draft-ietf-stir-rfc4474bis-07.txt   draft-ietf-stir-rfc4474bis-08.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: August 6, 2016 Cisco Expires: September 22, 2016 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
C. Wendt C. Wendt
Comcast Comcast
February 3, 2016 March 21, 2016
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-07.txt draft-ietf-stir-rfc4474bis-08.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 a identifying originators of SIP requests. It does so by defining a
SIP header field 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
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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 August 6, 2016. This Internet-Draft will expire on September 22, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 6 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Signature Generation and Validation . . . . . . . . . . . . . 7 4. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
4.1. Authentication Service Behavior . . . . . . . . . . . . . 7 5. Signature Generation and Validation . . . . . . . . . . . . . 7
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9 5.1. Authentication Service Behavior . . . . . . . . . . . . . 7
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10
5.1. Credential Use by the Authentication Service . . . . . . 11 6. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Credential Use by the Verification Service . . . . . . . 12 6.1. Credential Use by the Authentication Service . . . . . . 11
5.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 13 6.2. Credential Use by the Verification Service . . . . . . . 12
5.4. Credential System Requirements . . . . . . . . . . . . . 13 6.3. Handling 'info' parameter URIs . . . . . . . . . . . . . 13
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 15 6.4. Credential System Requirements . . . . . . . . . . . . . 14
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 15 7. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15 7.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 15
6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17 7.1.1. Canonicalization Procedures . . . . . . . . . . . . . 16
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18 7.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 18
8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 21 8. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Gatewaying to PASSporT for non-SIP Transit . . . . . . . . . 22 9. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 10. Gatewaying to PASSporT for non-SIP Transit . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 24 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23
11.1. Protected Request Fields . . . . . . . . . . . . . . . . 24 12. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11.1.1. Protection of the To Header and Retargeting . . . . 26 12.1. Protected Request Fields . . . . . . . . . . . . . . . . 25
11.2. Unprotected Request Fields . . . . . . . . . . . . . . . 26 12.1.1. Protection of the To Header and Retargeting . . . . 27
11.3. Malicious Removal of Identity Headers . . . . . . . . . 27 12.2. Unprotected Request Fields . . . . . . . . . . . . . . . 27
11.4. Securing the Connection to the Authentication Service . 28 12.3. Malicious Removal of Identity Headers . . . . . . . . . 28
11.5. Authorization and Transitional Strategies . . . . . . . 29 12.4. Securing the Connection to the Authentication Service . 28
11.6. Display-Names and Identity . . . . . . . . . . . . . . . 30 12.5. Authorization and Transitional Strategies . . . . . . . 29
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 12.6. Display-Names and Identity . . . . . . . . . . . . . . . 30
12.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 30 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
12.2. Identity-Info Algorithm Parameter Values . . . . . . . . 30 13.1. Identity-Info Parameters . . . . . . . . . . . . . . . . 31
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31 13.2. Identity-Info Algorithm Parameter Values . . . . . . . . 31
14. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 31 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 15. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 32
15.1. Normative References . . . . . . . . . . . . . . . . . . 31 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
15.2. Informative References . . . . . . . . . . . . . . . . . 32 16.1. Normative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 16.2. Informative References . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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
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requests in order to provide an 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. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919].
3. Background
Per [RFC7340], problems such as robocalling, voicemail hacking, and Per [RFC7340], problems such as robocalling, voicemail hacking, and
swatting are enabled by an attacker's ability to impersonate someone swatting are enabled by an attacker's ability to impersonate someone
else. The secure operation of most SIP applications and services else. The secure operation of most SIP applications and services
depends on authorizing the source of communications as it is depends on authorizing the source of communications as it is
represented in a SIP request. Such authorization policies can be represented in a SIP request. Such authorization policies can be
automated or be a part of human operation of SIP devices. An example automated or be a part of human operation of SIP devices. An example
of the former would be a voicemail service that compares the identity of the former would be a voicemail service that compares the identity
of the caller to a whitelist before determining whether it should of the caller to a whitelist before determining whether it should
allow the caller access to recorded messages. An example of the allow the caller access to recorded messages. An example of the
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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 constructing tokens based on It relies on the authentication service constructing tokens based on
the [ietf-stir-passport] format, a JSON [RFC7159] object comprising the PASSporT [I-D.ietf-stir-passport] format, a JSON [RFC7159] object
values copied from certain header field values in the SIP request. comprising values copied from certain header field values in the SIP
The authentication service then computes a signature over those JSON request. The authentication service then computes a signature over
object in a manner following PASSporT. That signature is then placed those JSON object in a manner following PASSporT. That signature is
in a SIP Identity header. In order to assist in the validation of then placed in a SIP Identity header. In order to assist in the
the Identity header, this specification also describes some metadata validation of the Identity header, this specification also describes
fields associated with the header that can be used by the recipient some metadata fields associated with the header that can be used by
of a request to recover the credentials of the signer. Note that the the recipient of a request to recover the credentials of the signer.
scope of this document is limited to providing this identity Note that the scope of this document is limited to providing this
assurance for SIP requests; solving this problem for SIP responses is identity assurance for SIP requests; solving this problem for SIP
outside the scope of this work (see [RFC4916]). Future work might responses is outside the scope of this work (see [RFC4916]). Future
specify ways that a SIP implementation could gateway PASSporT objects work might specify ways that a SIP implementation could gateway
to other protocols. PASSporT objects to other protocols.
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.
3. Overview of Operations 4. Overview of Operations
This section provides an informative (non-normative) high-level This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document. overview of the mechanisms described in this document.
Imagine a case where Alice, who has the home proxy of example.com and Imagine a case where Alice, who has the home proxy of example.com and
the address-of-record sip:alice@example.com, wants to communicate the address-of-record sip:alice@example.com, wants to communicate
with Bob at sip:bob@example.org. They have no prior relationship, with Bob at sip:bob@example.org. They have no prior relationship,
and Bob implements best practices to prevent impersonation attacks. and Bob implements best practices to prevent impersonation attacks.
Alice generates an INVITE and places her identity, in this case her Alice generates an INVITE and places her identity, in this case her
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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 5. Signature Generation and Validation
4.1. Authentication Service Behavior 5.1. 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, for example, by an intermediary such as a proxy server instantiated, for example, by an intermediary such as a proxy server
or by a user agent. Any entity that instantiates the authentication or by a user agent. Any entity that instantiates the authentication
service role MUST possess the private key of one or more credentials service role MUST possess the private key of one or more credentials
that can be used to sign for a domain or a telephone number (see that can be used to sign for a domain or a telephone number (see
Section 5.1). Intermediaries that instantiate this role MUST be Section 6.1). Intermediaries that instantiate this role MUST be
capable of authenticating one or more SIP users who can register for capable of authenticating one or more SIP users who can register for
that identity. Commonly, this role will be instantiated by a proxy that 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. An authentication service adds the Identity header to SIP requests.
