Internet DRAFT - draft-jennings-dispatch-rfc4474bis
draft-jennings-dispatch-rfc4474bis
Network Working Group J. Peterson
Internet-Draft NeuStar
Intended status: Standards Track C. Jennings
Expires: January 16, 2014 Cisco
E. Rescorla
RTFM, Inc.
July 15, 2013
Authenticated Identity Management in the Session Initiation Protocol
(SIP)
draft-jennings-dispatch-rfc4474bis-01
Abstract
The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining new
SIP header fields for conveying a signature used for validating the
identity, and for conveying a reference to the credentials of the
signer.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 16, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
Peterson, et al. Expires January 16, 2014 [Page 1]
Internet-Draft SIP Identity July 2013
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 6
4.1. Motivation for Changes . . . . . . . . . . . . . . . . . 7
4.2. Changes to the Identity-Info Header . . . . . . . . . . . 9
4.3. Changes to the Identity Header . . . . . . . . . . . . . 10
5. Overview of Operations . . . . . . . . . . . . . . . . . . . 10
6. Authentication Service Behavior . . . . . . . . . . . . . . . 11
6.1. Identity within a Dialog and Retargeting . . . . . . . . 15
7. Verifier Behavior . . . . . . . . . . . . . . . . . . . . . . 16
8. Considerations for User Agent . . . . . . . . . . . . . . . . 18
9. Considerations for Proxy Servers . . . . . . . . . . . . . . 18
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19
11. Compliance Tests and Examples . . . . . . . . . . . . . . . . 23
11.1. Identity-Info with a Singlepart MIME body . . . . . . . 23
11.2. Identity for a Request with No MIME Body or Contact . . 26
12. Identity and Telephone Numbers . . . . . . . . . . . . . . . 29
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 30
14. Security Considerations . . . . . . . . . . . . . . . . . . . 31
14.1. Handling of digest-string Elements . . . . . . . . . . . 31
14.2. Display-Names and Identity . . . . . . . . . . . . . . . 34
14.3. Securing the Connection to the Authentication Service . 35
14.4. Domain Names and Subordination . . . . . . . . . . . . . 36
14.5. Authorization and Transitional Strategies . . . . . . . 37
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
Peterson, et al. Expires January 16, 2014 [Page 2]
Internet-Draft SIP Identity July 2013
15.1. Header Field Names . . . . . . . . . . . . . . . . . . . 38
15.2. 428 'Use Identity Header' Response Code . . . . . . . . 39
15.3. 436 'Bad Identity-Info' Response Code . . . . . . . . . 39
15.4. 437 'Unsupported Certificate' Response Code . . . . . . 39
15.5. 438 'Invalid Identity Header' Response Code . . . . . . 40
15.6. Identity-Info Parameters . . . . . . . . . . . . . . . . 40
15.7. Identity-Info Algorithm Parameter Values . . . . . . . . 40
15.8. Acknowledgements . . . . . . . . . . . . . . . . . . . . 40
15.9. Original RFC 4474 Requirements . . . . . . . . . . . . . 40
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
16.1. Normative References . . . . . . . . . . . . . . . . . . 41
16.2. Informative References . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP, RFC 3261 [RFC3261]). An identity, for the purposes of this
document, is defined as either a SIP URI, commonly a canonical
address-of-record (AoR) employed to reach a user (such as
'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.
RFC 3261 [RFC3261] stipulates several places within a SIP request
where a user can express an identity for themselves, notably the
user-populated From header field. However, the recipient of a SIP
request has no way to verify that the From header field has been
populated appropriately, in the absence of some sort of cryptographic
authentication mechanism.
RFC 3261 [RFC3261] specifies a number of security mechanisms that can
be employed by SIP user agents (UAs), including Digest, Transport
Layer Security (TLS), and S/MIME (implementations may support other
security schemes as well). However, few SIP user agents today
support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and furthermore Digest
authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which
they have no previous association -- just as in the telephone network
today, one can receive a call from someone with whom one has no
previous association, and still have a reasonable assurance that the
person's displayed Caller-ID is accurate. A cryptographic approach,
like the one described in this document, can provide a much stronger
and less spoofable assurance of identity than the telephone network
provides today.
