draft-ietf-stir-rfc4474bis-04.txt   draft-ietf-stir-rfc4474bis-05.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: January 7, 2016 Cisco Expires: March 17, 2016 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
July 6, 2015 September 14, 2015
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-04.txt draft-ietf-stir-rfc4474bis-05.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
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2016. This Internet-Draft will expire on March 17, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . 6
4. Signature Generation and Validation . . . . . . . . . . . . . 6 4. Signature Generation and Validation . . . . . . . . . . . . . 7
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6 4.1. Authentication Service Behavior . . . . . . . . . . . . . 7
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9 4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 9
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Credential Use by the Authentication Service . . . . . . 11 5.1. Credential Use by the Authentication Service . . . . . . 11
5.2. Credential Use by the Verification Service . . . . . . . 12 5.2. Credential Use by the Verification Service . . . . . . . 12
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 13 5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 13
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 13 5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 13
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14 6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14 6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 14
6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15 6.1.1. Canonicalization Procedures . . . . . . . . . . . . . 15
6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . 17
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 21 8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10.1. Handling of digest-string Elements . . . . . . . . . . . 23 10.1. Protected Request Fields . . . . . . . . . . . . . . . . 23
10.1.1. Protection of the To Header and Retargeting . . . . 26 10.1.1. Protection of the To Header and Retargeting . . . . 24
10.2. Securing the Connection to the Authentication Service . 27 10.2. Unprotected Request Fields . . . . . . . . . . . . . . . 25
10.3. Authorization and Transitional Strategies . . . . . . . 28 10.3. Malicious Removal of Identity Headers . . . . . . . . . 26
10.4. Display-Names and Identity . . . . . . . . . . . . . . . 29 10.4. Securing the Connection to the Authentication Service . 26
10.5. Authorization and Transitional Strategies . . . . . . . 27
10.6. Display-Names and Identity . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29 11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29
11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 30 11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 29
11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 30 11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 29
11.4. Identity-Extension Names . . . . . . . . . . . . . . . . 30 11.4. Identity-Extension Names . . . . . . . . . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 31 13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 30
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
14.1. Normative References . . . . . . . . . . . . . . . . . . 31 14.1. Normative References . . . . . . . . . . . . . . . . . . 30
14.2. Informative References . . . . . . . . . . . . . . . . . 32 14.2. Informative References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 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
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[I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474 [I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474
did not deal well with telephone numbers as identifiers, despite did not deal well with telephone numbers as identifiers, despite
their enduring use in SIP deployments. RFC 4474 also provided a their enduring use in SIP deployments. RFC 4474 also provided a
signature over material that intermediaries in the field commonly signature over material that intermediaries in the field commonly
altered. This specification therefore revises RFC 4474 in light of altered. This specification therefore revises RFC 4474 in light of
recent reconsideration of the problem space to align with the threat recent reconsideration of the problem space to align with the threat
model in [RFC7375]. model in [RFC7375].
2. Background 2. Background
The secure operation of many SIP applications and services depends on Per [RFC7340], a key enabler of problems such as robocalling,
authorization policies. These policies may be automated, or they may voicemail hacking, and swatting lies in an attacker's ability to
be exercised manually by humans. An example of the latter would be impersonate someone else. This secure operation of most SIP
an Internet telephone application that displays the calling party applications and services depends on authorizing the source of
number (and/or Caller-ID) of a caller, which a human may review to communications as it is represented in a SIP request. Such
make a policy decision before answering a call. An example of the authorization policies can be automated or be a part of human
former would be a voicemail service that compares the identity of the operation of SIP devices. An example of the latter would be an
caller to a whitelist before determining whether it should allow the Internet telephone application that displays the calling party number
caller access to recorded messages. In both of these cases, (and/or Caller-ID) of a caller, which a human may review to make a
attackers might attempt to circumvent these authorization policies policy decision before answering a call. An example of the former
through impersonation. Since the primary identifier of the sender of would be a voicemail service that compares the identity of the caller
a SIP request, the From header field, can be populated arbitrarily by to a whitelist before determining whether it should allow the caller
the controller of a user agent, impersonation is very simple today. access to recorded messages. In both of these cases, attackers might
The mechanism described in this document provides a strong identity attempt to circumvent these authorization policies through
system for SIP requests for detecting attempted impersonation. impersonation. Since the primary identifier of the sender of a SIP
request, the From header field, can be populated arbitrarily by the
controller of a user agent, impersonation is very simple today. The
mechanism described in this document provides a strong identity
system for detecting attempted impersonation in SIP requests.
This document proposes an authentication architecture for SIP in This identity architecture for SIP depends on a logical
which requests are processed by a logical authentication service that "authentication service" which processes and signs requests; it may
may be implemented as part of a user agent or as a proxy server. be implemented either as part of a user agent or as a proxy server.
Once a message has been authenticated, the service then adds new Once the sender of the message has been authenticated, the service
cryptographic information to requests to communicate to other SIP then adds new cryptographic information to requests to communicate to
entities that the sending user has been authenticated and its use of other SIP entities that the sending user has been authenticated and
the From header field has been authorized. its claim of a particular identity has been authorized. A
"verification service" on the receiving end then validates this
signature and enables policy decisions to be made based on the
results of the verification.
But authorized by whom? Identities are issued to users by Identities are issued to users by authorities. When a new user
authorities. When a new user becomes associated with example.com, becomes associated with example.com, the administrator of the SIP
the administrator of the SIP service for that domain will issue them service for that domain can issue them an identity in that namespace,
an identity in that namespace, such as alice@example.com. Alice may such as alice@example.com. Alice may then send REGISTER requests to
then send REGISTER requests to example.com that make her user agents example.com that make her user agents eligible to receive requests
eligible to receive requests for sip:alice@example.com. In some for sip:alice@example.com. In some cases, Alice may be the owner of
cases, Alice may be the owner of the domain herself, and may issue the domain herself, and may issue herself identities as she chooses.
herself identities as she chooses. But ultimately, it is the But ultimately, it is the controller of the SIP service at
controller of the SIP service at example.com that must be responsible example.com that must be responsible for authorizing the use of names
for authorizing the use of names in the example.com domain. in the example.com domain. Therefore, for the purposes of baseline
Therefore, for the purposes of this specification, the credentials SIP, the credentials needed to prove a user is authorized to use a
needed to prove a user is authorized to use a particular From header particular From header field must ultimately derive from the domain
field must ultimately derive from the domain owner: either a user owner: either a user agent gives requests to the domain name owner in
agent gives requests to the domain name owner in order for them to be order for them to be signed by the domain owner's credentials, or the
signed by the domain owner's credentials, or the user agent must user agent must possess credentials that prove in some fashion that
possess credentials that prove in some fashion that the domain owner the domain owner has given the user agent the right to a name.
has given the user agent the right to a name.
