draft-ietf-stir-rfc4474bis-01.txt   draft-ietf-stir-rfc4474bis-02.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 5, 2015 Cisco Expires: April 25, 2015 Cisco
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
July 4, 2014 October 22, 2014
Authenticated Identity Management in the Session Initiation Protocol Authenticated Identity Management in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-stir-rfc4474bis-01.txt draft-ietf-stir-rfc4474bis-02.txt
Abstract Abstract
The baseline security mechanisms in the Session Initiation Protocol The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of (SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining new identifying originators of SIP requests. It does so by defining new
SIP header fields for conveying a signature used for validating the SIP header fields for conveying a signature used for validating the
identity, and for conveying a reference to the credentials of the identity, and for conveying a reference to the credentials of the
skipping to change at page 1, line 41 skipping to change at page 1, line 41
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This Internet-Draft will expire on January 5, 2015. This Internet-Draft will expire on April 25, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 5 3. Overview of Operations . . . . . . . . . . . . . . . . . . . 5
4. Signature Generation and Validation . . . . . . . . . . . . . 6 4. Signature Generation and Validation . . . . . . . . . . . . . 6
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6 4.1. Authentication Service Behavior . . . . . . . . . . . . . 6
4.1.1. Intermediary Authentication Services . . . . . . . . 9 4.1.1. Intermediary Authentication Services . . . . . . . . 9
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10 4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10
4.3. Identity within a Dialog and Retargeting . . . . . . . . 11 4.3. Identity within a Dialog and Retargeting . . . . . . . . 12
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Credential Use by the Authentication Service . . . . . . 12 5.1. Credential Use by the Authentication Service . . . . . . 13
5.2. Credential Use by the Verification Service . . . . . . . 13 5.2. Credential Use by the Verification Service . . . . . . . 14
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 14 5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 14
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 15 5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 15
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 16 6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 16 6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 16
6.2. Usernames with Domain Names . . . . . . . . . . . . . . . 18 6.2. Usernames with Domain Names . . . . . . . . . . . . . . . 18
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19 7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10.1. Handling of digest-string Elements . . . . . . . . . . . 23 10.1. Handling of digest-string Elements . . . . . . . . . . . 24
10.2. Securing the Connection to the Authentication Service . 26 10.2. Securing the Connection to the Authentication Service . 27
10.3. Authorization and Transitional Strategies . . . . . . . 27 10.3. Authorization and Transitional Strategies . . . . . . . 28
10.4. Display-Names and Identity . . . . . . . . . . . . . . . 28 10.4. Display-Names and Identity . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 28 11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 29
11.2. 428 'Use Identity Header' Response Code . . . . . . . . 29 11.2. Identity-Info Parameters . . . . . . . . . . . . . . . . 29
11.3. 436 'Bad Identity-Info' Response Code . . . . . . . . . 29 11.3. Identity-Info Algorithm Parameter Values . . . . . . . . 29
11.4. 437 'Unsupported Credential' Response Code . . . . . . . 29
11.5. 438 'Invalid Identity Header' Response Code . . . . . . 30
11.6. Identity-Info Parameters . . . . . . . . . . . . . . . . 30
11.7. Identity-Info Algorithm Parameter Values . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 31 13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 30
14. Informative References . . . . . . . . . . . . . . . . . . . 31 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
14.1. Normative References . . . . . . . . . . . . . . . . . . 30
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, RFC 3261 [RFC3261]). An identity, for the purposes of this (SIP, [RFC3261]). An identity, for the purposes of this document, is
document, is defined as either a SIP URI, commonly a canonical defined as either a SIP URI, commonly a canonical address-of-record
address-of-record (AoR) employed to reach a user (such as (AoR) employed to reach a user (such as
'sip:alice@atlanta.example.com'), or a telephone number, which can be 'sip:alice@atlanta.example.com'), or a telephone number, which can be
represented as either a TEL URI or as the user portion of a SIP URI. represented as either a TEL URI [RFC3966] or as the user portion of a
SIP URI.
RFC 3261 [RFC3261] stipulates several places within a SIP request [RFC3261] stipulates several places within a SIP request where users
where a user can express an identity for themselves, notably the can express an identity for themselves, primarily the user-populated
user-populated From header field. However, the recipient of a SIP From header field. However, the recipient of a SIP request has no
request has no way to verify that the From header field has been way to verify that the From header field has been populated
populated appropriately, in the absence of some sort of cryptographic appropriately, in the absence of some sort of cryptographic
authentication mechanism. authentication mechanism. This leaves SIP vulnerable to a category
of abuses, including impersonation attacks that enable robocalling
and related problems as described in [RFC7340].
RFC 3261 [RFC3261] specifies a number of security mechanisms that can [RFC3261] specifies a number of security mechanisms that can be
be employed by SIP user agents (UAs), including Digest, Transport employed by SIP user agents (UAs), including Digest, Transport Layer
Layer Security (TLS), and S/MIME (implementations may support other Security (TLS), and S/MIME (implementations may support other
security schemes as well). However, few SIP user agents today security schemes as well). However, few SIP user agents today
support the end-user certificates necessary to authenticate support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and furthermore Digest themselves (via S/MIME, for example), and furthermore Digest
authentication is limited by the fact that the originator and authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which user agents to be able to send requests to destinations with which
they have no previous association -- just as in the telephone network they have no previous association -- just as in the telephone network
today, one can receive a call from someone with whom one has no today, one can receive a call from someone with whom one has no
previous association, and still have a reasonable assurance that the previous association, and still have a reasonable assurance that the
person's displayed calling party number (and/or Caller-ID) is person's displayed calling party number (and/or Caller-ID) is
accurate. A cryptographic approach, like the one described in this accurate. A cryptographic approach, like the one described in this
document, can provide a much stronger and less spoofable assurance of document, can provide a much stronger and less spoofable assurance of
identity than the telephone network provides today. identity than the telephone network provides today.
