentication service then
   creates and adds an Identity header field to the request.  This



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   requires computing cryptographic information, including a digital
   signature over some components of messages, that lets other SIP
   entities verify that the sending user has been authenticated and its
   claim of a particular identity has been authorized.  These
   "verification services" validate the signature and enable policy
   decisions to be made based on the results of the validation.

   Policy decisions made after validation depend heavily on the
   verification service's trust for the credentials that the
   authentication service uses to sign requests.  As robocalling,
   voicemail hacking, and swatting usually involve impersonation of
   telephone numbers, credentials that will be trusted by relying
   parties to sign for telephone numbers are a key component of the
   architecture.  Authority over telephone numbers is, however, not as
   easy to establish on the Internet as authority over traditional
   domain names.  This document assumes the existence of credentials for
   establishing authority over telephone numbers, for cases where the
   telephone number is the identity of the user, but this document does
   not mandate or specify a credential system;
   [I-D.ietf-stir-certificates] describes a credential system compatible
   with this architecture.

   Although addressing the vulnerabilities in the STIR problem statement
   and threat model mostly requires dealing with telephone number as
   identities, SIP must also handle signing for SIP URIs as identities.
   This is typically easier to deal with, as these identities are issued
   by organizations that have authority over Internet domains.  When a
   new user becomes associated with example.com, for example, the
   administrator of the SIP service for that domain can issue them an
   identity in that namespace, such as sip: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 other
   cases, Alice may herself be the owner of her own domain, 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 SIP as defined in [RFC3261], the
   necessary 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 that the domain owner
   has given the user agent the right to a name.

   In order to share a cryptographic assurance of end-user SIP identity
   in an interdomain or intradomain context, an authentication service
   constructs tokens based on the PASSporT [I-D.ietf-stir-passport]
   format, which is special encoding of a JSON [RFC7159] object



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   comprising values derived from certain header field values in the SIP
   request.  The authentication service computes a signature over those
   JSON elements as PASSporT specifies.  An encoding of the resulting
   PASSporT is then placed in the SIP Identity header field.  In order
   to assist in the validation of the Identity header field, this
   specification also describes a parameter of the Identity header field
   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 an
   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
   PASSporTs to other protocols.

4.  Identity Header Field Syntax

   The Identity and Identity-Info header fields that were previously
   defined in RFC4474 are here deprecated.  This revised specification
   collapses the grammar of Identity-Info into the Identity header field
   via the "info" parameter.  Note that unlike the prior specification
   in RFC4474, the Identity header field is now allowed to appear more
   than one time in a SIP request.  The revised grammar for the Identity
   header field builds on the ABNF [RFC5234] in RFC 3261 [RFC3261]
   Section 25.  It is as follows:

      Identity = "Identity" HCOLON signed-identity-digest SEMI
          ident-info *( SEMI ident-info-params )
      signed-identity-digest = 1*(base64-char / ".")
      ident-info = "info" EQUAL ident-info-uri
      ident-info-uri = LAQUOT absoluteURI RAQUOT
      ident-info-params = ident-info-alg / ident-type /
          ident-info-extension
      ident-info-alg = "alg" EQUAL token
      ident-type = "ppt" EQUAL token
      ident-info-extension = generic-param

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


   In addition to the "info" parameter, and the "alg" parameter
   previously defined in RFC4474, this specification defines the
   optional "ppt" parameter (PASSporT Type).  The 'absoluteURI' portion
   of ident-info-uri MUST contain a URI; see Section 7.3 for more on
   choosing how to advertise credentials through this parameter.

   The signed-identity-digest contains a base64 encoding of a PASSporT
   [I-D.ietf-stir-passport], which secures the request with a signature



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   that PASSporT generates over the JSON header and payload objects;
   some of those header and claim element values will mirror values of
   the SIP request.

4.1.  PASSporT Construction

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

      First, per baseline [I-D.ietf-stir-passport], the JSON "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 field.  Note if the
      "alg" parameter is absent from the Identity header, the default
      value is "ES256".

      Third, the JSON key "x5u" MUST have a value equivalent to the
      quoted URI in the "info" parameter, per the simple string
      comparison rules of [RFC3986] section 6.2.1.

      Fourth, if a PASSporT extension is in use, then the optional JSON
      key "ppt" MUST be present and have a value equivalent to the
      quoted value of the "ppt" parameter of the Identity header field.

   An example of the PASSporT header JSON object without any extension
   is:

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

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

      First, the JSON "orig" object MUST be populated.  If the
      originating identity is a telephone number, then the array MUST be
      populated with a JSON object containing a "tn" element with a
      value set to the value of the quoted originating identity, a
      canonicalized telephone number (see Section 8.3).  Otherwise, the
      object MUST be populated with a JSON object containing "uri"
      element, set to the value of the AoR of the UA sending the message
      as taken from the addr-spec of the From header field, per the
      procedures in Section 8.5.





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      Second, the JSON "dest" array MUST be populated.  If the
      destination identity is a telephone number, then the array MUST be
      populated with a JSON object containing a "tn" element with a
      value set to the value of the quoted destination identity, a
      canonicalized telephone number (see Section 8.3).  Otherwise, the
      array MUST be populated with a JSON object containing a "uri"
      element, 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 8.5.  Multiple JSON objects are
      permitted in "dest" for future compatibility reasons.