The situation is however more complicated for telephone numbers. The situation is however more complicated for telephone numbers,
Authority over telephone numbers does not correspond directly to however. Authority over telephone numbers does not correspond
Internet domains. While a user could register at a SIP domain with a directly to Internet domains. While a user could register at a SIP
username that corresponds to a telephone number, any connection domain with a username that corresponds to a telephone number, any
between the administrator of that domain and the assignment of connection between the administrator of that domain and the
telephone numbers is not currently reflected on the Internet. assignment of telephone numbers is not currently reflected on the
Telephone numbers do not share the domain-scope property described Internet. Telephone numbers do not share the domain-scope property
above, as they are dialed without any domain component. This described above, as they are dialed without any domain component.
document thus assumes the existence of a separate means of This document thus assumes the existence of a separate means of
establishing authority over telephone numbers, for cases where the establishing authority over telephone numbers, for cases where the
telephone number is the identity of the user. As with SIP URIs, the telephone number is the identity of the user. As with SIP URIs, the
necessary credentials to prove authority for a name might reside necessary credentials to prove authority for a name might reside
either in the endpoint or at some intermediary. either in the endpoint or at some intermediary.
This document specifies a means of sharing a cryptographic assurance This document specifies a means of sharing a cryptographic assurance
of end-user SIP identity in an interdomain or intradomain context. of end-user SIP identity in an interdomain or intradomain context.
It relies on the authentication service adding to requests a SIP It relies on the authentication service adding to requests a SIP
header, the Identity header, which contains that cryptographic header, the Identity header, which contains that cryptographic
assurance. In order to assist in the validation of the Identity assurance. In order to assist in the validation of the Identity
header, this specification also describes an Identity-Info header header, this specification also describes an Identity-Info header
that can be used by the recipient of a request to recover the that can be used by the recipient of a request to recover the
credentials of the signer. Note that the scope of this document is credentials of the signer. Note that the scope of this document is
limited to providing this identity assurance for SIP requests; limited to providing this identity assurance for SIP requests;
solving this problem for SIP responses is outside the scope of this solving this problem for SIP responses is outside the scope of this
work (see [RFC4916]). work (see [RFC4916]).
This specification allows either a user agent or a proxy server to This specification allows either a user agent or a proxy server to
provide the authentication service function and/or to verify provide the authentication service function and/or the verification
identities. To maximize end-to-end security, it is obviously service function. To maximize end-to-end security, it is obviously
preferable for end-users to acquire their own credentials; if they preferable for end-users to acquire their own credentials; if they
do, their user agents can act as authentication services. However, do, their user agents can act as authentication services. However,
for some deployments end-user credentials may be neither practical for some deployments end-user credentials may be neither practical
nor affordable, given the potentially large number of SIP user agents nor affordable, given the potentially large number of SIP user agents
(phones, PCs, laptops, PDAs, gaming devices) that may be employed by (phones, PCs, laptops, PDAs, gaming devices) that may be employed by
a single user. In such environments, synchronizing keying material a single user. In such environments, synchronizing keying material
across multiple devices may be prohobitively complex and require across multiple devices may be prohibitively complex and require
quite a good deal of additional endpoint behavior. Managing several quite a good deal of additional endpoint behavior. Managing several
credentials for the various devices could also be burdensome. In credentials for the various devices could also be burdensome. In
these cases, implementation the authentication service at an these cases, implementation the authentication service at an
intermediary may be more practical. This trade-off needs to be intermediary may be more practical. This trade-off needs to be
understood by implementers of this specification. understood by implementers of this specification.
3. Overview of Operations 3. Overview of Operations
This section provides an informative (non-normative) high-level This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document. overview of the mechanisms described in this document.
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address-of-record, in the From header field of the request. She then address-of-record, in the From header field of the request. She then
sends an INVITE over TLS to an authentication service proxy for the sends an INVITE over TLS to an authentication service proxy for the
example.com domain. example.com domain.
The authentication service authenticates Alice (possibly by sending a The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized Digest authentication challenge) and validates that she is authorized
to assert the identity that she populated in the From header field. to assert the identity that she populated in the From header field.
This value is Alice's AoR, but in other cases it could be some This value is Alice's AoR, but in other cases it could be some
different value that the proxy server has authority over, such as a different value that the proxy server has authority over, such as a
telephone number. The proxy then computes a hash over some telephone number. The proxy then computes a hash over some
particular headers, including the From header field (and optionally particular headers and fields, including part of the From header
the body) of the message. This hash is signed with the appropriate field of the message. This hash is signed with the appropriate
credential for the identity (example.com, in the credential for the identity (example.com, in the
sip:alice@example.com case) and inserted in a new header field in the sip:alice@example.com case) and inserted in a new header field in the
SIP message, the 'Identity' header. SIP message, the 'Identity' header.
The proxy, as the holder of the private key for the example.com The proxy, as the holder of the private key for the example.com
domain, is asserting that the originator of this request has been domain, is asserting that the originator of this request has been
authenticated and that she is authorized to claim the identity that authenticated and that she is authorized to claim the identity that
appears in the From header field. The proxy also inserts a companion appears in the From header field. The proxy also inserts a companion
header field, Identity-Info, that tells Bob how to acquire keying header field, Identity-Info, that tells Bob how to acquire keying
material necessary to validate its credentials (a public key), if he material necessary to validate its credentials (a public key), in
doesn't already have it. case he doesn't already have it.
When Bob's domain receives the request, it verifies the signature When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validate that the provided in the Identity header, and thus can validate that the
authority over the identity in the From header field authenticated authority over the identity in the From header field authenticated
the user, and permitted the user to assert that From header field the user, and permitted the user to assert that From header field
value. This same validation operation may be performed by Bob's user value. This same validation operation may be performed by Bob's user
agent server (UAS). As the request has been validated, it is agent server (UAS). As the request has been validated, it is
rendered to Bob. If the validation was unsuccessful, some other rendered to Bob. If the validation was unsuccessful, some other
treatment would be applied by the receiving domain. treatment would be applied by the receiving domain.
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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.