[RFC4474] previously specified a means of signing portions of SIP
requests in order to provide that identity assurance. However, RFC
4474 was in several ways misaligned with deployment realities (see
[I-D.rosenberg-sip-rfc4474-concerns]). Most significantly, RFC 4474
did not deal well with telephone numbers as identifiers, despite
their enduring use in SIP deployments. RFC 4474 also provided a
signature over material that intermediaries in the field commonly
altered. This specification therefore revises RFC 4474 in light of
recent reconsideration of the problem space to align with the threat
model in [RFC 7375].
2. Background 2. Background
The usage of many SIP applications and services is governed by The secure operation of many SIP applications and services depends on
authorization policies. These policies may be automated, or they may authorization policies. These policies may be automated, or they may
be applied manually by humans. An example of the latter would be an be exercised manually by humans. An example of the latter would be
Internet telephone application that displays the calling party number an Internet telephone application that displays the calling party
(and/or Caller-ID) of a caller, which a human may review to make a number (and/or Caller-ID) of a caller, which a human may review to
policy decision before answering a call. An example of the former make a policy decision before answering a call. An example of the
would be a voicemail service that compares the identity of the caller former would be a voicemail service that compares the identity of the
to a whitelist before determining whether it should allow the caller caller to a whitelist before determining whether it should allow the
access to recorded messages. In both of these cases, attackers might caller access to recorded messages. In both of these cases,
attempt to circumvent these authorization policies through attackers might attempt to circumvent these authorization policies
impersonation. Since the primary identifier of the sender of a SIP through impersonation. Since the primary identifier of the sender of
request, the From header field, can be populated arbitrarily by the a SIP request, the From header field, can be populated arbitrarily by
controller of a user agent, impersonation is very simple today. The the controller of a user agent, impersonation is very simple today.
mechanism described in this document provides a strong identity The mechanism described in this document provides a strong identity
system for SIP requests in which authorization policies cannot be system for SIP requests in which authorization policies cannot be
circumvented by impersonation. circumvented by impersonation.
This document proposes an authentication architecture for SIP in This document proposes an authentication architecture for SIP in
which requests are processed by a logical authentication service that which requests are processed by a logical authentication service that
may be implemented as part of a user agent or as a proxy server. may be implemented as part of a user agent or as a proxy server.
Once a message has been authenticated, the service then adds new Once a message has been authenticated, the service then adds new
cryptographic information to requests to communicate to other SIP cryptographic information to requests to communicate to other SIP
entities that the sending user has been authenticated and its use of entities that the sending user has been authenticated and its use of
the From header field has been authorized. the From header field has been authorized.
But authorized by whom? Identities are issued to users by But authorized by whom? Identities are issued to users by
authorities. When a new user becomes associated with example.com, authorities. When a new user becomes associated with example.com,
the administrator of the SIP service for that domain will issue them the administrator of the SIP service for that domain will issue them
an identity in that namespace, such as alice@example.com. Alice may an identity in that namespace, such as alice@example.com. Alice may
then send REGISTER requests to example.com that make her user agents then send REGISTER requests to example.com that make her user agents
eligible to receive requests for sip:alice@example.com. In some eligible to receive requests for sip:alice@example.com. In some
cases, Alice may be the owner of the domain herself, and may issue cases, Alice may be the owner of the domain herself, and may issue
herself identities as she chooses. But ultimately, it is the herself identities as she chooses. But ultimately, it is the
controller of the SIP service at example.com that must be responsible controller of the SIP service at example.com that must be responsible
authorizing the use of names in the example.com domain. Therefore, for authorizing the use of names in the example.com domain.
the credentials needed to prove this authorization must ultimately Therefore, for the purposes of this specification, the credentials
derive from the domain owner: either a user agent gives requests to needed to prove a user is authorized to use a particular From header
the domain name owner in order for them to be signed by the domain field must ultimately derive from the domain owner: either a user
owner's credentials, or the user agent must possess credentials that agent gives requests to the domain name owner in order for them to be
prove in some fashion that the domain owner has given the user agent signed by the domain owner's credentials, or the user agent must
the right to a name. possess credentials that prove in some fashion that the domain owner
has given the user agent the right to a name.
The situation is however more complicated for telephone numbers. The situation is however more complicated for telephone numbers.
Authority over telephone numbers does not correspond directly to Authority over telephone numbers does not correspond directly to
Internet domains. While a user could register at a SIP domain with a Internet domains. While a user could register at a SIP domain with a
username that corresponds to a telephone number, any connection username that corresponds to a telephone number, any connection
between the administrator of that domain and the assignment of between the administrator of that domain and the assignment of
telephone numbers is not currently reflected on the Internet. telephone numbers is not currently reflected on the Internet.
