draft-ietf-stir-rfc4474bis-02.txt   draft-ietf-stir-rfc4474bis-03.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: April 25, 2015 Cisco Expires: September 9, 2015 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
October 22, 2014 March 8, 2015
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-02.txt draft-ietf-stir-rfc4474bis-03.txt
Abstract Abstract
The baseline security mechanisms in the Session Initiation Protocol The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of (SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining new identifying originators of SIP requests. It does so by defining new
SIP header fields for conveying a signature used for validating the SIP header fields 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
skipping to change at page 1, line 41 skipping to change at page 1, line 41
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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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 April 25, 2015. This Internet-Draft will expire on September 9, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 5 3. Overview of Operations . . . . . . . . . . . . . . . . . . . 5
4. Signature Generation and Validation . . . . . . . . . . . . . 6 4. Signature Generation and Validation . . . . . . . . . . . . . 6
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6 4.1. Authentication Service Behavior . . . . . . . . . . . . . 6
4.1.1. Intermediary Authentication Services . . . . . . . . 9 4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10 5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Identity within a Dialog and Retargeting . . . . . . . . 12 5.1. Credential Use by the Authentication Service . . . . . . 10
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2. Credential Use by the Verification Service . . . . . . . 12
5.1. Credential Use by the Authentication Service . . . . . . 13 5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 12
5.2. Credential Use by the Verification Service . . . . . . . 14 5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 13
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 14 6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 15 6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 16 6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 16 6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Usernames with Domain Names . . . . . . . . . . . . . . . 18 7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22 9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 9.1. Handling of digest-string Elements . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24 9.1.1. Protection of the To Header and Retargeting . . . . . 24
10.1. Handling of digest-string Elements . . . . . . . . . . . 24 9.2. Securing the Connection to the Authentication Service . . 25
10.2. Securing the Connection to the Authentication Service . 27 9.3. Authorization and Transitional Strategies . . . . . . . . 26
10.3. Authorization and Transitional Strategies . . . . . . . 28 9.4. Display-Names and Identity . . . . . . . . . . . . . . . 27
10.4. Display-Names and Identity . . . . . . . . . . . . . . . 29 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 10.1. Header Field Names . . . . . . . . . . . . . . . . . . . 27
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29 10.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 28
11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 29 10.3. Identity-Info Algorithm Parameter Values . . . . . . . . 28
11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 29 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 12. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 28
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 30 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 13.1. Normative References . . . . . . . . . . . . . . . . . . 29
14.1. Normative References . . . . . . . . . . . . . . . . . . 30 13.2. Informative References . . . . . . . . . . . . . . . . . 29
14.2. Informative References . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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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[RFC3261] specifies a number of security mechanisms that can be [RFC3261] specifies a number of security mechanisms that can be
employed by SIP user agents (UAs), including Digest, Transport Layer employed by SIP user agents (UAs), including Digest, Transport Layer
Security (TLS), and S/MIME (implementations may support other Security (TLS), and S/MIME (implementations may support other
security schemes as well). However, few SIP user agents today security schemes as well). However, few SIP user agents today
support the end-user certificates necessary to authenticate support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and furthermore Digest themselves (via S/MIME, for example), and furthermore Digest
authentication is limited by the fact that the originator and authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which user agents to be able to send requests to destinations with which
they have no previous association -- just as in the telephone network they have no previous association. A cryptographic approach, like
today, one can receive a call from someone with whom one has no the one described in this document, can provide a much stronger and
previous association, and still have a reasonable assurance that the less spoofable assurance of identity than the Caller ID services that
person's displayed calling party number (and/or Caller-ID) is the telephone network provides today.
accurate. A cryptographic approach, like the one described in this
document, can provide a much stronger and less spoofable assurance of
identity than the telephone network provides today.
[RFC4474] previously specified a means of signing portions of SIP [RFC4474] previously specified a means of signing portions of SIP
requests in order to provide that identity assurance. However, RFC requests in order to provide that 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 [RFC 7375]. model in [RFC7375].
2. Background 2. Background
The secure operation of many SIP applications and services depends on The secure operation of many SIP applications and services depends on
authorization policies. These policies may be automated, or they may authorization policies. These policies may be automated, or they may
be exercised manually by humans. An example of the latter would be be exercised manually by humans. An example of the latter would be
an Internet telephone application that displays the calling party an Internet telephone application that displays the calling party
number (and/or Caller-ID) of a caller, which a human may review to number (and/or Caller-ID) of a caller, which a human may review to
make a policy decision before answering a call. An example of the make a policy decision before answering a call. 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 allow the caller to a whitelist before determining whether it should allow the
caller access to recorded messages. In both of these cases, caller access to recorded messages. In both of these cases,
attackers might attempt to circumvent these authorization policies attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of through impersonation. Since the primary identifier of the sender of
a SIP request, the From header field, can be populated arbitrarily by a SIP request, the From header field, can be populated arbitrarily by
the controller of a user agent, impersonation is very simple today. the controller of a user agent, impersonation is very simple today.
The mechanism described in this document provides a strong identity The mechanism described in this document provides a strong identity
system for SIP requests in which authorization policies cannot be system for SIP requests for detecting attempted impersonation.
circumvented by impersonation.
This document proposes an authentication architecture for SIP in This document proposes an authentication architecture for SIP in
which requests are processed by a logical authentication service that which requests are processed by a logical authentication service that
may be implemented as part of a user agent or as a proxy server. may be implemented as part of a user agent or as a proxy server.
Once a message has been authenticated, the service then adds new Once a message has been authenticated, the service then adds new
cryptographic information to requests to communicate to other SIP cryptographic information to requests to communicate to other SIP
entities that the sending user has been authenticated and its use of entities that the sending user has been authenticated and its use of
the From header field has been authorized. the From header field has been authorized.
But authorized by whom? Identities are issued to users by But authorized by whom? Identities are issued to users by
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Intermediaries that instantiate this role MUST be capable of Intermediaries that instantiate this role MUST be capable of
authenticating one or more SIP users who can register for that authenticating one or more SIP users who can register for that
identity. Commonly, this role will be instantiated by a proxy identity. Commonly, this role will be instantiated by a proxy
server, since these entities are more likely to have a static server, since these entities are more likely to have a static
hostname, hold corresponding credentials, and have access to SIP hostname, hold corresponding credentials, and have access to SIP
registrar capabilities that allow them to authenticate users. It is registrar capabilities that allow them to authenticate users. It is
also possible that the authentication service role might be also possible that the authentication service role might be
instantiated by an entity that acts as a redirect server, but that is instantiated by an entity that acts as a redirect server, but that is
left as a topic for future work. left as a topic for future work.
SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present (see
Section 7 for information on how the Date header field assists
verifiers).
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:
The authentication service MUST extract the identity of the sender The authentication service MUST extract the identity of the sender
from the request. The authentication service takes this value from from the request. The authentication service takes this URI value
the From header field; this AoR will be referred to here as the from the addr-spec component of From header field; this URI will be
'identity field'. If the identity field contains a SIP or SIP Secure referred to here as the 'identity field'. If the identity field
(SIPS) URI, and the user portion is not a telephone number, the contains a SIP or SIP Secure (SIPS) URI, and the user portion is not
authentication service MUST extract the hostname portion of the a telephone number, the authentication service MUST extract the
identity field and compare it to the domain(s) for which it is hostname portion of the identity field and compare it to the
responsible (following the procedures in RFC 3261 [RFC3261], domain(s) for which it is responsible (following the procedures in
Section 16.4), used by a proxy server to determine the domain(s) for RFC 3261 [RFC3261], Section 16.4). If the identity field uses the
which it is responsible). If the identity field uses the TEL URI TEL URI scheme [RFC3966], or the identity field is a SIP or SIPS URI
scheme [RFC3966], or the identity field is a SIP or SIPS URI with a with a telephone number in the user portion, the authentication
telephone number in the user portion, the authentication service service determines whether or not it is responsible for this
determines whether or not it is responsible for this telephone telephone number; see Section 6.1 for more information. An
number; see Section 6.1 for more information. If the authentication authentication service proceeding with a signature over a telephone
service is not authoritative for the identity in question, it SHOULD number MAY add the optional 'canon' parameter to the request as
process and forward the request normally, but it MUST NOT following described in that section. If the authentication service is not
the steps below to add an Identity header; see below for more authoritative for the identity in question, it SHOULD process and
information on authentication service handling of an existing forward the request normally, but it MUST NOT follow the steps below
Identity header. [where?] to add an Identity header. An authentication service MUST NOT add an
Identity header to a request that already has one.