Peterson, et al. Expires January 16, 2014 [Page 3]
Internet-Draft SIP Identity July 2013
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919].
3. Background
The usage of many SIP applications and services is governed by
authorization policies. These policies may be automated, or they may
be applied manually by humans. An example of the latter would be an
Internet telephone application that displays the Caller-ID of a
caller, which a human may review before answering a call. An example
of the former would be a presence service that compares the identity
of potential subscribers to a whitelist before determining whether it
should accept or reject the subscription. In both of these cases,
attackers might attempt to circumvent these authorization policies
through 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 aspires to provide a strong
identity system for SIP requests in which authorization policies
cannot be circumvented by impersonation.
All RFC 3261 [RFC3261] compliant user agents support Digest
authentication, which utilizes a shared secret, as a means for
authenticating themselves to a SIP registrar. Registration allows a
user agent to express that it is an appropriate entity to which
requests should be sent for a particular SIP AoR URI (e.g.,
'sip:alice@atlanta.example.com').
For those SIP URIs, by the definition of identity used in this
document, registration is a proof of the identity of the user to a
registrar. However, the Digest credentials with which a user agent
proves its identity to a registrar cannot be validated by just any
user agent or proxy server -- these credentials are only shared
between the user agent and their domain administrator. So this
shared secret does not immediately help a user to authenticate to a
wide range of recipients. Recipients require a means of determining
whether or not the 'return address' identity of a non-REGISTER
request (i.e., the From header field value) has legitimately been
asserted.
The AoR URI used for registration is also the URI with which a UA
commonly populates the From header field of requests in order to
provide a 'return address' identity to recipients. From an
authorization perspective, if you can prove you are eligible to
Peterson, et al. Expires January 16, 2014 [Page 4]
Internet-Draft SIP Identity July 2013
register in a domain under a particular AoR, you can prove you can
legitimately receive requests for that AoR, and accordingly, when you
place that AoR in the From header field of a SIP request other than a
registration (like an INVITE), you are providing a 'return address'
where you can legitimately be reached. In other words, if you are
authorized to receive requests for that 'return address', logically,
it follows that you are also authorized to assert that 'return
address' in your From header field. This is of course only one
manner in which a domain might determine how a particular user is
authorized to populate the From header field; as an aside, for other
sorts of URIs in the From (like anonymous URIs), other authorization
policies would apply.
The situation is however different for telephone numbers. Authority
over telephone numbers does not correspond directly to Internet
domains. While a user could register at a SIP domain with a username
that corresponds to a telephone number, any connection between the
administrator of that domain and the assignment of telephone numbers
is not reflected on the Internet. Telephone numbers do not share the
'return address' property described above, as they are dialed without
any domain component. This document thus assumes the existence of a
separate means of establishing authority over telephone numbers, for
cases where the telephone number in the user part of a SIP URI is the
identity of the user.
Ideally, SIP user agents should have some way of proving to
recipients of SIP requests that the proper authority has
authenticated them and authorized the population of the From header
field. This document proposes a mediated authentication architecture
for SIP in which requests are sent through a logical authentication
service which authenticates such requests (which could use the same
practices by which the domain would authenticate REGISTER requests).
Once a message has been authenticated, the local domain then needs
some way to communicate to other SIP entities that the sending user
has been authenticated and its use of the From header field has been
authorized. This document addresses how that imprimatur of
authentication can be shared.
Peterson, et al. Expires January 16, 2014 [Page 5]
Internet-Draft SIP Identity July 2013
RFC 3261 [RFC3261] already describes an architecture very similar to
this in Section 26.3.2.2, in which a user agent authenticates itself
to a local proxy server, which in turn authenticates itself to a
remote proxy server via mutual TLS, creating a two-link chain of
transitive authentication between the originator and the remote
domain. While this works well in some architectures, there are a few
respects in which this is impractical. For one, transitive trust is
inherently weaker than an assertion that can be validated end-to-end.
It is possible for SIP requests to cross multiple intermediaries in
separate administrative domains, in which case transitive trust
becomes even less compelling.