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 First, the authentication service must determine whether it is
from the request. The authentication service takes this URI value authoritative for the identity of the sender of the request. In
from the addr-spec component of From header field; this URI will be ordinary operations, the authentication service decides this by
referred to here as the 'identity field'. If the identity field inspecting the URI value from the addr-spec component of From header
contains a SIP or SIP Secure (SIPS) URI, and the user portion is not field; this URI will be referred to here as the 'identity field'. If
a telephone number, the authentication service MUST extract the the identity field contains a SIP or SIP Secure (SIPS) URI, and the
hostname portion of the identity field and compare it to the user portion is not a telephone number, the authentication service
domain(s) for which it is responsible (following the procedures in MUST extract the hostname portion of the identity field and compare
RFC 3261 [RFC3261], Section 16.4). If the identity field uses the it to the domain(s) for which it is responsible (following the
TEL URI scheme [RFC3966], or the identity field is a SIP or SIPS URI procedures in RFC 3261 [RFC3261], Section 16.4). If the identity
with a telephone number in the user portion, the authentication field uses the TEL URI scheme [RFC3966], or the identity field is a
service determines whether or not it is responsible for this SIP or SIPS URI with a telephone number in the user portion, the
telephone number; see Section 6.1 for more information. An authentication service determines whether or not it is responsible
for this telephone number; see Section 6.1 for more information. An
authentication service proceeding with a signature over a telephone authentication service proceeding with a signature over a telephone
number MAY add the optional 'canon' parameter to the request as number MUST then follow the canonicaliaation procedures described in
described in that section. If the authentication service is not Section 6.1.1. If the authentication service is not authoritative
authoritative for the identity in question, it SHOULD process and for the identity in question, it SHOULD process and forward the
forward the request normally, but it MUST NOT follow the steps below request normally, but it MUST NOT follow the steps below to add an
to add an Identity header. An authentication service MUST NOT add an Identity header. An authentication service MUST NOT add an Identity
Identity header to a request that already has one. 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
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If the authentication service is instantiated by a SIP user agent, If the authentication service is instantiated by a SIP user agent,
a user agent may authenticate its own user through any system- a user agent may authenticate its own user through any system-
specific means, perhaps simply by virtue of having physical access specific means, perhaps simply by virtue of having physical access
to the user agent. to the user agent.
Authorization of the use of a particular username or telephone number Authorization of the use of a particular username or telephone number
in the user part of the From header field is a matter of local policy in the user part of the From header field is a matter of local policy
for the authentication service, see Section 5.1 for more information. for the authentication service, see Section 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
header field (e.g., the URI of the sender, like identity field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display- 'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta'). name portion of the From header field (e.g., 'Alice Atlanta').
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 could return a 403 response constraints, the authentication service could return a 403 response
code. In this case, the reason phrase should indicate the nature of code. In this case, the reason phrase should indicate the nature of
the problem; for example, "Inappropriate Display Name". However, the the problem; for example, "Inappropriate Display Name". However, the
display-name is not always present, and in many environments the display-name is not always present, and in many environments the
requisite operational procedures for display-name validation may not requisite operational procedures for display-name validation may not
exist, so no normative guidance is given here. For more information, exist, so no normative guidance is given here. For more information,
see Section 10.4. see Section 10.6.
Step 3: Step 3:
An authentication service MUST add a Date header field to SIP An authentication service MUST add a Date header field to SIP
requests if one is not already present. The authentication service requests if one is not already present. The authentication service
MUST ensure that any preexisting Date header in the request is MUST ensure that any preexisting Date header in the request is
accurate. Local policy can dictate precisely how accurate the Date accurate. Local policy can dictate precisely how accurate the Date
must be; a RECOMMENDED maximum discrepancy of sixty seconds will must be; a RECOMMENDED maximum discrepancy of sixty seconds will
ensure that the request is unlikely to upset any verifiers. If the ensure that the request is unlikely to upset any verifiers. If the
Date header contains a time different by more than one minute from Date header contains a time different by more than one minute from
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Identity header is not intended to provide a source of non- Identity header is not intended to provide a source of non-
repudiation or a perfect record of when messages are processed. repudiation or a perfect record of when messages are processed.
Finally, the authentication service MUST verify that the Date header Finally, the authentication service MUST verify that the Date header
falls within the validity period of its credential. falls within the validity period of its credential.
See Section 7 for information on how the Date header field assists See Section 7 for information on how the Date header field assists
verifiers. verifiers.
Step 4: Step 4:
The authentication service MAY form an identity-reliance signature The authentication service MAY add extend the identity mechanism by
and add an Identity-Reliance header to the request containing this adding one or more Identity-Extension headers to the request. Only
signature. The Identity-Reliance header provides body security implementations that extend this base mechanism MAY populate this
properties that are useful for non-INVITE transactions, and in
environments where body security of INVITE transactions is necessary.
Details on the generation of this header are provided in Section 7.
If the authentication service is adding an Identity-Reliance header,
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
they are hashing exactly as many bytes of message-body as the
authentication service when they verify the message.
Step 5:
The authentication service MAY an identity-extension signature and
add an Identity-Extension header to the request containing this
signature. The Identity-Extension header is created by this
specification, but the header field value is left undefined. Only
implementations that extend this base mechanism MAY popular this
header field and add this signature. See Section 8. header field and add this signature. See Section 8.
Step 6: Step 5:
The authentication service MUST form the identity signature and add Subsequently, the authentication service MUST form the identity
an Identity header to the request containing this signature. After signature and add an Identity header to the request containing this
the Identity header has been added to the request, the authentication signature. After the Identity header has been added to the request,
service MUST also add an Identity-Info header. The Identity-Info the authentication service MUST also add an Identity-Info header.
header contains a URI from which the authentication service's The Identity-Info header contains a URI from which the authentication
credential can be acquired; see Section 5.3 for more on credential service's credential can be acquired; see Section 5.3 for more on
acquisition. Details on the syntax of both of these headers are credential acquisition. Details on the syntax of both of these
provided in Section 7. headers are provided in Section 7.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
4.2. 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
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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-
sending-user policy), then a 428 'Use Identity Header' response MUST sending-user policy), then a 428 'Use Identity Header' response MUST
be sent. be sent.
