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Versions: (draft-jennings-stir-rfc4474bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 8224

Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                             C. Jennings
Expires: August 6, 2016                                            Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                                C. Wendt
                                                                 Comcast
                                                        February 3, 2016


  Authenticated Identity Management in the Session Initiation Protocol
                                 (SIP)
                   draft-ietf-stir-rfc4474bis-07.txt

Abstract

   The baseline security mechanisms in the Session Initiation Protocol
   (SIP) are inadequate for cryptographically assuring the identity of
   the end users that originate SIP requests, especially in an
   interdomain context.  This document defines a mechanism for securely
   identifying originators of SIP requests.  It does so by defining a
   SIP header field for conveying a signature used for validating the
   identity, and for conveying a reference to the credentials of the
   signer.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on August 6, 2016.

Copyright Notice

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




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   6
   4.  Signature Generation and Validation . . . . . . . . . . . . .   7
     4.1.  Authentication Service Behavior . . . . . . . . . . . . .   7
     4.2.  Verifier Behavior . . . . . . . . . . . . . . . . . . . .   9
   5.  Credentials . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Credential Use by the Authentication Service  . . . . . .  11
     5.2.  Credential Use by the Verification Service  . . . . . . .  12
     5.3.  Handling 'info' parameter URIs  . . . . . . . . . . . . .  13
     5.4.  Credential System Requirements  . . . . . . . . . . . . .  13
   6.  Identity Types  . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Telephone Numbers . . . . . . . . . . . . . . . . . . . .  15
       6.1.1.  Canonicalization Procedures . . . . . . . . . . . . .  15
     6.2.  Domain Names  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Header Syntax . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Gatewaying to PASSporT for non-SIP Transit  . . . . . . . . .  22
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  24
     11.1.  Protected Request Fields . . . . . . . . . . . . . . . .  24
       11.1.1.  Protection of the To Header and Retargeting  . . . .  26
     11.2.  Unprotected Request Fields . . . . . . . . . . . . . . .  26
     11.3.  Malicious Removal of Identity Headers  . . . . . . . . .  27
     11.4.  Securing the Connection to the Authentication Service  .  28
     11.5.  Authorization and Transitional Strategies  . . . . . . .  29
     11.6.  Display-Names and Identity . . . . . . . . . . . . . . .  30
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Identity-Info Parameters . . . . . . . . . . . . . . . .  30
     12.2.  Identity-Info Algorithm Parameter Values . . . . . . . .  30
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   14. Changes from RFC4474  . . . . . . . . . . . . . . . . . . . .  31
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     15.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34



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1.  Introduction

   This document provides enhancements to the existing mechanisms for
   authenticated identity management in the Session Initiation Protocol
   (SIP, [RFC3261]).  An identity, for the purposes of this document, is
   defined as either a SIP URI, commonly a canonical address-of-record
   (AoR) employed to reach a user (such as
   'sip:alice@atlanta.example.com'), or a telephone number, which can be
   represented as either a TEL URI [RFC3966] or as the user portion of a
   SIP URI.

   [RFC3261] stipulates several places within a SIP request where users
   can express an identity for themselves, primarily the user-populated
   From header field.  However, the recipient of a SIP request has no
   way to verify that the From header field has been populated
   appropriately, in the absence of some sort of cryptographic
   authentication mechanism.  This leaves SIP vulnerable to a category
   of abuses, including impersonation attacks that enable robocalling
   and related problems as described in [RFC7340].  Ideally, a
   cryptographic approach to identity can provide a much stronger and
   less spoofable assurance of identity than the Caller ID services that
   the telephone network provides today.

   [RFC3261] specifies a number of security mechanisms that can be
   employed by SIP user agents (UAs), including Digest authentication,
   Transport Layer Security (TLS), and S/MIME (implementations may
   support other security schemes as well).  However, few SIP user
   agents today support the end-user certificates necessary to
   authenticate themselves (via S/MIME, for example), and furthermore
   Digest authentication is limited by the fact that the originator and
   destination must share a prearranged secret.  It is desirable for SIP
   user agents to be able to send requests to destinations with which
   they have no previous association.

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







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2.  Background

   Per [RFC7340], problems such as robocalling, voicemail hacking, and
   swatting are enabled by an attacker's ability to impersonate someone
   else.  The secure operation of most SIP applications and services
   depends on authorizing the source of communications as it is
   represented in a SIP request.  Such authorization policies can be
   automated or be a part of human operation of SIP devices.  An example
   of the former would be a voicemail service that compares the identity
   of the caller to a whitelist before determining whether it should
   allow the caller access to recorded messages.  An example of the
   latter would be an Internet telephone application that displays the
   calling party number (and/or Caller-ID) of a caller, which a human
   may review to make a policy decision before answering a call.  In
   both of these cases, attackers might attempt to circumvent these
   authorization policies through impersonation.  Since the primary
   identifier of the sender of a SIP request, the From header field, can
   be populated arbitrarily by the controller of a user agent,
   impersonation is very simple today in many environments.  The
   mechanism described in this document provides a strong identity
   system for detecting attempted impersonation in SIP requests.

   This identity architecture for SIP depends on a logical
   "authentication service" which validates outgoing requests; the
   authentication service may be implemented either as part of a user
   agent or as a proxy server.  Once the sender of the message has been
   authenticated, the authentication service then computes and adds
   cryptographic information (including a digital signature over some
   components of messages) to requests to communicate to other SIP
   entities that the sending user has been authenticated and its claim
   of a particular identity has been authorized.  A "verification
   service" on the receiving end then validates this signature and
   enables policy decisions to be made based on the results of the
   verification.

   Identities are issued to users by authorities.  When a new user
   becomes associated with example.com, the administrator of the SIP
   service for that domain can issue them an identity in that namespace,
   such as alice@example.com.  Alice may then send REGISTER requests to
   example.com that make her user agents eligible to receive requests
   for sip:alice@example.com.  In some cases, Alice may be the owner of
   the domain herself, and may issue herself identities as she chooses.
   But ultimately, it is the controller of the SIP service at
   example.com that must be responsible for authorizing the use of names
   in the example.com domain.  Therefore, for the purposes of baseline
   SIP, the credentials needed to prove a user is authorized to use a
   particular From header field must ultimately derive from the domain
   owner: either a user agent gives requests to the domain name owner in



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   order for them to be signed by the domain owner's credentials, or the
   user agent must possess credentials that prove in some fashion that
   the domain owner has given the user agent the right to a name.