Telephone numbers do not share the domain-scope property described Telephone numbers do not share the domain-scope property described
above, as they are dialed without any domain component. This above, as they are dialed without any domain component. This
document thus assumes the existence of a separate means of 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.
that is based on the authentication service adding a SIP header, the It relies on the authentication service adding to requests a SIP
Identity header. In order to assist in the validation of this header, the Identity header, which contains that cryptographic
assurance, this specification also describes an Identity-Info header assurance. In order to assist in the validation of the Identity
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. 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 identity services and to verify identities. To maximize end- provide the authentication service function and/or to verify
to-end security, it is obviously preferable for end-users to acquire identities. To maximize end-to-end security, it is obviously
their own credentials; if they do, their user agents can act as an preferable for end-users to acquire their own credentials; if they
authentication service. However, end-user credentials may be neither do, their user agents can act as authentication services. However,
practical nor affordable, given the potentially large number of SIP for some deployments end-user credentials may be neither practical
user agents (phones, PCs, laptops, PDAs, gaming devices) that may be nor affordable, given the potentially large number of SIP user agents
employed by a single user. In such environments, synchronizing (phones, PCs, laptops, PDAs, gaming devices) that may be employed by
keying material across multiple devices may be very complex and a single user. In such environments, synchronizing keying material
requires quite a good deal of additional endpoint behavior. Managing across multiple devices may be prohobitively complex and require
several credentials for the various devices could also be burdensome. quite a good deal of additional endpoint behavior. Managing several
This trade-off needs to be understood by implementers of this credentials for the various devices could also be burdensome. In
specification. these cases, implementation the authentication service at an
intermediary may be more practical. This trade-off needs to be
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.
Imagine the case where Alice, who has the home proxy of example.com Imagine a case where Alice, who has the home proxy of example.com and
and the address-of-record sip:alice@example.com, wants to communicate the address-of-record sip:alice@example.com, wants to communicate
with sip:bob@example.org. with Bob at sip:bob@example.org. They have no prior relationship,
and Bob implements best practices to prevent impersonation attacks.
Alice generates an INVITE and places her identity in the From header Alice generates an INVITE and places her identity, in this case her
field of the request. She then sends an INVITE over TLS to an address-of-record, in the From header field of the request. She then
authentication service proxy for her domain. sends an INVITE over TLS to an authentication service proxy for the
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 is populated in the From header field. to assert the identity that she populated in the From header field.
This value may be Alice's AoR, or in other cases it may 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. It then computes a hash over some particular telephone number. The proxy then computes a hash over some
headers, including the From header field (and optionally the body) of particular headers, including the From header field (and optionally
the message. This hash is signed with the appropriate credential the body) of the message. This hash is signed with the appropriate
(example.com, in the sip:alice@example.com case) and inserted in a credential for the identity (example.com, in the
new header field in the SIP message, the 'Identity' header. sip:alice@example.com case) and inserted in a new header field in the
SIP message, the 'Identity' header.
The proxy, as the holder of the private key for its domain, is The proxy, as the holder of the private key for the example.com
asserting that the originator of this request has been authenticated domain, is asserting that the originator of this request has been
and that she is authorized to claim the identity (the SIP address- authenticated and that she is authorized to claim the identity that
of-record) that appears in the From header field. The proxy also appears in the From header field. The proxy also inserts a companion
inserts a companion header field, Identity-Info, that tells Bob how header field, Identity-Info, that tells Bob how to acquire keying
to acquire keying material necessary to validate its credentials, if material necessary to validate its credentials (a public key), if he
he doesn't already have it. 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). agent server (UAS). As the request has been validated, it is
rendered to Bob. If the validation was unsuccessful, some other
treatment would be applied by the receiving domain.
4. Signature Generation and Validation 4. Signature Generation and Validation
4.1. Authentication Service Behavior 4.1. Authentication Service Behavior
This document specifies a role for SIP entities called an This document specifies a role for SIP entities called an
authentication service. The authentication service role can be authentication service. The authentication service role can be
instantiated by an intermediary such as a proxy server or a user instantiated by an intermediary such as a proxy server or by a user
agent. Any entity that instantiates the authentication service role agent. Any entity that instantiates the authentication service role
MUST possess the private key of one or more credentials that can be MUST possess the private key of one or more credentials that can be
used to sign for a domain or a telephone number (see Section 5.1). used to sign for a domain or a telephone number (see Section 5.1).
Intermediaries that instantiate this role MUST be capable of Intermediaries that instantiate this role MUST be capable of
authenticating one or more SIP users who can register for that authenticating one or more SIP users who can register for that
identity. Commonly, this role will be instantiated by a proxy identity. Commonly, this role will be instantiated by a proxy
server, since these entities are more likely to have a static server, since these entities are more likely to have a static
hostname, hold corresponding credentials, and have access to SIP hostname, hold corresponding credentials, and have access to SIP
registrar capabilities that allow them to authenticate users. It is registrar capabilities that allow them to authenticate users. It is
also possible that the authentication service role might be also possible that the authentication service role might be
skipping to change at page 7, line 9 skipping to change at page 7, line 31
The authentication service MUST extract the identity of the sender The authentication service MUST extract the identity of the sender
from the request. The authentication service takes this value from from the request. The authentication service takes this value from
the From header field; this AoR will be referred to here as the the From header field; this AoR will be referred to here as the
'identity field'. If the identity field contains a SIP or SIP Secure 'identity field'. If the identity field contains a SIP or SIP Secure
(SIPS) URI, and the user portion is not a telephone number, the (SIPS) URI, and the user portion is not a telephone number, the
authentication service MUST extract the hostname portion of the authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is identity field and compare it to the domain(s) for which it is
responsible (following the procedures in RFC 3261 [RFC3261], responsible (following the procedures in RFC 3261 [RFC3261],
Section 16.4), used by a proxy server to determine the domain(s) for Section 16.4), used by a proxy server to determine the domain(s) for
which it is responsible). If the identity field uses the TEL URI which it is responsible). If the identity field uses the TEL URI
scheme, or the identity field is a SIP or SIPS URI with a telephone scheme [RFC3966], or the identity field is a SIP or SIPS URI with a
number in the user portion, the authentication service determines telephone number in the user portion, the authentication service
whether or not it is responsible for this telephone number; see determines whether or not it is responsible for this telephone
Section 6.1 for more information. If the authentication service is number; see Section 6.1 for more information. If the authentication
not authoritative for the identity in question, it SHOULD process and service is not authoritative for the identity in question, it SHOULD
forward the request normally, but it MUST NOT following the steps process and forward the request normally, but it MUST NOT following
below to add an Identity header; see below for more information on the steps below to add an Identity header; see below for more
authentication service handling of an existing Identity header. information on authentication service handling of an existing
[where?] Identity header. [where?]