The procedures below define the steps that must be taken when each an 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 header is added. More than one may appear in a single request, and
an authentication service may add an Identity header to a request an authentication service may add an Identity header to a request
that already contains one or more Identity headers. If the Identity that already contains one or more Identity headers. If the Identity
header added follows extended signing procedures beyond the baseline header added follows extended signing procedures beyond the baseline
given in Section 7, then it differentiates the header with a "type" given in Section 8, then it differentiates the header with a "ppt"
parameter per the fourth step below. 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
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inspecting the URI value from the addr-spec component of From header inspecting the URI value from the addr-spec component of From header
field; this URI will be referred to here as the 'identity field'. If field; this URI will be referred to here as the 'identity field'. If
the identity field contains a SIP or SIP Secure (SIPS) URI, and the 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 7.1 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 canonicalization procedures described in number MUST then follow the canonicalization procedures described in
Section 6.1.1. If the authentication service is not authoritative Section 7.1.1. 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 unless the local policy is to block such requests. request normally unless the local policy is to block such requests.
The authentication service MUST NOT follow the steps below to add an The authentication service MUST NOT add an Identity header if the
Identity header corresponding to an identity for which the authentication service does not hav ethe authority to make the claim
authentication service is not authoritative. it asserts.
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
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the authentication scheme used for registration in its domain the authentication scheme used for registration in its domain
(e.g., Digest authentication). (e.g., Digest authentication).
If the authentication service is instantiated by a SIP user agent, If the authentication service is instantiated by a SIP user agent,
a user agent may authenticate its own user through any system- a user agent may authenticate its own user through any system-
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 6.1 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'). For name portion of the From header field (e.g., 'Alice Atlanta'). For
more information, see Section 11.6. more information, see Section 12.6.
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 that do not have one. The authentication service MUST requests that do not have one. The authentication service MUST
ensure that any preexisting Date header in the request is accurate. ensure that any preexisting Date header in the request is accurate.
Local policy can dictate precisely how accurate the Date must be; a Local policy can dictate precisely how accurate the Date must be; a
RECOMMENDED maximum discrepancy of sixty seconds will ensure that the RECOMMENDED maximum discrepancy of sixty seconds will ensure that the
request is unlikely to upset any verifiers. If the Date header request is unlikely to upset any verifiers. If the Date header
contains a time different by more than one minute from the current contains a time different by more than one minute from the current
time noted by the authentication service, the authentication service time noted by the authentication service, the authentication service
SHOULD reject the request. This behavior is not mandatory because a SHOULD reject the request. This behavior is not mandatory because a
user agent client (UAC) could only exploit the Date header in order user agent client (UAC) could only exploit the Date header in order
to cause a request to fail verification; the Identity header is not to cause a request to fail verification; the Identity header is not
intended to provide a source of non-repudiation or a perfect record intended to provide a source of non-repudiation or a perfect record
of when messages are processed. Finally, the authentication service of when messages are processed. Finally, the authentication service
MUST verify that both the Date header and the current time fall MUST verify that both the Date header and the current time fall
within the validity period of its credential. within the validity period of its credential.
See Section 11 for information on how the Date header field assists See Section 12 for information on how the Date header field assists
verifiers. verifiers.
Step 4: Step 4:
Subsequently, the authentication service MUST form a PASSporT object Subsequently, the authentication service MUST form a PASSporT object
and add a corresponding an Identity header to the request containing and add a corresponding an Identity header to the request containing
this signature. For baseline PASSporT objects headers (without an this signature. For baseline PASSporT objects headers (without an
Identity header "type" parameter), this follows the procedures in Identity header "ppt" parameter), this follows the procedures in
Section 7; if the authentication service is using an alternative Section 8; if the authentication service is using an alternative
"type", it MUST add an appropriate "type" parameter and follow the "ppt", it MUST add an appropriate "ppt" parameter and follow the
procedures associated with it (see Section 8). After the Identity procedures associated with it (see Section 9). After the Identity
header has been added to the request, the authentication service MUST header has been added to the request, the authentication service MUST
also add a "info" parameter to the Identity header. The "info" also add a "info" parameter to the Identity header. The "info"
parameter contains a URI from which the authentication service's parameter contains a URI from which the authentication service's
credential can be acquired; see Section 5.3 for more on credential credential can be acquired; see Section 6.3 for more on credential
acquisition. acquisition.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
4.2. Verifier Behavior In some cases, a request sent through an authentication service will
be rejected by the verification service, and the authentication
service will receive a 4xx status code (such as 438) in the backwards
direction. If the authentication service did not originally send the
request with the "canon" parameter, it MAY retry a request once after
receiving a 438 response, this time including the "canon". The
information in "canon" is useful for debugging errors, and there are
some known causes of verification failures (such as the Date header
changing in transit, see Section 12.1 for more information) that can
be resolved by the inclusion of "canon".
5.2. 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 one or more Identity headers, it inspects the message containing one or more Identity headers, it inspects the
signature to verify the identity of the sender of the message. 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. process that is outside the scope of this document.
A SIP request may contain zero, one, or more Identity headers. A A SIP request may contain zero, one, or more Identity headers. A
verification service performs the procedures below on each Identity verification service performs the procedures below on each Identity
header that appears in a request. If the verifier does not support 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 an Identity header present in a request due to the presence of an
unsupported "type" parameter, or if no Identity header is present, unsupported "ppt" parameter, or if no Identity header is present, and
and the presence of an Identity header is required by local policy the presence of an Identity header is required by local policy (for
(for example, based on a per-sending-domain policy, or a per-sending- example, based on a per-sending-domain policy, or a per-sending-user
user policy), then a 428 'Use Identity Header' response MUST be sent policy), then a 428 'Use Identity Header' response MUST be sent in
in the backwards direction. 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:
The verifier MUST inspect any optional "type" parameter appearing the The verifier MUST inspect any optional "ppt" parameter appearing the
Identity request. If no "type" parameter is present, then the Identity request. If no "ppt" parameter is present, then the
verifier proceeds normally below. If a "type" parameter value is verifier proceeds normally below. If a "ppt" parameter value is
present, and the verifier does not support it, it MUST ignore the present, and the verifier does not support it, it MUST ignore the
Identity header. If a supported "type" parameter value is present, Identity header. If a supported "ppt" parameter value is present,
the verifier follows the procedures below, including the variations the verifier follows the procedures below, including the variations
described in Step 5. described in Step 5.
