draft-ietf-stir-rfc4474bis-05.txt   draft-ietf-stir-rfc4474bis-06.txt 
Network Working Group J. Peterson Network Working Group J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Intended status: Standards Track C. Jennings Intended status: Standards Track C. Jennings
Expires: March 17, 2016 Cisco Expires: April 21, 2016 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
September 14, 2015 C. Wendt
Comcast
October 19, 2015
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-05.txt draft-ietf-stir-rfc4474bis-06.txt
Abstract Abstract
The baseline security mechanisms in the Session Initiation Protocol The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of (SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining new identifying originators of SIP requests. It does so by defining a
SIP header fields for conveying a signature used for validating the SIP header field for conveying a signature used for validating the
identity, and for conveying a reference to the credentials of the identity, and for conveying a reference to the credentials of the
signer. signer.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 17, 2016. This Internet-Draft will expire on April 21, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 23 skipping to change at page 2, line 26
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 6 3. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
4. Signature Generation and Validation . . . . . . . . . . . . . 7 4. Signature Generation and Validation . . . . . . . . . . . . . 7
4.1. Authentication Service Behavior . . . . . . . . . . . . . 7 4.1. Authentication Service Behavior . . . . . . . . . . . . . 7
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9 4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Credential Use by the Authentication Service . . . . . . 11 5.1. Credential Use by the Authentication Service . . . . . . 11
5.2. Credential Use by the Verification Service . . . . . . . 12 5.2. Credential Use by the Verification Service . . . . . . . 12
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 13 5.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 13
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 13 5.4. Credential System Requirements . . . . . . . . . . . . . 13
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14 6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14 6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14
6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15 6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15
6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21 9. Gatewaying to JWT for non-SIP Transit . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
10.1. Protected Request Fields . . . . . . . . . . . . . . . . 23 11. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10.1.1. Protection of the To Header and Retargeting . . . . 24 11.1. Protected Request Fields . . . . . . . . . . . . . . . . 24
10.2. Unprotected Request Fields . . . . . . . . . . . . . . . 25 11.1.1. Protection of the To Header and Retargeting . . . . 25
10.3. Malicious Removal of Identity Headers . . . . . . . . . 26 11.2. Unprotected Request Fields . . . . . . . . . . . . . . . 26
10.4. Securing the Connection to the Authentication Service . 26 11.3. Malicious Removal of Identity Headers . . . . . . . . . 27
10.5. Authorization and Transitional Strategies . . . . . . . 27 11.4. Securing the Connection to the Authentication Service . 27
10.6. Display-Names and Identity . . . . . . . . . . . . . . . 29 11.5. Authorization and Transitional Strategies . . . . . . . 28
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 11.6. Display-Names and Identity . . . . . . . . . . . . . . . 29
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 29 12.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 30
11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 29 12.2. Identity-Info Algorithm Parameter Values . . . . . . . . 30
11.4. Identity-Extension Names . . . . . . . . . . . . . . . . 30 12.3. spec parameter Names . . . . . . . . . . . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 30 14. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 31
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
14.1. Normative References . . . . . . . . . . . . . . . . . . 30 15.1. Normative References . . . . . . . . . . . . . . . . . . 31
14.2. Informative References . . . . . . . . . . . . . . . . . 31 15.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
This document provides enhancements to the existing mechanisms for This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol authenticated identity management in the Session Initiation Protocol
(SIP, [RFC3261]). An identity, for the purposes of this document, is (SIP, [RFC3261]). An identity, for the purposes of this document, is
defined as either a SIP URI, commonly a canonical address-of-record defined as either a SIP URI, commonly a canonical address-of-record
(AoR) employed to reach a user (such as (AoR) employed to reach a user (such as
'sip:alice@atlanta.example.com'), or a telephone number, which can be '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 represented as either a TEL URI [RFC3966] or as the user portion of a
SIP URI. SIP URI.
[RFC3261] stipulates several places within a SIP request where users [RFC3261] stipulates several places within a SIP request where users
can express an identity for themselves, primarily the user-populated can express an identity for themselves, primarily the user-populated
From header field. However, the recipient of a SIP request has no From header field. However, the recipient of a SIP request has no
way to verify that the From header field has been populated way to verify that the From header field has been populated
appropriately, in the absence of some sort of cryptographic appropriately, in the absence of some sort of cryptographic
authentication mechanism. This leaves SIP vulnerable to a category authentication mechanism. This leaves SIP vulnerable to a category
of abuses, including impersonation attacks that enable robocalling of abuses, including impersonation attacks that enable robocalling
and related problems as described in [RFC7340]. 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 [RFC3261] specifies a number of security mechanisms that can be
employed by SIP user agents (UAs), including Digest, Transport Layer employed by SIP user agents (UAs), including Digest authentication,
Security (TLS), and S/MIME (implementations may support other Transport Layer Security (TLS), and S/MIME (implementations may
security schemes as well). However, few SIP user agents today support other security schemes as well). However, few SIP user
support the end-user certificates necessary to authenticate agents today support the end-user certificates necessary to
themselves (via S/MIME, for example), and furthermore Digest authenticate themselves (via S/MIME, for example), and furthermore
authentication is limited by the fact that the originator and Digest authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which user agents to be able to send requests to destinations with which
they have no previous association. A cryptographic approach, like they have no previous association.
the one described in this document, can provide a much stronger and
less spoofable assurance of identity than the Caller ID services that
the telephone network provides today.
[RFC4474] previously specified a means of signing portions of SIP [RFC4474] previously specified a means of signing portions of SIP
requests in order to provide that identity assurance. However, RFC requests in order to provide an identity assurance. However, RFC
4474 was in several ways misaligned with deployment realities (see 4474 was in several ways misaligned with deployment realities (see
[I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474 [I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474
did not deal well with telephone numbers as identifiers, despite did not deal well with telephone numbers as identifiers, despite
their enduring use in SIP deployments. RFC 4474 also provided a their enduring use in SIP deployments. RFC 4474 also provided a
signature over material that intermediaries in the field commonly signature over material that intermediaries in the field commonly
altered. This specification therefore revises RFC 4474 in light of altered. This specification therefore revises RFC 4474 in light of
recent reconsideration of the problem space to align with the threat recent reconsideration of the problem space to align with the threat
model in [RFC7375]. model in [RFC7375].
2. Background 2. Background
Per [RFC7340], a key enabler of problems such as robocalling, Per [RFC7340], problems such as robocalling, voicemail hacking, and
voicemail hacking, and swatting lies in an attacker's ability to swatting are enabled by an attacker's ability to impersonate someone
impersonate someone else. This secure operation of most SIP else. The secure operation of most SIP applications and services
applications and services depends on authorizing the source of depends on authorizing the source of communications as it is
communications as it is represented in a SIP request. Such represented in a SIP request. Such authorization policies can be
authorization policies can be automated or be a part of human automated or be a part of human operation of SIP devices. An example
operation of SIP devices. An example of the latter would be an of the former would be a voicemail service that compares the identity
Internet telephone application that displays the calling party number of the caller to a whitelist before determining whether it should
(and/or Caller-ID) of a caller, which a human may review to make a allow the caller access to recorded messages. An example of the
policy decision before answering a call. An example of the former latter would be an Internet telephone application that displays the
would be a voicemail service that compares the identity of the caller calling party number (and/or Caller-ID) of a caller, which a human
to a whitelist before determining whether it should allow the caller may review to make a policy decision before answering a call. In
access to recorded messages. In both of these cases, attackers might both of these cases, attackers might attempt to circumvent these
attempt to circumvent these authorization policies through authorization policies through impersonation. Since the primary
impersonation. Since the primary identifier of the sender of a SIP identifier of the sender of a SIP request, the From header field, can
request, the From header field, can be populated arbitrarily by the be populated arbitrarily by the controller of a user agent,
controller of a user agent, impersonation is very simple today. The impersonation is very simple today in many environments. The
mechanism described in this document provides a strong identity mechanism described in this document provides a strong identity
system for detecting attempted impersonation in SIP requests. system for detecting attempted impersonation in SIP requests.
This identity architecture for SIP depends on a logical This identity architecture for SIP depends on a logical
"authentication service" which processes and signs requests; it may "authentication service" which validates outgoing requests; the
be implemented either as part of a user agent or as a proxy server. authentication service may be implemented either as part of a user
Once the sender of the message has been authenticated, the service agent or as a proxy server. Once the sender of the message has been
then adds new cryptographic information to requests to communicate to authenticated, the authentication service then computes and adds
other SIP entities that the sending user has been authenticated and cryptographic information (including a digital signature over some
its claim of a particular identity has been authorized. A components of messages) to requests to communicate to other SIP
"verification service" on the receiving end then validates this entities that the sending user has been authenticated and its claim
signature and enables policy decisions to be made based on the of a particular identity has been authorized. A "verification
results of the 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 Identities are issued to users by authorities. When a new user
becomes associated with example.com, the administrator of the SIP becomes associated with example.com, the administrator of the SIP
service for that domain can issue them an identity in that namespace, service for that domain can issue them an identity in that namespace,
such as alice@example.com. Alice may then send REGISTER requests to such as alice@example.com. Alice may then send REGISTER requests to
example.com that make her user agents eligible to receive requests 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 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. the domain herself, and may issue herself identities as she chooses.