One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity
header) is given in [12]. However, this solution allows only hop-
by-hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict
mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment.
Accordingly, this document specifies a means of sharing a
cryptographic assurance of end-user SIP identity in an interdomain or
intradomain context that is based on the concept of an
'authentication service' and a new SIP header, the Identity header.
Note that the scope of this document is limited to providing this
identity assurance for SIP requests; solving this problem for SIP
responses is more complicated and is a subject for future work.
This specification allows either a user agent or a proxy server to
provide identity services and to verify identities. To maximize end-
to-end security, it is obviously preferable for end-users to acquire
their own certificates and corresponding private keys; if they do,
they can act as an authentication service. However, end-user
certificates may be neither practical nor affordable, given the
difficulties of establishing a Public Key Infrastructure (PKI) that
extends to end-users, and moreover, given the potentially large
number of SIP user agents (phones, PCs, laptops, PDAs, gaming
devices) that may be employed by a single user. In such
environments, synchronizing keying material across multiple devices
may be very complex and requires quite a good deal of additional
endpoint behavior. Managing several certificates for the various
devices is also quite problematic and unpopular with users.
Accordingly, in the initial use of this mechanism, it is likely that
intermediaries will instantiate the authentication service role.
4. Changes from RFC4474
Peterson, et al. Expires January 16, 2014 [Page 6]
Internet-Draft SIP Identity July 2013
4.1. Motivation for Changes
The original sip-identity drafts that lead to RFC 4474 [RFC4474] were
first published in 2002. Since that point many things have changed
that impact the design.
o The DNS root has been signed.
o SPAM continues to be a problem.
o It has become clear that B2BUAs will continue to be a major factor
in SIP deployments.
o Multipart MIME has failed as a SIP extension mechanism.
o Widespread identity providers such as Facebook have emerged.
o Techniques for non-carrier entities to verify phone numbers and
then use them for addressing (such as Apple's iMessage) have been
shown to be commercially feasible.
o Substantial portions of commercial, government, and personal voice
communications rely on SIP at some stage in the communications.
o The cost of operating large databases has fallen and outsourced
versions of these databases have become cheaply available.
o Extensive experience and user research has improved our
understanding of how to present security information to users.
o The world is in the middle of a huge transition to mobile devices.
Even the most limited modern mobile devices have user interface
and computational capabilities that greatly exceed a 2002-era SIP
phone.
The authors believe that the confluence of changing technology, the
evolution of mobile devices and internet, and a political will to
change make this the right time to consider an change of the scope of
4474 to solve the following problems:
o Assert strong identity for E.164 numbers such as +1 408 555-1212
o Continue to assert strong identity for domain scoped names such as
alice@example.com
o Work for calls crossing even the most adverse networks such as the
PSTN.
Peterson, et al. Expires January 16, 2014 [Page 7]
Internet-Draft SIP Identity July 2013
o Provide reliable information about who is calling before the call
is answered to help stop SPAM.
o Provide reliable information about who you are talking to.
o Work with evolving non SIP based communications systems such as
WebRTC.
o Potentially, as future work explore organization attributes (e.g.,
"this is a Bank").
We believe it is possible to solve all of these in a way that is
commercially viable, deployable, and provides a delightful user
experience.
The core problem in a global identity system with delegated names is
understanding who is authorized to make assertions about a given
name. The proposal is to solve that problem with a two pronged
approach. The design of such a system is outside the scope of this
draft, and perhaps of the IETF, but we believe it will have a twofold
character:
First, it will delegate responsibility for a number down from a root
in a series of delegation sub delegation towards the user. For
example, the North American Numbering Plan Administrator assigns a
portion of the +1 space to a service provider. That service provider
may assign a sub space to a company and that company may assign a
number to a user. At each level of delegation, cryptographic
credentials could be provided that allow the user to prove the space
was delegated to them given some common trust root. This approach is
referred to as "delegation" and effectively works from the top down.
The other prong to solving the problem is called "claims" and works
via a bottom up approach. The end user of a number basically claims
it and some trusted system validates this claim. The validation may
be as simple as sending a SMS to the number or more complicated such
as the VIPR system.