Step 2: Step 2:
In order to determine whether the signature for the identity field In order to determine whether the signature for the identity field
should be over the entire identity field URI or just a canonicalized should be over the entire identity field URI or just a canonicalized
telephone number, the verification service MUST follow the telephone number, the verification service MUST follow the
canonicalization process described in Section 6.1.1. That section canonicalization process described in Section 7.1.1. That section
also describes the procedures the verification service MUST follow to also describes the procedures the verification service MUST follow to
determine if the signer is authoritative for a telephone number. For determine if the signer is authoritative for a telephone number. For
domains, the verifier MUST follow the process described in domains, the verifier MUST follow the process described in
Section 6.2 to determine if the signer is authoritative for the Section 7.2 to determine if the signer is authoritative for the
identity field. identity field.
Step 3: 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 a URI in the "info" parameter of which usually involves dereferencing a URI in the "info" parameter of
the Identity header. See Section 5.2 for more information on these the Identity header. See Section 6.2 for more information on these
procedures. If the verifier does not suport the credential described procedures. If the verifier does not suport the credential described
in the "info" parameter, it MUST return a 437 "Unsupported in the "info" parameter, it MUST return a 437 "Unsupported
Certificate" response. Certificate" response.
Step 4: Step 4:
The verifier 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 11.1. this prevents, see Section 12.1.
Step 5: Step 5:
The verifier MUST validate the signature in the Identity header field The verifier MUST validate the signature in the Identity header field
over the PASSporT object. For baseline PASSporT objects (with no over the PASSporT object. For baseline PASSporT objects (with no
Identity header "type" parameter) the verifier MUST follow the Identity header "ppt" parameter) the verifier MUST follow the
procedures for generating the signature over a PASSporT object procedures for generating the signature over a PASSporT object
described in Section 7. If a "type" parameter is present, the described in Section 8. If a "ppt" parameter is present, the
verifier follows the procedures for that "type" (see Section 8). If verifier follows the procedures for that "ppt" (see Section 9). If a
a verifier determines that the signature on the message does not verifier determines that the signature on the message does not
correspond to the reconstructed signed-identity-digest, then a 438 correspond to the reconstructed signed-identity-digest, then a 438
'Invalid Identity Header' response MUST be returned. '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 6. Credentials
5.1. Credential Use by the Authentication Service 6.1. Credential Use by the Authentication Service
In order to act as an authentication service, a SIP entity must have In order to act as an authentication service, a SIP entity must have
access to the private keying material of one or more credentials that access to the private keying material of one or more credentials that
cover domain names or telephone numbers. These credentials may cover domain names or telephone numbers. These credentials may
represent authority over an entire domain (such as example.com) or represent authority over an entire domain (such as example.com) or
potentially a set of domains enumerated by the credential. potentially a set of domains enumerated by the credential.
Similarly, a credential may represent authority over a single Similarly, a credential may represent authority over a single
telephone number or a range of telephone numbers. The way that the telephone number or a range of telephone numbers. The way that the
scope of a credential is expressed is specific to the credential scope of a credential is expressed is specific to the credential
mechanism. mechanism.
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case, this is deferred to future work, but implementors should note case, this is deferred to future work, but implementors should note
this as a possible future direction. this as a possible future direction.
Exceptions to such authentication service policies arise for cases Exceptions to such authentication service policies arise for cases
like anonymity; if the AoR asserted in the From header field uses a like anonymity; if the AoR asserted in the From header field uses a
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 6.2. 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 URI found in the "info" parameter of the Identity Dereferencing the URI found in the "info" parameter of the Identity
header (as described in the next section) MUST be supported by all header (as described in the next section) MUST be supported by all
verification service implementations to create a baseline means of verification service implementations to create a baseline means of
credential acquisition. Provided that the credential used to sign a credential acquisition. Provided that the credential used to sign a
message is not previously known to the verifier, SIP entities SHOULD message is not previously known to the verifier, SIP entities SHOULD
discover this credential by dereferencing the "info" parameter, discover this credential by dereferencing the "info" parameter,
unless they have some more other implementation-specific way of unless they have some more other implementation-specific way of
acquiring the needed keying material, such as an offline store of acquiring the needed keying material, such as an offline store of
periodically-updated credentials. If the URI in the "info" parameter periodically-updated credentials. If the URI in the "info" parameter
cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST
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unless they have some more other implementation-specific way of unless they have some more other implementation-specific way of
acquiring the needed keying material, such as an offline store of acquiring the needed keying material, such as an offline store of
periodically-updated credentials. If the URI in the "info" parameter periodically-updated credentials. If the URI in the "info" parameter
cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST cannot be dereferenced, then a 436 'Bad Identity-Info' response MUST
be 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 6.4.
Verification service implementations supporting this specification Verification service implementations supporting this specification
may wish to have some means of retaining credentials (in accordance may wish to have some means of retaining credentials (in accordance
with normal practices for credential lifetimes and revocation) in with normal practices for credential lifetimes and revocation) in
order to prevent themselves from needlessly downloading the same order to prevent themselves from needlessly downloading the same
credential every time a request from the same identity is received. credential every time a request from the same identity is received.
Credentials cached in this manner may be indexed in accordance with Credentials cached in this manner may be indexed in accordance with
local policy: for example, by their scope, or the URI given in the local policy: for example, by their scope, or the URI given in the
"info" parameter value. Further consideration of how to cache "info" parameter value. Further consideration of how to cache
credentials is deferred to the credential mechanism specifications. credentials is deferred to the credential mechanism specifications.
5.3. Handling 'info' parameter URIs 6.3. Handling 'info' parameter URIs
An "info" parameter 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 "info parameter" be dereferencable by any essential that a URI in the "info parameter" be dereferencable by any
entity that could plausibly receive the request. For common cases, entity that could plausibly receive the request. For common cases,
this means that the URI must be dereferencable by any entity on the this means that the URI must be dereferencable by any entity on the
public Internet. In constrained deployment environments, a service public Internet. In constrained deployment environments, a service
private to the environment might be used instead. private to the environment might be used instead.
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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 "info" differentiate which one performed the signature. The "info"
parameter performs that function. parameter performs that function.