But ultimately, it is the controller of the SIP service at But ultimately, it is the controller of the SIP service at
example.com that must be responsible for authorizing the use of names example.com that must be responsible for authorizing the use of names
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Internet. Telephone numbers do not share the domain-scope property Internet. Telephone numbers do not share the domain-scope property
described above, as they are dialed without any domain component. described above, as they are dialed without any domain component.
This document thus assumes the existence of a separate means of This document thus assumes the existence of a separate means of
establishing authority over telephone numbers, for cases where the establishing authority over telephone numbers, for cases where the
telephone number is the identity of the user. As with SIP URIs, the telephone number is the identity of the user. As with SIP URIs, the
necessary credentials to prove authority for a name might reside necessary credentials to prove authority for a name might reside
either in the endpoint or at some intermediary. either in the endpoint or at some intermediary.
This document specifies a means of sharing a cryptographic assurance This document specifies a means of sharing a cryptographic assurance
of end-user SIP identity in an interdomain or intradomain context. of end-user SIP identity in an interdomain or intradomain context.
It relies on the authentication service adding to requests a SIP It relies on the authentication service constructing JSON [RFC7159]
header, the Identity header, which contains that cryptographic objects composed of certain header field values taken from the SIP
assurance. In order to assist in the validation of the Identity request. The authentication service then computes a signature over
header, this specification also describes an Identity-Info header those JSON object in a manner following JWT [RFC7519]. That
that can be used by the recipient of a request to recover the signature is then placed in a SIP Identity header. In order to
credentials of the signer. Note that the scope of this document is assist in the validation of the Identity header, this specification
limited to providing this identity assurance for SIP requests; also describes some metadata fields associated with the header that
solving this problem for SIP responses is outside the scope of this can be used by the recipient of a request to recover the credentials
work (see [RFC4916]). 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 specifications may detail how the JSON object
constructed and signed by this mechanism might be used with other
protocols, or further gateway operations upon this object.
This specification allows either a user agent or a proxy server to This specification allows either a user agent or a proxy server to
provide the authentication service function and/or the verification provide the authentication service function and/or the verification
service function. To maximize end-to-end security, it is obviously service function. To maximize end-to-end security, it is obviously
preferable for end-users to acquire their own credentials; if they preferable for end-users to acquire their own credentials; if they
do, their user agents can act as authentication services. However, do, their user agents can act as authentication services. However,
for some deployments end-user credentials may be neither practical for some deployments, end-user credentials may be neither practical
nor affordable, given the potentially large number of SIP user agents nor affordable, given the potentially large number of SIP user agents
(phones, PCs, laptops, PDAs, gaming devices) that may be employed by (phones, PCs, laptops, PDAs, gaming devices) that may be employed by
a single user. In such environments, synchronizing keying material a single user. In such environments, synchronizing keying material
across multiple devices may be prohibitively complex and require across multiple devices may be prohibitively complex and require
quite a good deal of additional endpoint behavior. Managing several quite a good deal of additional endpoint behavior. Managing several
credentials for the various devices could also be burdensome. In credentials for the various devices could also be burdensome. In
these cases, implementation the authentication service at an these cases, implementation the authentication service at an
intermediary may be more practical. This trade-off needs to be intermediary may be more practical. This trade-off needs to be
understood by implementers of this specification. understood by implementers of this specification.
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Alice generates an INVITE and places her identity, in this case her 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 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 sends an INVITE over TLS to an authentication service proxy for the
example.com domain. example.com domain.
The authentication service authenticates Alice (possibly by sending a The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized Digest authentication challenge) and validates that she is authorized
to assert the identity that she populated in the From header field. 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 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 different value that the proxy server has authority over, such as a
telephone number. The proxy then computes a hash over some telephone number. The proxy then constructs an object composed of
particular headers and fields, including part of the From header some particular headers and fields, including part of the From header
field of the message. This hash is signed with the appropriate field of the message, and generates a hash of the object. This hash
credential for the identity (example.com, in the is then signed with the appropriate credential for the identity
sip:alice@example.com case) and inserted in a new header field in the (example.com, in the sip:alice@example.com case) and the signature is
SIP message, the 'Identity' header. inserted into the Identity header field value of the request.
The proxy, as the holder of the private key for the example.com The proxy, as the holder of the private key for the example.com
domain, is asserting that the originator of this request has been domain, is asserting that the originator of this request has been
authenticated and that she is authorized to claim the identity that authenticated and that she is authorized to claim the identity that
appears in the From header field. The proxy also inserts a companion appears in the From header field. The proxy inserts an "info"
header field, Identity-Info, that tells Bob how to acquire keying parameter into the Identity header that tells Bob how to acquire
material necessary to validate its credentials (a public key), in keying material necessary to validate its credentials (a public key),
case he doesn't already have it. in case he doesn't already have it.
When Bob's domain receives the request, it verifies the signature When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validate that the provided in the Identity header, and thus can validate that the
authority over the identity in the From header field authenticated authority over the identity in the From header field authenticated
the user, and permitted the user to assert that From header field the user, and permitted the user to assert that From header field
value. This same validation operation may be performed by Bob's user value. This same validation operation may be performed by Bob's user
agent server (UAS). As the request has been validated, it is agent server (UAS). As the request has been validated, it is
rendered to Bob. If the validation was unsuccessful, some other rendered to Bob. If the validation was unsuccessful, some other
treatment would be applied by the receiving domain. treatment would be applied by the receiving domain.
4. Signature Generation and Validation 4. Signature Generation and Validation
4.1. Authentication Service Behavior 4.1. Authentication Service Behavior
This document specifies a role for SIP entities called an This document specifies a role for SIP entities called an
authentication service. The authentication service role can be authentication service. The authentication service role can be
instantiated by an intermediary such as a proxy server or by a user instantiated either by an intermediary such as a proxy server or by a
agent. Any entity that instantiates the authentication service role user agent. Any entity that instantiates the authentication service
MUST possess the private key of one or more credentials that can be role MUST possess the private key of one or more credentials that can
used to sign for a domain or a telephone number (see Section 5.1). be used to sign for a domain or a telephone number (see Section 5.1).
Intermediaries that instantiate this role MUST be capable of Intermediaries that instantiate this role MUST be capable of
authenticating one or more SIP users who can register for that authenticating one or more SIP users who can register for that
identity. Commonly, this role will be instantiated by a proxy identity. Commonly, this role will be instantiated by a proxy
server, since these entities are more likely to have a static server, since these entities are more likely to have a static
hostname, hold corresponding credentials, and have access to SIP hostname, hold corresponding credentials, and have access to SIP
registrar capabilities that allow them to authenticate users. It is registrar capabilities that allow them to authenticate users. It is
also possible that the authentication service role might be also possible that the authentication service role might be
instantiated by an entity that acts as a redirect server, but that is instantiated by an entity that acts as a redirect server, but that is
left as a topic for future work. 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 "spec"
parameter per the fourth step below.
Entities instantiating the authentication service role perform the Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP following steps, in order, to generate an Identity header for a SIP
request: request:
Step 1: Step 1:
First, the authentication service must determine whether it is First, the authentication service must determine whether it is
authoritative for the identity of the sender of the request. In authoritative for the identity of the sender of the request. In
ordinary operations, the authentication service decides this by ordinary operations, the authentication service decides this by
inspecting the URI value from the addr-spec component of From header inspecting the URI value from the addr-spec component of From header
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the identity field contains a SIP or SIP Secure (SIPS) URI, and the the identity field contains a SIP or SIP Secure (SIPS) URI, and the
user portion is not a telephone number, the authentication service user portion is not a telephone number, the authentication service
MUST extract the hostname portion of the identity field and compare MUST extract the hostname portion of the identity field and compare
it to the domain(s) for which it is responsible (following the it to the domain(s) for which it is responsible (following the
procedures in RFC 3261 [RFC3261], Section 16.4). If the identity procedures in RFC 3261 [RFC3261], Section 16.4). If the identity
field uses the TEL URI scheme [RFC3966], or the identity field is a 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 SIP or SIPS URI with a telephone number in the user portion, the
authentication service determines whether or not it is responsible authentication service determines whether or not it is responsible
for this telephone number; see Section 6.1 for more information. An for this telephone number; see Section 6.1 for more information. An
authentication service proceeding with a signature over a telephone authentication service proceeding with a signature over a telephone
number MUST then follow the canonicaliaation procedures described in number MUST then follow the canonicalization procedures described in
Section 6.1.1. If the authentication service is not authoritative Section 6.1.1. If the authentication service is not authoritative
for the identity in question, it SHOULD process and forward the for the identity in question, it SHOULD process and forward the
request normally, but it MUST NOT follow the steps below to add an request normally, but it MUST NOT follow the steps below to add an
Identity header. An authentication service MUST NOT add an Identity Identity header.
header to a request that already has one.