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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header cannot be dereferenced, then a 436 'Bad Identity-Info' header cannot be dereferenced, then a 436 'Bad Identity-Info'
response MUST be returned. response MUST be returned.
Verification service implementations supporting this specification Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials every time a request from the same identity is received. Credentials
cached in this manner max be indexed in accordance with local policy: cached in this manner max 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. header field value. Further consideration of how to cache
credentials is deferred to the credential mechanisms.
[TBD: Should we add some kind of hash or similar indication to the
Identity-Info header to make it easier for verifiers to ascertain
that they already possess a credential without dereferencing the
URI?]
5.3. Handling Identity-Info URIs 5.3. Handling Identity-Info URIs
An Identity-Info header MUST contain a URI which dereferences to a An Identity-Info header MUST contain a URI which dereferences to a
resource which contains the public key components of the credential resource which contains the public key components of the credential
used by the authentication service to sign a request. Much as is the used by the authentication service to sign a request. Much as is the
case with the trust anchor(s) required for deployments of this case with the trust anchor(s) required for deployments of this
specification, it is essential that a URI in the Identity-Info header specification, it is essential that a URI in the Identity-Info header
be dereferencable by any entity that could plausibly receive the be dereferencable by any entity that could plausibly receive the
request. For common cases, this means that the URI must be request. For common cases, this means that the URI must be
skipping to change at page 14, line 40 skipping to change at page 15, line 7
are potentially multiple authorities eligible to sign. For example, are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service imagine a case where a domain implements the authentication service
role for example.com, and a user agent belonging to Alice has role for example.com, and a user agent belonging to Alice has
acquired a credential for alice@example.com. Either would be acquired a credential for alice@example.com. Either would be
eligible to sign a SIP request from alice@example.com. Verification eligible to sign a SIP request from alice@example.com. Verification
services however need a means to differentiate which one performed services however need a means to differentiate which one performed
the signature. The Identity-Info header performs that function. the signature. The Identity-Info header performs that function.
If the optional "canon" parameter is present, it contains the result If the optional "canon" parameter is present, it contains the result
of the number canonicalization process performed by the of the number canonicalization process performed by the
authentication service (see Section 6.1). This value is provided authentication service (see Section 6.1) on the identity in the From.
purely informationally as an optimization for the verification This value is provided purely informationally as an optimization for
service. The verification service MAY compute its own the verification service. The verification service MAY compute its
canonicalization of the number and compare this to the value in the own canonicalization of the number and compare this to the value in
"canon" parameter before performing any cryptographic functions in the "canon" parameter before performing any cryptographic functions
order to ascertain whether or not the two ends agree on the canonical in order to ascertain whether or not the two ends agree on the
number form. canonical number form.
5.4. Credential Systems 5.4. Credential Systems
This document makes no specific recommendation for the use of any This document makes no specific recommendation for the use of any
credential system. Today, there are two primary credential systems credential system. Today, there are two primary credential systems
in place for proving ownership of domain names: certificates (e.g., in place for proving ownership of domain names: certificates (e.g.,
X.509 v3, see [RFC5280]) and the domain name system itself (e.g., X.509 v3, see [RFC5280]) and the domain name system itself (e.g.,
DANE, see [RFC6698]). It is envisioned that either could be used in DANE, see [RFC6698]). It is envisioned that either could be used in
the SIP context: an Identity-Info header could for example give an the SIP context: an Identity-Info header could for example give an
HTTP URL of the form 'application/pkix-cert' pointing to a HTTP URL of the form 'application/pkix-cert' pointing to a
certificate (following the conventions of [RFC2585]). The Identity- certificate (following the conventions of [RFC2585]). The Identity-
Info headers may use the DNS URL scheme (see [RFC4501]( to indicate Info headers may use the DNS URL scheme (see [RFC4501]( to indicate
keys in the DNS. keys in the DNS.
While no comparable public credentials exist for telephone numbers, While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential either approach could be applied to telephone numbers. A credential
system based on certificates is given in draft-peterson-stir- system based on certificates is given in
certificates [TBD - fix after submitting]. One based on the domain [I-D.peterson-stir-certificates]. One based on the domain name
name system is given in [I-D.kaplan-stir-cider]. system is given in [I-D.kaplan-stir-cider].