The delegation approach creates an easier user experience but is
harder to deploy from a business incentive point of view so our
approach is to do both and work down from the top and up from the
bottom with a meet in the middle approach to coverage of the full
name space. For the purposes of the current work, it is envisioned
that a certificate authority could encompass both approaches.
Authentication services that possess a credential (whether of the
delegation or claim variety) for a telephone number or domain name
can, in this mechanism, create one of two types of assertions: basic
Peterson, et al. Expires January 16, 2014 [Page 8]
Internet-Draft SIP Identity July 2013
assertions and reliance assertions. The basic assertion provides
replay protection, whereas the reliance assertion provides a broader
body protection. Some networks might modify the signaling in ways
that impact the reliance assertions but not the other, and thus the
reliance assertion is optional.
As in RFC4474, identity assertions are passed in-band in SIP from the
caller to the callee for verification. There are however some cases
where in-band signaling cannot survive the call path, such as when
the call passes through a gateway to the PSTN. This specification
assumes that other, out-of-band mechanisms may be used in cases where
in-band identity is not carried end-to-end, but those mechanisms are
outside the scope of this document.
4.2. Changes to the Identity-Info Header
RFC4474 restricted the subject of the certificate to a domain name,
and accordingly the RFC4474 Identity-Info header contains a URI which
designates a certificate whose subject (more precisely,
subjectAltName) must correspond to the domain of the URI in the From
header field value of the SIP request. Per the analysis in
[I-D.peterson-secure-origin-ps], this document relaxes that
constraint to allow designating an alternative authority for
telephone numbers, when telephone numbers appear in the From header
field value.
These changes will allow the Identity-Info URI to point to the
certificate with authority over the calling telephone number. A
verification service will therefore authorize a SIP request when the
telephone number in the From header field value agrees with the
subject of the certificate. Verification services must of course
trust the certificate authority that issued the certificate in
question. To implement this change to the Identity-Info header, we
must allow for two possibilities for the conveyance of a telephone
number in a request: appearing within a tel URI or appearing as the
user portion of a SIP URI. Therefore, we must prescribe the
verification service behind in the case where the From header field
value URI contains a telephone user part followed by a domain --
which should the verification service expect to find in a
certificate?
Future version of this document may explore alternate ways of
acquiring credentials, including the use of credentials other than
certificates. This might include implementing enough flexibility in
the URI to allow a model more like the IdP model described in
[I-D.rescorla-rtcweb-generic-idp]; this could be useful as RTCWeb
sees increasing deployment. We also should consider any implications
of the signing of the DNSSEC root and the DANE specifications to the
Peterson, et al. Expires January 16, 2014 [Page 9]
Internet-Draft SIP Identity July 2013
existing Identity-Info uses with domain name. At a high level, it is
not expected that the proposed changes will radically alter the
semantics of Identity-Info.
4.3. Changes to the Identity Header
Per the analysis in [I-D.peterson-secure-origin-ps], this document
changes the signature mechanism that RFC44474 specified for the
Identity header: in particular, to replace this signature mechanism
with one that is more likely to survive end-to-end in SIP networks
where intermediaries act as back-to-back user agents rather than
proxy servers.
To accomplish this, we here create two distinct signatures within SIP
requests: a basic assurance and a reliance assurance. The basic
assurance prevents impersonation attacks by providing a signature
over the From header field value and certain other headers which will
allow a verification service to detect some cut-and-paste attacks.
The reliance assurance protects against attackers changing other
parameters of the call: these include the entirely of the messaging
body, including the target IP address and ports in SDP which, if
unprotected, can allow an attacker to succeed with more sophisticated
cut-and-paste attacks. Authentication services behavior would change
to allow them to decide, based on their policy in a deployment
environment, whether only the basic assurance can realistically
survive network transit, or if the reliance assurance should be
available. There are several similar design choices in this space to
consider, and some analysis will be required to identify the best
option.
In cases where the From header field value of a SIP request contains
a SIP URI with a telephone number user part, we will also consider
replay assurance canonicalizations that do not cover the domain
portion of the URI.