If the optional "canon" parameter is present, it contains the bae64 If the optional "canon" parameter is present, it contains the bae64
encoded result of JSON object construction process performed by the encoded result of JSON object construction process performed by the
authentication service (see Section 6.1.1), including the authentication service (see Section 7.1.1), including the
canonicalization processes applied to the identity in the identity canonicalization processes applied to the identity in the identity
fields of the sender and intended recipient. The "canon" is provided fields of the sender and intended recipient. The "canon" is provided
purely as an optimization for the verification service. The purely as an optimization for the verification service. The
verification service MAY compute its own canonicalization of the verification service MAY compute its own canonicalization of the
numbers and compare them to the values in the "canon" parameter numbers and compare them to the values in the "canon" parameter
before performing any cryptographic functions in order to ascertain before performing any cryptographic functions in order to ascertain
whether or not the two ends agree on the canonical number form. whether or not the two ends agree on the canonical number form.
5.4. Credential System Requirements 6.4. 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 "info" parameter could for example give the SIP identity context: an "info" parameter could for example 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 "info" certificate (following the conventions of [RFC2585]). The "info"
parameter may use the DNS URL scheme (see [RFC4501]) to designate parameter may use the DNS URL scheme (see [RFC4501]) to designate
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Note that credential systems must address key lifecycle management Note that credential systems must address key lifecycle management
concerns: were a domain to change the credential available at the concerns: were a domain to change the credential available at the
Identity-Info URI before a verifier evaluates a request signed by an Identity-Info URI before a verifier evaluates a request signed by an
authentication service, this would cause obvious verifier failures. authentication service, this would cause obvious verifier failures.
When a rollover occurs, authentication services SHOULD thus provide When a rollover occurs, authentication services SHOULD thus provide
new Identity-Info URIs for each new credential, and SHOULD continue new Identity-Info URIs for each new credential, and SHOULD continue
to make older key acquisition URIs available for a duration longer to make older key acquisition URIs available for a duration longer
than the plausible lifetime of a SIP transaction (a minute would most than the plausible lifetime of a SIP transaction (a minute would most
likely suffice). likely suffice).
6. Identity Types 7. Identity Types
6.1. Telephone Numbers 7.1. Telephone Numbers
Since many SIP applications provide a Voice over IP (VoIP) service, Since many SIP applications provide a Voice over IP (VoIP) service,
telephone numbers are commonly used as identities in SIP deployments. telephone numbers are commonly used as identities in SIP deployments.
In order for telephone numbers to be used with the mechanism In order for telephone numbers to be used with the mechanism
described in this document, authentication services must enroll with described in this document, authentication services must enroll with
an authority that issues credentials for telephone numbers or an authority that issues credentials for telephone numbers or
telephone number ranges, and verification services must trust the telephone number ranges, and verification services must trust the
authority employed by the authentication service that signs a authority employed by the authentication service that signs a
request. Enrollment procedures and credential management are outside request. Enrollment procedures and credential management are outside
the scope of this document. the scope of this document.
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scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to
appear in the SIP To or From header field outside the context of a appear in the SIP To or From header field outside the context of a
SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases, SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases,
it's clear that the signer must have authority over the telephone it's clear that the signer must have authority over the telephone
number, not the domain name of the SIP URI. It is also possible, number, not the domain name of the SIP URI. It is also possible,
however, for requests to contain a URI like however, for requests to contain a URI like
'sip:7005551000@chicago.example.com'. It may be non-trivial for a 'sip:7005551000@chicago.example.com'. It may be non-trivial for a
service to ascertain in this case whether the URI contains a service to ascertain in this case whether the URI contains a
telephone number or not. telephone number or not.
6.1.1. Canonicalization Procedures 7.1.1. Canonicalization Procedures
In order to determine whether or not the user portion of a SIP URI is In order to determine whether or not the user portion of a SIP URI is
a telephone number, authentication services and verification services a telephone number, authentication services and verification services
must perform the following canonicalization procedure on any SIP URI must perform the following canonicalization procedure on any SIP URI
they inspect which contains a wholly numeric user part. Note that they inspect which contains a wholly numeric user part. Note that
the same procedures are followed for creating the canonical form of the same procedures are followed for creating the canonical form of
URIs found in both the From and To header field values; this section URIs found in both the From and To header field values; this section
also describes procedures for extracting the URI containing the also describes procedures for extracting the URI containing the
telephone number from the P-Asserted-Identity header field value for telephone number from the P-Asserted-Identity header field value for
environments where that is applicable. environments where that is applicable.
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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 "canon" from how it appears in the From header field, the use of the "canon"
parameter by authentication services is RECOMMENDED, but because parameter by authentication services is RECOMMENDED, but because
"canon" itself could then divulge information about users or "canon" itself could then divulge information about users or
networks, implementers should be mindful of the guidelines in networks, implementers should be mindful of the guidelines in
Section 10. Section 11.
First, implementations must assess if the user-portion of the URI First, implementations must assess if the user-portion of the URI
constitutes a telephone number. In some environments, numbers constitutes a telephone number. In some environments, numbers
will be explicitly labeled by the use of TEL URIs or the will be explicitly labeled by the use of TEL URIs or the
'user=phone' parameter, or implicitly by the presence of the '+' 'user=phone' parameter, or implicitly by the presence of the '+'
indicator at the start of the user-portion. Absent these indicator at the start of the user-portion. Absent these
indications, if there are numbers present in the user-portion, indications, if there are numbers present in the user-portion,
implementations may also detect that the user-portion of the URI implementations may also detect that the user-portion of the URI
contains a telephone number by determining whether or not those contains a telephone number by determining whether or not those
numbers would be dialable or routable in the local environment -- numbers would be dialable or routable in the local environment --
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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
signed-identity-string by both the authentication service and signed-identity-string by both the authentication service and
verification service. Practically, entities that perform the verification service. Practically, entities that perform the
authentication service role will sometimes alter the telephone authentication service role will sometimes alter the telephone
numbers that appear in the To and From header field values, numbers that appear in the To and From header field values,
converting them to this format (though note this is not a function converting them to this format (though note this is not a function
that [RFC3261] permits proxy servers to perform). The result of the that [RFC3261] permits proxy servers to perform). The result of the
canonicalization process of the From header field value may also be canonicalization process of the From header field value may also be
recorded through the use of the "canon" parameter of the Identity(see recorded through the use of the "canon" parameter of the Identity(see
Section 7). If the result of the canonicalization of the From header Section 8). If the result of the canonicalization of the From header
field value does not form a complete telephone number, the field value does not form a complete telephone number, the
authentication service and verification service should treat the authentication service and verification service should treat the
entire URI as a SIP URI, and apply a domain signature per the entire URI as a SIP URI, and apply a domain signature per the
procedures in Section 6.2. procedures in Section 7.2.