Step 2: Step 2:
The authentication service MUST then determine whether or not the The authentication service MUST then determine whether or not the
sender of the request is authorized to claim the identity given in sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include: which this authentication might be performed include:
If the authentication service is instantiated by a SIP If the authentication service is instantiated by a SIP
skipping to change at page 8, line 30 skipping to change at page 8, line 37
specific means, perhaps simply by virtue of having physical access specific means, perhaps simply by virtue of having physical access
to the user agent. to the user agent.
Authorization of the use of a particular username or telephone number 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 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. for the authentication service, see Section 5.1 for more information.
Note that this check is performed only on the addr-spec in the Note that this check is performed only on the addr-spec in the
identity field (e.g., the URI of the sender, like identity field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display- 'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta'). name portion of the From header field (e.g., 'Alice Atlanta'). For
Authentication services MAY check and validate the display-name as more information, see Section 11.6.
well, and compare it to a list of acceptable display-names that may
be used by the sender; if the display-name does not meet policy
constraints, the authentication service could return a 403 response
code. In this case, the reason phrase should indicate the nature of
the problem; for example, "Inappropriate Display Name". However, the
display-name is not always present, and in many environments the
requisite operational procedures for display-name validation may not
exist, so no normative guidance is given here. For more information,
see Section 10.6.
Step 3: Step 3:
An authentication service MUST add a Date header field to SIP An authentication service MUST add a Date header field to SIP
requests if one is not already present. The authentication service requests if one is not already present. The authentication service
MUST ensure that any preexisting Date header in the request is MUST ensure that any preexisting Date header in the request is
accurate. Local policy can dictate precisely how accurate the Date accurate. Local policy can dictate precisely how accurate the Date
must be; a RECOMMENDED maximum discrepancy of sixty seconds will must be; a RECOMMENDED maximum discrepancy of sixty seconds will
ensure that the request is unlikely to upset any verifiers. If the ensure that the request is unlikely to upset any verifiers. If the
Date header contains a time different by more than one minute from Date header contains a time different by more than one minute from
the current time noted by the authentication service, the the current time noted by the authentication service, the
authentication service SHOULD reject the request. This behavior is authentication service SHOULD reject the request. This behavior is
not mandatory because a user agent client (UAC) could only exploit not mandatory because a user agent client (UAC) could only exploit
the Date header in order to cause a request to fail verification; the the Date header in order to cause a request to fail verification; the
Identity header is not intended to provide a source of non- Identity header is not intended to provide a source of non-
repudiation or a perfect record of when messages are processed. repudiation or a perfect record of when messages are processed.
Finally, the authentication service MUST verify that the Date header Finally, the authentication service MUST verify that the Date header
falls within the validity period of its credential. falls within the validity period of its credential.
See Section 7 for information on how the Date header field assists See Section 11 for information on how the Date header field assists
verifiers. verifiers.
Step 4: Step 4:
The authentication service MAY add extend the identity mechanism by
adding one or more Identity-Extension headers to the request. Only
implementations that extend this base mechanism MAY populate this
header field and add this signature. See Section 8.
Step 5:
Subsequently, the authentication service MUST form the identity Subsequently, the authentication service MUST form the identity
signature and add an Identity header to the request containing this signature and add an Identity header to the request containing this
signature. After the Identity header has been added to the request, signature. For baseline Identity headers (without a "spec"
the authentication service MUST also add an Identity-Info header. parameter), this follows the procedures in Section 7; if the
The Identity-Info header contains a URI from which the authentication authentication service is using an alternative "spec", it MUST add an
service's credential can be acquired; see Section 5.3 for more on appropriate "spec" parameter and follow the procedures associated
credential acquisition. Details on the syntax of both of these with it (see Section 8). After the Identity header has been added to
headers are provided in Section 7. 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 Finally, the authentication service MUST forward the message
normally. normally.
4.2. Verifier Behavior 4.2. Verifier Behavior
This document specifies a logical role for SIP entities called a This document specifies a logical role for SIP entities called a
verification service, or verifier. When a verifier receives a SIP verification service, or verifier. When a verifier receives a SIP
message containing an Identity header, it inspects the signature to message containing one or more Identity headers, it inspects the
verify the identity of the sender of the message. Typically, the signature to verify the identity of the sender of the message. The
results of a verification are provided as input to an authorization results of a verification are provided as input to an authorization
process that is outside the scope of this document. If an Identity process that is outside the scope of this document.
header is not present in a request, and one is required by local
policy (for example, based on a per-sending-domain policy, or a per- A SIP request may contain zero, one, or more Identity headers. A
sending-user policy), then a 428 'Use Identity Header' response MUST verification service performs the procedures below on each Identity
be sent. 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 "spec" 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.
In order to verify the identity of the sender of a message, an entity 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 acting as a verifier MUST perform the following steps, in the order
here specified. here specified.
Step 1: Step 1:
In order to determine whether the signature for the identity field The verifier MUST inspect any optional "spec" parameter appearing the
should be over the entire URI or just a canonicalized telephone Identity request. If no "spec" parameter is present, then the
number, the verification service MUST follow the canonicalization verifier proceeds normally below. If a "spec" parameter value is
process described in Section 6.1.1. That section also describes the present, and the verifier does not support it, it MUST ignore the
procedures the verification service MUST follow to determine if the Identity header. If a supported "spec" parameter value is present,
signer is authoritative for a telephone number. For domains, the the verifier follows the procedures below, including the variations
verifier MUST follow the process described in Section 6.2 to described in Step 5.
determine if the signer is authoritative for the identity field.
Step 2: 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 The verifier must first ensure that it possesses the proper keying
material to validate the signature in the Identity header field, material to validate the signature in the Identity header field,
which usually involves dereferencing the Identity-Info header. See which usually involves dereferencing a URI in the "info" parameter of
Section 5.2 for more information on these procedures. the Identity header. See Section 5.2 for more information on these
procedures.
Step 3: Step 4:
The verifier MUST must furthermore ensure that the value of the Date The verifier MUST furthermore ensure that the value of the Date
header meets local policy for freshness (usually, within sixty header meets local policy for freshness (usually, within sixty
seconds) and that it falls within the validity period of the seconds) and that it falls within the validity period of the
credential used to sign the Identity header. For more on the attacks credential used to sign the Identity header. For more on the attacks
this prevents, see Section 10.1. this prevents, see Section 11.1.
Step 4:
The verifier MUST validate the signature in the Identity header
field, following the procedures for generating the hashed digest-
string described in Section 7. If a verifier determines that the
signature on the message does not correspond to the reconstructed
digest-string, then a 438 'Invalid Identity Header' response MUST be
returned.
Step 5: Step 5:
If the request contains one or more Identity-Extension headers, then The verifier MUST validate the signature in the Identity header
if the verifier supports the included extension(s), it SHOULD verify field. For baseline Identity headers (with no "spec" parameter) the
any associated fields following the procedures specified in that verifier MUST follow the procedures for generating the signature over
extension (see Section 8). If a verifier determines that such a hashed JSON objects described in Section 7. If a "spec" parameter is
signature in the message does not correspond to the reconstructed present, the verifier follows the procedures for that "spec" (see
digest-string, then a 438 'Invalid Identity Header' response SHOULD Section 8). If a verifier determines that the signature on the
be returned. If the verifier does not support the extension(s), then message does not correspond to the reconstructed digest-string, then
the verifier takes no further action. a 438 'Invalid Identity Header' response MUST be returned.
The handling of the message after the verification process depends on The handling of the message after the verification process depends on
how the implementation service is implemented, and on local policy. how the implementation service is implemented, and on local policy.
This specification does not propose any authorization policy for user This specification does not propose any authorization policy for user
agents or proxy servers to follow based on the presence of a valid agents or proxy servers to follow based on the presence of a valid
Identity header, the presence of an invalid Identity header, or the Identity header, the presence of an invalid Identity header, or the
absence of an Identity header, but it is anticipated that local absence of an Identity header, but it is anticipated that local
policies could involve making different forwarding decisions in policies could involve making different forwarding decisions in
intermediary implementations, or changing how the user is alerted, or intermediary implementations, or changing how the user is alerted, or
how identity is rendered, in user agent implementations. how identity is rendered, in user agent implementations.
5. Credentials 5. Credentials
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form like 'sip:anonymous@example.com' (see [RFC3323]), then the form like 'sip:anonymous@example.com' (see [RFC3323]), then the
'example.com' proxy might authenticate only that the user is a valid 'example.com' proxy might authenticate only that the user is a valid
user in the domain and insert the signature over the From header user in the domain and insert the signature over the From header
field as usual. field as usual.
5.2. Credential Use by the Verification Service 5.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a In order to act as a verification service, a SIP entity must have a
way to acquire and retain credentials for authorities over particular way to acquire and retain credentials for authorities over particular
domain names and/or telephone numbers or number ranges. domain names and/or telephone numbers or number ranges.
Dereferencing the Identity-Info header (as described in the next Dereferencing the URI found in the "info" parameter of the Identity
section) MUST be supported by all verification service header (as described in the next section) MUST be supported by all
implementations to create a baseline means of credential acquisition. verification service implementations to create a baseline means of
Provided that the credential used to sign a message is not previously credential acquisition. Provided that the credential used to sign a
known to the verifier, SIP entities SHOULD discover this credential message is not previously known to the verifier, SIP entities SHOULD
by dereferencing the Identity-Info header, unless they have some more discover this credential by dereferencing the "info" parameter,
other implementation-specific way of acquiring the needed keying unless they have some more other implementation-specific way of
material, such as an offline store of periodically-updated acquiring the needed keying material, such as an offline store of
credentials. If the URI in the Identity-Info header cannot be periodically-updated credentials. If the URI in the "info" parameter
dereferenced, then a 436 'Bad Identity-Info' response MUST be cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST
returned. be returned.