      Third, the JSON key "iat" MUST appear.  The authentication service
      SHOULD set the value of "iat" to an encoding of the value of the
      SIP Date header field as a JSON NumericDate (as UNIX time, per
      [RFC7519] Section 2), though an authentication service MAY set the
      value of "iat" to its own current clock time.  If the
      authentication service uses its own clock time then the use of the
      full form of PASSporT is REQUIRED.  In either case, the
      authentication service MUST NOT generate a PASSporT for a SIP
      request if the Date header is outside of its local policy for
      freshness (recommended sixty seconds).

      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 5.2.2.

   For example:

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

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

   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
   "dest" array may 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 align




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   multiple destination identities in PASSporT with such systems is left
   as a subject for future specification.

   After these two JSON objects, the header and the payload, have been
   constructed and base64-encoded, they must each be hashed and signed
   per [I-D.ietf-stir-passport] Section 6.  The header, payload and
   signature components comprise a full PASSporT object.  The resulting
   PASSporT may be carried in SIP in either a full form, which includes
   the header and payload as well as the signature, or a compact form
   which only carries the signature per [I-D.ietf-stir-passport]
   Section 7.  The hashing and signing algorithm is specified by the
   'alg' parameter of the Identity header field and the mirrored "alg"
   parameter of PASSporT.  All implementations of this specification
   MUST support the required signing algorithms of PASSporT.  At present
   there is one mandatory-to-support value for the 'alg' parameter:
   'ES256', as defined in [RFC7519], which connotes an ECDSA P-256
   digital signature.

4.1.1.  Example Full and Compact Forms of PASSporT in Identity

   As Appendix F of the JWS specification [RFC7515] notes, there are
   cases where "it is useful to integrity-protect content that is not
   itself contained in a JWS."  Since the fields that make up the
   majority of the PASSporT header and payload have values replicated in
   the SIP request, the SIP usage of PASSporT may exclude the base64
   encoded version of the header and payload JSON objects from the
   Identity header field and instead present a detached signature: what
   PASSporT calls its compact form, see [I-D.ietf-stir-passport]
   Section 7.

   When an authentication service constructs an Identity header, the
   contents of the signed-identity-digest field MUST contain either a
   full or compact PASSporT.  Use of the compact form is RECOMMENDED in
   order to reduce message size, but note that extensions often require
   the full form (see Section 9).

   For example, a full form of PASSporT in an Identity header might look
   as follows (backslashes shown for line folding only):

   Identity: eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1I \
   joiaHR0cHM6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJ \
   kZXN0Ijp7InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdC \
   I6IjE0NDMyMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.r \
   q3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
   ojNCpTzO3QfPOlckGaS6hEck7w;info=<https://biloxi.example.org \
   /biloxi.cert>





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   The compact form of the same PASSporT object would appear in the
   Identity header as:

   Identity: ..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qj \
   pjlk-cpFYpFYsojNCpTzO3QfPOlckGaS6hEck7w;                    \
   info=<https://biloxi.example.org/biloxi.cert>

5.  Example of Operations

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

   Imagine a case where Bob, who has the home proxy of example.com and
   the address-of-record sip:12155551212@example.com;user=phone, wants
   to communicate with Alice at sip:alice@example.org.  They have no
   prior relationship, and Alice implements best practices to prevent
   impersonation attacks.

   Bob's user agent generates an INVITE and places his address-of-record
   in the From header field of the request.  He then sends an INVITE to
   an authentication service proxy for his domain.

   ............................          ..............................
   .                          .          .                            .
   .                +-------+ .          . +-------+                  .
   .     Signs for  |       | .  Signed  . |       |                  .
   .     12125551xxx| Auth  |------------> | Verif |                  .
   .                |  Svc  | .  INVITE  . |  Svc  |                  .
   .                | Proxy | .          . | Proxy |                  .
   .              > +-------+ .          . +-------+ \                .
   .             /       |    .          ->           \               .
   .            /        |    .        --.             \              .
   .           /         |    .      --  .              \             .
   .          /          |    .    --    .               \            .
   .         /       +-------+.  --      .                \           .
   .        /        |       |.<-        .                 \          .
   .       /         | Cert  |.          .                  >         .
   .   +-------+     | Store |.          .                +-------+   .
   .   |       |     |       |.          .                |       |   .
   .   | Bob   |     +-------+.          .                | Alice |   .
   .   | UA    |              .          .                | UA    |   .
   .   |       |              .          .                |       |   .
   .   +-------+              .          .                +-------+   .
   .              Domain A    .          .   Domain B                 .
   ............................          ..............................





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   The proxy authenticates Bob, and validates that he is authorized to
   assert the identity that he populated in the From header field.  The
   proxy authentication service then constructs a PASSporT which
   contains a JSON representation of values which mirror certain parts
   of the SIP request, including the identity in the From header field
   value.  As a part of generating the PASSporT, the authentication
   service signs a hash of that JSON header and payload with the private
   key associated with the appropriate credential for the identity (in
   this example, a certificate with authority to sign for numbers in a
   range from 12155551000 to 121555519999), and the signature is
   inserted by the proxy server into the Identity header field value of
   the request as a compact form of PASSporT.  Alternatively, the JSON
   header and payload themselves might also have been included in the
   object when using the full form of PASSporT.