6.2. Domain Names 7.2. 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 "info" that is the subject of the credential acquired from the "info"
parameter. 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
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A verifier MUST evaluate the correspondence between the user's A verifier MUST evaluate the correspondence between the user's
identity and the signing credential by following the procedures identity and the signing credential by following the procedures
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 8. Header Syntax
The Identity and Identity-Info headers that were previously defined The Identity and Identity-Info headers that were previously defined
in RFC4474 are deprecated. This document collapses the grammar of in RFC4474 are deprecated. This revised specification collapses the
the Identity-Info into the Identity header via the "info" parameter. grammar of Identity-Info into the Identity header via the "info"
Note that unlike the prior specification in RFC4474, the Identity parameter. Note that unlike the prior specification in RFC4474, the
header is now allowed to appear more than one time in a SIP request. Identity header is now allowed to appear more than one time in a SIP
The revised grammar for the Identity header is (following the ABNF request. The revised grammar for the Identity header is (following
[RFC4234] in RFC 3261 [RFC3261]): the ABNF [RFC4234] in RFC 3261 [RFC3261]):
Identity = "Identity" HCOLON signed-identity-digest SEMI ident-info *( SEMI ident-info-params ) 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
ident-info = "info" EQUAL ident-info-uri ident-info = "info" EQUAL ident-info-uri
ident-info-uri = LAQUOT absoluteURI RAQUOT ident-info-uri = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-type / canonical-str / ident-info-extension ident-info-params = ident-info-alg / ident-type / canonical-str / ident-info-extension
ident-info-alg = "alg" EQUAL token ident-info-alg = "alg" EQUAL token
ident-type = "type" EQUAL token ident-type = "ppt" EQUAL token
canonical-str = "canon" EQUAL *base64-char canonical-str = "canon" EQUAL *base64-char
ident-info-extension = generic-param ident-info-extension = generic-param
base64-char = ALPHA / DIGIT / "/" / "+" base64-char = ALPHA / DIGIT / "/" / "+"
In addition to "info" parameter and the "alg" parameter defined in In addition to "info" parameter, and the "alg" parameter previously
RFC44744, this specification includes the optional "canon" and "type" defined in RFC4474, this specification includes the optional "canon"
parameters. Note that in RFC4474, the signed-identity-digest (see and "ppt" parameters. Note that in RFC4474, the signed-identity-
ABNF above) was given as quoted 32LHEX, whereas here it is given as a digest (see ABNF above) was given as quoted 32LHEX, whereas here it
quoted sequence of base64-char. is given as a quoted sequence of base64-char.
The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see
Section 5.3 for more on choosing how to advertise credentials through Section 6.3 for more on choosing how to advertise credentials through
this parameter. this parameter.
The signed-identity-digest is a signed hash of a [ietf-stir-passport] The signed-identity-digest is the signed hash component of a PASSporT
object, which is a pair of JSON objects generated from certain object [I-D.ietf-stir-passport], a signature which PASSporT generates
components of a SIP request. This first object contains header over a pair of JSON objects. The first PASSporT object contains
information, and the second contains claims, following the header information, and the second contains claims, following the
conventions of JWT [RFC7519]. Once these two JSON objects have been conventions of JWT [RFC7519]; some header and claim values will
generated, they will be encoded per the procedures of [ietf-stir- mirror elements of the SIP request. Once these two JSON objects have
passport], then hashed with a SHA-256 hash and then concatenated, been generated, they will be encoded, then hashed with a SHA-256
header then claims, into a string separated by a single "." per the hash. Those two hashes are then concatenated (header then claims)
conventions of baseline PASSporT. into a string separated by a single "." per baseline PASSporT.
To create the PASSporT object used in the construction of the signed- Finally, that string is signed to generate the signed-identity-digest
identity-digest of the Identity header, the following elements of a value of the Identity header.
SIP message MUST be placed in a first comma-separated JSON object, in
order:
First, the JSON key "typ" followed by a colon and then the quoted For SIP implementations to populate the PASSporT header object from a
string "PASSporT". SIP request, the following elements message MUST be placed as the
values corresponding to the designated JSON keys:
Second, the JSON key "alg" followed by a colon and then the quoted First, per baseline [I-D.ietf-stir-passport], the JSON key "typ"
value of the optional "alg" parameter in the Identity header. key MUST have the value "passport".
Note if the "alg" parameter is absent, the default value is
"RS256".
Third, the JSON key "x5u" followed by a colon and then the quoted Second, the JSON key "alg" MUST mirror the value of the optional
value of the URI in the "info" parameter. "alg" parameter in the SIP Identity header. Note if the "alg"
parameter is absent, the default value is "RS256".
Fourth, optionally the JSON key "type" followed by a colon and Third, the JSON key "x5u" MUST have a value equivalent to the
then the quoted value of the "type" parameter of the Identity quoted URI in the "info" parameter.
header. If the "type" parameter is absent from the header, the
"type" key will not appear in the JSON heaer object. Fourth, the optional JSON key "ppt", if present, MUST have a value
equivalent to the quoted value of the "ppt" parameter of the
Identity header. If the "ppt" parameter is absent from the
header, the "ppt" key MUST NOT not appear in the JSON heaer
object.
For example: For example:
{ "typ":"PASSporT", { "typ":"passport",
"alg":"RS256" "alg":"RS256",
"x5u":"https://www.example.com/cert.pkx" } "x5u":"https://www.example.com/cert.pkx" }
To create the PASSporT claims JSON object used in the construction of To populate the PASSporT claims JSON object, the following elements
the signed-identity-digest, the following elements of a SIP message MUST be placed as values corresponding to the designated JSON keys:
MUST be placed in a comma-separated JSON object, in order:
First, the JSON key "orig" followed by a colon and then the quoted First, if the originating identity is a telephone number, the JSON
identity. If the user part of the AoR in the From header field of key "otn" MUST be used, set to the value of the quoted originating
the request contains a telephone number, then the canonicalization identity, a canonicalized telephone number (see Section 7.1.1).
of that number goes into the first slot (see Section 6.1.1). Otherwise, the JSON key "ouri" MUST be used, set to the value of
Otherwise, the first slot contains the AoR of the UA sending the the AoR of the UA sending the message as taken from addr-spec of
message as taken from addr-spec of the From header field. the From header field.