[TBD: in order to preserve critical security parameters even in
adverse network conditions, should the basic assurance integrity
protection must always cover security parameters of the SDP required
to negotiate media-level security? There may be other exception
cases, or extensibility mechanisms, worth considering here. ]
5. Overview of Operations
This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document.
Peterson, et al. Expires January 16, 2014 [Page 10]
Internet-Draft SIP Identity July 2013
Imagine the case where Alice, who has the home proxy of example.com
and the address-of-record sip:alice@example.com, wants to communicate
with sip:bob@example.org.
Alice generates an INVITE and places her identity in the From header
field of the request. She then sends an INVITE over TLS to an
authentication service proxy for her domain.
The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized
to assert the identity that is populated in the From header field.
This value may be Alice's AoR, or it may be some other value that the
policy of the proxy server permits her to use, such as a telephone
number. It then computes a hash over some particular headers,
including the From header field (and, optionally the body) in the
message. This hash is signed with the appropriate certificate
(example.com, in Alice's case) and inserted in a new header field in
the SIP message, the 'Identity' header.
The proxy, as the holder of the private key of its domain, is
asserting that the originator of this request has been authenticated
and that she is authorized to claim the identity (the SIP address-
of-record) that appears in the From header field. The proxy also
inserts a companion header field, Identity-Info, that tells Bob how
to acquire its certificate, if he doesn't already have it.
When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validate that the
authority over the identity in the From header field authenticated
the user, and permitted the user to assert that From header field
value. This same validation operation may be performed by Bob's user
agent server (UAS).
6. Authentication Service Behavior
Peterson, et al. Expires January 16, 2014 [Page 11]
Internet-Draft SIP Identity July 2013
This document defines a role for SIP entities called an
authentication service. The authentication service role can be
instantiated by a proxy server or a user agent. Any entity that
instantiates the authentication service role MUST possess the private
key of a certificate that can be used to sign for a domain or a
telephone number. Intermediaries that instantiate this role MUST be
capable of authenticating one or more SIP users that can register for
that identity. Commonly, this role will be instantiated by a proxy
server, since these entities are more likely to have a static
hostname, hold a corresponding certificate, and have access to SIP
registrar capabilities that allow them to authenticate users. It is
also possible that the authentication service role might be
instantiated by an entity that acts as a redirect server, but that is
left as a topic for future work.
SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present (see
Section 10 for information on how the Date header field assists
verifiers). Similarly, authentication services MUST 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.
Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP
request:
Step 1:
The authentication service MUST extract the identity of the sender
from the request. The authentication service takes this value from
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
(SIPS) URI, and the user portion is not a telephone number, the
authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is
responsible (following the procedures in RFC 3261 [RFC3261],
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
scheme, or the identity field is a SIP or SIPS URI with a telephone
number in the user portion, the authentication service determines
whether or not it is responsible for this telephone number; see
Section 12 for more information. If the authentication service is
not responsible for the identity in question, it SHOULD process and
forward the request normally, but it MUST NOT add an Identity header;
see below for more information on authentication service handling of
an existing Identity header.
Peterson, et al. Expires January 16, 2014 [Page 12]
Internet-Draft SIP Identity July 2013
Step 2:
The authentication service MUST determine whether or not the sender
of the request is authorized to claim the identity given in the
identity field. In order to do so, the authentication service MUST
authenticate the sender of the message. Some possible ways in which
this authentication might be performed include:
If the authentication service is instantiated by a SIP
intermediary (proxy server), it may challenge the request with a
407 response code using the Digest authentication scheme (or
viewing a Proxy-Authentication header sent in the request, which
was sent in anticipation of a challenge using cached credentials,
as described in RFC 3261 [RFC3261], Section 22.3). Note that if
that proxy server is maintaining a TLS connection with the client
over which the client had previously authenticated itself using
Digest authentication, the identity value obtained from that
previous authentication step can be reused without an additional
Digest challenge.
If the authentication service is instantiated by a SIP user agent,
a user agent can be said to authenticate its user on the grounds
that the user can provision the user agent with the private key of
the certificate, or preferably by providing a password that
unlocks said private key.