   The proxy authentication service, as the holder of a private key with
   authority over Bob's telephone number, is asserting that the
   originator of this request has been authenticated and that he is
   authorized to claim the identity that appears in the From header
   field.  The proxy inserts an "info" parameter into the Identity
   header field that tells Alice how to acquire keying material
   necessary to validate its credentials (a public key), in case she
   doesn't already have it.

   When Alice's domain receives the request, a proxy verification
   service validates the signature provided in the Identity header
   field, and then determines that the authentication service
   credentials demonstrate authority over the identity in the From
   header field.  This same validation operation might be performed by a
   verification service in Alice's user agent server.  Ultimately, this
   valid request is rendered to Alice.  If the validation were
   unsuccessful, some other treatment could be applied by the receiving
   domain or Alice's user agent.

5.1.  Example Identity Header Construction

   For the following SIP request:














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    INVITE sip:bob@biloxi.example.org SIP/2.0
    Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
    To: Alice <sip:alice@example.com>
    From: Bob <sip:12155551212@example.com;user=phone>;tag=1928301774>
    Call-ID: a84b4c76e66710
    CSeq: 314159 INVITE
    Max-Forwards: 70
    Date: Fri, 25 Sep 2015 19:12:25 GMT
    Contact: <sip:12155551212gateway.example.com>
    Content-Type: application/sdp
    Content-Length: 147

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

   An authentication service will create a corresponding PASSporT
   object.  The properly-serialized PASSporT header and payload JSON
   objects would look as follows.  For the header, the values chosen by
   the authentication service at "example.org" might read:

   {"alg":"ES256","typ":"passport","x5u":"https://cert.example.org/
      passport.cer"}

   The serialized payload will derive values from the SIP request (the
   From, To, and Date header field values) as follows:

   {"dest":{"uri":["sip:alice@example.com"]},"iat":1443208345,
     "orig":{"tn":"12155551212"}}

   The authentication service would then generate the signature over the
   object following the procedures in [I-D.ietf-stir-passport]
   Section 6.  That signature would look as follows:

   rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
    ojNCpTzO3QfPOlckGaS6hEck7w

   An authentication service signing this request and using the compact
   form of PASSporT would thus generate and add to the request an
   Identity header field of the following form:

   Identity: ..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpj \
    lk-cpFYpFYsojNCpTzO3QfPOlckGaS6hEck7w; \
    info=<https://cert.example.org/passport.cer>



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6.  Signature Generation and Validation

   SIP entities that instantiate the authentication service and
   verification service roles will, respectively, generate and validate
   the Identity header and the signature it contains.

6.1.  Authentication Service Behavior

   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 7.1).  The
   authentication service role can be instantiated, for example, by an
   intermediary such as a proxy server or by a user agent.
   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 proxy servers 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 field to SIP
   requests.  The procedures below define the steps that must be taken
   when each Identity header field is added.  More than one Identity
   header field may appear in a single request, and an authentication
   service may add an Identity header field to a request that already
   contains one or more Identity header fields.

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

   Step 1: Check Authority for the Identity

   First, the authentication service must determine whether it is
   authoritative for the identity of the originator of the request.  The
   authentication service extracts the identity from the URI value from
   the "identity field"; in ordinary operations, that is the addr-spec
   component of From header field.  In order to determine whether the
   signature for the identity field should be over the entire identity
   field URI or just a telephone number, the authentication service MUST
   follow the process described in Section 8.1.  That section will
   either lead to the telephone number canonicalization procedures in
   Section 8.3 for telephone numbers, or to the URI normalization
   procedures described in Section 8.5 for domain names.  Whichever the
   result, if the authentication service is not authoritative for the



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   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 field if the
   authentication service does not have the authority to make the claim
   it asserts.

   Step 2: Authenticate the Originator

   The authentication service MUST then determine whether or not the
   originator 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 originator of the message.  Some possible ways
   in which this authentication might be performed include:

      If the authentication service is instantiated by a SIP
      intermediary (proxy server), it may authenticate the request with
      the authentication scheme used for registration in its domain
      (e.g., Digest authentication).

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

   Authorization of the use of a particular username or telephone number
   in the user part of the From header field is a matter of local policy
   for the authentication service; see Section 7.1 for more information.

   Note that this check is performed only on the addr-spec in the
   identity field (e.g., the URI of the originator, like
   'sip:alice@atlanta.example.com'); it does not cover the display-name
   portion of the From header field (e.g., 'Alice Atlanta').  For more
   information, see Section 12.6.

   Step 3: Verify Date is Present and Valid

   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 field 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 field value 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 field in order to cause a request to fail
   verification; the Identity header field is not intended to provide a



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   perfect record of when messages are processed.  Finally, the
   authentication service MUST verify that both the Date header field
   and the current time fall within the validity period of its
   credential.

   See Section 12.1 for information on how the Date header field assists
   verifiers.