Step 2: Step 2:
The authentication service MUST then determine whether or not the The authentication service MUST then determine whether or not the
sender of the request is authorized to claim the identity given in sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include: which this authentication might be performed include:
If the authentication service is instantiated by a SIP If the authentication service is instantiated by a SIP
intermediary (proxy server), it may challenge the request with a intermediary (proxy server), it may authenticate the request with
407 response code using the Digest authentication scheme (or the authentication scheme used for registration in its domain
viewing a Proxy-Authentication header sent in the request, which (e.g., Digest authentication).
was sent in anticipation of a challenge using cached credentials,
as described in RFC 3261 [RFC3261], Section 22.3). Note that if
that proxy server is maintaining a TLS connection with the client
over which the client had previously authenticated itself using
Digest authentication, the identity value obtained from that
previous authentication step can be reused without an additional
Digest challenge.
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 can be said to authenticate its user on the grounds a user agent may authenticate its own user through any system-
that the user can provision the user agent with the private key of specific means, perhaps simply by virtue of having physical access
the credential, or preferably by providing a password that unlocks to the user agent.
said private key.
Authorization of the use of a particular username or telephone number Authorization of the use of a particular username or telephone number
in the user part of the From header field is a matter of local policy in the user part of the From header field is a matter of local policy
for the authentication service, see Section 5.1 for more information. for the authentication service, see Section 5.1 for more information.
Note that this check is performed only on the addr-spec in the From Note that this check is performed only on the addr-spec in the From
header field (e.g., the URI of the sender, like header field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display- 'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta'). name portion of the From header field (e.g., 'Alice Atlanta').
Authentication services MAY check and validate the display-name as Authentication services MAY check and validate the display-name as
well, and compare it to a list of acceptable display-names that may well, and compare it to a list of acceptable display-names that may
be used by the sender; if the display-name does not meet policy be used by the sender; if the display-name does not meet policy
constraints, the authentication service MUST return a 403 response constraints, the authentication service could return a 403 response
code. The reason phrase should indicate the nature of the problem; code. In this case, the reason phrase should indicate the nature of
for example, "Inappropriate Display Name". However, the display-name the problem; for example, "Inappropriate Display Name". However, the
is not always present, and in many environments the requisite display-name is not always present, and in many environments the
operational procedures for display-name validation may not exist. requisite operational procedures for display-name validation may not
For more information, see Section 10.4. exist, so no normative guidance is given here. For more information,
see Section 9.4.
Step 3: Step 3:
The authentication service SHOULD ensure that any preexisting Date An authentication service MUST add a Date header field to SIP
header in the request is accurate. Local policy can dictate requests if one is not already present. The authentication service
precisely how accurate the Date must be; a RECOMMENDED maximum MUST ensure that any preexisting Date header in the request is
discrepancy of ten minutes will ensure that the request is unlikely accurate. Local policy can dictate precisely how accurate the Date
to upset any verifiers. If the Date header contains a time different must be; a RECOMMENDED maximum discrepancy of sixty seconds will
by more than ten minutes from the current time noted by the ensure that the request is unlikely to upset any verifiers. If the
authentication service, the authentication service SHOULD reject the Date header contains a time different by more than one minute from
request. This behavior is not mandatory because a user agent client the current time noted by the authentication service, the
(UAC) could only exploit the Date header in order to cause a request authentication service SHOULD reject the request. This behavior is
to fail verification; the Identity header is not intended to provide not mandatory because a user agent client (UAC) could only exploit
a source of non-repudiation or a perfect record of when messages are the Date header in order to cause a request to fail verification; the
processed. Finally, the authentication service MUST verify that the Identity header is not intended to provide a source of non-
Date header falls within the validity period of its credential. For repudiation or a perfect record of when messages are processed.
more information on the security properties associated with the Date Finally, the authentication service MUST verify that the Date header
header field value, see Section 7. falls within the validity period of its credential.
[TBD: Should consider a lower threshold than ten minutes? With the See Section 7 for information on how the Date header field assists
removal of other elements from the sig, that's a lot of leeway.] verifiers.
Step 4: Step 4:
The authentication service MAY form an identity-reliance signature The authentication service MAY form an identity-reliance signature
and add an Identity-Reliance header to the request containing this and add an Identity-Reliance header to the request containing this
signature. The Identity-Reliance header provides body security signature. The Identity-Reliance header provides body security
properties that are useful for non-INVITE transactions, and in properties that are useful for non-INVITE transactions, and in
environments where body security of INVITE transactions is necessary. environments where body security of INVITE transactions is necessary.
Details on the generation of this header is provided in Section 7. Details on the generation of this header are provided in Section 7.
If the authentication service is adding an Identity-Reliance header, If the authentication service is adding an Identity-Reliance header,
it MUST also add a Content-Length header field to SIP requests if one it MUST also add a Content-Length header field to SIP requests if one
is not already present; this can help verifiers to double-check that is not already present; this can help verifiers to double-check that
they are hashing exactly as many bytes of message-body as the they are hashing exactly as many bytes of message-body as the
authentication service when they verify the message. authentication service when they verify the message.
Step 5: Step 5:
The authentication service MUST form the identity signature and add The authentication service MUST form the identity signature and add
an Identity header to the request containing this signature. After an Identity header to the request containing this signature. After
the Identity header has been added to the request, the authentication the Identity header has been added to the request, the authentication
service MUST also add an Identity-Info header. The Identity-Info service MUST also add an Identity-Info header. The Identity-Info
header contains a URI from which its credential can be acquired; see header contains a URI from which the authentication service's
Section 5.3 for more on credential acquisition. Details on the credential can be acquired; see Section 5.3 for more on credential
syntax of both of these headers are provided in Section 7. acquisition. Details on the syntax of both of these headers are
provided in Section 7.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
4.1.1. Intermediary Authentication Services
In cases where a user agent does not possess its own credentials to
sign an Identity header, the user agent can send its request through
an intermediary that will provide a signed Identity header based on
the contents of the request. This requires, among other things, that
intermediaries have some means of authenticating the user agents
sending requests.
All RFC 3261 [RFC3261] compliant user agents support Digest
authentication, which utilizes a shared secret, as a means for
authenticating themselves to a SIP registrar. Registration allows a
user agent to express that it is an appropriate entity to which
requests should be sent for a particular SIP AoR URI (e.g.,
'sip:alice@atlanta.example.com'). For such SIP URIs, by the
definition of identity used in this document, registration proves the
identity of the user to a registrar. Similar checks might be
performed for telephone numbers as identities. This is of course
only one manner in which a domain might determine how a particular
user is authorized to populate the From header field; as an aside,
for other sorts of URIs in the From (like anonymous URIs), other
authorization policies would apply.