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
for the authentication service, one that depends greatly on the
manner in which authentication is performed. For non-telephone
number user parts, one policy might be as follows: the username given
in the 'username' parameter of the Proxy-Authorization header MUST
correspond exactly to the username in the From header field of the
SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use 'username' parameters in
Proxy-Authorization that do not correspond to the user-portion of SIP
From headers, or a user might manage multiple accounts in the same
administrative domain. In this latter case, a domain might maintain
a mapping between the values in the 'username' parameter of Proxy-
Authorization and a set of one or more SIP URIs that might
legitimately be asserted for that 'username'. For example, the
username can correspond to the 'private identity' as defined in Third
Generation Partnership Project (3GPP), in which case the From header
field can contain any one of the public identities associated with
this private identity. In this instance, another policy might be as
follows: the URI in the From header field MUST correspond exactly to
one of the mapped URIs associated with the 'username' given in the
Proxy-Authorization header. This is a suitable approach for
Peterson, et al. Expires January 16, 2014 [Page 13]
Internet-Draft SIP Identity July 2013
telephone numbers in particular. Various exceptions to such policies
might arise for cases like anonymity; if the AoR asserted in the From
header field uses a form like 'sip:anonymous@example.com', then the
'example.com' proxy should authenticate that the user is a valid user
in the domain and insert the signature over the From header field as
usual.
Note that this check is performed on the addr-spec in the From header
field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta').
Authentication services MAY check and validate the display-name as
well, and compare it to a list of acceptable display-names that may
be used by the sender; if the display-name does not meet policy
constraints, the authentication service MUST return a 403 response
code. The reason phrase should indicate the nature of the problem;
for example, "Inappropriate Display Name". However, the display-name
is not always present, and in many environments the requisite
operational procedures for display-name validation may not exist.
For more information, see Section 14.2.
Step 3:
The authentication service SHOULD ensure that any preexisting Date
header in the request is accurate. Local policy can dictate
precisely how accurate the Date must be; a RECOMMENDED maximum
discrepancy of ten minutes will ensure that the request is unlikely
to upset any verifiers. If the Date header contains a time different
by more than ten minutes from the current time noted by the
authentication service, the authentication service SHOULD reject the
request. This behavior is not mandatory because a user agent client
(UAC) could only exploit the Date header in order to cause a request
to fail verification; the Identity header is not intended to provide
a source of non-repudiation or a perfect record of when messages are
processed. Finally, the authentication service MUST verify that the
Date header falls within the validity period of its certificate. For
more information on the security properties associated with the Date
header field value, see Section 10.
[TBD: Should consider a lower threshold than ten minutes? With the
removal of other elements from the sig, that's a lot of leeway.]
Step 4:
The authentication service MAY form an identity-reliance signature
and add an Identity-Reliance header to the request containing this
signature. The Identity-Reliance header provides body security
properties that are useful for non-INVITE transactions, and in
Peterson, et al. Expires January 16, 2014 [Page 14]
Internet-Draft SIP Identity July 2013
environments where body security of INVITE transactions is necessary.
Details on the generation of this header is provided in Section 10.
Step 5:
The authentication service MUST form the identity signature and add
an Identity header to the request containing this signature. After
the Identity header has been added to the request, the authentication
service MUST also add an Identity-Info header. The Identity-Info
header contains a URI from which its certificate can be acquired.
Details on the generation of both of these headers are provided in
Section 10.
Finally, the authentication service MUST forward the message
normally.
6.1. Identity within a Dialog and Retargeting
Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting supplants the
original target URI with one that corresponds to a different user, a
user that is not authorized to register under the original target
URI. By this definition, retargeting does not include translation of
the Request-URI to a contact address of an endpoint that has
registered under the original target URI, for example.
When a dialog-forming request is retargeted, this can cause a few
wrinkles for the Identity mechanism when it is applied to requests
sent in the backwards direction within a dialog. This section
provides some non-normative considerations related to this case.
When a request is retargeted, it may reach a SIP endpoint whose user
is not identified by the URI designated in the To header field value.
The value in the To header field of a dialog-forming request is used
as the From header field of requests sent in the backwards direction
during the dialog, and is accordingly the header that would be signed
by an authentication service for requests sent in the backwards
direction. In retargeting cases, if the URI in the From header does
not identify the sender of the request in the backwards direction,
then clearly it would be inappropriate to provide an Identity
signature over that From header. As specified above, if the
authentication service is not responsible for the domain in the From
header field of the request, it MUST NOT add an Identity header to
the request, and it should process/forward the request normally.