   Step 4: Populate and Add the Identity Header

   Subsequently, the authentication service MUST form a PASSporT object
   and add a corresponding Identity header field to the request
   containing either the full or compact form of PASSporT.  For the
   baseline PASSporT header (headers containing no "ppt" parameter),
   this follows the procedures in Section 4; 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 field has
   been added to the request, the authentication service MUST also add a
   "info" parameter to the Identity header field.  The "info" parameter
   contains a URI from which the authentication service's credential can
   be acquired; see Section 7.3 for more on credential acquisition.

   An authentication service MAY use the full form of the PASSporT in
   the Identity header field.  The presence of the full form is OPTIONAL
   because the information carried in the baseline PASSporT headers and
   claims is usually redundant with information already carried
   elsewhere in the SIP request.  Using the compact form can
   significantly reduce SIP message size, especially when the PASSporT
   payload contains media keys.  The syntax of the compact form is given
   in [I-D.ietf-stir-passport] Section 7; essentially, it contains only
   the signature component of the PASSporT.

   Note that per the behavior specified in [I-D.ietf-stir-passport], use
   of the full form is mandatory when optional extensions are included.
   See Section 9.

6.1.1.  Handling Repairable Errors

   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
   full form of the PASSporT object in the Identity header field, it
   SHOULD retry the request with the full form after receiving a 438
   response; however implementations SHOULD NOT retry the request more
   than once.  Authentication services implemented at proxy servers



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   would retry such a request as a ssequential for, by re-processing the
   destination as a new target and handling it serially as described in
   Section 16.6 of [RFC3261].

   The information in the full form is useful on the verification side
   for debugging errors, and there are some known causes of verification
   failures (such as the Date header field value changing in transit,
   see Section 12.1 for more information) that can be resolved by the
   inclusion of the full form of PASSporT.

   Finally, the authentication service forwards the message normally.

6.2.  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 header fields, it inspects
   the signature(s) to verify the identity of the originator 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 header fields.
   A verification service performs the steps below on each Identity
   header field that appears in a request.  If a verification service
   cannot use any Identity header in a request, due to the absence of
   Identity headers or unsupported "ppt" parameters, and the presence of
   an Identity header field is required by local policy (for example,
   based on a per-sending-domain policy, or a per-sending-user policy),
   then a 428 'Use Identity Header' response MUST be sent in the
   backwards direction.  For more on this and other verifier responses,
   see Section 6.2.2.

   In order to verify an Identity header field 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 field contains a
   full form PASSporT object, the verifier MUST follow the additional
   procedures in Section 6.2.4.

   Step 1: Check for an Unsupported "ppt"

   The verifier MUST inspect any optional "ppt" parameter appearing in
   the Identity header.  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 field.  If a supported "ppt" parameter value is
   present, the verifier proceeds with Step 2, and will ultimately
   follow the "ppt" variations described in Step 5.




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   Step 2: Determine the Originator's Identity

   In order to determine whether the signature for the identity field
   should be over the entire identity field URI or just a telephone
   number, the verification service MUST follow the process described in
   Section 8.1.  That section will either lead to the telephone number
   canonicalization procedures in Section 8.3 for telephone numbers, or
   to the URI normalization procedures described in Section 8.5 for
   domain names.

   Step 3: Identify Credential for Validation

   The verifier must ensure that it has access to 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 field.  See Section 7.2 for more information on
   these procedures.  If the verifier does not support the credential
   described in the "info" parameter, then it treats the credential for
   this header field as unsupported.

   Step 4: Check the Freshness of Date

   The verifier furthermore ensures that the value of the Date header
   field of the request meets local policy for freshness (sixty seconds
   is RECOMMENDED) and that it falls within the validity period of the
   credential used to sign the Identity header field.  For more on the
   attacks this prevents, see Section 12.1.  If the full form of the
   PASSporT is present, the verifier SHOULD compare the "iat" value in
   the PASSporT to the Date header field value in the request.  If the
   two are different, and the "iat" value differs from the Date header
   field value but remains 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 header
   field value.

   Step 5: Validate the Signature

   The verifier MUST validate the signature in the Identity header field
   over the PASSporT object.  For baseline PASSporT objects (with no
   Identity header field "ppt" parameter) the verifier MUST follow the
   procedures for generating the signature over a PASSporT object
   described in Section 4.  If a "ppt" parameter is present (and per
   Step 1, is supported), the verifier follows the procedures for that
   "ppt" (see Section 9).  If a verifier determines that the signature
   in the Identity does not correspond to the reconstructed signed-
   identity-digest, then the Identity header field should be considered
   invalid.




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6.2.1.  Authorization of Requests

   The verification of an Identity header field does not entail any
   particular treatment of the request.  The handling of the message
   after the verification process depends on how the verification
   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 field, the
   presence of an invalid Identity header field, or the absence of an
   Identity header field, or a stale Date header field value, 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.

   The presence of multiple Identity header fields within a message
   raises the prospect that a verification services could receive a
   message containing some valid and some invalid Identity header
   fields.  As a guideline, this specification recommends that only if a
   verifier determines all Identity header fields within a message are
   invalid should the request be considered to have an invalid identity.
   If at least one Identity header field value is valid and from a
   trusted source, then relying parties can use that header for
   authorization decisions regardless of whether other untrusted or
   invalid Identity headers appear in a request.