This specification does not propose any particular policy for a This specification does not propose any particular policy for a
verification service to determine whether or not the holder of a verification service to determine whether or not the holder of a
credential is the appropriate party to sign for a given SIP identity. credential is the appropriate party to sign for a given SIP identity.
Guidance on this is deferred to the credential mechanism Guidance on this is deferred to the credential mechanism
specifications, which must meet the requirements in Section 5.4. specifications, which must meet the requirements in Section 5.4.
Verification service implementations supporting this specification Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials every time a request from the same identity is received. Credentials
cached in this manner may be indexed in accordance with local policy: cached in this manner may be indexed in accordance with local policy:
for example, by their scope, or the URI given in the Identity-Info for example, by their scope, or the URI given in the "info" parameter
header field value. Further consideration of how to cache value. Further consideration of how to cache credentials is deferred
credentials is deferred to the credential mechanism specifications. to the credential mechanism specifications.
5.3. Handling Identity-Info URIs 5.3. Handling 'info' parameter URIs
An Identity-Info header MUST contain a URI which dereferences to a An "info" parameter MUST contain a URI which dereferences to a
resource that contains the public key components of the credential resource that contains the public key components of the credential
used by the authentication service to sign a request. It is used by the authentication service to sign a request. It is
essential that a URI in the Identity-Info header be dereferencable by essential that a URI in the "info parameter" be dereferencable by any
any entity that could plausibly receive the request. For common entity that could plausibly receive the request. For common cases,
cases, this means that the URI must be dereferencable by any entity this means that the URI must be dereferencable by any entity on the
on the public Internet. In constrained deployment environments, a public Internet. In constrained deployment environments, a service
service private to the environment might be used instead. private to the environment might be used instead.
Beyond providing a means of accessing credentials for an identity, Beyond providing a means of accessing credentials for an identity,
the Identity-Info header further serves as a means of differentiating the "info" parameter further serves as a means of differentiating
which particular credential was used to sign a request, when there which particular credential was used to sign a request, when there
are potentially multiple authorities eligible to sign. For example, are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service imagine a case where a domain implements the authentication service
role for a range of telephone and a user agent belonging to Alice has role for a range of telephone and a user agent belonging to Alice has
acquired a credential for a single telephone number within that acquired a credential for a single telephone number within that
range. Either would be eligible to sign a SIP request for the number range. Either would be eligible to sign a SIP request for the number
in question. Verification services however need a means to in question. Verification services however need a means to
differentiate which one performed the signature. The Identity-Info differentiate which one performed the signature. The "info"
header performs that function. parameter performs that function.
If the optional "canon" parameter is present, it contains the result If the optional "canon" parameter is present, it contains the bae64
of the number canonicalization process performed by the encoded result of JSON object construction process performed by the
authentication service (see Section 6.1.1) on the identity in the authentication service (see Section 6.1.1), including the
From. This value is provided purely informationally as an canonicalization processes applied to the identity in the identity
optimization for the verification service. The verification service fields of the sender and intended recipient. The "canon" is provided
MAY compute its own canonicalization of the number and compare this purely informationally as an optimization for the verification
to the value in the "canon" parameter before performing any service. The verification service MAY compute its own
cryptographic functions in order to ascertain whether or not the two canonicalization of the numbers and compare them to the values in the
ends agree on the canonical number form. "canon" parameter before performing any cryptographic functions in
order to ascertain whether or not the two ends agree on the canonical
number form.
5.4. Credential Systems 5.4. Credential System Requirements
This document makes no recommendation for the use of any specific This document makes no recommendation for the use of any specific
credential system. Today, there are two primary credential systems credential system. Today, there are two primary credential systems
in place for proving ownership of domain names: certificates (e.g., in place for proving ownership of domain names: certificates (e.g.,
X.509 v3, see [RFC5280]) and the domain name system itself (e.g., X.509 v3, see [RFC5280]) and the domain name system itself (e.g.,
DANE, see [RFC6698]). It is envisioned that either could be used in DANE, see [RFC6698]). It is envisioned that either could be used in
the SIP identity context: an Identity-Info header could for example the SIP identity context: an "info" parameter could for example give
give an HTTP URL of the form 'application/pkix-cert' pointing to a an HTTP URL of the form 'application/pkix-cert' pointing to a
certificate (following the conventions of [RFC2585]). The Identity- certificate (following the conventions of [RFC2585]). The "info"
Info headers may use the DNS URL scheme (see [RFC4501]) to designate parameter may use the DNS URL scheme (see [RFC4501]) to designate
keys in the DNS. keys in the DNS.
While no comparable public credentials exist for telephone numbers, While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential either approach could be applied to telephone numbers. A credential
system based on certificates is given in system based on certificates is given in
[I-D.ietf-stir-certificates]. One based on the domain name system is [I-D.ietf-stir-certificates]. One based on the domain name system is
given in [I-D.kaplan-stir-cider]. given in [I-D.kaplan-stir-cider].
In order for a credential system to work with this mechanism, its In order for a credential system to work with this mechanism, its
specification must detail: specification must detail:
which URIs schemes the credential will use in the Identity-Info which URIs schemes the credential will use in the "info"
header, and any special procedures required to dereference the parameter, and any special procedures required to dereference the
URIs URIs
how the verifier can learn the scope of the credential how the verifier can learn the scope of the credential
any special procedures required to extract keying material from any special procedures required to extract keying material from
the resources designated by the URI the resources designated by the URI
any algorithms that would appear in the Identity-Info "alg" any algorithms that would appear in the Identity-Info "alg"
parameter other than 'rsa-sha256.' Note that per the IANA parameter other than 'RS256.' Note that per the IANA
Considerations of RFC 4474, new algorithms can only be specified Considerations of RFC 4474, new algorithms can only be specified
by Standards Action by Standards Action
SIP entities cannot reliably predict where SIP requests will SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar trust anchors is assured by business arrangements or similar
constraints. constraints.
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the sender of a request; while it is not envisioned that most 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 those networks would or should make use of the Identity mechanism
described in this specification, where they do, local policy might described in this specification, where they do, local policy might
therefore dictate that the canonical string derive from the P- therefore dictate that the canonical string derive from the P-
Asserted-Identity header field rather than the From. In any case Asserted-Identity header field rather than the From. In any case
where local policy canonicalizes the number into a form different where local policy canonicalizes the number into a form different
from how it appears in the From header field, the use of the from how it appears in the From header field, the use of the
"canon" parameter by authentication services is RECOMMENDED, but "canon" parameter by authentication services is RECOMMENDED, but
because "canon" itself could then divulge information about users because "canon" itself could then divulge information about users
or networks, implementers should be mindful of the guidelines in or networks, implementers should be mindful of the guidelines in
Section 9. Section 10.
The resulting canonical number string will be used as input to the The resulting canonical number string will be used as input to the
hash calculation during signing and verifying processes. hash calculation during signing and verifying processes.
The ABNF of this number string is: The ABNF of this number string is:
tn-spec = [ "#" / "*" ] 1*DIGIT tn-spec = [ "#" / "*" ] 1*DIGIT
If the result of this procedure forms a complete telephone number, If the result of this procedure forms a complete telephone number,
that number is used for the purpose of creating and signing the that number is used for the purpose of creating and signing the
digest-string by both the authentication service and verification digest-string by both the authentication service and verification
service. Practically, entities that perform the authentication service. Practically, entities that perform the authentication
service role will sometimes alter the telephone numbers that appear service role will sometimes alter the telephone numbers that appear
in the To and From header field values, converting them to this in the To and From header field values, converting them to this
format (though note this is not a function that [RFC3261] permits format (though note this is not a function that [RFC3261] permits
proxy servers to perform). The authentication service MAY also add proxy servers to perform). The result of the canonicalization
the result of the canonicalization process of the From header field process of the From header field value may also be recorded through
value to the "canon" parameter of the Identity-Info header. If the the use of the "canon" parameter of the Identity(see Section 7). If
result of the canonicalization of the From header field value does the result of the canonicalization of the From header field value
not form a complete telephone number, the authentication service and does not form a complete telephone number, the authentication service
verification service should treat the entire URI as a SIP URI, and and verification service should treat the entire URI as a SIP URI,
apply a domain signature per the procedures in Section 6.2. and apply a domain signature per the procedures in Section 6.2.