In order for a credential system to work with this mechanism, its In order for a credential system to work with this mechanism, its
specification must detail: specification must detail:
which URIs schemes the credential will use in the Identity-Info which URIs schemes the credential will use in the Identity-Info
header, and any special procedures required to dereference the header, and any special procedures required to dereference the
URIs URIs
how the verifier can learn the scope of the credential. how the verifier can learn the scope of the credential.
any special procedures required to extract keying material from any special procedures required to extract keying material from
the resources designated by the URI the resources designated by the URI
any algorithms that would appear in the Identity-Info "alg" any algorithms that would appear in the Identity-Info "alg"
parameter other than 'rsa-sha256.' Note that per the IANA parameter other than 'rsa-sha256.' Note that per the IANA
Considerations of this document (Section 11.7), new algorithms can Considerations of RFC 4474, new algorithms can only be specified
only be specified by Standards Action. by Standards Action.
SIP entities cannot reliably predict where SIP requests will SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar trust anchors is assured by business arrangements or similar
constraints. constraints.
Note that credential systems must address key lifecycle management Note that credential systems must address key lifecycle management
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specification, it is therefore essential that the trust anchor(s) for specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar trust anchors is assured by business arrangements or similar
constraints. constraints.
Note that credential systems must address key lifecycle management Note that credential systems must address key lifecycle management
concerns: were a domain to change the credential available at the concerns: were a domain to change the credential available at the
Identity-Info URI before a verifier evaluates a request signed by an Identity-Info URI before a verifier evaluates a request signed by an
authentication service, this would cause obvious verifier failures. authentication service, this would cause obvious verifier failures.
When a rollover occurs, authentication services SHOULD thus provide When a rollover occurs, authentication services SHOULD thus provide
new Identity-Info URIs for each new credential, and SHOULD continue new Identity-Info URIs for each new credential, and SHOULD continue
to make older key acquisition URIs available for a duration longer to make older key acquisition URIs available for a duration longer
than the plausible lifetime of a SIP message (an hour would most than the plausible lifetime of a SIP message (an hour would most
likely suffice). likely suffice).
[TBD: What will the normative language here be? Support for which
mechanisms?]
6. Identity Types 6. Identity Types
6.1. Telephone Numbers 6.1. Telephone Numbers
Since many SIP applications provide a Voice over IP (VoIP) service, Since many SIP applications provide a Voice over IP (VoIP) service,
telephone numbers are commonly used as identities in SIP deployments. telephone numbers are commonly used as identities in SIP deployments.
In order for telephone numbers to be used with the mechanism In order for telephone numbers to be used with the mechanism
described in this document, authentication services must enroll with described in this document, authentication services must enroll with
an authority that issues credentials for telephone numbers or an authority that issues credentials for telephone numbers or
telephone number ranges, and verification services must trust the telephone number ranges, and verification services must trust the
skipping to change at page 17, line 47 skipping to change at page 18, line 7
code and region code digits; another domain might prefix usernames code and region code digits; another domain might prefix usernames
with trunk- routing codes and need to remove the prefix. with trunk- routing codes and need to remove the prefix.
The resulting canonical number string will be used as input to the The resulting canonical number string will be used as input to the
hash calculation during signing and verifying processes. hash calculation during signing and verifying processes.
The ABNF of this number string is: The ABNF of this number string is:
tn-spec = [ "#" / "*" ] 1*DIGIT tn-spec = [ "#" / "*" ] 1*DIGIT
[TBD: do we need military codes A-D? Other codes?]
If the result of this procedure forms a complete telephone number, If the result of this procedure forms a complete telephone number,
that number is used for the purpose of creating and signing the that number is used for the purpose of creating and signing the
digest-string by both the authentication service and verification digest-string by both the authentication service and verification
service. If the result does not form a complete telephone number, service. Optionally, the entity instantiating the authentication
the authentication service and verification service should treat the service function MAY alter the telephone numbers that appear in the
entire URI as a SIP URI, and apply a domain signature per the To and From header field values, converting them to this format. The
procedures in Section 6.2. authentication service MAY also add the result of the
canonicalization process of the From header field value to the
"canon" parameter of the Identity-Info header. If the result of the
canonicalization of the From header field value does not form a
complete telephone number, the authentication service and
verification service should treat the entire URI as a SIP URI, and
apply a domain signature per the procedures in Section 6.2.
In the longer term, it is possible that some directory or other In the longer term, it is possible that some directory or other
discovery mechanism may provide a way to determine which discovery mechanism may provide a way to determine which
administrative domain is responsible for a telephone number, and this administrative domain is responsible for a telephone number, and this
may aid in the signing and verification of SIP identities that may aid in the signing and verification of SIP identities that
contain telephone numbers. This is a subject for future work. contain telephone numbers. This is a subject for future work.
6.2. Usernames with Domain Names 6.2. Usernames with Domain Names
When a verifier processes a request containing an Identity-Info When a verifier processes a request containing an Identity-Info
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Identity-Reliance = "Identity-Reliance" HCOLON signed-identity-reliance-digest Identity-Reliance = "Identity-Reliance" HCOLON signed-identity-reliance-digest
signed-identity-reliance-digest = LDQUOT 32LHEX RDQUOT signed-identity-reliance-digest = LDQUOT 32LHEX 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
[TBD: The version has the Identity-Reliance header covered under the
Identity signature. It is also possible to do this the other way
around, where the base Identity signature is generated first, and
Identity-Reliance would cover both the Identity header and the body.
This is a trade-off of whether the authentication service should
decide whether Identity-Reliance is needed or if the verification
service should decide. These have different properties, and some
investigation would be needed to decide between them.]