6.2.2.  Failure Response Codes Sent by a Verification Service

   RFC4474 originally defined four response codes for failure conditions
   specific to the Identity header field and its original mechanism.
   These status codes are retained in this specification, with some
   slight modifications.  Also, this specification details responding
   with 403 when a stale Date header field value is received.

   A 428 response will be sent (per Section 6.2) when an Identity header
   field is required, but no Identity header field without a "ppt"
   parameter, or with a supported "ppt" value, has been received.  In
   the case where one or more Identity header fields with unsupported
   "ppt" values have been received, then a verification service may send
   a 428 with a human-readable reason phrase like "Use Supported
   PASSporT Format".  Note however that this specification gives no
   guidance on how a verification service might decide to require an
   Identity header field for a particular SIP request.  Such
   authorization policies are outside the scope of this specification.

   The 436 'Bad Identity Info' response code indicates an inability to
   acquire the credentials needed by the verification service for
   validating the signature in an Identity header field.  Again, given



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   the potential presence of multiple Identity header fields, this
   response code should only be sent when the verification service is
   unable to deference the URIs and/or acquire the credentials
   associated with all Identity header fields 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 Credential' is sent when a verification service
   can acquire, or already holds, the credential represented by the
   'info' parameter of at least one Identity header field 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.

   The 438 'Invalid Identity Header' response indicates that of the set
   of Identity header fields in a request, no header field 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 field
   is grounds for rejecting a request.  Note that in some cases, an
   Identity header field 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 field of the request.  Sending
   a full form PASSporT can repair some of these conditions (see
   Section 6.2.4), so the recommended way to attempt to repair this
   failure is to retry the request with the full form of PASSporT if it
   had originally been sent with the compact form.  The alternative
   reason phrase 'Invalid PASSporT' can be used when an extended full
   form PASSporT lacks required headers or claims, or when an extended
   full form PASSporT signaled with the "ppt" parameter lacks required
   claims for that extension.  Sending a string along these lines will
   help humans debugging the sending system.

   Finally, a 403 response may be sent when the verification service
   receives a request with a Date header field value that is older than
   the local policy for freshness permits.  The same response may be
   used when the "iat" in the full form of a PASSporT has a value older
   than the local policy for freshness permits.  The reason phrase
   "Stale Date" can be sent to help humans debug the failure.

   Future specifications may explore ways, including Reason codes or
   Warning headers, to communicate further information that could be
   used to disambiguate the source of errors in cases with multiple
   Identity headers in a single request, or provide similar detailed
   feedback for debugging purposes.





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6.2.3.  Handling Retried Requests

   If a verification service sends a failure response in the backwards
   direction, the authentication service may retry the request as
   described in Section 6.1.1.  If the authentication service is
   instantiated at a proxy server, then it will retry the request as a
   sequential fork.  Verification services implemented at a proxy server
   will recognize this request as a spiral rather than a loop due to the
   proxy behavior fix documented in [RFC5393] Section 4.2.  However, if
   the verification service is implemented in an endpoint, the endpoint
   will need to override the default UAS behavior (in particular, the
   SHOULD in [RFC3261] Section 8.2.2.2) to accept this request as a
   spiral rather than a loop.

6.2.4.  Handling the full form of PASSporT

   If the full form of PASSporT is present in an Identity header, this
   permits the use of optional extensions as described in
   [I-D.ietf-stir-passport] Section 8.3.  Furthermore, the verification
   service can extract from the "orig" and "dest" elements of the
   PASSporT full form the canonical telephone numbers created by the
   authentication service, as well as an "iat" claim corresponding to
   the Date header field that the authentication service used.  These
   values may be used to debug canonicalization problems, or to avoid
   unnecessary signature breakage caused by intermediaries that alter
   certain SIP header field values in transit.

   However, the verification service MUST NOT treat the value in the
   "orig" of a full form PASSporT as the originating identity of the
   call: the originating identity of the call is always derived from the
   SIP signaling, and it is that value, per the procedures above in
   Section 6.2 Step 2, which is used to recompute the signature at the
   verification service.  That value, rather than the value inside the
   PASSporT object, is rendered to an end user in ordinary SIP
   operations, and if a verification service were to simply trust that
   the value in the "orig" corresponded to the call that it received
   without comparing it to the call signaling, this would enable various
   cut-and-paste attacks.  As an optimization, when the full form is
   present, the verification service MAY delay performing that
   cryptographic operation and first compute its own canonicalization of
   an originating telephone number to compare it to the values in the
   "orig" element of PASSporT.  This would allow the verification
   service to ascertain whether or not the two ends agree on the
   canonical number form; if they do not, then surely the signature
   validation would fail.






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7.  Credentials

   This section gives general guidance on the use of credential systems
   by authentication and verification services, as well as requirements
   that must be met by credential systems that conform with this
   architecture.  It does not mandate any specific credential system.