RFC 3261 [RFC3261] already describes an intermediary architecture
very similar to the one proposed in this document in
Section 26.3.2.2, in which a user agent authenticates itself to a
local proxy server, which in turn authenticates itself to a remote
proxy server via mutual TLS, creating a two-link chain of transitive
authentication between the originator and the remote domain. While
this works well in some architectures, there are a few respects in
which this is impractical. For one, transitive trust is inherently
weaker than an assertion that can be validated end-to-end. It is
possible for SIP requests to cross multiple intermediaries in
separate administrative domains, in which case transitive trust
becomes even less compelling.
This specification assumes that UACs will have an appropriate means
to discover an authentication service that can sign with a credential
corresponding to the UAC's identity. Most likely, this information
will simply be provisioned in UACs.
One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity
header) is given in RFC 3325 [RFC3325]. However, this solution
allows only hop- by-hop trust between intermediaries, not end-to-end
cryptographic authentication, and it assumes a managed network of
nodes with strict mutual trust relationships, an assumption that is
incompatible with widespread Internet deployment.
4.2. Verifier Behavior 4.2. Verifier Behavior
This document specifies a logical role for SIP entities called a This document specifies a logical role for SIP entities called a
verification service, or verifier. When a verifier receives a SIP verification service, or verifier. When a verifier receives a SIP
message containing an Identity header, it inspects the signature to message containing an Identity header, it inspects the signature to
verify the identity of the sender of the message. Typically, the verify the identity of the sender of the message. Typically, the
results of a verification are provided as input to an authorization results of a verification are provided as input to an authorization
process that is outside the scope of this document. If an Identity process that is outside the scope of this document. If an Identity
header is not present in a request, and one is required by local header is not present in a request, and one is required by local
policy (for example, based on a per-sending-domain policy, or a per- policy (for example, based on a per-sending-domain policy, or a per-
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In order to verify the identity of the sender of a message, an entity In order to verify the identity of the sender of a message, an entity
acting as a verifier MUST perform the following steps, in the order acting as a verifier MUST perform the following steps, in the order
here specified. here specified.
Step 1: Step 1:
In order to determine whether the signature for the URI in the From In order to determine whether the signature for the URI in the From
header field value should be over the entire URI or just a header field value should be over the entire URI or just a
canonicalized telephone number, the verification service must follow canonicalized telephone number, the verification service must follow
the process described in Section 6.1. That section also describes the canonicalization process described in Section 6.1.1. That
the procedures the verification service must follow to determine if section also describes the procedures the verification service must
the signer is authoritative for a telephone number. For domains, the follow to determine if the signer is authoritative for a telephone
verifier MUST follow the process described in Section 6.2 to number. For domains, the verifier MUST follow the process described
determine if the signer is authoritative for the URI in the From in Section 6.2 to determine if the signer is authoritative for the
header field. URI in the From header field.
Step 2: Step 2:
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. See material to validate the signature in the Identity header field,
which usually involves dereferencing the Identity-Info header. See
Section 5.2 for more information on these procedures. Section 5.2 for more information on these procedures.
Step 3: Step 3:
The verifier MUST verify the signature in the Identity header field, The verifier MUST validate the signature in the Identity header
following the procedures for generating the hashed digest-string field, following the procedures for generating the hashed digest-
described in Section 7. If a verifier determines that the signature string described in Section 7. If a verifier determines that the
on the message does not correspond to the reconstructed digest- signature on the message does not correspond to the reconstructed
string, then a 438 'Invalid Identity Header' response MUST be digest-string, then a 438 'Invalid Identity Header' response MUST be
returned. returned.
Step 4: Step 4:
If the request contains an Identity-Reliance header, the verifier If the request contains an Identity-Reliance header, the verifier
SHOULD verify the signature in the Identity-Reliance header field, SHOULD verify the signature in the Identity-Reliance header field,
following the procedures for generating the hashed reliance-digest- following the procedures for generating the hashed reliance-digest-
string described in Section 7. If a verifier determines that the string described in Section 7. The Identity-Reliance header provides
signature on the message does not correspond to the reconstructed important protections for non-INVITE transactions (such as MESSAGE or
digest-string, then a 438 'Invalid Identity Header' response SHOULD NOTIFY), but verifiers MAY elect not to verify Identity-Reliance when
be returned. it protects SDP. If a verifier determines that the signature on the
message does not correspond to the reconstructed digest-string, then
a 438 'Invalid Identity Header' response SHOULD be returned.
Step 5: Step 5:
The verifier MUST validate the Date header in the manner described in The verifier MUST must furthermore ensure that the value of the Date
Section 10.1; recipients that wish to verify Identity signatures MUST header meets local policy for freshness (usually, within sixty
support all of the operations described there. It must furthermore seconds) and that it falls within the validity period of the
ensure that the value of the Date header falls within the validity credential used to sign the Identity header. For more on the attacks
period of the credential used to sign the Identity header. this prevents, see Section 9.1.
4.3. Identity within a Dialog and Retargeting
The mechanism in this document provides a signature over the URI in
the To header field value in order to allow recipient policy to
detect replay attacks where a request originally sent to one
recipient is forwarded to another, unrelated recipient by an
attacker. The recipient of a request may compare the To header field
value to their own identity in order to determine whether or not the
identity information in this call might have been replayed.
Retargeting, however, complicates this evaluation.
Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting replaces the
original target URI with one that corresponds to a different user,
potentially a user that would not be not authorized to register under
the original target URI. By this definition, retargeting does not
include translation of the Request-URI to a contact address of an
endpoint that has registered under the original target URI.
When a request is retargeted, it may reach a SIP endpoint whose user
is not identified by the URI designated in the To header field value.
This can cause complications for securing requests sent in the
backwards direction in a dialog; while this is not in the scope of
impersonation protection for this document, some considerations are
given here for future work that attempts to tackle that problem by
building on this mechanism. In the absence of "from-change" option
provided in [RFC4916], the value in the To header field of a dialog-
forming request is used as the From header field of requests sent in
the backwards direction during the dialog, and is accordingly the
header that would be signed by an authentication service for requests
sent in the backwards direction. But in retargeting cases, if the
URI in the From header does not identify the sender of the request in
the backwards direction, then clearly it would be inappropriate to
provide an Identity signature over that From header. As specified
above, if the authentication service is not responsible for the
domain in the From header field of the request, it MUST NOT add an
Identity header to the request, and it should process/forward the
request normally.
Any means of anticipating retargeting, and so on, is outside the
scope of this document, and likely to have equal applicability to
response identity as it does to requests in the backwards direction
within a dialog. Consequently, no special guidance is given for
implementers here regarding the 'connected party' problem;
authentication service behavior is unchanged if retargeting has
occurred for a dialog-forming request. Ultimately, the
authentication service provides an Identity header for requests in
the backwards dialog when the user is authorized to assert the
identity given in the From header field, and if they are not, an
Identity header is not provided.
For further information on the problems of response identity see
[I-D.peterson-sipping-retarget].
5. Credentials 5. Credentials
5.1. Credential Use by the Authentication Service 5.1. 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 URIs, domain names or telephone numbers. These credentials may cover domain names or telephone numbers. These credentials may
represent authority over only a single name (such as represent authority over an entire domain (such as example.com) or
alice@example.com), an entire domain (such as example.com), or potentially a set of domains enumerated by the credential.
potentially a set of domains. Similarly, a credential may represent Similarly, a credential may represent authority over a single
authority over a single telephone number or a range of telephone telephone number or a range of telephone numbers. The way that the
numbers. The way that the scope of a credential is expressed is scope of a credential is expressed is specific to the credential
specific to the credential mechanism. mechanism.