The signed-identity-reliance-digest is a signed hash of a canonical The signed-identity-reliance-digest is a signed hash of a canonical
string generated from certain components of a SIP request. Creating string generated from certain components of a SIP request. Creating
this hash and the Identity-Reliance header field to contain it is this hash and the Identity-Reliance header field to contain it is
OPTIONAL, and its usage is a matter of local policy for OPTIONAL, and its usage is a matter of local policy for
authentication services. To create the contents of the signed- authentication services. To create the contents of the signed-
identity-reliance-digest, the following element of a SIP message MUST identity-reliance-digest, the following element of a SIP message MUST
be placed in a bit-exact string: be placed in a bit-exact string:
The body content of the message with the bits exactly as they are 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 in the message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that includes all components of multipart message bodies. Note that
the message-body does NOT include the CRLF separating the SIP the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that headers from the message-body, but does include everything that
follows that CRLF. follows that CRLF.
[TBD: Explore alternatives to including the whole body for INVITE
requests; should there be a special case for security parameters that
would appear in SDP?]
The signed-identity-digest is a signed hash of a canonical string The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character: specified here, separated by a vertical line, "|" or %x7C, character:
First, the identity. If the user part of the AoR in the From First, the identity. If the user part of the AoR in the From
header field of the request contains a telephone number, then the header field of the request contains a telephone number, then the
canonicalization of that number goes into the first slot (see canonicalization of that number goes into the first slot (see
Section 6.1). Otherwise, the first slot contains the AoR of the Section 6.1). Otherwise, the first slot contains the AoR of the
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BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for BNF of RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for
weekday and month is a choice amongst a set of tokens. The RFC weekday and month is a choice amongst a set of tokens. The RFC
4234 [RFC4234] rules for the BNF specify that tokens are case 4234 [RFC4234] rules for the BNF specify that tokens are case
sensitive. However, when used to construct the canonical string sensitive. However, when used to construct the canonical string
defined here, the first letter of each week and month MUST be defined here, the first letter of each week and month MUST be
capitalized, and the remaining two letters must be lowercase. capitalized, and the remaining two letters must be lowercase.
This matches the capitalization provided in the definition of each This matches the capitalization provided in the definition of each
token. All requests that use the Identity mechanism MUST contain token. All requests that use the Identity mechanism MUST contain
a Date header. a Date header.
Fifth, the Identity-Reliance header field value, if there is an Fifth, if the request contains an SDP message body, and if that
SDP contains an "a=fingerprint" attribute, the value of the
attribute. The attribute value consists of all characters
following the colon after "a=fingerprint" including the algorithm
description and hexadecimal key representation, any whitespace,
carriage returns, and "/" line break indicators. If the SDP body
contains no "a=fingerprint" attribute, the fifth element MUST be
empty, containing no whitespace, resulting in a "||" in the
signed-identity-digest.
Sixth, the Identity-Reliance header field value, if there is an
Identity-Reliance field in the request. If the message has no Identity-Reliance field in the request. If the message has no
body, or no Identity-Reliance header, then the fifth slot will be body, or no Identity-Reliance header, then the fifth slot will be
empty, and the final "|" will not be followed by any additional empty, and the final "|" will not be followed by any additional
characters. characters.
For more information on the security properties of these headers, and For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 10 and why their inclusion mitigates replay attacks, see Section 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 "|" [ signed-identity-reliance-digest ] Method "|" SIP-date "|" [ sdp-fingerprint ] "|" [ signed-identity-reliance-digest ]
sdp-fingerprint = byte-string
For the definition of 'tn-spec' see Section 6.1. For the definition of 'tn-spec' see Section 6.1.
After the digest-string or reliance-digest-string is formed, each After the digest-string or reliance-digest-string is formed, each
MUST be hashed and signed with the certificate of authority over the MUST be hashed and signed with the certificate of authority over the
identity. The hashing and signing algorithm is specified by the identity. The hashing and signing algorithm is specified by the
'alg' parameter of the Identity-Info header (see below for more 'alg' parameter of the Identity-Info header (see below for more
information on Identity-Info header parameters). This document information on Identity-Info header parameters). This document
defines only one value for the 'alg' parameter: 'rsa-sha256'; further defines only one value for the 'alg' parameter: 'rsa-sha256'; further
values MUST be defined in a Standards Track RFC, see Section 14.7 for values MUST be defined in a Standards Track RFC, see Section 14.7 for
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CANCEL would be significantly limited. The Identity and Identity- CANCEL would be significantly limited. The Identity and Identity-
Info header MUST NOT appear in CANCEL. Note as well that the use of Info header MUST NOT appear in CANCEL. Note as well that the use of
Identity with REGISTER is consequently a subject for future study, Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility although it is left as optional here for forward-compatibility
reasons. reasons.
8. Examples 8. Examples
9. Privacy Considerations 9. Privacy Considerations
The purpose of this mechanism is to provide a strong identification
of the originator of a SIP request, specifically a cryptographic
assurance that the URI given in the From header field value can
legitimately be claimed by the originator. This URI may contain a
variety of personally identifying information, including the name of
a human being, their place of work or service provider, and possibly
further details. The intrinsic privacy risks associated with that
URI are, however, no different from those of baseline SIP. Per the
guidance in [RFC6973], implementors should make users aware of the
privacy trade-off of providing secure identity.