Second, the JSON key "term" followed by a colon and the quoted Second, if the destination identity is a telephone number, the
target. If the user part of the AoR in the To header field of the JSON key "dtn" MUST be used, set to the value of the quoted
request contains a telephone number, then the canonicalization of destination identity, a canonicalized telephone number (see
that number goes into the second slot (again, see Section 6.1.1). Section 7.1.1). Otherwise, the JSON key "duri" MUST be used, set
Otherwise, the second slot contains the addr-spec component of the to the value of the addr-spec component of the To header field,
To header field, which is the AoR to which the request is being which is the AoR to which the request is being sent.
sent.
Third, the JSON key "iat" followed by a colon and then a quoted Third, the JSON key "iat" MUST appear, set to the value of a
encoding of the value of the SIP Date header field as a JSON quoted encoding of the value of the SIP Date header field as a
NumericDate (as UNIX time, per [RFC7519] Section 2). JSON NumericDate (as UNIX time, per [RFC7519] Section 2).
Fourth, if the request contains an SDP message body, and if that Fourth, if the request contains an SDP message body, and if that
SDP contains one or more "a=fingerprint" attributes, then the JSON 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 key "mky" MUST appear with the quoted value(s) of the fingerprint
fingerprint attributes (if they differ). Each attribute value attributes (if they differ). Each attribute value consists of all
consists of all characters following the colon after characters following the colon after "a=fingerprint" including the
"a=fingerprint" including the algorithm description and algorithm description and hexadecimal key representation, after
hexadecimal key representation, any whitespace, carriage returns, removing any whitespace, carriage returns, and "/" line break
and "/" line break indicators. If multiple non-identical indicators. If multiple non-identical "a=fingerprint" attributes
"a=fingerprint" attributes appear in an SDP body, then all non- appear in an SDP body, then all non-identical attributes values
identical attributes values MUST be concatenated, with no MUST be concatenated, with no separating character, after sorting
separating character, after sorting the values in alphanumeric the values in alphanumeric order. If the SDP body contains no
order. If the SDP body contains no "a=fingerprint" attribute, "a=fingerprint" attribute, then no JSON "mky" key is added to the
then no JSON "mky" key is added to the object. object.
For example: For example:
{ "orig":"12155551212", { "otn":"12155551212",
"term":"12155551213", "dtn":"12155551213",
"iat": "1443208345", "iat":"1443208345" }
For more information on the security properties of these headers, and For more information on the security properties of these SIP message
why their inclusion mitigates replay attacks, see Section 11 and elements, and why their inclusion mitigates replay attacks, see
[RFC3893]. Section 12 and [RFC3893].
After these two JSON objects, the header and the claims, have been After these two JSON objects, the header and the claims, have been
constructed as a PASSporT object, they must be hashed and signed; constructed, they must each be hashed per [I-D.ietf-stir-passport]
this then becomes the signed-identity-string. The hashing and Section 3.3. The signed value of those concatenated hashes then
signing algorithm is specified by the 'alg' parameter of the Identity becomes the signed-identity-string of the Identity header. The
header. This document defines only one value for the 'alg' hashing and signing algorithm is specified by the 'alg' parameter of
parameter: 'RS256', as defined in [RFC7519], which connotes a SHA-256 the Identity header and the mirrored "alg" parameter of PASSporT.
hash followed by a RSASSA-PKCS1-v1_5 signature. All implementations This specification defines only one value for the 'alg' parameter:
of this specification MUST support 'RS256'. Any further 'alg' values 'RS256', as defined in [RFC7519], which connotes a SHA-256 hash
MUST be defined in a Standards Track RFC, see Section 12.2 for more followed by a RSASSA-PKCS1-v1_5 signature. All implementations of
information. The result of the hash and signing of the two this specification MUST support 'RS256'. Any further 'alg' values
concatenated JSON objects is placed in the Identity header field. MUST be defined in a Standards Track RFC, see Section 13.2 for more
information.
For example: The complete form of the Identity header will therefore look like the
following example:
Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo Identity: "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \ pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="; \
info=<https://biloxi.example.org/biloxi.cer>;alg=RS256 info=<https://biloxi.example.org/biloxi.cer>;alg=RS256
In a departure from JWT practice, the base64 encoded version of the In a departure from JWT practice, the SIP usage of PASSporT MAY NOT
JSON objects is not included in the Identity header: only the include the base64 encoded version of the JSON objects in the
signature component of the PASSporT is. Optionally, as an debugging Identity header: only the signature component of the PASSporT is
measure or optimization, the base64 encoded concatenation of the JSON REQUIRED. Optionally, as a debugging measure or optimization, the
header and claims may be included as the value of a "canon" parameter base64 encoded concatenation of the JSON header and claims may be
of the Identity header. Note that this may be lengthy string. included as the value of a "canon" parameter of the Identity header.
Note that this may be lengthy string.
8. Extensibility 9. 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 "type" Identity mechanism, this specification defines an optional "ppt"
parameter of the Identity header. The "type" parameter value MUST parameter of the Identity header, which mirrors the "ppt" header key
consist of a token containing an extension specification, which in PASSporT. The "ppt" parameter value MUST consist of a token
denotes an alternative set of signed claims per the type containing an extension specification, which denotes an extended set
extensibility mechanism specified in [ietf-stir-passport] of one or more signed claims per the type extensibility mechanism
specified in [I-D.ietf-stir-passport].
An authentication service cannot assume that verifiers will An authentication service cannot assume that verifiers will
understand any given extension. Verifiers that do support an understand any given extension. Verifiers that do support an
extension may then trigger appropriate application-level behavior in extension may then trigger appropriate application-level behavior in
the presence of an extension; authors of extensions should provide the presence of an extension; authors of extensions should provide
appropriate extension-specific guidance to application developers on appropriate extension-specific guidance to application developers on
this point. this point.
If any claim in an extension contains a JSON value that does not If any claim in an extension contains a JSON value that does not
correspond to any field of the SIP request, but then the optional correspond to any field of the SIP request, but then the optional
"canon" parameter MUST be used for the Identity header containing "canon" parameter MUST be used for the Identity header containing
that extension. that extension.