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 identity field
header field value should be over the entire URI or just a should be over the entire URI or just a canonicalized telephone
canonicalized telephone number, the verification service must follow number, the verification service MUST follow the canonicalization
the canonicalization process described in Section 6.1.1. That process described in Section 6.1.1. That section also describes the
section also describes the procedures the verification service must procedures the verification service MUST follow to determine if the
follow to determine if the signer is authoritative for a telephone signer is authoritative for a telephone number. For domains, the
number. For domains, the verifier MUST follow the process described verifier MUST follow the process described in Section 6.2 to
in Section 6.2 to determine if the signer is authoritative for the determine if the signer is authoritative for the identity 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, material to validate the signature in the Identity header field,
which usually involves dereferencing the Identity-Info header. See 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 must furthermore ensure that the value of the Date
header meets local policy for freshness (usually, within sixty
seconds) and that it falls within the validity period of the
credential used to sign the Identity header. For more on the attacks
this prevents, see Section 10.1.
Step 4:
The verifier MUST validate the signature in the Identity header The verifier MUST validate the signature in the Identity header
field, following the procedures for generating the hashed digest- field, following the procedures for generating the hashed digest-
string described in Section 7. If a verifier determines that the string described in Section 7. If a verifier determines that the
signature on the message does not correspond to the reconstructed signature on the message does not correspond to the reconstructed
digest-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:
If the request contains an Identity-Reliance header, the verifier
SHOULD verify the signature in the Identity-Reliance header field,
following the procedures for generating the hashed reliance-digest-
string described in Section 7. The Identity-Reliance header provides
important protections for non-INVITE transactions (such as MESSAGE or
NOTIFY), but verifiers MAY elect not to verify Identity-Reliance when
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:
If the request contains an Identity-Extension header, then if If the request contains one or more Identity-Extension headers, then
verifier supports the extension specified in the Identity-Extension if the verifier supports the included extension(s), it SHOULD verify
header field, it SHOULD verify any associated following the any associated fields following the procedures specified in that
procedures specified in that extension. See Section 8. If a extension (see Section 8). If a verifier determines that such a
verifier determines that such a signature in the message does not signature in the message does not correspond to the reconstructed
correspond to the reconstructed digest-string, then a 438 'Invalid digest-string, then a 438 'Invalid Identity Header' response SHOULD
Identity Header' response SHOULD be returned. If the verifier does be returned. If the verifier does not support the extension(s), then
not support the extension, then the contents of the Identity- the verifier takes no further action.
Extension header may be ignored.
Step 6: The handling of the message after the verification process depends on
how the implementation service is implemented, and on local policy.
The verifier MUST must furthermore ensure that the value of the Date This specification does not propose any authorization policy for user
header meets local policy for freshness (usually, within sixty agents or proxy servers to follow based on the presence of a valid
seconds) and that it falls within the validity period of the Identity header, the presence of an invalid Identity header, or the
credential used to sign the Identity header. For more on the attacks absence of an Identity header, but it is anticipated that local
this prevents, see Section 10.1. policies could involve making different forwarding decisions in
intermediary implementations, or changing how the user is alerted, or
how identity is rendered, in user agent implementations.
5. Credentials 5. 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 domain names or telephone numbers. These credentials may cover domain names or telephone numbers. These credentials may
represent authority over an entire domain (such as example.com) or represent authority over an entire domain (such as example.com) or
potentially a set of domains enumerated by the credential. potentially a set of domains enumerated by the credential.
Similarly, a credential may represent authority over a single Similarly, a credential may represent authority over a single
telephone number or a range of telephone numbers. The way that the telephone number or a range of telephone numbers. The way that the
scope of a credential is expressed is specific to the credential scope of a credential is expressed is specific to the credential
mechanism. mechanism.
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 identity field is a matter of local policy for the
for the authentication service, one that depends greatly on the authentication service, one that depends greatly on the manner in
manner in which authentication is performed. For non-telephone which authentication is performed. For non-telephone number user
number user parts, one policy might be as follows: the username given parts, one policy might be as follows: the username given in the
in the 'username' parameter of the Proxy-Authorization header MUST '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
Proxy-Authorization that do not correspond to the user-portion of SIP Proxy-Authorization that do not correspond to the user-portion of SIP
From headers, or a user might manage multiple accounts in the same From headers, or a user might manage multiple accounts in the same
administrative domain. In this latter case, a domain might maintain administrative domain. In this latter case, a domain might maintain
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
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In order to act as a verification service, a SIP entity must have a In order to act as a verification service, a SIP entity must have a
way to acquire and retain credentials for authorities over particular way to acquire and retain credentials for authorities over particular
domain names and/or telephone numbers or number ranges. domain names and/or telephone numbers or number ranges.
Dereferencing the Identity-Info header (as described in the next Dereferencing the Identity-Info header (as described in the next
section) MUST be supported by all verification service section) MUST be supported by all verification service
implementations to create a baseline means of credential acquisition. implementations to create a baseline means of credential acquisition.
Provided that the credential used to sign a message is not previously Provided that the credential used to sign a message is not previously
known to the verifier, SIP entities SHOULD discover this credential known to the verifier, SIP entities SHOULD discover this credential
by dereferencing the Identity-Info header, unless they have some more by dereferencing the Identity-Info header, unless they have some more
other implementation-specific way of acquiring the needed other implementation-specific way of acquiring the needed keying
certificates, such as an offline store of periodically-updated material, such as an offline store of periodically-updated
credentials. If the URI in the Identity-Info header cannot be credentials. If the URI in the Identity-Info header cannot be
dereferenced, then a 436 'Bad Identity-Info' response MUST be dereferenced, then a 436 'Bad Identity-Info' response MUST be
returned. returned.
This specification does not propose any particular policy for a
verification service to determine whether or not the holder of a
credential is the appropriate party to sign for a given SIP identity.
Guidance on this is deferred to the credential mechanism
specifications, which must meet the requirements in Section 5.4.
Verification service implementations supporting this specification Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials every time a request from the same identity is received. Credentials
cached in this manner may be indexed in accordance with local policy: cached in this manner may be indexed in accordance with local policy:
for example, by their scope, or the URI given in the Identity-Info for example, by their scope, or the URI given in the 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 mechanism specifications.
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 that contains the public key components of the credential resource that contains the public key components of the credential
used by the authentication service to sign a request. It is used by the authentication service to sign a request. It is
essential that a URI in the Identity-Info header be dereferencable by essential that a URI in the Identity-Info header be dereferencable by
any entity that could plausibly receive the request. For common any entity that could plausibly receive the request. For common
cases, this means that the URI must be dereferencable by any entity cases, this means that the URI must be dereferencable by any entity
on the public Internet. In constrained deployment environments, a on the public Internet. In constrained deployment environments, a
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identity and the signing credential by following the procedures identity and the signing credential by following the procedures
defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818] defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818]
deals with the use of HTTP in TLS and is specific to certificates, deals with the use of HTTP in TLS and is specific to certificates,
the procedures described are applicable to verifying identity if one the procedures described are applicable to verifying identity if one
substitutes the "hostname of the server" in HTTP for the domain substitutes the "hostname of the server" in HTTP for the domain
portion of the user's identity in the From header field of a SIP portion of the user's identity in the From header field of a SIP
request with an Identity header. request with an Identity header.