   The situation is however more complicated for telephone numbers,
   however.  Authority over telephone numbers does not correspond
   directly to Internet domains.  While a user could register at a SIP
   domain with a username that corresponds to a telephone number, any
   connection between the administrator of that domain and the
   assignment of telephone numbers is not currently reflected on the
   Internet.  Telephone numbers do not share the domain-scope property
   described above, as they are dialed without any domain component.
   This document thus assumes the existence of a separate means of
   establishing authority over telephone numbers, for cases where the
   telephone number is the identity of the user.  As with SIP URIs, the
   necessary credentials to prove authority for a name might reside
   either in the endpoint or at some intermediary.

   This document specifies a means of sharing a cryptographic assurance
   of end-user SIP identity in an interdomain or intradomain context.
   It relies on the authentication service constructing tokens based on
   the [ietf-stir-passport] format, a JSON [RFC7159] object comprising
   values copied from certain header field values in the SIP request.
   The authentication service then computes a signature over those JSON
   object in a manner following PASSporT.  That signature is then placed
   in a SIP Identity header.  In order to assist in the validation of
   the Identity header, this specification also describes some metadata
   fields associated with the header that can be used by the recipient
   of a request to recover the credentials of the signer.  Note that the
   scope of this document is limited to providing this identity
   assurance for SIP requests; solving this problem for SIP responses is
   outside the scope of this work (see [RFC4916]).  Future work might
   specify ways that a SIP implementation could gateway PASSporT objects
   to other protocols.

   This specification allows either a user agent or a proxy server to
   provide the authentication service function and/or the verification
   service function.  To maximize end-to-end security, it is obviously
   preferable for end-users to acquire their own credentials; if they
   do, their user agents can act as authentication services.  However,
   for some deployments, end-user credentials may be neither practical
   nor affordable, given the potentially large number of SIP user agents
   (phones, PCs, laptops, PDAs, gaming devices) that may be employed by
   a single user.  In such environments, synchronizing keying material
   across multiple devices may be prohibitively complex and require
   quite a good deal of additional endpoint behavior.  Managing several
   credentials for the various devices could also be burdensome.  In
   these cases, implementation the authentication service at an



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   intermediary may be more practical.  This trade-off needs to be
   understood by implementers of this specification.

3.  Overview of Operations

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

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

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

   The authentication service authenticates Alice (possibly by sending a
   Digest authentication challenge) and validates that she is authorized
   to assert the identity that she populated in the From header field.
   This value is Alice's AoR, but in other cases it could be some
   different value that the proxy server has authority over, such as a
   telephone number.  The authentication service then constructs a JSON
   PASSporT object that mirrors particular SIP headers and fields,
   including part of the From header field of the message, and generates
   a hash of the object.  This hash is then signed with the appropriate
   credential for the identity (example.com, in the
   sip:alice@example.com case) and the signature is inserted by the
   proxy server into the Identity header field value of the request.

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

   When Bob's domain receives the request, it verifies the signature
   provided in the Identity header, and thus can validate that the
   authority over the identity in the From header field authenticated
   the user, and permitted the user to assert that From header field
   value.  This same validation operation may be performed by Bob's user
   agent server (UAS).  As the request has been validated, it is
   rendered to Bob. If the validation was unsuccessful, some other
   treatment would be applied by the receiving domain.




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4.  Signature Generation and Validation

4.1.  Authentication Service Behavior

   This document specifies a role for SIP entities called an
   authentication service.  The authentication service role can be
   instantiated, for example, by an intermediary such as a proxy server
   or by a user agent.  Any entity that instantiates the authentication
   service role MUST possess the private key of one or more credentials
   that can be used to sign for a domain or a telephone number (see
   Section 5.1).  Intermediaries that instantiate this role MUST be
   capable of authenticating one or more SIP users who can register for
   that identity.  Commonly, this role will be instantiated by a proxy
   server, since these entities are more likely to have a static
   hostname, hold corresponding credentials, and have access to SIP
   registrar capabilities that allow them to authenticate users.  It is
   also possible that the authentication service role might be
   instantiated by an entity that acts as a redirect server, but that is
   left as a topic for future work.

   An authentication service adds the Identity header to SIP requests.
   The procedures below define the steps that must be taken when each an
   header is added.  More than one may appear in a single request, and
   an authentication service may add an Identity header to a request
   that already contains one or more Identity headers.  If the Identity
   header added follows extended signing procedures beyond the baseline
   given in Section 7, then it differentiates the header with a "type"
   parameter per the fourth step below.

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

   Step 1:

   First, the authentication service must determine whether it is
   authoritative for the identity of the sender of the request.  In
   ordinary operations, the authentication service decides this by
   inspecting the URI value from the addr-spec component of From header
   field; this URI will be referred to here as the 'identity field'.  If
   the identity field contains a SIP or SIP Secure (SIPS) URI, and the
   user portion is not a telephone number, the authentication service
   MUST extract the hostname portion of the identity field and compare
   it to the domain(s) for which it is responsible (following the
   procedures in RFC 3261 [RFC3261], Section 16.4).  If the identity
   field uses the TEL URI scheme [RFC3966], or the identity field is a
   SIP or SIPS URI with a telephone number in the user portion, the
   authentication service determines whether or not it is responsible



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   for this telephone number; see Section 6.1 for more information.  An
   authentication service proceeding with a signature over a telephone
   number MUST then follow the canonicalization procedures described in
   Section 6.1.1.  If the authentication service is not authoritative
   for the identity in question, it SHOULD process and forward the
   request normally unless the local policy is to block such requests.
   The authentication service MUST NOT follow the steps below to add an
   Identity header corresponding to an identity for which the
   authentication service is not authoritative.

   Step 2:

   The authentication service MUST then determine whether or not the
   sender of the request is authorized to claim the identity given in
   the identity field.  In order to do so, the authentication service
   MUST authenticate the sender of the message.  Some possible ways in
   which this authentication might be performed include:

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

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

   Authorization of the use of a particular username or telephone number
   in the user part of the From header field is a matter of local policy
   for the authentication service; see Section 5.1 for more information.