6.2. Domain Names 6.2. Domain Names
When a verifier processes a request containing an Identity-Info When a verifier processes a request containing an Identity-Info
header with a domain signature, it must compare the domain portion of 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 the URI in the From header field of the request with the domain name
that is the subject of the credential acquired from the Identity-Info that is the subject of the credential acquired from the "info"
header. While it might seem that this should be a straightforward parameter. While it might seem that this should be a straightforward
process, it is complicated by two deployment realities. In the first process, it is complicated by two deployment realities. In the first
place, credentials have varying ways of describing their subjects, place, credentials have varying ways of describing their subjects,
and may indeed have multiple subjects, especially in 'virtual and may indeed have multiple subjects, especially in 'virtual
hosting' cases where multiple domains are managed by a single hosting' cases where multiple domains are managed by a single
application. Secondly, some SIP services may delegate SIP functions application. Secondly, some SIP services may delegate SIP functions
to a subordinate domain and utilize the procedures in RFC 3263 to a subordinate domain and utilize the procedures in RFC 3263
[RFC3263] that allow requests for, say, 'example.com' to be routed to [RFC3263] that allow requests for, say, 'example.com' to be routed to
'sip.example.com'. As a result, a user with the AoR 'sip.example.com'. As a result, a user with the AoR
'sip:jon@example.com' may process requests through a host like '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 'sip.example.com', and it may be that latter host that acts as an
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defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818] 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, deals with the use of HTTP in TLS and is specific to certificates,
the procedures described are applicable to verifying identity if one the procedures described are applicable to verifying identity if one
substitutes the "hostname of the server" in HTTP for the domain 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 portion of the user's identity in the From header field of a SIP
request with an Identity header. request with an Identity header.
7. Header Syntax 7. Header Syntax
Baseline RFC4474 defined the Identity and Identity-Info headers. Baseline RFC4474 defined the Identity and Identity-Info headers.
This document updates that specification and adds the optional This document deprecates the Identity-Info header, collapsing its
Identity-Extension header (the grammar for which appears in grammar into the Identity header. Note that unlike baseline RFC4474,
Section 8). Identity and Identity-Info are REQUIRED for securing the Identity header is now allowed to appear more than one time in a
requests with this specification, and may appear only once in a SIP request. The revised grammar for the Identity header is
request, while Identity-Extension can be present multiple times. The (following the ABNF [RFC4234] in RFC 3261 [RFC3261]):
grammar for the first two headers is (following the ABNF [RFC4234] in
RFC 3261 [RFC3261]):
Identity = "Identity" HCOLON signed-identity-digest Identity = "Identity" HCOLON signed-identity-digest SEMI ident-info *( SEMI ident-info-params )
signed-identity-digest = LDQUOT *base64-char RDQUOT signed-identity-digest = LDQUOT *base64-char RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / canonical-str / ident-info-extension ident-info-params = ident-info-alg / ident-spec / canonical-str
ident-info-alg = "alg" EQUAL token ident-info-alg = "alg" EQUAL token
canonical-str = "canon" EQUAL tn-spec ident-spec = "spec" EQUAL token
canonical-str = "canon" EQUAL *base64-char
ident-info-extension = generic-param ident-info-extension = generic-param
base64-char = ALPHA / DIGIT / "/" / "+" base64-char = ALPHA / DIGIT / "/" / "+"
This follows the original specification of Identity and Identity-Info In addition to the parameters of Identity-Info in RFC4474, this
in RFC4474, except for the addition of the "canon" parameter. Note specification includes the optional "canon" and "spec" parameters.
that in RFC4474, the signed-identity-digest was given as quoted Note that in RFC4474, the signed-identity-digest was given as quoted
32LHEX, whereas here it is given as a quoted sequence of base64-char. 32LHEX, whereas here it is given as a quoted sequence of base64-char.
The signed-identity-digest is a signed hash of a canonical string The 'absoluteURI' portion of ident-info MUST contain a URI; see
generated from certain components of a SIP request. To create the Section 5.3 for more on choosing how to advertise credentials through
contents of the signed-identity-digest, the following elements of a this parameter.
SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character:
First, the identity. If the user part of the AoR in the From The signed-identity-digest is a signed hash of a pair of JSON objects
header field of the request contains a telephone number, then the generated from certain components of a SIP request. This first
canonicalization of that number goes into the first slot (see object contains header information, and the second contains claims,
Section 6.1.1). Otherwise, the first slot contains the AoR of the following the conventions of JWT [RFC7519]. Once these two JSON
UA sending the message as taken from addr-spec of the From header objects have been generated, they will be stripped of all unquoted
field. whitespace and linefeeds, and each will be separately base64 encoded,
hashed with a SHA-256 hash and then concatenated, header then claims,
into a string separated by a single "." per the conventions of
baseline JWT.
Second, the target. If the user part of the AoR in the To header To create the header JSON object used in the construction of the
field of the request contains a telephone number, then the signed-identity-digest of the Identity header, the following elements
canonicalization of that number goes into the second slot (again, of a SIP message MUST be placed in a comma-separated JSON object, in
see Section 6.1.1). Otherwise, the second slot contains the addr- order:
spec component of the To header field, which is the AoR to which
the request is being sent.
Third, the request method. First, the JSON key "typ" followed by a colon and then the quoted
string "JWT".
Fourth, the Date header field, with exactly one space each for Second, the JSON key "alg" followed by a colon and then the quoted
each SP and the weekday and month items case set as shown in the value of the optional "alg" parameter in the Identity header.
BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for Note if the "alg" parameter is absent it is assumed to the value
weekday and month is a choice amongst a set of tokens. The RFC "RS256".
4234 [RFC4234] rules for the BNF specify that tokens are case
sensitive. However, when used to construct the canonical string
defined here, the first letter of each week and month MUST be
capitalized, and the remaining two letters must be lowercase.
This matches the capitalization provided in the definition of each
token. All requests that use the Identity mechanism MUST contain
a Date header.
Fifth, if the request contains an SDP message body, and if that Third, the JSON key "x5u" followed by a colon and then the quoted
SDP contains one or more "a=fingerprint" attributes, the value(s) value of the URI in the "info" parameter.
of the attributes if they differ. Each attribute value consists
of all characters following the colon after "a=fingerprint"
including the algorithm description and hexadecimal key
representation, any whitespace, carriage returns, and "/" line
break indicators. If multiple non-identical "a=fingerprint"
attributes appear in an SDP body, then all non-identical
attributes values MUST be concatenated, with no separating
character, after sorting the values in alphanumeric order. If the
SDP body contains no "a=fingerprint" attribute, the fifth element
MUST be empty, containing no whitespace, resulting in a "||" in
the signed-identity-digest.
Sixth, the Identity-Extension header field value(s), if there is Fourth, optionally the JSON key "spec" followed by a colon and
at least one Identity-Extension header field in the request. If then the quoted value of the "spec" parameter of the Identity
multiple Identity-Extension header fields are in the request, they header. If the "spec" parameter is absent from the header, the
MUST be concatenated after sorting the header field values in "spec" key will not appear in the JSON heaer object.
alphanumeric order, with each entry separated by a vertical line,
"|" or %x7C. If the message contains no Identity-Extension For example:
header, then the sixth slot MUST be empty, containing no
whitespace, resulting in a "||" in the signed-identity-digest. { "typ":"JWT",
characters. "alg":"RS256"
"x5u":"https://www.example.com/cert.pkx" }
To create the 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.
For example:
{ "orig":"12155551212",
"term":"12155551213",
"iat": "1443208345",
For more information on the security properties of these headers, and For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 10 and why their inclusion mitigates replay attacks, see Section 11 and
[RFC3893]. The precise formulation of this digest-string is, [RFC3893]. The precise formulation of this digest-string is,
therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]): therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):
digest-string = ( addr-spec / tn-spec ) "|" ( addr-spec / tn-spec ) "|" After these two JSON objects, the header and the claims, have been
Method "|" SIP-date "|" [ sdp-fingerprint ] "|" constructed, they must be hashed and signed. The hashing and signing
[ signed-identity-extension-digest ] algorithm is specified by the 'alg' parameter of the Identity header.
This document defines only one value for the 'alg' parameter:
sdp-fingerprint = byte-string 'RS256', as defined in [RFC7519], which connotes a SHA-256 hash
followed by a RSASSA-PKCS1-v1_5 signature. Any further values MUST
be defined in a Standards Track RFC, see Section 12.2 for more
information. All implementations of this specification MUST support
'RS256'. The result of the hash and signing of the two concatenated
JSON objects is placed in the Identity header field.
For the definition of 'tn-spec' see Section 6.1.1. For example:
After the digest-string is formed, it MUST be hashed and signed with Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
the certificate of authority over the identity. The hashing and eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
signing algorithm is specified by the 'alg' parameter of the pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \
Identity-Info header (see below for more information on Identity-Info info=<https://biloxi.example.org/biloxi.cer>;alg=RS256
header parameters). This document defines only one value for the
'alg' parameter: 'rsa-sha256'; further values MUST be defined in a
Standards Track RFC, see Section 11.3 for more information. All
implementations of this specification MUST support 'rsa-sha256'.
When the 'rsa-sha256' algorithm is specified in the 'alg' parameter
of Identity-Info, the hash and signature MUST be generated as
follows: compute the results of signing this string with
sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and base64
encode the results as specified in RFC 3548 [RFC3548]. A 2048-bit or
longer RSA key MUST be used. The result of the digest-string hash is
placed in the Identity header field.