7. Header Syntax 7. Header Syntax
This document specifies four SIP headers: Identity, Identity- Baseline RFC4474 defined the Identity and Identity-Info headers.
Reliance, Identity- Info, and Identity-Extension. Each of these This document updates that specification and adds the optional
headers can appear only once in a SIP request; Identity-Reliance are Identity-Extension header (the grammar for which appears in
Identity-Extension are OPTIONAL, while Identity and Identity-Info are Section 8). Identity and Identity-Info are REQUIRED for securing
REQUIRED for securing requests with this specification. The grammar requests with this specification, and may appear only once in a
for the first three headers is (following the ABNF [RFC4234] in RFC request, while Identity-Extension can be present multiple times. The
3261 [RFC3261]): grammar for the first two headers is (following the ABNF [RFC4234] in
RFC 3261 [RFC3261]):
Identity = "Identity" HCOLON signed-identity-digest Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT *base64-char RDQUOT signed-identity-digest = LDQUOT *base64-char RDQUOT
Identity-Reliance = "Identity-Reliance" HCOLON signed-identity-reliance-digest
signed-identity-reliance-digest = LDQUOT *base64-char RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params ) *( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / canonical-str / ident-info-extension ident-info-params = ident-info-alg / canonical-str / ident-info-extension
ident-info-alg = "alg" EQUAL token ident-info-alg = "alg" EQUAL token
canonical-str = "canon" EQUAL tn-spec canonical-str = "canon" EQUAL tn-spec
ident-info-extension = generic-param ident-info-extension = generic-param
base64-char = ALPHA / DIGIT / "/" / "+" base64-char = ALPHA / DIGIT / "/" / "+"
The grammar for the Identity-Extension header field is given in This follows the original specification of Identity and Identity-Info
Section 8. in RFC4474, except for the addition of the "canon" parameter. Note
that in RFC4474, the signed-identity-digest was given as quoted
The signed-identity-reliance-digest is a signed hash of a canonical 32LHEX, whereas here it is given as a quoted sequence of base64-char.
string generated from certain components of a SIP request. Creating
this hash and the Identity-Reliance header field to contain it is
OPTIONAL, and its usage is a matter of local policy for
authentication services. To create the contents of the signed-
identity-reliance-digest, the following element of a SIP message MUST
be placed in a bit-exact string:
The body content of the message with the bits exactly as they are
in the message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that
the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that
follows that CRLF.
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
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including the algorithm description and hexadecimal key including the algorithm description and hexadecimal key
representation, any whitespace, carriage returns, and "/" line representation, any whitespace, carriage returns, and "/" line
break indicators. If multiple non-identical "a=fingerprint" break indicators. If multiple non-identical "a=fingerprint"
attributes appear in an SDP body, then all non-identical attributes appear in an SDP body, then all non-identical
attributes values MUST be concatenated, with no separating attributes values MUST be concatenated, with no separating
character, after sorting the values in alphanumeric order. If the character, after sorting the values in alphanumeric order. If the
SDP body contains no "a=fingerprint" attribute, the fifth element SDP body contains no "a=fingerprint" attribute, the fifth element
MUST be empty, containing no whitespace, resulting in a "||" in MUST be empty, containing no whitespace, resulting in a "||" in
the signed-identity-digest. the signed-identity-digest.
Sixth, the Identity-Extension header field value, if there is an Sixth, the Identity-Extension header field value(s), if there is
Identity-Extension header field in the request. If the message at least one Identity-Extension header field in the request. If
has no Identity-Extension header, then the sixth slot MUST be multiple Identity-Extension header fields are in the request, they
empty, containing no whitespace, resulting in a "||" in the MUST be concatenated after sorting the header field values in
signed-identity-digest. characters. alphanumeric order, with each entry separated by a vertical line,
"|" or %x7C. If the message contains no Identity-Extension
Seventh, the Identity-Reliance header field value, if there is an header, then the sixth slot MUST be empty, containing no
Identity-Reliance header field in the request. If the message has whitespace, resulting in a "||" in the signed-identity-digest.
no body, or no Identity-Reliance header, then the seventh slot characters.
MUST be empty, and the final "|" will not be followed by any
additional 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 10 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 ] "|" Method "|" SIP-date "|" [ sdp-fingerprint ] "|"
[ signed-identity-extension-digest ] "|" [ signed-identity-extension-digest ]
[ 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 6.1.1.
After the digest-string or reliance-digest-string is formed, each After the digest-string is formed, it MUST be hashed and signed with
MUST be hashed and signed with the certificate of authority over the the certificate of authority over the identity. The hashing and
identity. The hashing and signing algorithm is specified by the signing algorithm is specified by the 'alg' parameter of the
'alg' parameter of the Identity-Info header (see below for more Identity-Info header (see below for more information on Identity-Info
information on Identity-Info header parameters). This document header parameters). This document defines only one value for the
defines only one value for the 'alg' parameter: 'rsa-sha256'; further 'alg' parameter: 'rsa-sha256'; further values MUST be defined in a
values MUST be defined in a Standards Track RFC, see Section 11.3 for Standards Track RFC, see Section 11.3 for more information. All
more information. All implementations of this specification MUST implementations of this specification MUST support 'rsa-sha256'.
support 'rsa-sha256'. When the 'rsa-sha256' algorithm is specified When the 'rsa-sha256' algorithm is specified in the 'alg' parameter
in the 'alg' parameter of Identity-Info, the hash and signature MUST of Identity-Info, the hash and signature MUST be generated as
be generated as follows: compute the results of signing this string follows: compute the results of signing this string with
with sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and base64
base64 encode the results as specified in RFC 3548 [RFC3548]. A encode the results as specified in RFC 3548 [RFC3548]. A 2048-bit or
2048-bit or longer RSA key MUST be used. The result of the digest- longer RSA key MUST be used. The result of the digest-string hash is
string hash is placed in the Identity header field; the optional placed in the Identity header field.
reliance-digest-string hash goes in the Identity-Reliance header.
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.