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, one that depends greatly on the for the authentication service, one that depends greatly on the
manner in which authentication is performed. For non-telephone manner in which authentication is performed. For non-telephone
number user parts, one policy might be as follows: the username given number user parts, one policy might be as follows: the username given
in the 'username' parameter of the Proxy-Authorization header MUST in the 'username' parameter of the Proxy-Authorization header MUST
correspond exactly to the username in the From header field of the correspond exactly to the username in the From header field of the
SIP message. However, there are many cases in which this is too SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use 'username' parameters in limiting or inappropriate; a realm might use 'username' parameters in
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a mapping between the values in the 'username' parameter of Proxy- a mapping between the values in the 'username' parameter of Proxy-
Authorization and a set of one or more SIP URIs that might Authorization and a set of one or more SIP URIs that might
legitimately be asserted for that 'username'. For example, the legitimately be asserted for that 'username'. For example, the
username can correspond to the 'private identity' as defined in Third username can correspond to the 'private identity' as defined in Third
Generation Partnership Project (3GPP), in which case the From header Generation Partnership Project (3GPP), in which case the From header
field can contain any one of the public identities associated with field can contain any one of the public identities associated with
this private identity. In this instance, another policy might be as this private identity. In this instance, another policy might be as
follows: the URI in the From header field MUST correspond exactly to follows: the URI in the From header field MUST correspond exactly to
one of the mapped URIs associated with the 'username' given in the one of the mapped URIs associated with the 'username' given in the
Proxy-Authorization header. This is a suitable approach for Proxy-Authorization header. This is a suitable approach for
telephone numbers in particular. Various exceptions to such policies telephone numbers in particular.
might arise for cases like anonymity; if the AoR asserted in the From
header field uses a form like 'sip:anonymous@example.com', then the This specification could also be used with credentials that cover a
'example.com' proxy should authenticate that the user is a valid user single name or URI, such as alice@example.com or
in the domain and insert the signature over the From header field as sip:alice@example.com. This would require a modification to
usual. authentication service behavior to operate on a whole URI rather than
a domain name. Because this is not believed to be a pressing use
case, this is deferred to future work, but implementors should note
this as a possible future direction.
Exceptions to such authentication service policies arise for cases
like anonymity; if the AoR asserted in the From header field uses a
form like 'sip:anonymous@example.com' (see [RFC3323]), then the
'example.com' proxy might authenticate only that the user is a valid
user in the domain and insert the signature over the From header
field as usual.
5.2. Credential Use by the Verification Service 5.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a In order to act as a verification service, a SIP entity must have a
way to acquire and retain credentials for authorities over particular way to acquire and retain credentials for authorities over particular
URIs, domain names and/or telephone numbers. The Identity-Info domain names and/or telephone numbers or number ranges.
header (as described in the next section) is supported by all Dereferencing the Identity-Info header (as described in the next
verification service implementations to create a baseline means of section) MUST be supported by all verification service
credential acquisition. Provided that the credential used to sign a implementations to create a baseline means of credential acquisition.
message is not previously known to the verifier, SIP entities SHOULD Provided that the credential used to sign a message is not previously
discover this credential by dereferencing the Identity-Info header, known to the verifier, SIP entities SHOULD discover this credential
unless they have some more efficient implementation-specific way of by dereferencing the Identity-Info header, unless they have some more
acquiring certificates. If the URI scheme in the Identity-Info other implementation-specific way of acquiring the needed
header cannot be dereferenced, then a 436 'Bad Identity-Info' certificates, such as an offline store of periodically-updated
response MUST be returned. credentials. If the URI in the Identity-Info header cannot be
dereferenced, then a 436 'Bad Identity-Info' response MUST be
returned.
Verification service implementations supporting this specification Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials every time a request from the same identity is received. Credentials
cached in this manner max be indexed in accordance with local policy: cached in this manner may be indexed in accordance with local policy:
for example, by their scope, or the URI given in the Identity-Info for example, by their scope, or the URI given in the Identity-Info
header field value. Further consideration of how to cache header field value. Further consideration of how to cache
credentials is deferred to the credential mechanisms. credentials is deferred to the credential mechanisms.
5.3. Handling Identity-Info URIs 5.3. Handling Identity-Info URIs
An Identity-Info header MUST contain a URI which dereferences to a An Identity-Info header MUST contain a URI which dereferences to a
resource which 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. Much as is the used by the authentication service to sign a request. It is
case with the trust anchor(s) required for deployments of this essential that a URI in the Identity-Info header be dereferencable by
specification, it is essential that a URI in the Identity-Info header any entity that could plausibly receive the request. For common
be dereferencable by any entity that could plausibly receive the cases, this means that the URI must be dereferencable by any entity
request. For common cases, this means that the URI must be on the public Internet. In constrained deployment environments, a
dereferencable by any entity on the public Internet. In constrained service private to the environment might be used instead.
deployment environments, a service private to the environment might
be used instead.
Beyond providing a means of accessing credentials for an identity, Beyond providing a means of accessing credentials for an identity,
the Identity-Info header further services a means of differentiating the Identity-Info header further serves as a means of differentiating
which particular credential was used to sign a request, when there which particular credential was used to sign a request, when there
are potentially multiple authorities eligible to sign. For example, are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service imagine a case where a domain implements the authentication service
role for example.com, 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 alice@example.com. Either would be acquired a credential for a single telephone number within that
eligible to sign a SIP request from alice@example.com. Verification range. Either would be eligible to sign a SIP request for the number
services however need a means to differentiate which one performed in question. Verification services however need a means to
the signature. The Identity-Info header performs that function. differentiate which one performed the signature. The Identity-Info
header performs that function.
If the optional "canon" parameter is present, it contains the result If the optional "canon" parameter is present, it contains the result
of the number canonicalization process performed by the of the number canonicalization process performed by the
authentication service (see Section 6.1) on the identity in the From. authentication service (see Section 6.1.1) on the identity in the
This value is provided purely informationally as an optimization for From. This value is provided purely informationally as an
the verification service. The verification service MAY compute its optimization for the verification service. The verification service
own canonicalization of the number and compare this to the value in MAY compute its own canonicalization of the number and compare this
the "canon" parameter before performing any cryptographic functions to the value in the "canon" parameter before performing any
in order to ascertain whether or not the two ends agree on the cryptographic functions in order to ascertain whether or not the two
canonical number form. ends agree on the canonical number form.
5.4. Credential Systems 5.4. Credential Systems
This document makes no specific recommendation for the use of any 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 context: an Identity-Info header could for example give an the SIP identity context: an Identity-Info header could for example
HTTP URL of the form 'application/pkix-cert' pointing to a give an HTTP URL of the form 'application/pkix-cert' pointing to a
certificate (following the conventions of [RFC2585]). The Identity- certificate (following the conventions of [RFC2585]). The Identity-
Info headers may use the DNS URL scheme (see [RFC4501]( to indicate Info headers may use the DNS URL scheme (see [RFC4501]) to designate
keys in the DNS. keys in the DNS.
While no comparable public credentials exist for telephone numbers, While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential either approach could be applied to telephone numbers. A credential
system based on certificates is given in system based on certificates is given in
[I-D.peterson-stir-certificates]. One based on the domain name [I-D.ietf-stir-certificates]. One based on the domain name system is
system is given in [I-D.kaplan-stir-cider]. given in [I-D.kaplan-stir-cider].
In order for a credential system to work with this mechanism, its In order for a credential system to work with this mechanism, its
specification must detail: specification must detail:
which URIs schemes the credential will use in the Identity-Info which URIs schemes the credential will use in the Identity-Info
header, and any special procedures required to dereference the header, and any special procedures required to dereference the
URIs URIs
how the verifier can learn the scope of the credential. how the verifier can learn the scope of the credential
any special procedures required to extract keying material from any special procedures required to extract keying material from
the resources designated by the URI the resources designated by the URI
any algorithms that would appear in the Identity-Info "alg" any algorithms that would appear in the Identity-Info "alg"
parameter other than 'rsa-sha256.' Note that per the IANA parameter other than 'rsa-sha256.' Note that per the IANA
Considerations of RFC 4474, new algorithms can only be specified Considerations of RFC 4474, new algorithms can only be specified
by Standards Action. by Standards Action
SIP entities cannot reliably predict where SIP requests will SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar trust anchors is assured by business arrangements or similar
constraints. constraints.