The identity mechanism presented in this document is compatible with The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in RFC 3323 the standard SIP practices for privacy described in RFC 3323
[RFC3323]. A SIP proxy server can act both as a privacy service and [RFC3323]. A SIP proxy server can act both as a privacy service and
as an authentication service. Since a user agent can provide any as an authentication service. Since a user agent can provide any
From header field value that the authentication service is willing to From header field value that the authentication service is willing to
authorize, there is no reason why private SIP URIs that contain authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity by an authentication service. The construction of the Identity
header is the same for private URIs as it is for any other sort of header is the same for private URIs as it is for any other sort of
URIs. URIs.
skipping to change at page 24, line 39 skipping to change at page 25, line 11
Without the method, an INVITE request could be cut- and-pasted by an Without the method, an INVITE request could be cut- and-pasted by an
attacker and transformed into a MESSAGE request without changing any attacker and transformed into a MESSAGE request without changing any
fields covered by the Identity header, and moreover requests within a fields covered by the Identity header, and moreover requests within a
certain transaction could be replayed in potentially confusing or certain transaction could be replayed in potentially confusing or
malicious ways. malicious ways.
RFC4474 originally had protections for the Contact, Call-ID and CSeq. RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing against the these headers does not seem impactful to preventing the simple
simple unauthorized claiming of a From header field value, which is unauthorized claiming of a From header field value, which is the
the primary scope of the current document. primary scope of the current document.
It might seem attractive to provide a signature over some of the It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example, information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header, 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- 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 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 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, over the topmost Via header does not prevent attacks of this nature,
since the attacker could leave the topmost Via intact and merely since the attacker could leave the topmost Via intact and merely
insert a new Via header field directly after it, which would cause insert a new Via header field directly after it, which would cause
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guarantee that no Via hops are inserted between the sending user guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented. inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS. known practice for securing Via is the use of SIPS.
This mechanism also provides an optional signature over the bodies of When signing a request that contains a fingerprint of keying material
SIP requests. This can help to protect non-INVITE transactions such in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
as MESSAGE or NOTIFY, as well as INVITEs in those environments where signature over that fingerprint. This signature prevents certain
intermediaries do not change SDP. While this is not strictly classes of impersonation attacks in which an attacker forwards or
necessary to prevent the impersonation attacks, there is little cut-and-pastes a legitimate request: although the target of the
purpose in establishing the identity of the user that originated a attack may accept the request, the attacker will be unable to
SIP request if this assurance is not coupled with a comparable exchange media with the target as they will not possess a key
assurance over the contents of the message. There are furthermore corresponding to the fingerprint. For example there are some baiting
some baiting attacks (where the attacker receives a request from the attacks (where the attacker receives a request from the target and
target and reoriginates it to a third party) that might not be reoriginates it to a third party) that might not be prevented by only
prevented by only a signature over the From, To and Date, but could a signature over the From, To and Date, but could be prevented by
be prevented by securing SDP. Note, however, that this is not securing a fingerprint for DTLS-SRTP. While this is a different form
perfect end-to-end security. The authentication service itself, when of interpretation than is commonly needed for robocalling, ultimately
instantiated at an intermediary, could conceivably change the body there is little purpose in establishing the identity of the user that
(and SIP headers, for that matter) before providing a signature. originated a SIP request if this assurance is not coupled with a
Thus, while this mechanism reduces the chance that a replayer or man- comparable assurance over the contents of the subsequent
in-the-middle will modify bodies, it does not eliminate it entirely. communication. This signature also, per [RFC7258], reduces the
Since it is a foundational assumption of this mechanism that the potential for passive monitoring attacks against the SIP media. In
users trust their local domain to vouch for their security, they must environments where DTLS-SRTP is unsupported, however, this mechanism
also trust the service not to violate the integrity of their message is not exercised and no protections are provided.
without good reason.
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- In the end analysis, the Identity, Identity-Reliance and Identity-
Info headers cannot protect themselves. Any attacker could remove Info headers cannot protect themselves. Any attacker could remove
these headers from a SIP request, and modify the request arbitrarily these headers from a SIP request, and modify the request arbitrarily
afterwards. However, this mechanism is not intended to protect afterwards. However, this mechanism is not intended to protect
requests from men-in-the- middle who interfere with SIP messages; it requests from men-in-the- middle who interfere with SIP messages; it
is intended only to provide a way that the originators of SIP 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 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 stripping identity information from a request, a man-in-the-middle
could make it impossible to distinguish any illegitimate messages he could make it impossible to distinguish any illegitimate messages he
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As a matter of interface design, SIP user agents might render the As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this component of the From header field value is outside the scope of this
document, but may be the subject of future work. document, but may be the subject of future work.
11. IANA Considerations 11. IANA Considerations
[TBD: update for rfc4474bis or remove?] This document relies on the headers and response codes defined in RFC
4474. It also retains the requirements for the specification of new
This document requests changes to the header and response-code sub- algorithms or headers related to the mechanisms described in that
registries of the SIP parameters IANA registry, and requests the document.
creation of two new registries for parameters for the Identity-Info
header.
11.1. Header Field Names 11.1. Header Field Names
This document specifies three SIP headers: Identity, Identity- This document specifies one new SIP header called Identity-Reliance.