8. Extensibility 8. Extensibility
As future requirements may warrant increasing the scope of the As future requirements may warrant increasing the scope of the
Identity mechanism, this specification defines an optional Identity- Identity mechanism, this specification defines an optional Identity-
Extension header. Each extension header field value MUST consist of Extension header. Each extension header field value MUST consist of
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developers on this point. developers on this point.
Identity-Extension = "Identity-Extension" HCOLON identity-extension-string Identity-Extension = "Identity-Extension" HCOLON identity-extension-string
identity-extension-string = identity-extension-name EQUAL *base64-char identity-extension-string = identity-extension-name EQUAL *base64-char
identity-extension-name = token identity-extension-name = token
signed-identity-extension-digest = LDQUOT *base64-char RDQUOT signed-identity-extension-digest = LDQUOT *base64-char RDQUOT
Defining a new Identity-Extension requires a Standards Action; see Defining a new Identity-Extension requires a Standards Action; see
Section 11.4. Section 11.4.
No provision is made in this specification for multiple extensions to
appear in a single SIP request.
9. Privacy Considerations 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
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process would fail, using P-Asserted-Identity with the Identity process would fail, using P-Asserted-Identity with the Identity
"canon" parameter in this fashion is NOT RECOMMENDED outside of "canon" parameter in this fashion is NOT RECOMMENDED outside of
environments where SIP requests will never leave the trust domain. environments where SIP requests will never leave the trust domain.
Finally, note that unlike [RFC3325], the mechanism described in this Finally, note that unlike [RFC3325], the mechanism described in this
specification adds no information to SIP requests that has privacy specification adds no information to SIP requests that has privacy
implications. implications.
10. Security Considerations 10. Security Considerations
10.1. 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 of SIP requests, parts of the To and From
From header fields of SIP requests, as well as optional protections header fields, the request method, and when present any media keying
for the message body. While a signature over the From header field material in the message body. In general, the considerations related
would be sufficient to secure a URI alone, the additional headers to the security of these headers are the same as those given in
provide replay protection and reference integrity necessary to make [RFC3261] for including headers in tunneled 'message/sip' MIME bodies
sure that the Identity header will not be replayed in cut-and-paste (see Section 23 in particular). The following section details the
attacks. In general, the considerations related to the security of individual security properties obtained by including each of these
these headers are the same as those given in [RFC3261] for including header fields within the signature; collectively, this set of header
headers in tunneled 'message/sip' MIME bodies (see Section 23 in fields provides the necessary properties to prevent impersonation.
particular). The following section details the individual security It adddresses the solution-specific attacks again in-band solutions
properties obtained by including each of these header fields within enumerated in [RFC7375] Section 4.1.
the signature; collectively, this set of header fields provides the
necessary properties to prevent impersonation.
The From header field indicates the identity of the sender of the 10.1. Protected Request Fields
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
the purposes of this document. The To header field provides the
identity of the SIP user that this request targets. Providing the To
header field in the Identity signature serves two purposes: first, it
prevents cut-and-paste attacks in which an Identity header from
legitimate request for one user is cut-and-pasted into a request for
a different user; second, it preserves the starting URI scheme of the
request, which helps prevent downgrade attacks against the use of
SIPS.
The Date header field provides replay protection, as described in The From header field value indicates the identity of the sender of
[RFC3261], Section 23.4.2. Implementations of this specification the message, and the SIP address-of-record URI, or an embedded
MUST NOT deem valid a request with an outdated Date header field (the telephone number, in the From header field is the identity of a SIP
RECOMMENDED interval is that the Date header must indicate a time user, for the purposes of this document. This is the key piece of
within 60 seconds of the receipt of a message). The result of this information that this mechanism secures; the remainder of the signed
is that if an Identity header is replayed within the Date interval, parts of a SIP request are present to provide reference integrity and
verifiers will recognize that it is invalid; if an Identity header is to prevent certain types of cut-and-paste attacks.
replayed after the Date interval, verifiers will recognize that it is
invalid because the Date is stale.
Without the method, an INVITE request could be cut- and-pasted by an The Date header field value protects against cut-and-paste attacks,
attacker and transformed into a MESSAGE request without changing any as described in [RFC3261], Section 23.4.2. Implementations of this
fields covered by the Identity header, and moreover requests within a specification MUST NOT deem valid a request with an outdated Date
transaction (for example, a re-INVITE) could be replayed in header field (the RECOMMENDED interval is that the Date header must
potentially confusing or malicious ways. indicate a time within 60 seconds of the receipt of a message). Note
that per baseline [RFC3261] behavior, servers keep state of recently
received requests, and thus if an Identity header is replayed by an
attacker within the Date interval, verifiers can detect that it is
spoofed; because a message with an identical Date from the same
source had recently been received.
RFC4474 originally had protections for the Contact, Call-ID and CSeq. The To header field value provides the identity of the SIP user that
These are removed from RFC4474bis. The absence of these header this request originally targeted. Providing the To header field in
values creates some opportunities for determined attackers to the Identity signature serves two purposes: first, it prevents cut-
impersonate based on cut-and-paste attacks; however, the absence of and-paste attacks in which an Identity header from legitimate request
these headers does not seem impactful to preventing the simple for one user is cut-and-pasted into a request for a different user;
unauthorized claiming of a From header field value, which is the second, it preserves the starting URI scheme of the request, which
primary scope of the current document. helps prevent downgrade attacks against the use of SIPS. The To
offers additional protection against cut-and-paste attacks beyond the
Date header field: for example, without a signature over the To, an
attacker who receives a call from a target could immediately forward
the INVITE to the target's voicemail service within the Date
interval, and the voicemail service would have no way knowing that
the Identity header it received had been originally signed for a call
intended for a different number. However, note the caveats below in
Section 10.1.1.
It might seem attractive to provide a signature over some of the Without the request method, an INVITE request could be cut- and-
information present in the Via header field value(s). For example, pasted by an attacker and transformed into a MESSAGE request without
without a signature over the sent-by field of the topmost Via header, changing any fields covered by the Identity header, and moreover
an attacker could remove that Via header and insert its own in a cut- requests within a transaction (for example, a re-INVITE) could be
and-paste attack, which would cause all responses to the request to replayed in potentially confusing or malicious ways to enable
be routed to a host of the attacker's choosing. However, a signature impersonation.
over the topmost Via header does not prevent attacks of this nature,
since the attacker could leave the topmost Via intact and merely
insert a new Via header field directly after it, which would cause
responses to be routed to the attacker's host "on their way" to the
valid host, which has exactly the same end result. Although it is
possible that an intermediary-based authentication service could
guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS.