   Furthermore, this specification allows either a user agent or a proxy
   server to provide the authentication service function and/or the
   verification service function.  For the purposes of end-to-end
   security, it is obviously preferable for end systems to acquire their
   own credentials; in this case 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.  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.  Thus,
   for reasons of credential management alone, implementing the
   authentication service at an intermediary may be more practical.
   This trade-off needs to be understood by implementers of this
   specification.

7.1.  Credential Use by the Authentication Service

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

   Authorization of the use of a particular username or telephone number
   in the From header field value 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 field 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 header field that do not correspond to the user-
   portion of From header fields, 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'



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

7.2.  Credential Use by the Verification Service

   In order to act as a verification service, a SIP entity must have a
   way to acquire credentials for authorities over particular domain
   names, telephone numbers and/or number ranges.  Dereferencing the URI
   found in the "info" parameter of the Identity header field (as
   described Section 7.3) 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
   implementation-specific way of acquiring the needed keying material,
   such as an offline store of periodically-updated credentials.  The
   436 'Bad Identity Info' response exists for cases where the
   verification service cannot deference the URI in the "info"
   parameter.

   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 credential mechanism specifications.




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

7.3.  '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 SHOULD be dereferencable by any entity on the
   public Internet.  In constrained deployment environments, a service
   private to the environment MAY 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 numbers 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.

7.4.  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 might use the DNS URL scheme (see [RFC4501]) to designate
   keys in the DNS.





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   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].

   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), and

      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 header field "info" parameter 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 "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).








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8.  Identity Types

   The problem statement of STIR [RFC7340] 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, the Identity
   header mechanism also works with SIP URIs without telephone numbers
   (of the form "sip:user@host"), and potentially other identifiers when
   SIP interworks with other protocols.

   Authentication services confirm the identity of the originator of a
   call, which is typically found in the From header field value.  The
   guidance in this specification 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
   trusted 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 originator of a request; where it does, local
   policy might therefore dictate that the canonical identity derives
   from the P-Asserted-Identity header field rather than the From header
   field.

   Ultimately, in any case where local policy canonicalizes the identity
   into a form different from how it appears in the From header field,
   the use of the full form of PASSporT by authentication services is
   RECOMMENDED, but because the "orig" claim of PASSporT itself could
   then divulge information about users or networks, implementers should
   be mindful of the guidelines in Section 11.

8.1.  Differentiating Telephone Numbers from URIs

   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 a URI found in the
   From header field as they are for one found in the To header field or
   the P-Asserted-Identity header field.

   First, implementations will ascertain if 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 SIP URIs with the "user=phone" parameter conforms to the
   syntax of the TEL URI scheme (RFC 3966 [RFC3966]).  It is also
   possible for a TEL URI to appear in SIP header fields outside the
   context of a SIP or SIPS URI (e.g., 'tel:+17005551008').  Thus, in




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   standards-compliant environments, numbers will be explicitly labeled
   by the use of TEL URIs or the 'user=phone' parameter.

   Alternatively, implementations in environments that do not conform to
   those standards MAY follow local policies for identifying telephone
   numbers.  For example, implementations could infer that the user part
   is a telephone number due to the presence of the '+' indicator at the
   start of the user-portion.  Absent even that indication, if there are
   numbers present in the user-portion, implementations might
   conceivably 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.
   Implementations could also rely on external hints: for example, a
   verification service implementation could infer from the type of
   credential that signed a request that the signature must be over a
   telephone number.

   Regardless of how the implementation detects telephone numbers, once
   a telephone number has been detected, implementations SHOULD follow
   the procedures in Section 8.3.  If the URI field does not contain a
   telephone number, or if the result of the canonicalization of the
   From header field value does not form a valid E.164 telephone number,
   the authentication service and/or verification service SHOULD treat
   the entire URI as a SIP URI, and apply the procedures in Section 8.5.
   These URI normalization procedures are invoked to canonicalize the
   URI before it is included in a PASSporT object in, for example, a
   "uri" claim.  See Section 8.5 for that behavior.

8.2.  Authority for Telephone Numbers

   In order for telephone numbers to be used with the mechanism
   described in this document, authentication services must receive
   credentials from an authority 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 7.4, 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].

8.3.  Telephone Number Canonicalization Procedures

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




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      Implementations MUST drop any "+"s, any internal dashes,
      parentheses or other non-numeric characters, excepting only the
      "#" 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, calls to those numbers are routed in a very strict
      fashion which ordinarily prevents them from reaching entities that
      don't understand the numbers.

      Some domains may need to take unique steps to convert their
      numbers into a global format, and such transformations during
      canonicalization can also 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
      header field user portions to E.164 by prepending country-code and
      region code digits; another domain might have prefixed usernames
      with trunk-routing codes, in which case the canonicalization will
      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*tn-char
             tn-char = "#" / "*" / DIGIT

   The resulting number string is used in the construction of the
   telephone number field(s) in a PASSporT object.

8.4.  Authority for Domain Names

   To use a SIP URI as an identity in this mechanism requires
   authentication and verification systems to support standard




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   mechanisms for proving authority over a domain name: that is, the
   domain name in the host portion of the SIP URI.