Reliance and Identity- Info. Their syntax is given in Section 7. Its syntax is given in Section 7. This header is defined by the
These headers are defined by the following information, which has following information, which has been added to the header sub-
been added to the header sub-registry under registry under http://www.iana.org/assignments/sip-parameters
http://www.iana.org/assignments/sip-parameters
Header Name: Identity
Compact Form: y
Header Name: Identity-Info
Compact Form: n
Header Name: Identity-Reliance
Compact Form:
11.2. 428 'Use Identity Header' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is sent when a verifier receives a SIP request that
lacks an Identity header in order to indicate that the request should
be re-sent with an Identity header. This response code is defined by
the following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
Response Code Number: 428
Default Reason Phrase: Use Identity Header
11.3. 436 'Bad Identity-Info' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the Identity-Info header contains a URI
that cannot be dereferenced by the verifier (either the URI scheme is
unsupported by the verifier, or the resource designated by the URI is
otherwise unavailable). This response code is defined by the
following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
Response Code Number: 436
Default Reason Phrase: Bad Identity-Info
11.4. 437 'Unsupported Credential' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the verifier cannot validate the
credential referenced by the URI of the Identity-Info header,
because, for example, the credential is self-signed, or signed by an
authority for whom the verifier does not trust. This response code
is defined by the following information, which has been added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters
Response Code Number: 437
Default Reason Phrase: Unsupported Credential
11.5. 438 'Invalid Identity Header' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the verifier receives a message with an
Identity signature that does not correspond to the digest-string
calculated by the verifier. This response code is defined by the
following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
Response Code Number: 438 Header Name: Identity-Reliance
Default Reason Phrase: Invalid Identity Header Compact Form: N/A
11.6. Identity-Info Parameters 11.2. Identity-Info Parameters
The IANA has created a registry for Identity-Info headers. This The IANA has already created a registry for Identity-Info headers
registry is to be prepopulated with a single entry for a parameter parameters. This specification defines a new value called "canon" as
called 'alg', which describes the algorithm used to create the defined in Section 5.3.
signature that appears in the Identity header. Registry entries must
contain the name of the parameter and the specification in which the
parameter is defined. New parameters for the Identity-Info header
may be defined only in Standards Track RFCs.
11.7. Identity-Info Algorithm Parameter Values 11.3. Identity-Info Algorithm Parameter Values
The IANA has created a registry for Identity-Info 'alg' parameter The IANA has already created a registry for Identity-Info 'alg'
values. This registry is to be prepopulated with a single entry for parameter values. This registry is to be prepopulated with a single
a value called 'rsa-sha256', which describes the algorithm used to entry for a value called 'rsa-sha256', which describes the algorithm
create the signature that appears in the Identity header. Registry used to create the signature that appears in the Identity header.
entries must contain the name of the 'alg' parameter value and the Registry entries must contain the name of the 'alg' parameter value
specification in which the value is described. New values for the and the specification in which the value is described. New values
'alg' parameter may be defined only in Standards Track RFCs. for the 'alg' parameter may be defined only in Standards Track RFCs.
A previous version of this specification defined the 'rsa-sha1' value RFC4474 defined the 'rsa-sha1' value for this registry. That value
for this registry. That value is hereby deprecated, and should be is hereby deprecated, and should be treated as such. It is not
removed. It is not believed that any implementations are making use believed that any implementations are making use of this value.
of this value.
[TBD - consider EC for smaller credential sizes?] Future specifications may consider elliptical curves for smaller key
sizes.
12. Acknowledgments 12. Acknowledgments
Lots of people made significant contributions to this document. The authors would like to thank Stephen Kent, Brian Rosen, Alex
Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin
Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov,
Pierce Gorman, David Schwartz, Philippe Fouquart, Michael Hamer,
Henning Schulzrinne, and Richard Barnes for their comments.
13. Changes from RFC4474 13. Changes from RFC4474
Lots of people made significant contributions to this document. The following are salient changes from the original RFC 4474:
Generalized the credential mechanism; credential enrollment and Generalized the credential mechanism; credential enrollment and
acquisition is now outside the scope of this document acquisition is now outside the scope of this document
Reduced the scope of the Identity signature to remove CSeq, Call- Reduced the scope of the Identity signature to remove CSeq, Call-
ID, Contact, and the message body. ID, Contact, and the message body
Added any DTLS-SRTP fingerprint in SDP as a mandatory element of
the digest-string
Added the Identity-Reliance header Added the Identity-Reliance header
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
[TBD - more] 14. References
14. Informative References
[I-D.ietf-stir-problem-statement]
Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
draft-ietf-stir-problem-statement-05 (work in progress),
May 2014.
[I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013.
[I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key 14.1. Normative References
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002. 2002.
[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. April 2002.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
14.2. Informative References
[I-D.kaplan-stir-cider]
Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013.
[I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
[I-D.peterson-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-peterson-stir-
certificates-00 (work in progress), February 2014.
[I-D.rosenberg-sip-rfc4474-concerns]
Rosenberg, J., "Concerns around the Applicability of RFC
4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
progress), February 2008.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[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, November 2002.
[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. November 2002.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003. Encodings", RFC 3548, July 2003.
[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. September 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
[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, October 2005.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4501] Josefsson, S., "Domain Name System Uniform Resource [RFC4501] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, May 2006. Identifiers", RFC 4501, May 2006.
[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, June 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
Housley, R., and W. Polk, "Internet X.509 Public Key for Establishing a Secure Real-time Transport Protocol
Infrastructure Certificate and Certificate Revocation List (SRTP) Security Context Using Datagram Transport Layer
(CRL) Profile", RFC 5280, May 2008. Security (DTLS)", RFC 5763, May 2010.
[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, August 2012.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, September 2014.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
Neustar, Inc. Neustar, Inc.
1800 Sutter St Suite 570 1800 Sutter St Suite 570
Concord, CA 94520 Concord, CA 94520
US US
Email: jon.peterson@neustar.biz Email: jon.peterson@neustar.biz
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