When signing a request that contains a fingerprint of keying material When signing a request that contains a fingerprint of keying material
in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
signature over that fingerprint. This signature prevents certain signature over that fingerprint. This signature prevents certain
classes of impersonation attacks in which an attacker forwards or classes of impersonation attacks in which an attacker forwards or
cut-and-pastes a legitimate request: although the target of the cut-and-pastes a legitimate request: although the target of the
attack may accept the request, the attacker will be unable to attack may accept the request, the attacker will be unable to
exchange media with the target as they will not possess a key exchange media with the target as they will not possess a key
corresponding to the fingerprint. For example there are some baiting corresponding to the fingerprint. For example there are some baiting
attacks (where the attacker receives a request from the target and attacks, launched with the REFER method or through social
reoriginates it to a third party) that might not be prevented by only engineering, where the attacker receives a request from the target
a signature over the From, To and Date, but could be prevented by and reoriginates it to a third party: these might not be prevented by
securing a fingerprint for DTLS-SRTP. While this is a different form only a signature over the From, To and Date, but could be prevented
of interpretation than is commonly needed for robocalling, ultimately by securing a fingerprint for DTLS-SRTP. While this is a different
there is little purpose in establishing the identity of the user that form of impersonation than is commonly used for robocalling,
originated a SIP request if this assurance is not coupled with a ultimately there is little purpose in establishing the identity of
comparable assurance over the contents of the subsequent the user that originated a SIP request if this assurance is not
communication. This signature also, per [RFC7258], reduces the coupled with a comparable assurance over the contents of the
potential for passive monitoring attacks against the SIP media. In subsequent communication. This signature also, per [RFC7258],
environments where DTLS-SRTP is unsupported, however, this mechanism reduces the potential for passive monitoring attacks against the SIP
is not exercised and no protections are provided. media. In environments where DTLS-SRTP is unsupported, however, no
field is signed and no protections are provided.
This mechanism also provides an optional full signature over the
bodies of SIP requests. This can help to protect non-INVITE
transactions such as MESSAGE or NOTIFY, as well as INVITEs in those
environments where intermediaries do not change SDP. Note, however,
that this is not perfect end-to-end security. The authentication
service itself, when instantiated at an intermediary, could
conceivably change the body (and SIP headers, for that matter) before
providing a signature. Thus, while this mechanism reduces the chance
that a replayer or man-in-the-middle will modify bodies, it does not
eliminate it entirely. Since it is a foundational assumption of this
mechanism that the users trust their local domain to vouch for their
security, they must also trust the service not to violate the
integrity of their message without good reason.
In the end analysis, the Identity, Identity-Reliance and Identity-
Info headers cannot protect themselves. Any attacker could remove
these headers from a SIP request, and modify the request arbitrarily
afterwards. However, this mechanism is not intended to protect
requests from men-in-the- middle who interfere with SIP messages; it
is intended only to provide a way that the originators of SIP
requests can prove that they are who they claim to be. At best, by
stripping identity information from a request, a man-in-the-middle
could make it impossible to distinguish any illegitimate messages he
would like to send from those messages sent by an authorized user.
However, it requires a considerably greater amount of energy to mount
such an attack than it does to mount trivial impersonations by just
copying someone else's From header field. This mechanism provides a
way that an authorized user can provide a definitive assurance of his
identity that an unauthorized user, an impersonator, cannot.
One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. Once again, it is
important to note that preventing men-in-the-middle is not the
primary impetus for this mechanism. Moreover, changing the 'alg'
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'
parameter is defined in this document and that thus there is
currently no weaker algorithm to which the mechanism can be bid down.
'alg' has been incorporated into this mechanism for forward-
compatibility reasons in case the current algorithm exhibits
weaknesses, and requires swift replacement, in the future.
10.1.1. Protection of the To Header and Retargeting 10.1.1. Protection of the To Header and Retargeting
The mechanism in this document provides a signature over the identity The mechanism in this document provides a signature over the identity
information in the To header field value of requests. This provides information in the To header field value of requests. This provides
a means for verifiers to detect replay attacks where a signed request a means for verifiers to detect replay attacks where a signed request
originally sent to one target is modified and then forwarded by an originally sent to one target is modified and then forwarded by an
attacker to another, unrelated target. Armed with the original value attacker to another, unrelated target. Armed with the original value
of the To header field, the recipient of a request may compare it to of the To header field, the recipient of a request may compare it to
their own identity in order to determine whether or not the identity their own identity in order to determine whether or not the identity
skipping to change at page 27, line 13 skipping to change at page 25, line 28
guidance is given for implementers here regarding the 'connected guidance is given for implementers here regarding the 'connected
party' problem (see [RFC4916]); authentication service behavior is party' problem (see [RFC4916]); authentication service behavior is
unchanged if retargeting has occurred for a dialog-forming request. unchanged if retargeting has occurred for a dialog-forming request.
Ultimately, the authentication service provides an Identity header Ultimately, the authentication service provides an Identity header
for requests in the backwards dialog when the user is authorized to for requests in the backwards dialog when the user is authorized to
assert the identity given in the From header field, and if they are assert the identity given in the From header field, and if they are
not, an Identity header is not provided. And per the threat model of not, an Identity header is not provided. And per the threat model of
[RFC7375], resolving problems with 'connected' identity has little [RFC7375], resolving problems with 'connected' identity has little
bearing on detecting robocalling or related impersonation attacks. bearing on detecting robocalling or related impersonation attacks.
10.2. Securing the Connection to the Authentication Service 10.2. Unprotected Request Fields
RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing the simple
unauthorized claiming of an identity for the purposes of robocalling,
voicemail hacking, or swatting, which is the primary scope of the
current document.
It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert its own in a cut-
and-paste attack, which would cause all responses to the request to
be routed to a host of the attacker's choosing. However, a signature
over the topmost Via header does not prevent attacks of this nature,
since the attacker could leave the topmost Via intact and merely
insert a new Via header field directly after it, which would cause
responses to be routed to the attacker's host "on their way" to the
valid host, which has exactly the same end result. Although it is
possible that an intermediary-based authentication service could
guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS.
10.3. Malicious Removal of Identity Headers
In the end analysis, the Identity, Identity-Info and Identity-
Extension headers cannot protect themselves. Any attacker could
remove these headers from a SIP request, and modify the request
arbitrarily afterwards. However, this mechanism is not intended to
protect requests from men-in-the-middle who interfere with SIP
messages; it is intended only to provide a way that the originators
of SIP requests can prove that they are who they claim to be. At
best, by stripping identity information from a request, a man-in-the-
middle could make it impossible to distinguish any illegitimate
messages he would like to send from those messages sent by an
authorized user. However, it requires a considerably greater amount
of energy to mount such an attack than it does to mount trivial
impersonations by just copying someone else's From header field.