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 message (an hour would most than the plausible lifetime of a SIP transaction (a minute would most
likely suffice). likely suffice).
6. Identity Types 6. Identity Types
6.1. Telephone Numbers 6.1. 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.
Given the existence of such authorities, authentication and In the longer term, it is possible that some directory or other
verification services must identify when a request should be signed discovery mechanism may provide a way to determine which
by an authority for a telephone number, and when it should be signed administrative domain is responsible for a telephone number, and this
by an authority for a domain. Telephone numbers most commonly appear may aid in the signing and verification of SIP identities that
in SIP header field values in the username portion of a SIP URI contain telephone numbers. This is a subject for future work.
(e.g., 'sip:+17005551008@chicago.example.com;user=phone'). The user
part of that URI conforms to the syntax of the TEL URI scheme (RFC
3966 [RFC3966]). It is also possible for a TEL URI to appear in the
SIP To or From header field outside the context of a SIP or SIPS URI
(e.g., 'tel:+17005551008'). In both of these cases, it's clear that
the signer must have authority over the telephone number, not the
domain name of the SIP URI. It is also possible, however, for
requests to contain a URI like 'sip:7005551000@chicago.example.com'.
It may be non-trivial for a service to ascertain in this case whether
the URI contains a telephone number or not.
To address this problem, the authentication service and verification In order to work with any such authorities, authentication and
service both must perform the following canonicalization procedure on verification services must be able to identify when a request should
any SIP URI they inspect which contains a wholly numeric user part. be signed by an authority for a telephone number, and when it should
Note that the same procedures are followed for creating the canonical be signed by an authority for a domain. Telephone numbers most
form of URIs found in both the From and To header field values. commonly appear in SIP header field values in the username portion of
a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone').
The user part of that URI conforms to the syntax of the TEL URI
scheme (RFC 3966 [RFC3966]). It is also possible for a TEL URI to
appear in the SIP To or From header field outside the context of a
SIP or SIPS URI (e.g., 'tel:+17005551008'). In both of these cases,
it's clear that the signer must have authority over the telephone
number, not the domain name of the SIP URI. It is also possible,
however, for requests to contain a URI like
'sip:7005551000@chicago.example.com'. It may be non-trivial for a
service to ascertain in this case whether the URI contains a
telephone number or not.
6.1.1. Canonicalization Procedures
In order to determine whether or not the user portion of a SIP URI is
a telephone number, authentication services and verification services
must perform the following canonicalization procedure on any SIP URI
they inspect which contains a wholly numeric user part. Note that
the same procedures are followed for creating the canonical form of
URIs found in both the From and To header field values.
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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characters, excepting only the leading "#" or "*" keys used in characters, excepting only the leading "#" or "*" keys used in
some special service numbers (typically, these will appear only in some special service numbers (typically, these will appear only in
the To header field value). This MUST result in an ASCII string the To header field value). This MUST result in an ASCII string
limited to "#", "*" and digits without whitespace or visual limited to "#", "*" and digits without whitespace or visual
separators. separators.
Next, an implementation must assess if the number string is a Next, an implementation must assess if the number string is a
valid, globally-routable number with a leading country code. If valid, globally-routable number with a leading country code. If
not, implementations SHOULD convert the number into E.164 format, not, implementations SHOULD convert the number into E.164 format,
adding a country code if necessary; this may involve transforming adding a country code if necessary; this may involve transforming
the number from a dial string (see RFC3966 [RFC3966]), removing the number from a dial string (see [RFC3966]), removing any
any national or international dialing prefixes or performing national or international dialing prefixes or performing similar
similar procedures. It is only in the case that an implementation procedures. It is only in the case that an implementation cannot
cannot determine how to convert the number to a globally-routable determine how to convert the number to a globally-routable format
format that this step may be skipped. that this step may be skipped.
In some cases, further transformations MAY be made in accordance In some cases, further transformations MAY be made in accordance
with specific policies used within the local domain. For example, with specific policies used within the local domain. For example,
one domain may only use local number formatting and need to one domain may only use local number formatting and need to
convert all To/From user portions to E.164 by prepending country- convert all To/From user portions to E.164 by prepending country-
code and region code digits; another domain might prefix usernames code and region code digits; another domain might prefix usernames
with trunk- routing codes and need to remove the prefix. with trunk-routing codes and need to remove the prefix.
The resulting canonical number string will be used as input to the The resulting canonical number string will be used as input to the
hash calculation during signing and verifying processes. hash calculation during signing and verifying processes.
The ABNF of this number string is: The ABNF of this number string is:
tn-spec = [ "#" / "*" ] 1*DIGIT tn-spec = [ "#" / "*" ] 1*DIGIT
If the result of this procedure forms a complete telephone number, If the result of this procedure forms a complete telephone number,
that number is used for the purpose of creating and signing the that number is used for the purpose of creating and signing the
digest-string by both the authentication service and verification digest-string by both the authentication service and verification
service. Optionally, the entity instantiating the authentication service. Practically, entities that perform the authentication
service function MAY alter the telephone numbers that appear in the service role will sometimes alter the telephone numbers that appear
To and From header field values, converting them to this format. The in the To and From header field values, converting them to this
authentication service MAY also add the result of the format (though note this is not a function that [RFC3261] permits
canonicalization process of the From header field value to the proxy servers to perform). The authentication service MAY also add
"canon" parameter of the Identity-Info header. If the result of the the result of the canonicalization process of the From header field
canonicalization of the From header field value does not form a value to the "canon" parameter of the Identity-Info header. If the
complete telephone number, the authentication service and result of the canonicalization of the From header field value does
not form a complete telephone number, the authentication service and
verification service should treat the entire URI as a SIP URI, and verification service should treat the entire URI as a SIP URI, and
apply a domain signature per the procedures in Section 6.2. apply a domain signature per the procedures in Section 6.2.
In the longer term, it is possible that some directory or other 6.2. Domain Names
discovery mechanism may provide a way to determine which
administrative domain is responsible for a telephone number, and this
may aid in the signing and verification of SIP identities that
contain telephone numbers. This is a subject for future work.
6.2. Usernames with Domain Names
When a verifier processes a request containing an Identity-Info When a verifier processes a request containing an Identity-Info
header with a domain signature, it must compare the domain portion of header with a domain signature, it must compare the domain portion of
the URI in the From header field of the request with the domain name the URI in the From header field of the request with the domain name
that is the subject of the credential acquired from the Identity-Info that is the subject of the credential acquired from the Identity-Info
header. While this might seem that this should be a straightforward header. While it might seem that this should be a straightforward
process, it is complicated by two deployment realities. In the first process, it is complicated by two deployment realities. In the first
place, credentials have varying ways of describing their subjects, place, credentials have varying ways of describing their subjects,
and may indeed have multiple subjects, especially in 'virtual and may indeed have multiple subjects, especially in 'virtual
hosting' cases where multiple domains are managed by a single hosting' cases where multiple domains are managed by a single
application. Secondly, some SIP services may delegate SIP functions application. Secondly, some SIP services may delegate SIP functions
to a subordinate domain and utilize the procedures in RFC 3263 to a subordinate domain and utilize the procedures in RFC 3263
[RFC3263] that allow requests for, say, 'example.com' to be routed to [RFC3263] that allow requests for, say, 'example.com' to be routed to
'sip.example.com'. As a result, a user with the AoR 'sip.example.com'. As a result, a user with the AoR
'sip:jon@example.com' may process requests through a host like 'sip:jon@example.com' may process requests through a host like
'sip.example.com', and it may be that latter host that acts as an 'sip.example.com', and it may be that latter host that acts as an
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The signed-identity-digest is a signed hash of a canonical string The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character: specified here, separated by a vertical line, "|" or %x7C, character:
First, the identity. If the user part of the AoR in the From First, the identity. If the user part of the AoR in the From
header field of the request contains a telephone number, then the header field of the request contains a telephone number, then the
canonicalization of that number goes into the first slot (see canonicalization of that number goes into the first slot (see
Section 6.1). Otherwise, the first slot contains the AoR of the Section 6.1.1). Otherwise, the first slot contains the AoR of the
UA sending the message, or addr-spec of the From header field. UA sending the message as taken from addr-spec of the From header
field.