   A verifier MUST evaluate the correspondence between the user's
   identity and the signing credential by following the procedures
   defined in [RFC5922], Section 7.2.  While [RFC5922] deals with the
   use of TLS and is specific to certificates, the procedures described
   are applicable to verifying identity if one substitutes the "hostname
   of the server" for the domain portion of the user's identity in the
   From header field of a SIP request with an Identity header field.

   This process 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 (see [RFC5922] Section 7.8).  Secondly, some SIP
   services may delegate SIP functions to a subordinate domain and
   utilize the procedures in [RFC3263] that allow requests for, say,
   'example.com' to be routed to 'sip.example.com'.  As a result, a user
   with the AoR 'sip:alice@example.com' may process requests through a
   host like 'sip.example.com', and it may be that latter host that acts
   as an authentication service.

   To address the second of these problems, a domain that deploys an
   authentication service on a subordinate host might 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:alice@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.








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8.5.  URI Normalization

   Just as telephone numbers may undergo a number of syntactic
   transformations 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 escaped headers, from its syntax.  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-
   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.  Implementations MUST perform
   case normalization (rendering the scheme, user, and host all
   lowercase) and percent-encoding normalization (decoding any percent-
   encoded octet that corresponds to an unreserved character, per
   [RFC3986] Section 2.3).  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.







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9.  Extensibility

   As future requirements may warrant increasing the scope of the
   Identity mechanism, this specification specifies an optional "ppt"
   parameter of the Identity header field, which mirrors the "ppt"
   header 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] Section 8.  Note that
   per the guidance in that section, "ppt" is used only to enforce a
   mandatory extension: optional claims may be added to any PASSporT
   object without requiring the use of "ppt", but the compact form of
   PASSporT MUST NOT be used when optional claims are present in the
   PASSporT payload.

   The potential for extensions is one the primary motivations for
   allowing the presence of multiple Identity header fields in the same
   SIP request.  It is envisioned that future extensions might allow for
   alternate information to be signed, or to explicitly allow different
   parties to provide the signatures than the authorities envisioned by
   baseline STIR.  A request might, for example, have one Identity added
   by an authentication service at the originating administrative
   domain, and then another Identity header field added by some further
   intermediary using a PASSporT extension.  While this specification
   does not define any such specific purpose for multiple Identity
   header fields, implementations MUST support receiving multiple header
   fields for future compatibility reasons.

   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.

10.  Backwards Compatibility with RFC4474

   This specification introduces several significant changes from the
   RFC4474 version of the Identity header field.  However, due to the
   problems enumerated in [I-D.rosenberg-sip-rfc4474-concerns], it is
   not believed that the original Identity header field 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



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   field, it would likely fail the request with a 436 response code due
   to the absence of an Identity-Info header field.  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 reliable identification
   of the originator of a SIP request, specifically a cryptographic
   assurance that an authority asserts the originator can claim the URI
   the identity stipulated in the request.  This URI may contain or
   imply 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 RFC3323 privacy service and as an
   authentication service.  Since a user agent can provide any From
   header field value that the authentication service is willing to
   authorize, there is no reason why private SIP URIs that contain
   legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
   by an authentication service.  The construction of the Identity
   header field 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 authentication 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 value of
   the request; accordingly, using domains like 'anonymous.invalid' will
   not be usable by privacy services that simultaneously 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



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   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 field.  The sort of assertion provided by
   the P-Asserted-Identity header field is very different from the
   Identity header field presented in this document.  It contains
   additional information about the originator of a message that may go
   beyond what appears in the From header field; P-Asserted-Identity
   holds a definitive identity for the originator 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 field, and privacy
   services MUST NOT remove the Identity header field when a priv-value
   of "id" appears in a Privacy header field.

   The full form of the PASSporT object provides the complete JSON
   objects used to generate the signed-identity-digest of the Identity
   header field value, 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 full form
   of PASSporT 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 the object would need to be converted to the
   compact form 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 originator's identity could not
   be reassembled by a verifier, and thus the Identity signature
   validation process would fail, using P-Asserted-Identity with the
   full form of PASSporT 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.





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   Finally, note that unlike [RFC3325], the mechanism described in this
   specification adds no information to SIP requests that has privacy
   implications - apart from disclosing that an authentication service
   is willing to sign for an originator.

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 header fields are the same as those given in [RFC3261] for
   including header fields 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 originator 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 originator may reside in P-
   Asserted-Id instead.  The originator'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.  That specification
   recommends that implementations notify the user of a potential
   security issue if the signed Date header field value is stale by an
   hour or more.  To prevent cut-and-paste of recently-observed
   messages, this specification instead RECOMMENDS a shorter interval of
   sixty seconds.  Implementations of this specification MUST NOT deem
   valid a request with an outdated Date header field.  Note that per
   [RFC3893] Section 10 behavior, servers can keep state of recently
   received requests, and thus if an Identity header field 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



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   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 the full form of PASSporT:
   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 full form of the PASSporT object 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 field
   from a 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 cut-and-paste the
   Identity and From header field value from that INVITE into a new
   request to the target's voicemail service within the Date interval,
   and the voicemail service would have no way knowing that the Identity
   header field 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
   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 reduces the
   potential for active eavesdropping attacks against the SIP media.  In



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   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