The 'absoluteURI' portion of the Identity-Info header MUST contain a In a departure from JWT practice, the base64 encoded version of the
URI; see Section 5.3 for more on choosing how to advertise JSON objects is not included in the Identity header: only the
credentials through Identity-Info. signature component of the JWT 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 8. Extensibility
As future requirements may warrant increasing the scope of the As future requirements may warrant increasing the scope of the
Identity mechanism, this specification defines an optional Identity- Identity mechanism, this specification defines an optional "spec"
Extension header. Each extension header field value MUST consist of parameter of the Identity header. The "spec" parameter value MUST
a right hand side identifying the extension, an equals sign, and then consist of a token containing an extension specification, which
a left hand side consisting of a signature over an element in a SIP denotes an alternative set of signed claims.
request.
Future specifications that define extensions to the Identity Future specifications that define extensions to the Identity
mechanism must explicitly designate which elements of a SIP request mechanism MUST explicitly designate what claims they include, the
are to be signed, how a canonical string of those elements is order in which they will appear, and any further information
generated by both the authentication service and the verifier, and necessary to implement the extension at the authentication service
the mechanism and algorithms used to generate the signature (it is and the verification service. All extensions MUST incorporate the
RECOMMENDED that these follow the algorithm choice of this baseline JWT elements specified in Section 7; claims may only be
specification). Note that per verifier behavior in Section 4.2, appended to the claims object specified in there, they can never be
verifying an extension is always optional. An authentication service subtracted re-ordered. Specifying new claims follows the baseline
cannot assume that verifiers will understand any given extension. JWT procedures ([RFC7519] Section 10.1). Note that per the verifier
Verifiers that do support an extension may then trigger appropriate behavior in Section 4.2, understanding a "spec" extension is always
application-level behavior in the presence of a signature over optional for compliance with this specification (though future
additional part of the SIP request; authors of Identity extensions specifications or profiles for deployment environments may make other
"spec" values mandatory). 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 should provide appropriate extension-specific guidance to application
developers on this point. developers on this point.
Identity-Extension = "Identity-Extension" HCOLON identity-extension-string If any claim in an extension contains a JSON value that does not
identity-extension-string = identity-extension-name EQUAL *base64-char correspond to any field of the SIP request, but then the optional
identity-extension-name = token "canon" parameter MUST be used for the Identity header containing
signed-identity-extension-digest = LDQUOT *base64-char RDQUOT that extension.
Defining a new Identity-Extension requires a Standards Action; see The IANA procedure for registering new "spec" parameters is given in
Section 11.4. Section 12.3; no values are defined in this document. If no "spec"
value appears in an Identity header, then the signature covers the
baseline claims specified in Section 7.
9. Privacy Considerations Purely as an example, were a "calling name" extension to be
registered, it might choose the spec name "cna". Implementations
supporting "cna" could, for example, define a new JWT field called
"cna" with the semantics that it contains a value authorized by the
signer a display-name component for the From or P-Asserted-Identity
header field value.
9. Gatewaying to JWT 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 JWT
token. This is so that a valid JWT can be generated based on a SIP
request containing an Identity header. This JWT 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, 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.
Alternative transports for this JWT and their requirements are left
to future specifications.
10. Privacy Considerations
The purpose of this mechanism is to provide a strong identification The purpose of this mechanism is to provide a strong identification
of the originator of a SIP request, specifically a cryptographic of the originator of a SIP request, specifically a cryptographic
assurance that the URI given in the From header field value can assurance that the URI given in the From header field value can
legitimately be claimed by the originator. This URI may contain a legitimately be claimed by the originator. This URI may contain a
variety of personally identifying information, including the name of variety of personally identifying information, including the name of
a human being, their place of work or service provider, and possibly a human being, their place of work or service provider, and possibly
further details. The intrinsic privacy risks associated with that further details. The intrinsic privacy risks associated with that
URI are, however, no different from those of baseline SIP. Per the URI are, however, no different from those of baseline SIP. Per the
guidance in [RFC6973], implementors should make users aware of the guidance in [RFC6973], implementors should make users aware of the
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the sender of a message that may go beyond what appears in the From 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 header field; P-Asserted-Identity holds a definitive identity for the
sender that is somehow known to a closed network of intermediaries sender that is somehow known to a closed network of intermediaries
that presumably the network will use this identity for billing or that presumably the network will use this identity for billing or
security purposes. The danger of this network-specific information security purposes. The danger of this network-specific information
leaking outside of the closed network motivated the "id" priv-value leaking outside of the closed network motivated the "id" priv-value
token. The "id" priv-value token has no implications for the token. The "id" priv-value token has no implications for the
Identity header, and privacy services MUST NOT remove the Identity Identity header, and privacy services MUST NOT remove the Identity
header when a priv-value of "id" appears in a Privacy header. header when a priv-value of "id" appears in a Privacy header.
The optional "canon" parameter of the Identity-Info header specified The optional "canon" parameter of the Identity header specified in
in this document provides a canonicalized form of the telephone this document provides the complete JSON objects used to generate the
number of the originator of a call. In some contexts, local policy digest-string of the Identity header, including the canonicalized
may be used to populate a "canon" that may differ substantially from form of the telephone number of the originator of a call. In some
the original From header field. Depending on those policies, contexts, local policy may require a canonicalization which differs
potentially the "canon" parameter might divulge information about the substantially from the original From header field. Depending on
originating network or user that might not appear elsewhere in the those policies, potentially the "canon" parameter might divulge
SIP request. Were it to be used to reflect the contents of the P- information about the originating network or user that might not
Asserted-Identity header field, for example, then "canon" would need appear elsewhere in the SIP request. Were it to be used to reflect
to be removed when the P-Asserted-Identity header is removed to avoid the contents of the P-Asserted-Identity header field, for example,
any such leakage outside of a trust domain. Since, in those then "canon" would need to be removed when the P-Asserted-Identity
contexts, the canonical form of the sender's identity could not be header is removed to avoid any such leakage outside of a trust
reassembled by a verifier, and thus the Identity signature validation domain. Since, in those contexts, the canonical form of the sender's
process would fail, using P-Asserted-Identity with the Identity identity could not be reassembled by a verifier, and thus the
"canon" parameter in this fashion is NOT RECOMMENDED outside of Identity signature validation process would fail, using P-Asserted-
environments where SIP requests will never leave the trust domain. Identity with the Identity "canon" parameter in this fashion is NOT
RECOMMENDED outside of environments where SIP requests will never
leave the trust domain.
Finally, note that unlike [RFC3325], the mechanism described in this Finally, note that unlike [RFC3325], the mechanism described in this
specification adds no information to SIP requests that has privacy specification adds no information to SIP requests that has privacy
implications. implications.
10. Security Considerations 11. Security Considerations
This document describes a mechanism that provides a signature over This document describes a mechanism that provides a signature over
the Date header field of SIP requests, parts of the To and From the Date header field of SIP requests, parts of the To and From
header fields, the request method, and when present any media keying header fields, the request method, and when present any media keying
material in the message body. In general, the considerations related material in the message body. In general, the considerations related
to the security of these headers are the same as those given in to the security of these headers are the same as those given in
[RFC3261] for including headers in tunneled 'message/sip' MIME bodies [RFC3261] for including headers in tunneled 'message/sip' MIME bodies
(see Section 23 in particular). The following section details the (see Section 23 in particular). The following section details the
individual security properties obtained by including each of these individual security properties obtained by including each of these
header fields within the signature; collectively, this set of header header fields within the signature; collectively, this set of header
fields provides the necessary properties to prevent impersonation. fields provides the necessary properties to prevent impersonation.
It adddresses the solution-specific attacks again in-band solutions It addresses the solution-specific attacks again in-band solutions
enumerated in [RFC7375] Section 4.1. enumerated in [RFC7375] Section 4.1.
10.1. Protected Request Fields 11.1. Protected Request Fields
The From header field value indicates the identity of the sender of The From header field value (in ordinary operations) indicates the
the message, and the SIP address-of-record URI, or an embedded identity of the sender of the message, and the SIP address-of-record
telephone number, in the From header field is the identity of a SIP URI, or an embedded telephone number, in the From header field is the
user, for the purposes of this document. This is the key piece of 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-
Asserted-Id instead. The sender's identity is the key piece of
information that this mechanism secures; the remainder of the signed information that this mechanism secures; the remainder of the signed
parts of a SIP request are present to provide reference integrity and parts of a SIP request are present to provide reference integrity and
to prevent certain types of cut-and-paste attacks. to prevent certain types of cut-and-paste attacks.
The Date header field value protects against 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 as described in [RFC3261], Section 23.4.2. Implementations of this
specification MUST NOT deem valid a request with an outdated Date specification MUST NOT deem valid a request with an outdated Date
header field (the RECOMMENDED interval is that the Date header must header field (the RECOMMENDED interval is that the Date header must
indicate a time within 60 seconds of the receipt of a message). Note indicate a time within 60 seconds of the receipt of a message). Note
that per baseline [RFC3261] behavior, servers keep state of recently that per baseline [RFC3261] behavior, servers keep state of recently
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for one user is cut-and-pasted into a request for a different user; 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 second, it preserves the starting URI scheme of the request, which
helps prevent downgrade attacks against the use of SIPS. The To helps prevent downgrade attacks against the use of SIPS. The To
offers additional protection against cut-and-paste attacks beyond the offers additional protection against cut-and-paste attacks beyond the
Date header field: for example, without a signature over the To, an Date header field: for example, without a signature over the To, an
attacker who receives a call from a target could immediately forward attacker who receives a call from a target could immediately forward
the INVITE to the target's voicemail service within the Date the INVITE to the target's voicemail service within the Date
interval, and the voicemail service would have no way knowing that interval, and the voicemail service would have no way knowing that
the Identity header it received had been originally signed for a call the Identity header it received had been originally signed for a call
intended for a different number. However, note the caveats below in intended for a different number. However, note the caveats below in
Section 10.1.1. Section 11.1.1.