Second, the target. If the user part of the AoR in the To header Second, the target. If the user part of the AoR in the To header
field of the request contains a telephone number, then the field of the request contains a telephone number, then the
canonicalization of that number goes into the second slot (see canonicalization of that number goes into the second slot (again,
Section 6.1). Otherwise, the second slot contains the addr-spec see Section 6.1.1). Otherwise, the second slot contains the addr-
component of the To header field, which is the AoR to which the spec component of the To header field, which is the AoR to which
request is being sent. the request is being sent.
Third, the request method. Third, the request method.
Fourth, the Date header field, with exactly one space each for Fourth, the Date header field, with exactly one space each for
each SP and the weekday and month items case set as shown in the each SP and the weekday and month items case set as shown in the
BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for
weekday and month is a choice amongst a set of tokens. The RFC weekday and month is a choice amongst a set of tokens. The RFC
4234 [RFC4234] rules for the BNF specify that tokens are case 4234 [RFC4234] rules for the BNF specify that tokens are case
sensitive. However, when used to construct the canonical string sensitive. However, when used to construct the canonical string
defined here, the first letter of each week and month MUST be defined here, the first letter of each week and month MUST be
capitalized, and the remaining two letters must be lowercase. capitalized, and the remaining two letters must be lowercase.
This matches the capitalization provided in the definition of each This matches the capitalization provided in the definition of each
token. All requests that use the Identity mechanism MUST contain token. All requests that use the Identity mechanism MUST contain
a Date header. a Date header.
Fifth, if the request contains an SDP message body, and if that Fifth, if the request contains an SDP message body, and if that
SDP contains an "a=fingerprint" attribute, the value of the SDP contains one or more "a=fingerprint" attributes, the value(s)
attribute. The attribute value consists of all characters of the attributes if they differ. Each attribute value consists
following the colon after "a=fingerprint" including the algorithm of all characters following the colon after "a=fingerprint"
description and hexadecimal key representation, any whitespace, including the algorithm description and hexadecimal key
carriage returns, and "/" line break indicators. If the SDP body representation, any whitespace, carriage returns, and "/" line
contains no "a=fingerprint" attribute, the fifth element MUST be break indicators. If multiple non-identical "a=fingerprint"
empty, containing no whitespace, resulting in a "||" in the attributes appear in an SDP body, then all non-identical
signed-identity-digest. attributes values MUST be concatenated, with no separating
character, after sorting the values in alphanumeric order. If the
SDP body contains no "a=fingerprint" attribute, the fifth element
MUST be empty, containing no whitespace, resulting in a "||" in
the signed-identity-digest.
Sixth, the Identity-Reliance header field value, if there is an Sixth, the Identity-Reliance header field value, if there is an
Identity-Reliance field in the request. If the message has no Identity-Reliance field in the request. If the message has no
body, or no Identity-Reliance header, then the fifth slot will be body, or no Identity-Reliance header, then the fifth slot will be
empty, and the final "|" will not be followed by any additional empty, and the final "|" will not be followed by any additional
characters. characters.
For more information on the security properties of these headers, and For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 10 and why their inclusion mitigates replay attacks, see Section 9 and
[RFC3893]. The precise formulation of this digest-string is, [RFC3893]. The precise formulation of this digest-string is,
therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]): therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):
digest-string = ( addr-spec / tn-spec ) "|" ( addr-spec / tn-spec ) "|" digest-string = ( addr-spec / tn-spec ) "|" ( addr-spec / tn-spec ) "|"
Method "|" SIP-date "|" [ sdp-fingerprint ] "|" [ signed-identity-reliance-digest ] Method "|" SIP-date "|" [ sdp-fingerprint ] "|"
[ signed-identity-reliance-digest ]
sdp-fingerprint = byte-string sdp-fingerprint = byte-string
For the definition of 'tn-spec' see Section 6.1.1.
For the definition of 'tn-spec' see Section 6.1.
After the digest-string or reliance-digest-string is formed, each After the digest-string or reliance-digest-string is formed, each
MUST be hashed and signed with the certificate of authority over the MUST be hashed and signed with the certificate of authority over the
identity. The hashing and signing algorithm is specified by the identity. The hashing and signing algorithm is specified by the
'alg' parameter of the Identity-Info header (see below for more 'alg' parameter of the Identity-Info header (see below for more
information on Identity-Info header parameters). This document information on Identity-Info header parameters). This document
defines only one value for the 'alg' parameter: 'rsa-sha256'; further defines only one value for the 'alg' parameter: 'rsa-sha256'; further
values MUST be defined in a Standards Track RFC, see Section 14.7 for values MUST be defined in a Standards Track RFC, see Section 10.3 for
more information. All implementations of this specification MUST more information. All implementations of this specification MUST
support 'rsa-sha256'. When the 'rsa-sha256' algorithm is specified support 'rsa-sha256'. When the 'rsa-sha256' algorithm is specified
in the 'alg' parameter of Identity-Info, the hash and signature MUST in the 'alg' parameter of Identity-Info, the hash and signature MUST
be generated as follows: compute the results of signing this string be generated as follows: compute the results of signing this string
with sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and with sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and
base64 encode the results as specified in RFC 3548 [RFC3548]. A base64 encode the results as specified in RFC 3548 [RFC3548]. A
2048-bit or longer RSA key MUST be used. The result of the digest- 2048-bit or longer RSA key MUST be used. The result of the digest-
string hash is placed in the Identity header field; the optional string hash is placed in the Identity header field; the optional
reliance-digest-string hash goes in the Identity-Reliance header. reliance-digest-string hash goes in the Identity-Reliance header.
For detailed examples of the usage of this algorithm, see Section 8.
The 'absoluteURI' portion of the Identity-Info header MUST contain a The 'absoluteURI' portion of the Identity-Info header MUST contain a
URI; see Section 5.3 for more on choosing how to advertise URI; see Section 5.3 for more on choosing how to advertise
credentials through Identity-Info. credentials through Identity-Info.
This document adds (or amends) the following entries to Table 2 of 8. Privacy Considerations
RFC 3261 [RFC3261] (this repeats the registrations of RFC4474):
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Info R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Reliance R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Note, in the table above, that this mechanism does not protect the
CANCEL method. The CANCEL method cannot be challenged, because it is
hop-by-hop, and accordingly authentication service behavior for
CANCEL would be significantly limited. The Identity and Identity-
Info header MUST NOT appear in CANCEL. Note as well that the use of
Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility
reasons.