   Note that this check is performed only on the addr-spec in the
   identity field (e.g., the URI of the sender, like
   'sip:alice@atlanta.example.com'); it does not convert the display-
   name portion of the From header field (e.g., 'Alice Atlanta').  For
   more information, see Section 11.6.

   Step 3:

   An authentication service MUST add a Date header field to SIP
   requests that do not have one.  The authentication service MUST
   ensure that any preexisting Date header in the request is accurate.
   Local policy can dictate precisely how accurate the Date must be; a
   RECOMMENDED maximum discrepancy of sixty seconds will ensure that the
   request is unlikely to upset any verifiers.  If the Date header
   contains a time different by more than one minute from the current
   time noted by the authentication service, the authentication service



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   SHOULD reject the request.  This behavior is not mandatory because a
   user agent client (UAC) could only exploit the Date header in order
   to cause a request to fail verification; the Identity header is not
   intended to provide a source of non-repudiation or a perfect record
   of when messages are processed.  Finally, the authentication service
   MUST verify that both the Date header and the current time fall
   within the validity period of its credential.

   See Section 11 for information on how the Date header field assists
   verifiers.

   Step 4:

   Subsequently, the authentication service MUST form a PASSporT object
   and add a corresponding an Identity header to the request containing
   this signature.  For baseline PASSporT objects headers (without an
   Identity header "type" parameter), this follows the procedures in
   Section 7; if the authentication service is using an alternative
   "type", it MUST add an appropriate "type" parameter and follow the
   procedures associated with it (see Section 8).  After the Identity
   header has been added to the request, the authentication service MUST
   also add a "info" parameter to the Identity header.  The "info"
   parameter contains a URI from which the authentication service's
   credential can be acquired; see Section 5.3 for more on credential
   acquisition.

   Finally, the authentication service MUST forward the message
   normally.

4.2.  Verifier Behavior

   This document specifies a logical role for SIP entities called a
   verification service, or verifier.  When a verifier receives a SIP
   message containing one or more Identity headers, it inspects the
   signature to verify the identity of the sender of the message.  The
   results of a verification are provided as input to an authorization
   process that is outside the scope of this document.

   A SIP request may contain zero, one, or more Identity headers.  A
   verification service performs the procedures below on each Identity
   header that appears in a request.  If the verifier does not support
   an Identity header present in a request due to the presence of an
   unsupported "type" parameter, or if no Identity header is present,
   and the presence of an Identity header is required by local policy
   (for example, based on a per-sending-domain policy, or a per-sending-
   user policy), then a 428 'Use Identity Header' response MUST be sent
   in the backwards direction.




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

   Step 1:

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

   Step 2:

   In order to determine whether the signature for the identity field
   should be over the entire identity field URI or just a canonicalized
   telephone number, the verification service MUST follow the
   canonicalization process described in Section 6.1.1.  That section
   also describes the procedures the verification service MUST follow to
   determine if the signer is authoritative for a telephone number.  For
   domains, the verifier MUST follow the process described in
   Section 6.2 to determine if the signer is authoritative for the
   identity field.

   Step 3:

   The verifier must first ensure that it possesses the proper keying
   material to validate the signature in the Identity header field,
   which usually involves dereferencing a URI in the "info" parameter of
   the Identity header.  See Section 5.2 for more information on these
   procedures.  If the verifier does not suport the credential described
   in the "info" parameter, it MUST return a 437 "Unsupported
   Certificate" response.

   Step 4:

   The verifier MUST furthermore ensure that the value of the Date
   header meets local policy for freshness (usually, within sixty
   seconds) and that it falls within the validity period of the
   credential used to sign the Identity header.  For more on the attacks
   this prevents, see Section 11.1.

   Step 5:

   The verifier MUST validate the signature in the Identity header field
   over the PASSporT object.  For baseline PASSporT objects (with no



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   Identity header "type" parameter) the verifier MUST follow the
   procedures for generating the signature over a PASSporT object
   described in Section 7.  If a "type" parameter is present, the
   verifier follows the procedures for that "type" (see Section 8).  If
   a verifier determines that the signature on the message does not
   correspond to the reconstructed signed-identity-digest, then a 438
   'Invalid Identity Header' response MUST be returned.

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

5.  Credentials

5.1.  Credential Use by the Authentication Service

   In order to act as an authentication service, a SIP entity must have
   access to the private keying material of one or more credentials that
   cover domain names or telephone numbers.  These credentials may
   represent authority over an entire domain (such as example.com) or
   potentially a set of domains enumerated by the credential.
   Similarly, a credential may represent authority over a single
   telephone number or a range of telephone numbers.  The way that the
   scope of a credential is expressed is specific to the credential
   mechanism.

   Authorization of the use of a particular username or telephone number
   in the identity field is a matter of local policy for the
   authentication service, one that depends greatly on the manner in
   which authentication is performed.  For non-telephone number user
   parts, one policy might be as follows: the username given in the
   'username' parameter of the Proxy-Authorization header MUST
   correspond exactly to the username in the From header field of the
   SIP message.  However, there are many cases in which this is too
   limiting or inappropriate; a realm might use 'username' parameters in
   Proxy-Authorization that do not correspond to the user-portion of SIP
   From headers, or a user might manage multiple accounts in the same
   administrative domain.  In this latter case, a domain might maintain
   a mapping between the values in the 'username' parameter of Proxy-
   Authorization and a set of one or more SIP URIs that might
   legitimately be asserted for that 'username'.  For example, the
   username can correspond to the 'private identity' as defined in Third



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   Generation Partnership Project (3GPP), in which case the From header
   field can contain any one of the public identities associated with
   this private identity.  In this instance, another policy might be as
   follows: the URI in the From header field MUST correspond exactly to
   one of the mapped URIs associated with the 'username' given in the
   Proxy-Authorization header.  This is a suitable approach for
   telephone numbers in particular.

   This specification could also be used with credentials that cover a
   single name or URI, such as 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 implementors should note
   this as a possible future direction.

   Exceptions to such authentication service policies arise for cases
   like anonymity; if the AoR asserted in the From header field uses a
   form like '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.