Without the request method, an INVITE request could be cut- and-
pasted by an attacker and transformed into a MESSAGE request without
changing any fields covered by the Identity header, and moreover
requests within a transaction (for example, a re-INVITE) could be
replayed in potentially confusing or malicious ways to enable
impersonation.
When signing a request that contains a fingerprint of keying material When signing a request that contains a fingerprint of keying material
in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
signature over that fingerprint. This signature prevents certain signature over that fingerprint. This signature prevents certain
classes of impersonation attacks in which an attacker forwards or classes of impersonation attacks in which an attacker forwards or
cut-and-pastes a legitimate request: although the target of the cut-and-pastes a legitimate request: although the target of the
attack may accept the request, the attacker will be unable to attack may accept the request, the attacker will be unable to
exchange media with the target as they will not possess a key exchange media with the target as they will not possess a key
corresponding to the fingerprint. For example there are some baiting corresponding to the fingerprint. For example there are some baiting
attacks, launched with the REFER method or through social attacks, launched with the REFER method or through social
engineering, where the attacker receives a request from the target engineering, where the attacker receives a request from the target
and reoriginates it to a third party: these might not be prevented by 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 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 by securing a fingerprint for DTLS-SRTP. While this is a different
form of impersonation than is commonly used for robocalling, form of impersonation than is commonly used for robocalling,
ultimately there is little purpose in establishing the identity of ultimately there is little purpose in establishing the identity of
the user that originated a SIP request if this assurance is not the user that originated a SIP request if this assurance is not
coupled with a comparable assurance over the contents of the coupled with a comparable assurance over the contents of the
subsequent communication. This signature also, per [RFC7258], subsequent media communication. This signature also, per [RFC7258],
reduces the potential for passive monitoring attacks against the SIP reduces the potential for passive monitoring attacks against the SIP
media. In environments where DTLS-SRTP is unsupported, however, no media. In environments where DTLS-SRTP is unsupported, however, no
field is signed and no protections are provided. field is signed and no protections are provided.
10.1.1. Protection of the To Header and Retargeting 11.1.1. Protection of the To Header and Retargeting
The mechanism in this document provides a signature over the identity The mechanism in this document provides a signature over the identity
information in the To header field value of requests. This provides information in the To header field value of requests. This provides
a means for verifiers to detect replay attacks where a signed request a means for verifiers to detect replay attacks where a signed request
originally sent to one target is modified and then forwarded by an originally sent to one target is modified and then forwarded by an
attacker to another, unrelated target. Armed with the original value attacker to another, unrelated target. Armed with the original value
of the To header field, the recipient of a request may compare it to 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 their own identity in order to determine whether or not the identity
information in this call might have been replayed. However, any information in this call might have been replayed. However, any
request may be legitimately retargeted as well, and as a result request may be legitimately retargeted as well, and as a result
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guidance is given for implementers here regarding the 'connected guidance is given for implementers here regarding the 'connected
party' problem (see [RFC4916]); authentication service behavior is party' problem (see [RFC4916]); authentication service behavior is
unchanged if retargeting has occurred for a dialog-forming request. unchanged if retargeting has occurred for a dialog-forming request.
Ultimately, the authentication service provides an Identity header Ultimately, the authentication service provides an Identity header
for requests in the backwards dialog when the user is authorized to 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 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 not, an Identity header is not provided. And per the threat model of
[RFC7375], resolving problems with 'connected' identity has little [RFC7375], resolving problems with 'connected' identity has little
bearing on detecting robocalling or related impersonation attacks. bearing on detecting robocalling or related impersonation attacks.
10.2. Unprotected Request Fields 11.2. Unprotected Request Fields
RFC4474 originally had protections for the Contact, Call-ID and CSeq. RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing the simple these headers does not seem impactful to preventing the simple
unauthorized claiming of an identity for the purposes of robocalling, unauthorized claiming of an identity for the purposes of robocalling,
voicemail hacking, or swatting, which is the primary scope of the voicemail hacking, or swatting, which is the primary scope of the
current document. current document.
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guarantee that no Via hops are inserted between the sending user guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented. inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS. known practice for securing Via is the use of SIPS.
10.3. Malicious Removal of Identity Headers 11.3. Malicious Removal of Identity Headers
In the end analysis, the Identity, Identity-Info and Identity-
Extension headers cannot protect themselves. Any attacker could
remove these headers from a SIP request, and modify the request
arbitrarily afterwards. However, this mechanism is not intended to
protect requests from men-in-the-middle who interfere with SIP
messages; it is intended only to provide a way that the originators
of SIP requests can prove that they are who they claim to be. At
best, by stripping identity information from a request, a man-in-the-
middle could make it impossible to distinguish any illegitimate
messages he would like to send from those messages sent by an
authorized user. However, it requires a considerably greater amount
of energy to mount such an attack than it does to mount trivial
impersonations by just copying someone else's From header field.
This mechanism provides a way that an authorized user can provide a
definitive assurance of his identity that an unauthorized user, an
impersonator, cannot.
One additional respect in which the Identity-Info header cannot In the end analysis, the Identity header cannot protect itself. Any
protect itself is the 'alg' parameter. The 'alg' parameter is not attacker could remove the header from a SIP request, and modify the
included in the digest-string, and accordingly, a man-in-the-middle request arbitrarily afterwards. However, this mechanism is not
might attempt to modify the 'alg' parameter. Once again, it is intended to protect requests from men-in-the-middle who interfere
important to note that preventing men-in-the-middle is not the with SIP messages; it is intended only to provide a way that the
motivation for this mechanism. Moreover, changing the 'alg' would at originators of SIP requests can prove that they are who they claim to
worst result in some sort of bid-down attack, and at best cause a be. At best, by stripping identity information from a request, a
failure in the verifier. Note that only one valid 'alg' parameter is man-in-the-middle could make it impossible to distinguish any
defined in this document and that thus there is currently no weaker illegitimate messages he would like to send from those messages sent
algorithm to which the mechanism can be bid down. 'alg' has been by an authorized user. However, it requires a considerably greater
incorporated into this mechanism for forward-compatibility reasons in amount of energy to mount such an attack than it does to mount
case the current algorithm exhibits weaknesses, and requires swift trivial impersonations by just copying someone else's From header
replacement, in the future. 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.
10.4. Securing the Connection to the Authentication Service 11.4. Securing the Connection to the Authentication Service
In the absence of user agent-based authentication services, the In the absence of user agent-based authentication services, the
assurance provided by this mechanism is strongest when a user agent assurance provided by this mechanism is strongest when a user agent
forms a direct connection, preferably one secured by TLS, to an forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are intermediary-based authentication service. The reasons for this are
twofold: twofold:
If a user does not receive a certificate from the authentication If a user does not receive a certificate from the authentication
service over the TLS connection that corresponds to the expected service over the TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a domain (especially when the user receives a challenge via a
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arrives at an authentication service. Accordingly, a prior arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message legitimate or illegitimate intermediary could modify the message
arbitrarily. arbitrarily.
Of these two concerns, the first is most material to the intended Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is possible that applications relying on replay attacks. However, it is possible that applications relying on
the presence of the Identity header could leverage this integrity the presence of the Identity header could leverage this integrity
protection, especially body integrity, for services other than replay protection for services other than replay protection.
protection.
Accordingly, direct TLS connections SHOULD be used between the UAC Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly, UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS are not possible, a UAC should use when a direct connection and TLS are not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course; domain request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association policy must identify those cases where the UAC's security association
with the authentication service is too weak. with the authentication service is too weak.
10.5. Authorization and Transitional Strategies 11.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the authentication service is limited by the security practices of the authentication service
that issues the assurance. Relying on an Identity header generated that issues the assurance. Relying on an Identity header generated
by a remote administrative domain assumes that the issuing domain by a remote administrative domain assumes that the issuing domain
uses recommended administrative practices to authenticate its users. uses recommended administrative practices to authenticate its users.
However, it is possible that some authentication services will However, it is possible that some authentication services will
implement policies that effectively make users unaccountable (e.g., implement policies that effectively make users unaccountable (e.g.,
ones that accept unauthenticated registrations from arbitrary users). ones that accept unauthenticated registrations from arbitrary users).