9. Gatewaying to PASSporT for non-SIP Transit 10. Gatewaying to PASSporT for non-SIP Transit
As defined in this specification, the signature in the Identity As defined in this specification, the signature in the Identity
header is equivalent to the signature that would appear in a PASSporT header is equivalent to the signature that would appear in a PASSporT
token. This is so that a valid PASSporT can be generated based on a token. This is so that a valid PASSporT can be generated based on a
SIP request containing an Identity header. This PASSporT could then SIP request containing an Identity header. This PASSporT could then
be transported in alternate protocols, stored in a repository and be transported in alternate protocols, stored in a repository and
later accessed, or similarly used outside the context of establishing later accessed, or similarly used outside the context of establishing
an end-to-end SIP session. an end-to-end SIP session. Third-party services could also generate
PASSporT tokens which could be transformed into Identity headers and
added to SIP requests in transit by authentication services.
Because the base64 encoding the JSON objects containing headers and Because the base64 encoding of JSON objects containing headers and
claims can be quite long, and because the information it contains is claims can be quite long, and because the information baseline
necessarily redundant with information in the header field values of PASSporT contains is necessarily redundant with information in the
the SIP request itself, SIP does not require implementations to carry header field values of the SIP request itself, SIP does not require
the base64 encodings of those objects. The optional "canon" implementations to carry the base64 encodings of those objects. The
parameter of the Identity-Info, if present, contains the encoded optional "canon" parameter of the Identity-Info, if present, contains
objects used to generate the hash and signature (see Section 7), but the encoded objects used to generate the hash and signature (see
if the "canon" parameter is not present, the contents of the objects Section 8), but if the "canon" parameter is not present, the contents
can be regenerated by constructing the object anew from the SIP of the objects can be regenerated by constructing the object anew
header fields received. from the SIP header fields received.
Alternative transports for this PASSporT and their requirements are Alternative transports for PASSporT and their requirements are left
left to future specifications. to future specifications.
10. Privacy Considerations 11. 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 a cryptographically-assured authority asserts the assurance that an authority asserts the orginator can claim the URI
orginator can claim the URI given in the From header field. This URI given in the From header field. This URI may contain a variety of
may contain a variety of personally identifying information, personally identifying information, including the name of a human
including the name of a human being, their place of work or service being, their place of work or service provider, and possibly further
provider, and possibly further details. The intrinsic privacy risks details. The intrinsic privacy risks associated with that URI are,
associated with that URI are, however, no different from those of however, no different from those of baseline SIP. Per the guidance
baseline SIP. Per the guidance in [RFC6973], implementors should in [RFC6973], implementors should make users aware of the privacy
make users aware of the privacy trade-off of providing secure trade-off of providing secure identity.
identity.
The identity mechanism presented in this document is compatible with The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in [RFC3323]. A SIP the standard SIP practices for privacy described in [RFC3323]. A SIP
proxy server can act both as a privacy service and as an proxy server can act both as a privacy service and as an
authentication service. Since a user agent can provide any From authentication service. Since a user agent can provide any From
header field value that the authentication service is willing to header field value that the authentication service is willing to
authorize, there is no reason why private SIP URIs that contain authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity by an authentication service. The construction of the Identity
header is the same for private URIs as it is for any other sort of header is the same for private URIs as it is for any other sort of
URIs. URIs. Similar practices could be used to support opportunistic
signing of SIP requests for UA-integrated authentications services
with self-signed certificates, though that is outside the scope of
this specification and is left as a matter for future investigation.
Note, however, that even when using anonymous SIP URIs, an Note, however, that even when using anonymous SIP URIs, an
authentication service must possess a certificate corresponding to authentication service must possess a certificate corresponding to
the host portion of the addr-spec of the From header field of the the host portion of the addr-spec of the From header field of the
request; accordingly, using domains like 'anonymous.invalid' will not request; accordingly, using domains like 'anonymous.invalid' will not
be possible for privacy services that also act as authentication be possible for privacy services that also act as authentication
services. The assurance offered by the usage of anonymous URIs with services. The assurance offered by the usage of anonymous URIs with
a valid domain portion is "this is a known user in my domain that I a valid domain portion is "this is a known user in my domain that I
have authenticated, but I am keeping its identity private". have authenticated, but I am keeping its identity private".
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Identity with the Identity "canon" parameter in this fashion is NOT Identity with the Identity "canon" parameter in this fashion is NOT
RECOMMENDED outside of environments where SIP requests will never RECOMMENDED outside of environments where SIP requests will never
leave the trust domain. As a side note, history shows that closed leave the trust domain. As a side note, history shows that closed
networks never stay closed and one should design their implementation networks never stay closed and one should design their implementation
assuming connectivity to the broader Internet. assuming connectivity to the broader Internet.
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.
11. Security Considerations 12. 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, and when present any media keying material in the
material in the message body. In general, the considerations related message body. In general, the considerations related to the security
to the security of these headers are the same as those given in of these headers are the same as those given in [RFC3261] for
[RFC3261] for including headers in tunneled 'message/sip' MIME bodies including headers in tunneled 'message/sip' MIME bodies (see
(see Section 23 of RFC3261 in particular). The following section Section 23 of RFC3261 in particular). The following section details
details the individual security properties obtained by including each the individual security properties obtained by including each of
of these header fields within the signature; collectively, this set these header fields within the signature; collectively, this set of
of header fields provides the necessary properties to prevent header fields provides the necessary properties to prevent
impersonation. It addresses the solution-specific attacks against impersonation. It addresses the solution-specific attacks against
in-band solutions enumerated in [RFC7375] Section 4.1. in-band solutions enumerated in [RFC7375] Section 4.1.
11.1. Protected Request Fields 12.1. Protected Request Fields
The From header field value (in ordinary operations) indicates the The From header field value (in ordinary operations) indicates the
identity of the sender of the message. The SIP address-of-record identity of the sender of the message. The SIP address-of-record
URI, or an embedded telephone number, in the From header field is the URI, or an embedded telephone number, in the From header field is the
identity of a SIP user, for the purposes of this document. Note that 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- in some deployments the identity of the sender may reside in P-
Asserted-Id instead. The sender's identity is the key piece of 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.
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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
received requests, and thus if an Identity header is replayed by an received requests, and thus if an Identity header is replayed by an
attacker within the Date interval, verifiers can detect that it is attacker within the Date interval, verifiers can detect that it is
spoofed because a message with an identical Date from the same source spoofed because a message with an identical Date from the same source
had recently been received. had recently been received.
It has been observed in the wild that some networks change the Date
header field value of SIP requests in transit, and that alternative
behavior might be necessary to accomodate that use case.