5.2.  Credential Use by the Verification Service

   In order to act as a verification service, a SIP entity must have a
   way to acquire and retain credentials for authorities over particular
   domain names and/or telephone numbers or number ranges.
   Dereferencing the URI found in the "info" parameter of the Identity
   header (as described in the next section) MUST be supported by all
   verification service implementations to create a baseline means of
   credential acquisition.  Provided that the credential used to sign a
   message is not previously known to the verifier, SIP entities SHOULD
   discover this credential by dereferencing the "info" parameter,
   unless they have some more other implementation-specific way of
   acquiring the needed keying material, such as an offline store of
   periodically-updated credentials.  If the URI in the "info" parameter
   cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST
   be returned.

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

   Verification service implementations supporting this specification
   may wish to have some means of retaining credentials (in accordance



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   with normal practices for credential lifetimes and revocation) in
   order to prevent themselves from needlessly downloading the same
   credential every time a request from the same identity is received.
   Credentials cached in this manner may be indexed in accordance with
   local policy: for example, by their scope, or the URI given in the
   "info" parameter value.  Further consideration of how to cache
   credentials is deferred to the credential mechanism specifications.

5.3.  Handling 'info' parameter URIs

   An "info" parameter MUST contain a URI which dereferences to a
   resource that contains the public key components of the credential
   used by the authentication service to sign a request.  It is
   essential that a URI in the "info parameter" be dereferencable by any
   entity that could plausibly receive the request.  For common cases,
   this means that the URI must be dereferencable by any entity on the
   public Internet.  In constrained deployment environments, a service
   private to the environment might be used instead.

   Beyond providing a means of accessing credentials for an identity,
   the "info" parameter further serves as a means of differentiating
   which particular credential was used to sign a request, when there
   are potentially multiple authorities eligible to sign.  For example,
   imagine a case where a domain implements the authentication service
   role for a range of telephone and a user agent belonging to Alice has
   acquired a credential for a single telephone number within that
   range.  Either would be eligible to sign a SIP request for the number
   in question.  Verification services however need a means to
   differentiate which one performed the signature.  The "info"
   parameter performs that function.

   If the optional "canon" parameter is present, it contains the bae64
   encoded result of JSON object construction process performed by the
   authentication service (see Section 6.1.1), including the
   canonicalization processes applied to the identity in the identity
   fields of the sender and intended recipient.  The "canon" is provided
   purely as an optimization for the verification service.  The
   verification service MAY compute its own canonicalization of the
   numbers and compare them to the values in the "canon" parameter
   before performing any cryptographic functions in order to ascertain
   whether or not the two ends agree on the canonical number form.

5.4.  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.,



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   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 form 'application/pkix-cert' pointing to a
   certificate (following the conventions of [RFC2585]).  The "info"
   parameter may use the DNS URL scheme (see [RFC4501]) to designate
   keys in the DNS.

   While no comparable public credentials exist for telephone numbers,
   either approach could be applied to telephone numbers.  A credential
   system based on certificates is given in
   [I-D.ietf-stir-certificates].  One based on the domain name system is
   given in [I-D.kaplan-stir-cider].

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

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

      how the verifier can learn the scope of the credential

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

      any algorithms that would appear in the Identity-Info "alg"
      parameter other than 'RS256.'  Note that the policy for adding
      algorithms to this registry requires Standards Action

   SIP entities cannot reliably predict where SIP requests will
   terminate.  When choosing a credential scheme for deployments of this
   specification, it is therefore essential that the trust anchor(s) for
   credentials be widely trusted, or that deployments restrict the use
   of this mechanism to environments where the reliance on particular
   trust anchors is assured by business arrangements or similar
   constraints.

   Note that credential systems must address key lifecycle management
   concerns: were a domain to change the credential available at the
   Identity-Info URI before a verifier evaluates a request signed by an
   authentication service, this would cause obvious verifier failures.
   When a rollover occurs, authentication services SHOULD thus provide
   new Identity-Info URIs for each new credential, and SHOULD continue
   to make older key acquisition URIs available for a duration longer
   than the plausible lifetime of a SIP transaction (a minute would most
   likely suffice).





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6.  Identity Types

6.1.  Telephone Numbers

   Since many SIP applications provide a Voice over IP (VoIP) service,
   telephone numbers are commonly used as identities in SIP deployments.
   In order for telephone numbers to be used with the mechanism
   described in this document, authentication services must enroll with
   an authority that issues credentials for telephone numbers or
   telephone number ranges, and verification services must trust the
   authority employed by the authentication service that signs a
   request.  Enrollment procedures and credential management are outside
   the scope of this document.

   In the longer term, it is possible that some directory or other
   discovery mechanism may provide a way to determine which
   administrative domain is responsible for a telephone number, and this
   may aid in the signing and verification of SIP identities that
   contain telephone numbers.  This is a subject for future work.

   In order to work with any such authorities, authentication and
   verification services must be able to identify when a request should
   be signed by an authority for a telephone number, and when it should
   be signed by an authority for a domain.  Telephone numbers most
   commonly appear in SIP header field values in the username portion of
   a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone').
   The user part of that URI conforms to the syntax of the TEL URI
   scheme (RFC 3966 [RFC3966]).  It is also possible for a TEL URI to
   appear in the SIP To or From header field outside the context of a
   SIP or SIPS URI (e.g., 'tel:+17005551008').  In both of these cases,
   it's clear that the signer must have authority over the telephone
   number, not the domain name of the SIP URI.  It is also possible,
   however, for requests to contain a URI like
   'sip:7005551000@chicago.example.com'.  It may be non-trivial for a
   service to ascertain in this case whether the URI contains a
   telephone number or not.

6.1.1.  Canonicalization Procedures

   In order to determine whether or not the user portion of a SIP URI is
   a telephone number, authentication services and verification services
   must perform the following canonicalization procedure on any SIP URI
   they inspect which contains a wholly numeric user part.  Note that
   the same procedures are followed for creating the canonical form of
   URIs found in both the From and To header field values; this section
   also describes procedures for extracting the URI containing the
   telephone number from the P-Asserted-Identity header field value for
   environments where that is applicable.



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   In some networks, the P-Asserted-Identity header field value is used
   in lieu of the From header field to convey the telephone number of
   the sender of a request; while it is not envisioned that most of
   those networks would or should make use of the Identity mechanism
   described in this specification, where they do, local policy might
   therefore dictate that the canonical string derive from the P-
   Asserted-Identity header field rather than the From.  In any case
   where local policy canonicalizes the number into a form different
   from how it appears in the From header field, the use of the "canon"
   parameter by authentication services is RECOMMENDED, but because
   "canon" itself could then divulge information about users or
   networks, implementers should be mindful of the guidelines in
   Section 10.

