path: root/doc/rfc/rfc8224.txt

Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 8224                                       NeuStar
Obsoletes: 4474                                              C. Jennings
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                              E. Rescorla
                                                              RTFM, Inc.
                                                                C. Wendt
                                                                 Comcast
                                                           February 2018


                   Authenticated Identity Management
                in the Session Initiation Protocol (SIP)

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.

   This document obsoletes RFC 4474.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8224.











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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://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 ..........................................5
   4. Identity Header Field Syntax ....................................7
      4.1. PASSporT Construction ......................................8
           4.1.1. Example Full and Compact Forms of PASSporT
                  in Identity ........................................10
   5. Example of Operations ..........................................11
      5.1. Example Identity Header Construction ......................13
   6. Signature Generation and Validation ............................14
      6.1. Authentication Service Behavior ...........................14
           6.1.1. Handling Repairable Errors .........................16
      6.2. Verifier Behavior .........................................17
           6.2.1. Authorization of Requests ..........................19
           6.2.2. Failure Response Codes Sent by a
                  Verification Service ...............................19
           6.2.3. Handling Retried Requests ..........................21
           6.2.4. Handling the Full Form of PASSporT .................21
   7. Credentials ....................................................22
      7.1. Credential Use by the Authentication Service ..............22
      7.2. Credential Use by the Verification Service ................23
      7.3. "info" Parameter URIs .....................................24
      7.4. Credential System Requirements ............................25
   8. Identity Types .................................................26
      8.1. Differentiating Telephone Numbers from URIs ...............26
      8.2. Authority for Telephone Numbers ...........................27
      8.3. Telephone Number Canonicalization Procedures ..............28
      8.4. Authority for Domain Names ................................29
      8.5. URI Normalization .........................................30
   9. Extensibility ..................................................31
   10. Backwards Compatibility with RFC 4474 .........................32



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   11. Privacy Considerations ........................................32
   12. Security Considerations .......................................34
      12.1. Protected Request Fields .................................34
           12.1.1. Protection of the To Header and Retargeting .......36
      12.2. Unprotected Request Fields ...............................37
      12.3. Malicious Removal of Identity Headers ....................37
      12.4. Securing the Connection to the Authentication Service ....38
      12.5. Authorization and Transitional Strategies ................39
      12.6. Display-Names and Identity ...............................40
   13. IANA Considerations ...........................................40
      13.1. SIP Header Fields ........................................40
      13.2. SIP Response Codes .......................................41
      13.3. Identity-Info Parameters .................................41
      13.4. Identity-Info Algorithm Parameter Values .................41
   14. Changes from RFC 4474 .........................................41
   15. References ....................................................42
      15.1. Normative References .....................................42
      15.2. Informative References ...................................43
   Acknowledgments ...................................................46
   Authors' Addresses ................................................46

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

   o  a canonical address-of-record (AoR) SIP URI employed to reach a
      user (such as "sip:alice@atlanta.example.com") or

   o  a telephone number, which commonly appears either in 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 such as 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 assurance of identity than the
   Caller ID services that the telephone network provides today, and one
   less vulnerable to spoofing.





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   [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 UAs 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 UAs need to be able to securely communicate their users'
   identities 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 [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 syntax and behavior specified
   by [RFC4474], reconsidering the problem space in light of the threat
   model in [RFC7375] and aligning the signature format with PASSporT
   (Personal Assertion Token) [RFC8225].  Backwards-compatibility
   considerations are given in Section 10.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119].

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

   o  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.

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

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





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3.  Architectural Overview

   The identity architecture for SIP defined in this specification
   depends on a logical "authentication service" that validates outgoing
   requests.  An authentication service may be implemented either as
   part of a UA or as a proxy server; typically, it is a component of a
   network intermediary like a proxy to which originating UAs send
   unsigned requests.  Once the originator of the message has been
   authenticated, through prearranged means with the authentication
   service, the authentication service then creates and adds an Identity
   header field to the request.  This 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 does not mandate or
   specify a credential system; [RFC8226] describes a credential system
   compatible with this architecture.

   Although addressing the vulnerabilities in the Secure Telephone
   Identity Revisited (STIR) problem statement [RFC7340] 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 UAs 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



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   credentials needed to prove that a user is authorized to use a
   particular From header field must ultimately derive from the domain
   owner: either (1) a UA gives requests to the domain name owner in
   order for them to be signed by the domain owner's credentials or
   (2) the UA must possess credentials that prove that the domain owner
   has given the UA 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 format [RFC8225], which is
   special encoding of a JSON [RFC8259] object 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.


