Verification services that observe a signature validation failure MAY
therefore reconstruct the Date header field component of the
signature from the "iat" carried in PASSporT via the "canon"
parameter: provided that time recorded by "iat" falls within the
local policy for freshness that would ordinarily apply to the Date
header, the verification service MAY treat the signature as valid,
provided it keeps adequate state to detect recent replays. Note that
this will require the inclusion of the "canon" parameter by
authentication services in networks where such failures are observed.
The To header field value provides the identity of the SIP user that The To header field value provides the identity of the SIP user that
this request originally targeted. Providing the To header field in this request originally targeted. Providing the To header field in
the Identity signature serves two purposes. First, it prevents cut- the Identity signature serves two purposes. First, it prevents cut-
and-paste attacks in which an Identity header from legitimate request and-paste attacks in which an Identity header from legitimate request
for one user is cut-and-pasted into a request for a different user. 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 11.1.1. Section 12.1.1.
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 corresponding to the fingerprint. For example, there are some
baiting attacks, launched with the REFER method or through social baiting attacks, launched with the REFER method or through social
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prevented by securing a fingerprint for DTLS-SRTP. While this is a prevented by securing a fingerprint for DTLS-SRTP. While this is a
different form of impersonation than is commonly used for different form of impersonation than is commonly used for
robocalling, ultimately there is little purpose in establishing the robocalling, ultimately there is little purpose in establishing the
identity of the user that originated a SIP request if this assurance identity of the user that originated a SIP request if this assurance
is not coupled with a comparable assurance over the contents of the is not coupled with a comparable assurance over the contents of the
subsequent media 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.
11.1.1. Protection of the To Header and Retargeting 12.1.1. 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.
11.2. Unprotected Request Fields 12.2. 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.
11.3. Malicious Removal of Identity Headers 12.3. Malicious Removal of Identity Headers
In the end analysis, the Identity header cannot protect itself. Any In the end analysis, the Identity header cannot protect itself. Any
attacker could remove the header from a SIP request, and modify the attacker could remove the header from a SIP request, and modify the
request arbitrarily afterwards. However, this mechanism is not request arbitrarily afterwards. However, this mechanism is not
intended to protect requests from men-in-the-middle who interfere intended to protect requests from men-in-the-middle who interfere
with SIP messages; it is intended only to provide a way that the 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 originators of SIP requests can prove that they are who they claim to
be. At best, by stripping identity information from a request, a be. At best, by stripping identity information from a request, a
man-in-the-middle could make it impossible to distinguish any man-in-the-middle could make it impossible to distinguish any
illegitimate messages he would like to send from those messages sent illegitimate messages he would like to send from those messages sent
by an authorized user. However, it requires a considerably greater by an authorized user. However, it requires a considerably greater
amount of energy to mount such an attack than it does to mount amount of energy to mount such an attack than it does to mount
trivial impersonations by just copying someone else's From header trivial impersonations by just copying someone else's From header
field. This mechanism provides a way that an authorized user can field. This mechanism provides a way that an authorized user can
provide a definitive assurance of his identity that an unauthorized provide a definitive assurance of his identity that an unauthorized
user, an impersonator, cannot. user, an impersonator, cannot.
11.4. Securing the Connection to the Authentication Service 12.4. 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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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.
11.5. Authorization and Transitional Strategies 12.5. 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
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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.
11.6. Display-Names and Identity 12.6. 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 through the document, but may be the subject of future work, such as through the
"type" name mechanism. "ppt" name mechanism.
In the absence of signing the display-name, authentication services In the absence of signing the display-name, authentication services
might check and validate it, and compare it to a list of acceptable 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 display-names that may be used by the sender; if the display-name
does not meet policy constraints, the authentication service could does not meet policy constraints, the authentication service could
return a 403 response code. In this case, the reason phrase should return a 403 response code. In this case, the reason phrase should
indicate the nature of the problem; for example, "Inappropriate indicate the nature of the problem; for example, "Inappropriate
Display Name". However, the display-name is not always present, and Display Name". However, the display-name is not always present, and
in many environments the requisite operational procedures for in many environments the requisite operational procedures for
display-name validation may not exist, so no normative guidance is display-name validation may not exist, so no normative guidance is
given here. given here.
12. IANA Considerations 13. 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.
12.1. Identity-Info Parameters 13.1. Identity-Info Parameters
The IANA has already created a registry for Identity-Info parameters. The IANA has already created a registry for Identity-Info parameters.
This specification defines a new value called "canon" as defined in This specification defines a new value called "canon" as defined in
Section 5.3. Note however that unlike in RFC4474, Identity-Info Section 6.3. Note however that unlike in RFC4474, Identity-Info
parameters now appear in the Identity header. parameters now appear in the Identity header.
12.2. Identity-Info Algorithm Parameter Values 13.2. 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 populated with a value for parameter values. This registry is to be populated with a value for
'RS256', which describes the algorithm used to create the signature 'RS256', which describes the algorithm used to create the signature
that appears in the Identity header. Registry entries must contain that appears in the Identity header. Registry entries must contain
the name of the 'alg' parameter value and the specification in which the name of the 'alg' parameter value and the specification in which
the value is described. New values for the 'alg' parameter may be the value is described. New values 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.
Note that the Identity-Info header is also deprecated by this Note that the Identity-Info header is also deprecated by this
specification, and thus the "alg" parameter is now a value of the specification, and thus the "alg" parameter is now a value of the
Identity header, not Identity-Info. Identity header, not Identity-Info.
13. Acknowledgments 14. 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.
14. Changes from RFC4474 15. 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, Generalized the credential mechanism; credential enrollment,
acquisition and trust 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 Removed the Identity-Info header and relocated its components into
parameters of the Identity header 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 PASSporT the PASSporT
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
Changed the signed-identity-digest format for compatibility with Changed the signed-identity-digest format for compatibility with
PASSporT PASSporT
15. References 16. References
15.1. Normative References 16.1. Normative References
[I-D.wendt-verified-token] [I-D.ietf-stir-passport]
Wendt, C., "Verified Token", draft-wendt-verified-token-00 Wendt, C. and J. Peterson, "Persona Assertion Token",
(work in progress), October 2015. draft-ietf-stir-passport-00 (work in progress), February
2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [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 32, line 40 skipping to change at page 33, line 36
[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>.
15.2. Informative References [RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 6919,
DOI 10.17487/RFC6919, April 2013,
<http://www.rfc-editor.org/info/rfc6919>.
16.2. Informative References
[I-D.ietf-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.
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