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

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

      Next, an implementation must assess if the number string is a
      valid, globally-routable number with a leading country code.  If
      not, implementations SHOULD convert the number into E.164 format,
      adding a country code if necessary; this may involve transforming
      the number from a dial string (see [RFC3966]), removing any
      national or international dialing prefixes or performing similar
      procedures.  It is only in the case that an implementation cannot
      determine how to convert the number to a globally-routable format
      that this step may be skipped.  This will be the case, for
      example, for nationally-specific service numbers (e.g. 911, 112);
      however, the routing procedures associated with those numbers will




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      likely make sure that the verification service understands the
      context of their use.

      Oher transformations during canonicalization MAY be made in
      accordance with specific policies used within a local domain.  For
      example, one domain may only use local number formatting and need
      to convert all To/From user portions to E.164 by prepending
      country-code and region code digits; another domain might prefix
      usernames with trunk-routing codes and need to remove the prefix.
      This specification cannot anticipate all of the potential
      transformations that might be useful.

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

   The ABNF of this number string is:

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

   If the result of this procedure forms a complete telephone number,
   that number is used for the purpose of creating and signing the
   signed-identity-string by both the authentication service and
   verification service.  Practically, entities that perform the
   authentication service role will sometimes alter the telephone
   numbers that appear in the To and From header field values,
   converting them to this format (though note this is not a function
   that [RFC3261] permits proxy servers to perform).  The result of the
   canonicalization process of the From header field value may also be
   recorded through the use of the "canon" parameter of the Identity(see
   Section 7).  If the result of the canonicalization of the From header
   field value does not form a complete telephone number, the
   authentication service and verification service should treat the
   entire URI as a SIP URI, and apply a domain signature per the
   procedures in Section 6.2.

6.2.  Domain Names

   When a verifier processes a request containing an Identity-Info
   header with a domain signature, it must compare the domain portion of
   the URI in the From header field of the request with the domain name
   that is the subject of the credential acquired from the "info"
   parameter.  While it might seem that this should be a straightforward
   process, it is complicated by two deployment realities.  In the first
   place, credentials have varying ways of describing their subjects,
   and may indeed have multiple subjects, especially in 'virtual
   hosting' cases where multiple domains are managed by a single
   application.  Secondly, some SIP services may delegate SIP functions
   to a subordinate domain and utilize the procedures in RFC 3263



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   [RFC3263] that allow requests for, say, 'example.com' to be routed to
   'sip.example.com'.  As a result, a user with the AoR
   'sip:jon@example.com' may process requests through a host like
   'sip.example.com', and it may be that latter host that acts as an
   authentication service.

   To meet the second of these problems, a domain that deploys an
   authentication service on a subordinate host MUST be willing to
   supply that host with the private keying material associated with a
   credential whose subject is a domain name that corresponds to the
   domain portion of the AoRs that the domain distributes to users.
   Note that this corresponds to the comparable case of routing inbound
   SIP requests to a domain.  When the NAPTR and SRV procedures of RFC
   3263 are used to direct requests to a domain name other than the
   domain in the original Request-URI (e.g., for 'sip:jon@example.com',
   the corresponding SRV records point to the service
   'sip1.example.org'), the client expects that the certificate passed
   back in any TLS exchange with that host will correspond exactly with
   the domain of the original Request-URI, not the domain name of the
   host.  Consequently, in order to make inbound routing to such SIP
   services work, a domain administrator must similarly be willing to
   share the domain's private key with the service.  This design
   decision was made to compensate for the insecurity of the DNS, and it
   makes certain potential approaches to DNS-based 'virtual hosting'
   unsecurable for SIP in environments where domain administrators are
   unwilling to share keys with hosting services.

   A verifier MUST evaluate the correspondence between the user's
   identity and the signing credential by following the procedures
   defined in RFC 2818 [RFC2818], Section 3.1.  While RFC 2818 [RFC2818]
   deals with the use of HTTP in TLS and is specific to certificates,
   the procedures described are applicable to verifying identity if one
   substitutes the "hostname of the server" in HTTP for the domain
   portion of the user's identity in the From header field of a SIP
   request with an Identity header.

7.  Header Syntax

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






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

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


   In addition to "info" parameter and the "alg" parameter defined in
   RFC44744, this specification includes the optional "canon" and "type"
   parameters.  Note that in RFC4474, the signed-identity-digest (see
   ABNF above) was given as quoted 32LHEX, whereas here it is given as a
   quoted sequence of base64-char.

   The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see
   Section 5.3 for more on choosing how to advertise credentials through
   this parameter.

   The signed-identity-digest is a signed hash of a [ietf-stir-passport]
   object, which is a pair of JSON objects generated from certain
   components of a SIP request.  This first object contains header
   information, and the second contains claims, following the
   conventions of JWT [RFC7519].  Once these two JSON objects have been
   generated, they will be encoded per the procedures of [ietf-stir-
   passport], then hashed with a SHA-256 hash and then concatenated,
   header then claims, into a string separated by a single "." per the
   conventions of baseline PASSporT.

   To create the PASSporT object used in the construction of the signed-
   identity-digest of the Identity header, the following elements of a
   SIP message MUST be placed in a first comma-separated JSON object, in
   order:

      First, the JSON key "typ" followed by a colon and then the quoted
      string "PASSporT".

      Second, the JSON key "alg" followed by a colon and then the quoted
      value of the optional "alg" parameter in the Identity header.
      Note if the "alg" parameter is absent, the default value is
      "RS256".

      Third, the JSON key "x5u" followed by a colon and then the quoted
      value of the URI in the "info" parameter.



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      Fourth, optionally the JSON key "type" followed by a colon and
      then the quoted value of the "type" parameter of the Identity
      header.  If the "type" parameter is absent from the header, the
      "type" key will not appear in the JSON heaer object.