8. Examples
9. Privacy Considerations
The purpose of this mechanism is to provide a strong identification The purpose of this mechanism is to provide a strong identification
of the originator of a SIP request, specifically a cryptographic of the originator of a SIP request, specifically a cryptographic
assurance that the URI given in the From header field value can assurance that the URI given in the From header field value can
legitimately be claimed by the originator. This URI may contain a legitimately be claimed by the originator. This URI may contain a
variety of personally identifying information, including the name of variety of personally identifying information, including the name of
a human being, their place of work or service provider, and possibly a human being, their place of work or service provider, and possibly
further details. The intrinsic privacy risks associated with that further details. The intrinsic privacy risks associated with that
URI are, however, no different from those of baseline SIP. Per the URI are, however, no different from those of baseline SIP. Per the
guidance in [RFC6973], implementors should make users aware of the guidance in [RFC6973], implementors should make users aware of the
privacy trade-off of providing secure identity. privacy trade-off of providing secure 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 RFC 3323 the standard SIP practices for privacy described in [RFC3323]. A SIP
[RFC3323]. A SIP proxy server can act both as a privacy service and proxy server can act both as a privacy service and as an
as an authentication service. Since a user agent can provide any authentication service. Since a user agent can provide any From
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.
Note, however, that for using anonymous SIP URIs, an authentication Note, however, that even when using anonymous SIP URIs, an
service must possess a certificate corresponding to the host portion authentication service must possess a certificate corresponding to
of the addr-spec of the From header field of the request; the host portion of the addr-spec of the From header field of the
accordingly, using domains like 'anonymous.invalid' will not be request; accordingly, using domains like 'anonymous.invalid' will not
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". The use have authenticated, but I am keeping its identity private". The use
of the domain 'anonymous.invalid' entails that no corresponding of the domain 'anonymous.invalid' entails that no corresponding
authority for the domain can exist, and as a consequence, authority for the domain can exist, and as a consequence,
authentication service functions are meaningless. authentication service functions for that domain are meaningless.
RFC 3325 [RFC3325] defines the "id" priv-value token, which is [RFC3325] defines the "id" priv-value token, which is specific to the
specific to the P-Asserted-Identity header. The sort of assertion P-Asserted-Identity header. The sort of assertion provided by the P-
provided by the P-Asserted-Identity header is very different from the Asserted-Identity header is very different from the Identity header
Identity header presented in this document. It contains additional presented in this document. It contains additional information about
information about the sender of a message that may go beyond what the sender of a message that may go beyond what appears in the From
appears in the From header field; P-Asserted-Identity holds a header field; P-Asserted-Identity holds a definitive identity for the
definitive identity for the sender that is somehow known to a closed sender that is somehow known to a closed network of intermediaries
network of intermediaries that presumably the network will use this that presumably the network will use this identity for billing or
identity for billing or security purposes. The danger of this security purposes. The danger of this network-specific information
network-specific information leaking outside of the closed network leaking outside of the closed network motivated the "id" priv-value
motivated the "id" priv-value token. The "id" priv-value token has token. The "id" priv-value token has no implications for the
no implications for the Identity header, and privacy services MUST Identity header, and privacy services MUST NOT remove the Identity
NOT remove the Identity header when a priv-value of "id" appears in a header when a priv-value of "id" appears in a Privacy header.
Privacy header.
Finally, note that unlike RFC 3325 [RFC3325], the mechanism described Finally, note that unlike [RFC3325], the mechanism described in this
in this specification adds no information to SIP requests that has specification adds no information to SIP requests that has privacy
privacy implications. implications.
10. Security Considerations 9. Security Considerations
10.1. Handling of digest-string Elements 9.1. Handling of digest-string Elements
This document describes a mechanism that provides a signature over This document describes a mechanism that provides a signature over
the Date header field, and either the whole or part of the To and the Date header field, and either the whole or part of the To and
From header fields of SIP requests, as well as optional protections From header fields of SIP requests, as well as optional protections
for the message body. While a signature over the From header field for the message body. While a signature over the From header field
would be sufficient to secure a URI alone, the additional headers would be sufficient to secure a URI alone, the additional headers
provide replay protection and reference integrity necessary to make provide replay protection and reference integrity necessary to make
sure that the Identity header will not be replayed in cut-and-paste sure that the Identity header will not be replayed in cut-and-paste
attacks. In general, the considerations related to the security of attacks. In general, the considerations related to the security of
these headers are the same as those given in RFC 3261 [RFC3261] for these headers are the same as those given in [RFC3261] for including
including headers in tunneled 'message/sip' MIME bodies (see headers in tunneled 'message/sip' MIME bodies (see Section 23 in
Section 23 in particular). The following section details the particular). The following section details the individual security
individual security properties obtained by including each of these properties obtained by including each of these header fields within
header fields within the signature; collectively, this set of header the signature; collectively, this set of header fields provides the
fields provides the necessary properties to prevent impersonation. necessary properties to prevent impersonation.
The From header field indicates the identity of the sender of the The From header field indicates the identity of the sender of the
message, and the SIP address-of-record URI, or an embedded telephone message, and the SIP address-of-record URI, or an embedded telephone
number, in the From header field is the identity of a SIP user, for number, in the From header field is the identity of a SIP user, for
the purposes of this document. The To header field provides the the purposes of this document. The To header field provides the
identity of the SIP user that this request targets. Providing the To identity of the SIP user that this request targets. Providing the To
header field in the Identity signature serves two purposes: first, it header field in the Identity signature serves two purposes: first, it
prevents cut-and-paste attacks in which an Identity header from prevents cut-and-paste attacks in which an Identity header from
legitimate request for one user is cut-and-pasted into a request for legitimate request for one user is cut-and-pasted into a request for
a different user; second, it preserves the starting URI scheme of the a different user; second, it preserves the starting URI scheme of the
request, which helps prevent downgrade attacks against the use of request, which helps prevent downgrade attacks against the use of
SIPS. SIPS.
The Date header field provides replay protection, as described in RFC The Date header field provides replay protection, as described in
3261 [RFC3261], Section 23.4.2. Implementations of this [RFC3261], Section 23.4.2. Implementations of this specification
specification MUST NOT deem valid a request with an outdated Date MUST NOT deem valid a request with an outdated Date header field (the
header field (the RECOMMENDED interval is that the Date header must RECOMMENDED interval is that the Date header must indicate a time
indicate a time within 3600 seconds of the receipt of a message). within 60 seconds of the receipt of a message). The result of this
The result of this is that if an Identity header is replayed within is that if an Identity header is replayed within the Date interval,
the Date interval, verifiers will recognize that it is invalid; if an verifiers will recognize that it is invalid; if an Identity header is
Identity header is replayed after the Date interval, verifiers will replayed after the Date interval, verifiers will recognize that it is
recognize that it is invalid because the Date is stale. invalid because the Date is stale.
Without the method, an INVITE request could be cut- and-pasted by an Without the method, an INVITE request could be cut- and-pasted by an
attacker and transformed into a MESSAGE request without changing any attacker and transformed into a MESSAGE request without changing any
fields covered by the Identity header, and moreover requests within a fields covered by the Identity header, and moreover requests within a
certain transaction could be replayed in potentially confusing or transaction (for example, a re-INVITE) could be replayed in
malicious ways. potentially confusing or malicious ways.
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 a From header field value, which is the unauthorized claiming of a From header field value, which is the
primary scope of the current document. primary scope of the current document.
It might seem attractive to provide a signature over some of the It might seem attractive to provide a signature over some of the
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important to note that preventing men-in-the-middle is not the important to note that preventing men-in-the-middle is not the
primary impetus for this mechanism. Moreover, changing the 'alg' primary impetus for this mechanism. Moreover, changing the 'alg'
would at worst result in some sort of bid-down attack, and at best would at worst result in some sort of bid-down attack, and at best
cause a failure in the verifier. Note that only one valid 'alg' cause a failure in the verifier. Note that only one valid 'alg'
parameter is defined in this document and that thus there is parameter is defined in this document and that thus there is
currently no weaker algorithm to which the mechanism can be bid down. currently no weaker algorithm to which the mechanism can be bid down.