   Armed with the original value of the To header field, the recipient
   of a request may be tempted 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
   field for requests in the dialog only when the user is authorized to
   assert the identity given in the From header field, and if they are
   not, an Identity header field 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 field
   values creates some opportunities for determined attackers to
   impersonate based on cut-and-paste attacks; however, the absence of
   these header field values 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



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   field, an attacker could remove that Via header field 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 field 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 field cannot protect itself.
   Any attacker could remove the header field from a SIP request, and
   modify the request arbitrarily afterwards.  However, this mechanism
   is not intended to protect requests from men-in-the-middle who
   interfere with SIP messages; it is intended only to provide a way
   that the originators of SIP requests can prove that they are who they
   claim to be.  At best, by stripping identity information from a
   request, a man-in-the-middle could make it impossible to distinguish
   any illegitimate messages he would like to send from those messages
   sent by an authorized user.  However, it requires a considerably
   greater amount of energy to mount such an attack than it does to
   mount trivial impersonations by just copying someone else's From
   header field.  This mechanism provides a way that an authorized user
   can provide a definitive assurance of his identity that an
   unauthorized user, an impersonator, cannot.

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



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      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
   parts of the header and body is provided by this mechanism only to
   prevent replay attacks.  However, it is possible that applications
   relying on the presence of the Identity header field 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
   the SIPS mechanism, Digest 'auth-int' for body integrity, or both
   when it can.  The ultimate decision to add an Identity header field
   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
   field is limited by the security practices of the authentication
   service that issues the assurance.  Relying on an Identity header
   field 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
   field from such authentication services is questionable.  While there
   is no magic way for a verifier to distinguish "good" from "bad"



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   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 field 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 originator'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 source without an Identity header
      field with skepticism.

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

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

   Finally, it is worth noting that the presence or absence of the
   Identity header fields 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 header fields 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.




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   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 originator; 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 contains a number of actions for IANA.  Primarily, the
   previous references to RFC4474 in the sip-parameters registry should,
   unless specified otherwise below, be updated to point to [RFCthis].

13.1.  SIP Header Fields

   The Identity-Info header in the SIP Header Fields registry should be
   marked as deprecated by [RFCThis].

   Also, the Identity-Info header reserved the compact form "n" at its
   time of registration.  Please remove that compact form from the
   registry.  The Identity header however retains the compact form "y"
   reserved by RFC4474.

13.2.  SIP Response Codes

   The Reason phrase for the 436 response default reason phrase should
   be changed from "Bad Identity-Info" to "Bad Identity Info" in the SIP
   Response Code registry.

   The 437 "Unsupported Certificate" default reason phrase should be
   changed to "Unsupported Credential".

13.3.  Identity-Info Parameters

   The IANA manages a registry for Identity-Info parameters.  The
   specification asks the IANA to change the name of this registry to
   "Identity Parameters".

   This specification defines one new value for the registry: "info" as
   defined in this specification in Section 7.3.







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13.4.  Identity-Info Algorithm Parameter Values

   This IANA manages an Identity-Info Algorithm Parameter Values
   registry which this specification deprecates.  We request that the
   IANA deprecate and close this registry.  Since the algorithms for
   signing PASSporTs are defined in [I-D.ietf-stir-passport] rather than
   in this specification, there is no longer a need for an algorithm
   parameter registry for the Identity header field.

14.  Acknowledgments

   The authors would like to thank Adam Roach, Jim Schaad, Ning Zhang,
   Syed Ali, Olle Jacobson, Dave Frankel, Robert Sparks, Dave Crocker,
   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, Christer Holmberg, 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; signing of key fingerprints in
      SDP is now included

      Deprecated the Identity-Info header field and relocated its
      components into parameters of the Identity header field (which
      obsoletes the previous version of the header field)

      The Identity header field can now appear multiple times in one
      request

      Replaced previous signed-identity-digest format with PASSporT
      (signing algorithms now defined in a separate specification)

      Revised status code descriptions

16.  References








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16.1.  Normative References

   [E.164]    ITU-T, "The international public telecommunication
              numbering plan", E 164, February 2005,
              <https://www.itu.int/rec/T-REC-E.164/en>.

   [I-D.ietf-stir-passport]
              Wendt, C. and J. Peterson, "Personal Assertion Token
              (PASSporT)", draft-ietf-stir-passport-10 (work in
              progress), October 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>.

   [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>.

   [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>.

   [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>.

   [RFC5922]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, DOI 10.17487/RFC5922, June 2010,
              <http://www.rfc-editor.org/info/rfc5922>.





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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-11 (work in progress), October 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>.

   [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>.

   [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>.



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   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5393]  Sparks, R., Ed., Lawrence, S., Hawrylyshen, A., and B.
              Campen, "Addressing an Amplification Vulnerability in
              Session Initiation Protocol (SIP) Forking Proxies",
              RFC 5393, DOI 10.17487/RFC5393, December 2008,
              <http://www.rfc-editor.org/info/rfc5393>.

   [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>.

   [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>.






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   [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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [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@cisco.com


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

   Email: ekr@rtfm.com



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   Chris Wendt
   Comcast
   One Comcast Center
   Philadelphia, PA  19103
   USA

   Email: chris-ietf@chriswendt.net












































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