   For example:

   { "typ":"PASSporT",
     "alg":"RS256"
     "x5u":"https://www.example.com/cert.pkx" }

   To create the PASSporT claims JSON object used in the construction of
   the signed-identity-digest, the following elements of a SIP message
   MUST be placed in a comma-separated JSON object, in order:

      First, the JSON key "orig" followed by a colon and then the quoted
      identity.  If the user part of the AoR in the From header field of
      the request contains a telephone number, then the canonicalization
      of that number goes into the first slot (see Section 6.1.1).
      Otherwise, the first slot contains the AoR of the UA sending the
      message as taken from addr-spec of the From header field.

      Second, the JSON key "term" followed by a colon and the quoted
      target.  If the user part of the AoR in the To header field of the
      request contains a telephone number, then the canonicalization of
      that number goes into the second slot (again, see Section 6.1.1).
      Otherwise, the second slot contains the addr-spec component of the
      To header field, which is the AoR to which the request is being
      sent.

      Third, the JSON key "iat" followed by a colon and then a quoted
      encoding of the value of the SIP Date header field as a JSON
      NumericDate (as UNIX time, per [RFC7519] Section 2).

      Fourth, if the request contains an SDP message body, and if that
      SDP contains one or more "a=fingerprint" attributes, then the JSON
      key "mky" followed by a colon and then the quoted value(s) of the
      fingerprint attributes (if they differ).  Each attribute value
      consists of all characters following the colon after
      "a=fingerprint" including the algorithm description and
      hexadecimal key representation, any whitespace, carriage returns,
      and "/" line break indicators.  If multiple non-identical
      "a=fingerprint" attributes appear in an SDP body, then all non-
      identical attributes values MUST be concatenated, with no
      separating character, after sorting the values in alphanumeric
      order.  If the SDP body contains no "a=fingerprint" attribute,
      then no JSON "mky" key is added to the object.




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   For example:

      { "orig":"12155551212",
        "term":"12155551213",
        "iat": "1443208345",

   For more information on the security properties of these headers, and
   why their inclusion mitigates replay attacks, see Section 11 and
   [RFC3893].

   After these two JSON objects, the header and the claims, have been
   constructed as a PASSporT object, they must be hashed and signed;
   this then becomes the signed-identity-string.  The hashing and
   signing algorithm is specified by the 'alg' parameter of the Identity
   header.  This document defines only one value for the 'alg'
   parameter: 'RS256', as defined in [RFC7519], which connotes a SHA-256
   hash followed by a RSASSA-PKCS1-v1_5 signature.  All implementations
   of this specification MUST support 'RS256'.  Any further 'alg' values
   MUST be defined in a Standards Track RFC, see Section 12.2 for more
   information.  The result of the hash and signing of the two
   concatenated JSON objects is placed in the Identity header field.

   For example:

  Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
      eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
      pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \
          info=<https://biloxi.example.org/biloxi.cer>;alg=RS256

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

8.  Extensibility

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

   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



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

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

9.  Gatewaying to PASSporT for non-SIP Transit

   As defined in this specification, the signature in the Identity
   header is equivalent to the signature that would appear in a PASSporT
   token.  This is so that a valid PASSporT can be generated based on a
   SIP request containing an Identity header.  This PASSporT could then
   be transported in alternate protocols, stored in a repository and
   later accessed, or similarly used outside the context of establishing
   an end-to-end SIP session.

   Because the base64 encoding the JSON objects containing headers and
   claims can be quite long, and because the information it contains is
   necessarily redundant with information in the header field values of
   the SIP request itself, SIP does not require implementations to carry
   the base64 encodings of those objects.  The optional "canon"
   parameter of the Identity-Info, if present, contains the encoded
   objects used to generate the hash and signature (see Section 7), but
   if the "canon" parameter is not present, the contents of the objects
   can be regenerated by constructing the object anew from the SIP
   header fields received.

   Alternative transports for this PASSporT and their requirements are
   left to future specifications.

10.  Privacy Considerations

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





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   The identity mechanism presented in this document is compatible with
   the standard SIP practices for privacy described in [RFC3323].  A SIP
   proxy server can act both as a privacy service and as an
   authentication service.  Since a user agent can provide any From
   header field value that the authentication service is willing to
   authorize, there is no reason why private SIP URIs that contain
   legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
   by an authentication service.  The construction of the Identity
   header is the same for private URIs as it is for any other sort of
   URIs.

   Note, however, that even when using anonymous SIP URIs, an
   authentication service must possess a certificate corresponding to
   the host portion of the addr-spec of the From header field of the
   request; accordingly, using domains like 'anonymous.invalid' will not
   be possible for privacy services that also act as authentication
   services.  The assurance offered by the usage of anonymous URIs with
   a valid domain portion is "this is a known user in my domain that I
   have authenticated, but I am keeping its identity private".

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

   [RFC3325] defines the "id" priv-value token, which is specific to the
   P-Asserted-Identity header.  The sort of assertion provided by the P-
   Asserted-Identity header is very different from the Identity header
   presented in this document.  It contains additional information about
   the sender of a message that may go beyond what appears in the From
   header field; P-Asserted-Identity holds a definitive identity for the
   sender that is somehow known to a closed network of intermediaries.
   Presumably, that network will use this identity for billing or
   security purposes.  The danger of this network-specific information
   leaking outside of the closed network motivated the "id" priv-value
   token.  The "id" priv-value token has no implications for the
   Identity header, and privacy services MUST NOT remove the Identity
   header when a priv-value of "id" appears in a Privacy header.




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   The optional "canon" parameter of the Identity header specified in
   this document provides the complete JSON objects used to generate the
   signed-identity-digest of the Identity header, including the
   canonicalized form of the telephone number of the originator of a
   call, if the signature is over a telephone number.  In some contexts,
   local policy may require a canonicalization which differs
   substantially from the original From header field.  Depending on
   those policies, potentially the "canon" parameter might divulge
   information about the originating network or user that might not
   appear elsewhere in the SIP request.  Were it to be used to reflect
   the contents of the P-Asserted-Identity header field, for example,
   then "canon" would need to be removed when the P-Asserted-Identity
   header is removed to avoid any such leakage outside of a trust
   domain.  Since, in those contexts, the canonical form of the sender's
   identity could not be reassembled by a verifier, and thus the
   Identity signature validation process would fail, using P-Asserted-
   Identity with the Identity "canon" parameter in this fashion is NOT
   RECOMMENDED outside of environments where SIP requests will never
   leave the trust domain.  As a side note, history shows that closed
   networks never stay closed and one should design their implementation
   assuming connectivity to the broader Internet.