The value of an Identity header from such authentication services is The value of an Identity header from such authentication services is
questionable. While there is no magic way for a verifier to questionable. While there is no magic way for a verifier to
distinguish "good" from "bad" signers by inspecting a SIP request, it distinguish "good" from "bad" signers by inspecting a SIP request, it
is expected that further work in authorization practices could be is expected that further work in authorization practices could be
built on top of this identity solution; without such an identity built on top of this identity solution; without such an identity
solution, many promising approaches to authorization policy are solution, many promising approaches to authorization policy are
impossible. That much said, it is RECOMMENDED that authentication impossible. That much said, it is RECOMMENDED that authentication
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The value of an Identity header from such authentication services is The value of an Identity header from such authentication services is
questionable. While there is no magic way for a verifier to questionable. While there is no magic way for a verifier to
distinguish "good" from "bad" signers by inspecting a SIP request, it distinguish "good" from "bad" signers by inspecting a SIP request, it
is expected that further work in authorization practices could be is expected that further work in authorization practices could be
built on top of this identity solution; without such an identity built on top of this identity solution; without such an identity
solution, many promising approaches to authorization policy are solution, many promising approaches to authorization policy are
impossible. That much said, it is RECOMMENDED that authentication impossible. That much said, it is RECOMMENDED that authentication
services based on proxy servers employ strong authentication services based on proxy servers employ strong authentication
practices. practices.
One cannot expect the Identity and Identity-Info headers to be One cannot expect the Identity header to be supported by every SIP
supported by every SIP entity overnight. This leaves the verifier in entity overnight. This leaves the verifier in a compromising
a compromising position; when it receives a request from a given SIP position; when it receives a request from a given SIP user, how can
user, how can it know whether or not the sender's domain supports it know whether or not the sender's domain supports Identity? In the
Identity? In the absence of ubiquitous support for identity, some absence of ubiquitous support for identity, some transitional
transitional strategies are necessary. strategies are necessary.
A verifier could remember when it receives a request from a domain A verifier could remember when it receives a request from a domain
or telephone number that uses Identity, and in the future, view or telephone number that uses Identity, and in the future, view
messages received from that sources without Identity headers with messages received from that sources without Identity headers with
skepticism. skepticism.
A verifier could consult some sort of directory that indications A verifier could consult some sort of directory that indications
whether a given caller should have a signed identity. There are a whether a given caller should have a signed identity. There are a
number of potential ways in which this could be implemented. This number of potential ways in which this could be implemented. This
is left as a subject for future work. is left as a subject for future work.
skipping to change at page 29, line 5 skipping to change at page 29, line 36
Finally, it is worth noting that the presence or absence of the Finally, it is worth noting that the presence or absence of the
Identity headers cannot be the sole factor in making an authorization Identity headers cannot be the sole factor in making an authorization
decision. Permissions might be granted to a message on the basis of 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 the specific verified Identity or really on any other aspect of a SIP
request. Authorization policies are outside the scope of this request. Authorization policies are outside the scope of this
specification, but this specification advises any future specification, but this specification advises any future
authorization work not to assume that messages with valid Identity authorization work not to assume that messages with valid Identity
headers are always good. headers are always good.
10.6. Display-Names and Identity 11.6. Display-Names and Identity
As a matter of interface design, SIP user agents might render the 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 display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this component of the From header field value is outside the scope of this
document, but may be the subject of future work, such as an extension. document, but may be the subject of future work, such as through the
"spec" name mechanism.
11. IANA Considerations 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 This document relies on the headers and response codes defined in RFC
4474. It also retains the requirements for the specification of new 4474. It also retains the requirements for the specification of new
algorithms or headers related to the mechanisms described in that algorithms or headers related to the mechanisms described in that
document. document.
11.1. Header Field Names 12.1. Identity-Info Parameters
This document also specifies a new SIP header called Identity-
Extension. Its syntax is given in Section 8. A registry for
Identity-Extension names is defined in Section 11.4.
Header Name: Identity-Extension
Compact Form: N/A
11.2. Identity-Info Parameters
The IANA has already created a registry for Identity-Info header The IANA has already created a registry for Identity-Info parameters.
parameters. This specification defines a new value called "canon" as This specification defines a new value called "canon" as defined in
defined in Section 5.3. Section 5.3. Note however that unlike in RFC4474, Identity-Info
parameters now appear in the Identity header.
11.3. Identity-Info Algorithm Parameter Values 12.2. Identity-Info Algorithm Parameter Values
The IANA has already created a registry for Identity-Info 'alg' The IANA has already created a registry for Identity-Info "alg"
parameter values. This registry is to be prepopulated with a single parameter values. This registry is to be populated with a value for
entry for a value called 'rsa-sha256', which describes the algorithm 'RS256', which describes the algorithm used to create the signature
used to create the signature that appears in the Identity header. that appears in the Identity header. Registry entries must contain
Registry entries must contain the name of the 'alg' parameter value the name of the 'alg' parameter value and the specification in which
and the specification in which the value is described. New values the value is described. New values for the 'alg' parameter may be
for the 'alg' parameter may be defined only in Standards Track RFCs. defined only in Standards Track RFCs.
RFC4474 defined the 'rsa-sha1' value for this registry. That value RFC4474 defined the 'rsa-sha1' value for this registry. That value
is hereby deprecated, and should be treated as such. It is not is hereby deprecated, and should be treated as such. It is not
believed that any implementations are making use of this value. believed that any implementations are making use of this value.
Future specifications may consider elliptical curves for smaller key Future specifications may consider elliptical curves for smaller key
sizes. sizes.
11.4. Identity-Extension Names 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.
12.3. spec parameter Names
This specification requests that the IANA create a new registry for This specification requests that the IANA create a new registry for
Identity-Extension names. The registry will consist solely of a list spec parameter names. The registry will consist solely of a list of
of names mapped to the Standards Track RFCs in which those extensions names mapped to any specification where the procedures are defined
are defined. (or "N/A" if no specification is available).
The syntax of Identity-Extension names is given in Section 8. The syntax of "spec" names is given in Section 8. Registering a new
Registering a new Identity-Extension name requires a Standards "spec" name is on a First Come First Serve basis.
Action.
This specification does not provide any initial values for Identity- This specification does not provide any initial values for "spec"
Extension names. names.
12. Acknowledgments 13. Acknowledgments
The authors would like to thank Stephen Kent, Brian Rosen, Alex The authors would like to thank Stephen Kent, Brian Rosen, Alex
Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov, Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer, Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer,
Henning Schulzrinne, and Richard Barnes for their comments. Henning Schulzrinne, and Richard Barnes for their comments.
13. Changes from RFC4474 14. Changes from RFC4474
The following are salient changes from the original RFC 4474: The following are salient changes from the original RFC 4474:
Generalized the credential mechanism; credential enrollment and Generalized the credential mechanism; credential enrollment,
acquisition is now outside the scope of this document acquisition and trust is now outside the scope of this document
Reduced the scope of the Identity signature to remove CSeq, Call- Reduced the scope of the Identity signature to remove CSeq, Call-
ID, Contact, and the message body 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 Added any DTLS-SRTP fingerprint in SDP as a mandatory element of
the digest-string the digest-string
Added the Identity-Extension header and extensibility mechanism
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
14. References Changed the identity-digest-string format for compatibility with
JWT
14.1. Normative References 15. References
15.1. Normative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>. <http://www.rfc-editor.org/info/rfc2818>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002, DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>. <http://www.rfc-editor.org/info/rfc3261>.
skipping to change at page 31, line 36 skipping to change at page 32, line 30
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004, RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>. <http://www.rfc-editor.org/info/rfc3966>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <http://www.rfc-editor.org/info/rfc5280>.
14.2. Informative References 15.2. Informative References
[I-D.ietf-stir-certificates] [I-D.ietf-stir-certificates]
Peterson, J., "Secure Telephone Identity Credentials: Peterson, J., "Secure Telephone Identity Credentials:
Certificates", draft-ietf-stir-certificates-02 (work in Certificates", draft-ietf-stir-certificates-02 (work in
progress), July 2015. progress), July 2015.
[I-D.kaplan-stir-cider] [I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00 Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013. (work in progress), July 2013.
skipping to change at page 33, line 22 skipping to change at page 34, line 16
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>. 2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013, DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>. <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 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>. 2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements", Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014, RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>. <http://www.rfc-editor.org/info/rfc7340>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model", [RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014, RFC 7375, DOI 10.17487/RFC7375, October 2014,
<http://www.rfc-editor.org/info/rfc7375>. <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 Authors' Addresses
Jon Peterson Jon Peterson
Neustar, Inc. Neustar, Inc.
1800 Sutter St Suite 570 1800 Sutter St Suite 570
Concord, CA 94520 Concord, CA 94520
US US
Email: jon.peterson@neustar.biz Email: jon.peterson@neustar.biz
Cullen Jennings Cullen Jennings
Cisco Cisco
400 3rd Avenue SW, Suite 350 400 3rd Avenue SW, Suite 350
Calgary, AB T2P 4H2 Calgary, AB T2P 4H2
Canada Canada
Email: fluffy@iii.ca Email: fluffy@iii.ca
Eric Rescorla Eric Rescorla
RTFM, Inc. RTFM, Inc.
2064 Edgewood Drive 2064 Edgewood Drive
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com Email: ekr@rtfm.com
Chris Wendt
Comcast
One Comcast Center
Philadelphia, PA 19103
USA
Email: chris-ietf@chriswendt.net
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