This mechanism provides a way that an authorized user can provide a
definitive assurance of his identity that an unauthorized user, an
impersonator, cannot.
One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. Once again, it is
important to note that preventing men-in-the-middle is not the
motivation for this mechanism. Moreover, changing the 'alg' 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' parameter is
defined in this document and that thus there is currently no weaker
algorithm to which the mechanism can be bid down. 'alg' has been
incorporated into this mechanism for forward-compatibility reasons in
case the current algorithm exhibits weaknesses, and requires swift
replacement, in the future.
10.4. Securing the Connection to the Authentication Service
In the absence of user agent-based authentication services, the In the absence of user agent-based authentication services, the
assurance provided by this mechanism is strongest when a user agent assurance provided by this mechanism is strongest when a user agent
forms a direct connection, preferably one secured by TLS, to an forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are intermediary-based authentication service. The reasons for this are
twofold: twofold:
If a user does not receive a certificate from the authentication If a user does not receive a certificate from the authentication
service over the TLS connection that corresponds to the expected service over the TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a domain (especially when the user receives a challenge via a
skipping to change at page 28, line 11 skipping to change at page 27, line 45
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 10.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the authentication service is limited by the security practices of the authentication service
that issues the assurance. Relying on an Identity header generated that issues the assurance. Relying on an Identity header generated
by a remote administrative domain assumes that the issuing domain by a remote administrative domain assumes that the issuing domain
uses recommended administrative practices to authenticate its users. uses recommended administrative practices to authenticate its users.
However, it is possible that some authentication services will However, it is possible that some authentication services will
implement policies that effectively make users unaccountable (e.g., implement policies that effectively make users unaccountable (e.g.,
ones that accept unauthenticated registrations from arbitrary users). ones that accept unauthenticated registrations from arbitrary users).
The value of an Identity header from such authentication services is The value of an Identity header from such authentication services is
questionable. While there is no magic way for a verifier to questionable. While there is no magic way for a verifier to
distinguish "good" from "bad" signers by inspecting a SIP request, it distinguish "good" from "bad" signers by inspecting a SIP request, it
is expected that further work in authorization practices could be is expected that further work in authorization practices could be
built on top of this identity solution; without such an identity built on top of this identity solution; without such an identity
solution, many promising approaches to authorization policy are solution, many promising approaches to authorization policy are
impossible. That much said, it is RECOMMENDED that authentication impossible. That much said, it is RECOMMENDED that authentication
skipping to change at page 29, line 14 skipping to change at page 29, line 5
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 10.6. Display-Names and Identity
As a matter of interface design, SIP user agents might render the As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this component of the From header field value is outside the scope of this
document, but may be the subject of future work. document, but may be the subject of future work, such as an extension.
11. IANA Considerations 11. 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 11.1. Header Field Names
This document specifies a new SIP header called Identity-Reliance.
Its syntax is given in Section 7. This header is defined by the
following information, which has been added to the header sub-
registry under http://www.iana.org/assignments/sip-parameters
Header Name: Identity-Reliance
Compact Form: N/A
This document also specifies a new SIP header called Identity- This document also specifies a new SIP header called Identity-
Extension. Its syntax is given in Section 8. A registry for Extension. Its syntax is given in Section 8. A registry for
Identity-Extension names is defined in Section 11.4. Identity-Extension names is defined in Section 11.4.
Header Name: Identity-Extension Header Name: Identity-Extension
Compact Form: N/A Compact Form: N/A
11.2. Identity-Info Parameters 11.2. Identity-Info Parameters
The IANA has already created a registry for Identity-Info header The IANA has already created a registry for Identity-Info header
skipping to change at page 31, line 18 skipping to change at page 30, line 40
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-Extension header and extensibility mechanism Added the Identity-Extension header and extensibility mechanism
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
14. References 14. References
14.1. Normative References 14.1. Normative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [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. DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[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,
2002. DOI 10.17487/RFC3263, June 2002,
<http://www.rfc-editor.org/info/rfc3263>.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280, Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002. DOI 10.17487/RFC3280, April 2002,
<http://www.rfc-editor.org/info/rfc3280>.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002. Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
<http://www.rfc-editor.org/info/rfc3370>.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
3966, December 2004. RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
14.2. Informative References 14.2. Informative References
[I-D.ietf-stir-certificates] [I-D.ietf-stir-certificates]
Peterson, J., "Secure Telephone Identity Credentials: Peterson, J., "Secure Telephone Identity Credentials:
Certificates", draft-ietf-stir-certificates-01 (work in Certificates", draft-ietf-stir-certificates-02 (work in
progress), March 2015. progress), July 2015.
[I-D.kaplan-stir-cider] [I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00 Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013. (work in progress), July 2013.
[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.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",
2585, May 1999. RFC 2585, DOI 10.17487/RFC2585, May 1999,
<http://www.rfc-editor.org/info/rfc2585>.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session [RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002. Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<http://www.rfc-editor.org/info/rfc3323>.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325, Asserted Identity within Trusted Networks", RFC 3325,
November 2002. DOI 10.17487/RFC3325, November 2002,
<http://www.rfc-editor.org/info/rfc3325>.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC3548] Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003. Encodings", RFC 3548, DOI 10.17487/RFC3548, July 2003,
<http://www.rfc-editor.org/info/rfc3548>.
[RFC3893] Peterson, J., "Session Initiation Protocol (SIP) [RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893, Authenticated Identity Body (AIB) Format", RFC 3893,
September 2004. DOI 10.17487/RFC3893, September 2004,
<http://www.rfc-editor.org/info/rfc3893>.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
October 2005, <http://www.rfc-editor.org/info/rfc4234>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for [RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006. Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
[RFC4501] Josefsson, S., "Domain Name System Uniform Resource [RFC4501] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, May 2006. Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<http://www.rfc-editor.org/info/rfc4501>.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation [RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007. Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
2007, <http://www.rfc-editor.org/info/rfc4916>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer (SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010. Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012. Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July Considerations for Internet Protocols", RFC 6973,
2013. DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[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, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements", Telephone Identity Problem Statement and Requirements",
RFC 7340, September 2014. RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model", [RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, October 2014. RFC 7375, DOI 10.17487/RFC7375, October 2014,
<http://www.rfc-editor.org/info/rfc7375>.
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
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