'alg' has been incorporated into this mechanism for forward- 'alg' has been incorporated into this mechanism for forward-
compatibility reasons in case the current algorithm exhibits compatibility reasons in case the current algorithm exhibits
weaknesses, and requires swift replacement, in the future. weaknesses, and requires swift replacement, in the future.
10.2. Securing the Connection to the Authentication Service 9.1.1. Protection of the To Header and Retargeting
The mechanism in this document provides a signature over the identity
information in the To header field value of requests. This provides
a means for verifiers to detect replay attacks where a signed request
originally sent to one target is modified and then forwarded by an
attacker to another, unrelated target. Armed with the original value
of the To header field, the recipient of a request may compare it to
their own identity in order to determine whether or not the identity
information in this call might have been replayed. However, any
request may be legitimately retargeted as well, and as a result
legitimate requests may reach a SIP endpoint whose user is not
identified by the URI designated in the To header field value. It is
therefore difficult for any verifier to decide whether or not some
prior retargeting was "legitimate." Retargeting can also cause
confusion when identity information is provided for requests sent in
the backwards in a dialog, as the dialog identifiers may not match
credentials held by the ultimate target of the dialog. For further
information on the problems of response identity see
[I-D.peterson-sipping-retarget].
Any means for authentication services or verifiers to anticipate
retargeting is outside the scope of this document, and likely to have
equal applicability to response identity as it does to requests in
the backwards direction within a dialog. Consequently, no special
guidance is given for implementers here regarding the 'connected
party' problem (see [RFC4916]); authentication service behavior is
unchanged if retargeting has occurred for a dialog-forming request.
Ultimately, the authentication service provides an Identity header
for requests in the backwards dialog when the user is authorized to
assert the identity given in the From header field, and if they are
not, an Identity header is not provided. And per the threat model of
[RFC7375], resolving problems with 'connected' identity has little
bearing on detecting robocalling or related impersonation attacks.
9.2. 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 this 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
mechanism such as Digest), then it is possible that a rogue server mechanism such as Digest), then it is possible that a rogue server
is attempting to pose as an authentication service for a domain is attempting to pose as an authentication service for a domain
that it does not control, possibly in an attempt to collect shared that it does not control, possibly in an attempt to collect shared
secrets for that domain. A similar practice could be used for secrets for that domain. A similar practice could be used for
telephone numbers, though the application of certificates for telephone numbers, though the application of certificates for
telephone numbers to TLS is left as a matter for future study. telephone numbers to TLS is left as a matter for future study.
Without TLS, the various header field values and the body of the Without TLS, the various header field values and the body of the
request will not have integrity protection when the request request will not have integrity protection when the request
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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.
10.3. Authorization and Transitional Strategies 9.3. 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.
10.4. Display-Names and Identity 9.4. 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. document, but may be the subject of future work.
11. IANA Considerations 10. IANA Considerations
This document relies on the headers and response codes defined in RFC This document relies on the headers and response codes defined in RFC
4474. It also retains the requirements for the specification of new 4474. It also retains the requirements for the specification of new
algorithms or headers related to the mechanisms described in that algorithms or headers related to the mechanisms described in that
document. document.
11.1. Header Field Names 10.1. Header Field Names
This document specifies one new SIP header called Identity-Reliance. This document specifies one new SIP header called Identity-Reliance.
Its syntax is given in Section 7. This header is defined by the Its syntax is given in Section 7. This header is defined by the
following information, which has been added to the header sub- following information, which has been added to the header sub-
registry under http://www.iana.org/assignments/sip-parameters registry under http://www.iana.org/assignments/sip-parameters
Header Name: Identity-Reliance Header Name: Identity-Reliance
Compact Form: N/A Compact Form: N/A
11.2. Identity-Info Parameters 10.2. Identity-Info Parameters
The IANA has already created a registry for Identity-Info headers The IANA has already created a registry for Identity-Info header
parameters. This specification defines a new value called "canon" as parameters. This specification defines a new value called "canon" as
defined in Section 5.3. defined in Section 5.3.
11.3. Identity-Info Algorithm Parameter Values 10.3. Identity-Info Algorithm Parameter Values
The IANA has already created a registry for Identity-Info 'alg' The IANA has already created a registry for Identity-Info 'alg'
parameter values. This registry is to be prepopulated with a single parameter values. This registry is to be prepopulated with a single
entry for a value called 'rsa-sha256', which describes the algorithm entry for a value called 'rsa-sha256', which describes the algorithm
used to create the signature that appears in the Identity header. used to create the signature that appears in the Identity header.
Registry entries must contain the name of the 'alg' parameter value Registry entries must contain the name of the 'alg' parameter value
and the specification in which the value is described. New values and the specification in which the value is described. New values
for the 'alg' parameter may be defined only in Standards Track RFCs. for the 'alg' parameter may be 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.
12. Acknowledgments 11. Acknowledgments
The authors would like to thank Stephen Kent, Brian Rosen, Alex The authors would like to thank Stephen Kent, Brian Rosen, Alex
Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov, Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer, Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer,
Henning Schulzrinne, and Richard Barnes for their comments. Henning Schulzrinne, and Richard Barnes for their comments.
13. Changes from RFC4474 12. Changes from RFC4474
The following are salient changes from the original RFC 4474: The following are salient changes from the original RFC 4474:
Generalized the credential mechanism; credential enrollment and Generalized the credential mechanism; credential enrollment and
acquisition is now outside the scope of this document acquisition 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
Added any DTLS-SRTP fingerprint in SDP as a mandatory element of Added any DTLS-SRTP fingerprint in SDP as a mandatory element of
the digest-string the digest-string
Added the Identity-Reliance header Added the Identity-Reliance header
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
14. References 13. References
14.1. Normative References 13.1. Normative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[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,
June 2002. June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June Protocol (SIP): Locating SIP Servers", RFC 3263, June
skipping to change at page 31, line 21 skipping to change at page 29, line 36
Algorithms", RFC 3370, August 2002. Algorithms", RFC 3370, August 2002.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004. 3966, December 2004.
[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, May 2008. (CRL) Profile", RFC 5280, May 2008.
14.2. Informative References 13.2. Informative References
[I-D.ietf-stir-certificates]
Peterson, J., "Secure Telephone Identity Credentials:
Certificates", draft-ietf-stir-certificates-00 (work in
progress), October 2014.
[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.
[I-D.peterson-sipping-retarget] [I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping- Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005. retarget-00 (work in progress), February 2005.
[I-D.peterson-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-peterson-stir-
certificates-00 (work in progress), February 2014.
[I-D.rosenberg-sip-rfc4474-concerns] [I-D.rosenberg-sip-rfc4474-concerns]
Rosenberg, J., "Concerns around the Applicability of RFC Rosenberg, J., "Concerns around the Applicability of RFC
4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in 4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
progress), February 2008. progress), February 2008.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999. 2585, May 1999.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session [RFC3323] Peterson, J., "A Privacy Mechanism for the Session
skipping to change at page 33, line 5 skipping to change at page 31, line 17
Considerations for Internet Protocols", RFC 6973, July Considerations for Internet Protocols", RFC 6973, July
2013. 2013.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014. Attack", BCP 188, RFC 7258, May 2014.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements", Telephone Identity Problem Statement and Requirements",
RFC 7340, September 2014. RFC 7340, September 2014.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, October 2014.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
Neustar, Inc. Neustar, Inc.
1800 Sutter St Suite 570 1800 Sutter St Suite 570
Concord, CA 94520 Concord, CA 94520
US US
Email: jon.peterson@neustar.biz Email: jon.peterson@neustar.biz
 End of changes. 82 change blocks. 
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