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4.  Identity Header Field Syntax

   The Identity and Identity-Info header fields that were previously
   defined in [RFC4474] are deprecated by this document.  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
   [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
   [RFC8225], which secures the request with a signature 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.
















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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:

   o  First, per the baseline PASSporT specification [RFC8225], the JSON
      "typ" key MUST have the value "passport".

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

   o  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.

   o  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:

   o  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 a "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.

   o  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



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      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.

   o  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 (sixty seconds is RECOMMENDED).

   o  Fourth, if the request contains a Session Description Protocol
      (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 [RFC8225],
      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
   multiple destination identities in PASSporT with such systems is left
   as a subject for future specifications.






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   After these two JSON objects, the header and the payload, have been
   constructed and base64-encoded, they must each be hashed and signed
   per [RFC8225], Section 6.  The header, payload, and signature
   components comprise a full PASSporT object.  The resulting PASSporT
   may be carried in SIP in either (1) a full form, which includes the
   header and payload as well as the signature or (2) a compact form,
   which only carries the signature per [RFC8225], 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 Elliptic Curve Digital
   Signature Algorithm (ECDSA) P-256 digital signature.

4.1.1.  Example Full and Compact Forms of PASSporT in Identity

   As Appendix F of the JSON Web Signature (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 [RFC8225],
   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 AoR sip:12155551212@example.com;user=phone, wants to communicate
   with Alice at sip:alice@example.com.  They have no prior
   relationship, and Alice implements best practices to prevent
   impersonation attacks.

   Bob's UA generates an INVITE and places his AoR 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 that contains
   a JSON representation of values that 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 12155551999), 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 UA server (UAS).  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 UA.


















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5.1.  Example Identity Header Construction

   For the following SIP request:

    INVITE sip:alice@example.com 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:12155551212@gateway.example.com>
    Content-Type: application/sdp
    Content-Length: ...

    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.com" 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 [RFC8225], Section 6.  That
   signature would look as follows:

   rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
    ojNCpTzO3QfPOlckGaS6hEck7w







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   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>

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 UA.  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 the From header field.  In order to determine whether



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   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.  The
   information in that section will lead to either the telephone number
   canonicalization procedures in Section 8.3 for telephone numbers or
   the URI normalization procedures described in Section 8.5 for domain
   names.  Whichever the result, if the authentication service is not
   authoritative for the identity in question, it SHOULD process and
   forward the request normally unless the local policy is to block such
   requests.  The authentication service MUST NOT add an Identity header
   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 the
   following:

   o  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).

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

   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



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   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.  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
   an "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 [RFC8225], Section 7; essentially, it contains only the signature
   component of the PASSporT.

   Note that per the behavior specified in [RFC8225], 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 ("Invalid Identity Header")
   response indicating a verification failure.  If the authentication



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   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 would retry such a request as a
   sequential fork, by reprocessing 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; this role is
   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
   specified below.  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 with Steps 2 through 5.  If a "ppt"
   parameter value is present and the verifier does not support it,



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   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.

   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.  The information in that section will lead to either the
   telephone number canonicalization procedures in Section 8.3 for
   telephone numbers or 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; this
   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 as
   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



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   in the Identity header field does not correspond to the reconstructed
   signed-identity-digest, then the Identity header field should be
   considered invalid.

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 UAs or proxy servers to
   follow based on the presence of a valid Identity header field, the
   presence of an invalid Identity header field, the absence of an
   Identity header field, or the presence of a stale Date header field
   value.  However, 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 UA implementations.

   The presence of multiple Identity header fields within a message
   raises the prospect that a verification service could receive a
   message containing both valid and invalid Identity header fields.  As
   a guideline, this specification recommends that only if a verifier
   determines that 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 a 403 "Forbidden" response when a stale Date header
   field value is received; see below.

   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





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   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
   the potential presence of multiple Identity header fields, this
   response code should only be sent when the verification service is
   unable to dereference 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" response (previously
   "Unsupported Certificate"; see Section 13.2) 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 certification authority
   (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.









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   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 to provide similar detailed
   feedback for debugging purposes.

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 [RFC8225],
   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, that 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



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   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.