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

11.  Security Considerations

   This document describes a mechanism that provides a signature over
   the Date header field of SIP requests, parts of the To and From
   header fields, the request method, and when present any media keying
   material in the message body.  In general, the considerations related
   to the security of these headers are the same as those given in
   [RFC3261] for including headers in tunneled 'message/sip' MIME bodies
   (see Section 23 of RFC3261 in particular).  The following section
   details the individual security properties obtained by including each
   of these header fields within the signature; collectively, this set
   of header fields provides the necessary properties to prevent
   impersonation.  It addresses the solution-specific attacks against
   in-band solutions enumerated in [RFC7375] Section 4.1.

11.1.  Protected Request Fields

   The From header field value (in ordinary operations) indicates the
   identity of the sender of the message.  The SIP address-of-record
   URI, or an embedded telephone number, in the From header field is the
   identity of a SIP user, for the purposes of this document.  Note that
   in some deployments the identity of the sender may reside in P-



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   Asserted-Id instead.  The sender's identity is the key piece of
   information that this mechanism secures; the remainder of the signed
   parts of a SIP request are present to provide reference integrity and
   to prevent certain types of cut-and-paste attacks.

   The Date header field value protects against cut-and-paste attacks,
   as described in [RFC3261], Section 23.4.2.  Implementations of this
   specification MUST NOT deem valid a request with an outdated Date
   header field (the RECOMMENDED interval is that the Date header must
   indicate a time within 60 seconds of the receipt of a message).  Note
   that per baseline [RFC3261] behavior, servers keep state of recently
   received requests, and thus if an Identity header is replayed by an
   attacker within the Date interval, verifiers can detect that it is
   spoofed because a message with an identical Date from the same source
   had recently been received.

   The To header field value provides the identity of the SIP user that
   this request originally targeted.  Providing the To header field in
   the Identity signature serves two purposes.  First, it prevents cut-
   and-paste attacks in which an Identity header from legitimate request
   for one user is cut-and-pasted into a request for a different user.
   Second, it preserves the starting URI scheme of the request, which
   helps prevent downgrade attacks against the use of SIPS.  The To
   offers additional protection against cut-and-paste attacks beyond the
   Date header field.  For example, without a signature over the To, an
   attacker who receives a call from a target could immediately forward
   the INVITE to the target's voicemail service within the Date
   interval, and the voicemail service would have no way knowing that
   the Identity header it received had been originally signed for a call
   intended for a different number.  However, note the caveats below in
   Section 11.1.1.

   When signing a request that contains a fingerprint of keying material
   in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
   signature over that fingerprint.  This signature prevents certain
   classes of impersonation attacks in which an attacker forwards or
   cut-and-pastes a legitimate request.  Although the target of the
   attack may accept the request, the attacker will be unable to
   exchange media with the target as they will not possess a key
   corresponding to the fingerprint.  For example, there are some
   baiting attacks, launched with the REFER method or through social
   engineering, where the attacker receives a request from the target
   and reoriginates it to a third party.  These might not be prevented
   by only a signature over the From, To and Date, but could be
   prevented by securing a fingerprint for DTLS-SRTP.  While this is a
   different form of impersonation than is commonly used for
   robocalling, ultimately there is little purpose in establishing the
   identity of the user that originated a SIP request if this assurance



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   is not coupled with a comparable assurance over the contents of the
   subsequent media communication.  This signature also, per [RFC7258],
   reduces the potential for passive monitoring attacks against the SIP
   media.  In environments where DTLS-SRTP is unsupported, however, no
   field is signed and no protections are provided.

11.1.1.  Protection of the To Header and Retargeting

   The mechanism in this document provides a signature over the identity
   information in the To header field value of requests.  This provides
   a means for verifiers to detect replay attacks where a signed request
   originally sent to one target is modified and then forwarded by an
   attacker to another, unrelated target.  Armed with the original value
   of the To header field, the recipient of a request may compare it to
   their own identity in order to determine whether or not the identity
   information in this call might have been replayed.  However, any
   request may be legitimately retargeted as well, and as a result
   legitimate requests may reach a SIP endpoint whose user is not
   identified by the URI designated in the To header field value.  It is
   therefore difficult for any verifier to decide whether or not some
   prior retargeting was "legitimate."  Retargeting can also cause
   confusion when identity information is provided for requests sent in
   the backwards direction in a dialog, as the dialog identifiers may
   not match credentials held by the ultimate target of the dialog.  For
   further information on the problems of response identity see
   [I-D.peterson-sipping-retarget].

   Any means for authentication services or verifiers to anticipate
   retargeting is outside the scope of this document, and likely to have
   equal applicability to response identity as it does to requests in
   the backwards direction within a dialog.  Consequently, no special
   guidance is given for implementers here regarding the 'connected
   party' problem (see [RFC4916]); authentication service behavior is
   unchanged if retargeting has occurred for a dialog-forming request.
   Ultimately, the authentication service provides an Identity header
   for requests in the backwards dialog when the user is authorized to
   assert the identity given in the From header field, and if they are
   not, an Identity header is not provided.  And per the threat model of
   [RFC7375], resolving problems with 'connected' identity has little
   bearing on detecting robocalling or related impersonation attacks.

11.2.  Unprotected Request Fields

   RFC4474 originally had protections for the Contact, Call-ID and CSeq.
   These are removed from RFC4474bis.  The absence of these header
   values creates some opportunities for determined attackers to
   impersonate based on cut-and-paste attacks; however, the absence of
   these headers does not seem impactful to preventing the simple



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   unauthorized claiming of an identity for the purposes of robocalling,
   voicemail hacking, or swatting, which is the primary scope of the
   current document.