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 UA 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, UAs 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 UAs (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.






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   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
   the 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"
   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 by the Third Generation Partnership Project
   (3GPP) [TS-3GPP.23.228], 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 in Section 7.3) MUST be supported by all verification
   service implementations to create a baseline means of credential



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   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 dereference 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.

   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 by 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 that 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 UA 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.




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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.,
   DNS-Based Authentication of Named Entities (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.

   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 [RFC8226], but this
   specification can work with other credential systems; for example,
   using the DNS was proposed in [CIDER].

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

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

   o  how the verifier can learn the scope of the credential,

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

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

   o  how the associated credentials will support the mandatory signing
      algorithm(s) required by PASSporT [RFC8225].

   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



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   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).

8.  Identity Types

   The STIR problem statement [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 AoR 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 that
   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 the procedures used for a URI 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



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   user part of SIP URIs with the "user=phone" parameter conforms to the
   syntax of the tel URI scheme [RFC3966].  It is also possible for a
   tel URI to appear in SIP header fields outside the context of a SIP
   or Session Initiation Protocol Secure (SIPS) URI (e.g.,
   "tel:+17005551008").  Thus, in 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 [E.164], 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 [RFC8226].






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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:

   o  Implementations MUST drop any "+"s, internal dashes, parentheses,
      or other non-numeric characters, except for 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.

   o  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.

   o  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.

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










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   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
   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 [RFC3263] 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.



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   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.

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 that captures the AoR 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 the 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 [IRI-COMPARISON]) and that
   perfect normalization of URIs may not be possible in those
   environments.




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   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 could eliminate the
   scheme component.  A more exact definition is left to future
   specifications.

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 [RFC8225], 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 of 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 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 reasons of future compatibility.

   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.










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10.  Backwards Compatibility with RFC 4474

   This specification introduces several significant changes from the
   version of the Identity header field defined by [RFC4474].  However,
   due to the problems enumerated in [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 implementations
   compliant with [RFC4474], nor does it contain any related backwards-
   compatibility provisions.  Hypothetically, were an implementation
   compliant with [RFC4474] to receive messages containing this revised
   version of the Identity header field, it would likely fail the
   request with a 436 response code due to the absence of an
   Identity-Info header field (Section 4).  Implementations of this
   specification, for debugging purposes, might interpret a 436 with a
   reason phrase of "Bad Identity Info" (previously "Bad Identity-Info";
   see Section 13.2) as an indication that the request has failed
   because it reached a (hypothetical) verification service that is
   compliant with [RFC4474].

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 as both a privacy service as described in
   [RFC3323] and an authentication service.  Since a UA 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.



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   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
   and authenticated.  The use of the domain "anonymous.invalid" entails
   that no corresponding authority for the domain can exist, and as a
   consequence, authentication service functions for that domain are
   meaningless.  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 that 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



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   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.

   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).  This 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; for the purposes of this
   document, either the SIP AoR URI or an embedded telephone number
   provides the identity of a SIP user.  Note that in some deployments
   the identity of the originator may reside in P-Asserted-Identity
   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



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   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
   the behavior described in [RFC3893], Section 10, 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; to accommodate that
   type of scenario, alternative behavior might be necessary.
   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; this 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 of 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



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   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 they 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
   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 to 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 [SIP-RETARGET].

   Any means for authentication services or verifiers to anticipate
   retargeting is outside the scope of this document and is likely to
   have the same 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.





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12.2.  Unprotected Request Fields

   [RFC4474] originally provided protections for Contact, Call-ID, and
   CSeq.  This document removes protection for these fields.  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 the primary focus of this document, which is the
   prevention of the simple unauthorized claiming of an identity for the
   purposes of robocalling, voicemail hacking, or swatting.

   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
   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; the end result would be exactly the
   same.  Although it is possible that an intermediary-based
   authentication service could guarantee that no Via hops are inserted
   between the sending UA 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




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   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 UA-based authentication services, the assurance
   provided by this mechanism is strongest when a UA forms a direct
   connection, preferably one secured by TLS, to an intermediary-based
   authentication service.  The reasons for this are twofold:

   o  If a user does not receive a certificate from the authentication
      service over the TLS connection that corresponds to the expected
      domain (especially when the user receives a challenge via a
      mechanism such as Digest), then it is possible that a rogue server
      is attempting to pose as an authentication service for a domain
      that it does not control, possibly in an attempt to collect shared
      secrets for that domain.  A similar practice could be used for
      telephone numbers, though the application of certificates for
      telephone numbers to TLS is left as a matter for future study.

   o  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
   UA client (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.



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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"
   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 difficult
   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.

   o  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.

   o  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 SIP; 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




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   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 UAs might render the
   display-name portion of the From header field of a caller as the
   identity of the caller; there is a significant precedent in email
   user interfaces for this practice.  Securing the display-name
   component of the From header field value is outside the scope of this
   document but may be the subject of future work, such as through the
   "ppt" name mechanism.

   In the absence of signing the display-name, authentication services
   might check and validate it, and compare it to a list of acceptable
   display-names that may be used by the 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

   IANA has completed a number of actions described in this document.
   Primarily, the previous references to [RFC4474] in the "Session
   Initiation Protocol (SIP) Parameters" registry have been updated to
   point to this document, unless specified otherwise below.

13.1.  SIP Header Fields

   The Identity-Info header in the SIP "Header Fields" registry has been
   marked as deprecated by this document.

   Also, the Identity-Info header reserved the compact form "n" at its
   time of registration.  That compact form has been removed from the
   registry.  The Identity header, however, retains the compact form "y"
   reserved by [RFC4474].










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13.2.  SIP Response Codes

   The 436 "Bad Identity-Info" default reason phrase has been changed to
   "Bad Identity Info" in the SIP "Response Codes" registry.

   The 437 "Unsupported Certificate" default reason phrase has been
   changed to "Unsupported Credential".

13.3.  Identity-Info Parameters

   IANA manages a registry for Identity-Info parameters.  Per this
   specification, IANA has changed the name of this registry to
   "Identity Parameters".

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

13.4.  Identity-Info Algorithm Parameter Values

   IANA managed an "Identity-Info Algorithm Parameter Values" registry;
   per this specification, IANA has deprecated and closed this registry.
   Since the algorithms for signing PASSporTs are defined in [RFC8225]
   rather than in this specification, there is no longer a need for an
   algorithm parameter registry for the Identity header field.

14.  Changes from RFC 4474

   The following are salient changes from the original RFC 4474:

   o  The credential mechanism has been generalized; credential
      enrollment, acquisition, and trust are now outside the scope of
      this document.

   o  This document reduces 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.

   o  The Identity-Info header field has been deprecated, and its
      components have been relocated into parameters of the Identity
      header field (which obsoletes the previous version of the header
      field).

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







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   o  The previous signed-identity-digest format has been replaced with
      PASSporT (signing algorithms are now defined in a separate
      specification).

   o  Status code descriptions have been revised.

15.  References

15.1.  Normative References

   [E.164]    International Telecommunication Union, "The international
              public telecommunication numbering plan",
              ITU-T Recommendation E.164, November 2010,
              <https://www.itu.int/rec/T-REC-E.164/en>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3263>.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3986>.

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







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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5922>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

15.2.  Informative References

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

   [IRI-COMPARISON]
              Masinter, L. and M. Duerst, "Comparison, Equivalence and
              Canonicalization of Internationalized Resource
              Identifiers", Work in Progress, draft-ietf-iri-
              comparison-02, October 2012.

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

   [RFC4501]  Josefsson, S., "Domain Name System Uniform Resource
              Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
              <https://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, <https://www.rfc-editor.org/info/rfc4916>.

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

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

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





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   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,
              <https://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, <https://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,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

   [SIP-RETARGET]
              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements", Work in Progress,
              draft-peterson-sipping-retarget-00, February 2005.

   [SIP-RFC4474-CONCERNS]
              Rosenberg, J., "Concerns around the Applicability of
              RFC 4474", Work in Progress, draft-rosenberg-sip-rfc4474-
              concerns-00, February 2008.

   [TS-3GPP.23.228]
              3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP
              TS 23.228 7.7.0, March 2007,
              <http://www.3gpp.org/ftp/Specs/html-info/23228.htm>.




















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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 Kyzivat, 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.

Authors' Addresses

   Jon Peterson
   Neustar, Inc.
   1800 Sutter St. Suite 570
   Concord, CA  94520
   United States of America

   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
   United States of America

   Email: ekr@rtfm.com


   Chris Wendt
   Comcast
   One Comcast Center
   Philadelphia, PA  19103
   United States of America

   Email: chris-ietf@chriswendt.net





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