   It might seem attractive to provide a signature over some of the
   information present in the Via header field value(s).  For example,
   without a signature over the sent-by field of the topmost Via header,
   an attacker could remove that Via header and insert its own in a cut-
   and-paste attack, which would cause all responses to the request to
   be routed to a host of the attacker's choosing.  However, a signature
   over the topmost Via header does not prevent attacks of this nature,
   since the attacker could leave the topmost Via intact and merely
   insert a new Via header field directly after it, which would cause
   responses to be routed to the attacker's host "on their way" to the
   valid host, which has exactly the same end result.  Although it is
   possible that an intermediary-based authentication service could
   guarantee that no Via hops are inserted between the sending user
   agent and the authentication service, it could not prevent an
   attacker from adding a Via hop after the authentication service, and
   thereby preempting responses.  It is necessary for the proper
   operation of SIP for subsequent intermediaries to be capable of
   inserting such Via header fields, and thus it cannot be prevented.
   As such, though it is desirable, securing Via is not possible through
   the sort of identity mechanism described in this document; the best
   known practice for securing Via is the use of SIPS.

11.3.  Malicious Removal of Identity Headers

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








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11.4.  Securing the Connection to the Authentication Service

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

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

      Without TLS, the various header field values and the body of the
      request will not have integrity protection when the request
      arrives at an authentication service.  Accordingly, a prior
      legitimate or illegitimate intermediary could modify the message
      arbitrarily.

   Of these two concerns, the first is most material to the intended
   scope of this mechanism.  This mechanism is intended to prevent
   impersonation attacks, not man-in-the-middle attacks; integrity over
   the header and bodies is provided by this mechanism only to prevent
   replay attacks.  However, it is possible that applications relying on
   the presence of the Identity header could leverage this integrity
   protection for services other than replay protection.

   Accordingly, direct TLS connections SHOULD be used between the UAC
   and the authentication service whenever possible.  The opportunistic
   nature of this mechanism, however, makes it very difficult to
   constrain UAC behavior, and moreover there will be some deployment
   architectures where a direct connection is simply infeasible and the
   UAC cannot act as an authentication service itself.  Accordingly,
   when a direct connection and TLS are not possible, a UAC should use
   the SIPS mechanism, Digest 'auth-int' for body integrity, or both
   when it can.  The ultimate decision to add an Identity header to a
   request lies with the authentication service, of course; domain
   policy must identify those cases where the UAC's security association
   with the authentication service is too weak.







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11.5.  Authorization and Transitional Strategies

   Ultimately, the worth of an assurance provided by an Identity header
   is limited by the security practices of the authentication service
   that issues the assurance.  Relying on an Identity header generated
   by a remote administrative domain assumes that the issuing domain
   uses recommended administrative practices to authenticate its users.
   However, it is possible that some authentication services will
   implement policies that effectively make users unaccountable (e.g.,
   ones that accept unauthenticated registrations from arbitrary users).
   The value of an Identity header from such authentication services is
   questionable.  While there is no magic way for a verifier to
   distinguish "good" from "bad" signers by inspecting a SIP request, it
   is expected that further work in authorization practices could be
   built on top of this identity solution; without such an identity
   solution, many promising approaches to authorization policy are
   impossible.  That much said, it is RECOMMENDED that authentication
   services based on proxy servers employ strong authentication
   practices.

   One cannot expect the Identity header to be supported by every SIP
   entity overnight.  This leaves the verifier in a compromising
   position; when it receives a request from a given SIP user, how can
   it know whether or not the sender's domain supports Identity?  In the
   absence of ubiquitous support for identity, some transitional
   strategies are necessary.

      A verifier could remember when it receives a request from a domain
      or telephone number that uses Identity, and in the future, view
      messages received from that sources without Identity headers with
      skepticism.

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

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

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



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   authorization work not to assume that messages with valid Identity
   headers are always good.

11.6.  Display-Names and Identity

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

   In the absence of signing the display-name, authentication services
   might check and validate it, and compare it to a list of acceptable
   display-names that may be used by the sender; if the display-name
   does not meet policy constraints, the authentication service could
   return a 403 response code.  In this case, the reason phrase should
   indicate the nature of the problem; for example, "Inappropriate
   Display Name".  However, the display-name is not always present, and
   in many environments the requisite operational procedures for
   display-name validation may not exist, so no normative guidance is
   given here.

12.  IANA Considerations

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

12.1.  Identity-Info Parameters

   The IANA has already created a registry for Identity-Info parameters.
   This specification defines a new value called "canon" as defined in
   Section 5.3.  Note however that unlike in RFC4474, Identity-Info
   parameters now appear in the Identity header.

12.2.  Identity-Info Algorithm Parameter Values

   The IANA has already created a registry for Identity-Info "alg"
   parameter values.  This registry is to be populated with a value for
   'RS256', which describes the algorithm used to create the signature
   that appears in the Identity header.  Registry entries must contain
   the name of the 'alg' parameter value and the specification in which
   the value is described.  New values for the 'alg' parameter may be
   defined only in Standards Track RFCs.




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

   Future specifications may consider elliptical curves for smaller key
   sizes.

   Note that the Identity-Info header is also deprecated by this
   specification, and thus the "alg" parameter is now a value of the
   Identity header, not Identity-Info.

13.  Acknowledgments

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

14.  Changes from RFC4474

   The following are salient changes from the original RFC 4474:

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

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

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

      Added any DTLS-SRTP fingerprint in SDP as a mandatory element of
      the PASSporT

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

      Changed the signed-identity-digest format for compatibility with
      PASSporT

15.  References

15.1.  Normative References

   [I-D.wendt-verified-token]
              Wendt, C., "Verified Token", draft-wendt-verified-token-00
              (work in progress), October 2015.




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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <http://www.rfc-editor.org/info/rfc3261>.

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

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              DOI 10.17487/RFC3280, April 2002,
              <http://www.rfc-editor.org/info/rfc3280>.

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

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

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

15.2.  Informative References

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

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





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

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

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,
              <http://www.rfc-editor.org/info/rfc2585>.

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

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              DOI 10.17487/RFC3325, November 2002,
              <http://www.rfc-editor.org/info/rfc3325>.

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

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

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

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,
              <http://www.rfc-editor.org/info/rfc4474>.

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



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

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <http://www.rfc-editor.org/info/rfc5763>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <http://www.rfc-editor.org/info/rfc6973>.

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

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

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

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

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

Authors' Addresses








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

   Email: jon.peterson@neustar.biz


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

   Email: fluffy@iii.ca


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

   Email: ekr@rtfm.com


   Chris Wendt
   Comcast
   One Comcast Center
   Philadelphia, PA  19103
   USA

   Email: chris-ietf@chriswendt.net

















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