Any means of anticipating retargeting, and so on, is outside the
scope of this document, and likely to have equal applicability to
response identity as it does to requests in the backwards direction
Peterson, et al. Expires January 16, 2014 [Page 15]
Internet-Draft SIP Identity July 2013
within a dialog. Consequently, no special guidance is given for
implementers here regarding the 'connected party' problem;
authentication service behavior is unchanged if retargeting has
occurred for a dialog-forming request. Ultimately, the
authentication service provides an Identity header for requests in
the backwards dialog when the user is authorized to assert the
identity given in the From header field, and if they are not, an
Identity header is not provided.
For further information on the problems of response identity and the
potential solution spaces, see [15].
7. Verifier Behavior
This document introduces a new logical role for SIP entities called a
server. When a verifier receives a SIP message containing an
Identity header, it may inspect the signature to verify the identity
of the sender of the message. Typically, the results of a
verification are provided as input to an authorization process that
is outside the scope of this document. If an Identity header is not
present in a request, and one is required by local policy (for
example, based on a per-sending-domain policy, or a per-sending-user
policy), then a 428 'Use Identity Header' response MUST be sent.
In 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
here specified.
Step 1:
The verifier MUST acquire the certificate for the signer.
Implementations supporting this specification SHOULD have some means
of retaining certificates (in accordance with normal practices for
certificate lifetimes and revocation) in order to prevent themselves
from needlessly downloading the same certificate every time a request
from the same identity is received. Certificates cached in this
manner should be indexed by the URI given in the Identity-Info header
field value.
Provided that the certificate used to sign this message is not
previously known to the verifier, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header, unless they
have some more efficient implementation-specific way of acquiring
certificates. If the URI scheme in the Identity-Info header cannot
be dereferenced, then a 436 'Bad Identity-Info' response MUST be
returned. The verifier processes this certificate in the usual ways,
including checking that it has not expired, that the chain is valid
back to a trusted certification authority (CA), and that it does not
Peterson, et al. Expires January 16, 2014 [Page 16]
Internet-Draft SIP Identity July 2013
appear on revocation lists. Once the certificate is acquired, it
MUST be validated following the procedures in RFC 3280 [RFC3280]. If
the certificate cannot be validated (it is self-signed and untrusted,
or signed by an untrusted or unknown certificate authority, expired,
or revoked), the verifier MUST send a 437 'Unsupported Certificate'
response.
Step 2:
In order to determine whether the signature for the URI in the From
header field value should be over the entire URI or just a
canonicalized telephone number, the verification service must follow
the process described in Section 12. That section also describes the
procedures the verification service must follow to determine if the
signer is authoritative for a telephone number. For domains, the
verifier MUST follow the process described in Section 14.4 to
determine if the signer is authoritative for the URI in the From
header field.
Step 3:
The verifier MUST verify the signature in the Identity header field,
following the procedures for generating the hashed digest-string
described in Section 10. If a verifier determines that the signature
on the message does not correspond to the reconstructed digest-
string, then a 438 'Invalid Identity Header' response MUST be
returned.
Step 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 10. 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:
The verifier MUST validate the Date header in the manner described in
Section 14.1; recipients that wish to verify Identity signatures MUST
support all of the operations described there. It must furthermore
ensure that the value of the Date header falls within the validity
period of the certificate whose corresponding private key was used to
sign the Identity header.
Peterson, et al. Expires January 16, 2014 [Page 17]
Internet-Draft SIP Identity July 2013
8. Considerations for User Agent
This mechanism can be applied opportunistically to existing SIP
deployments; accordingly, it requires no change to SIP user agent
behavior in order for it to be effective. However, because this
mechanism does not provide integrity protection between the UAC and
the authentication service, a UAC SHOULD implement some means of
providing this integrity. TLS would be one such mechanism, which is
attractive because it MUST be supported by SIP proxy servers, but is
potentially problematic because it is a hop-by-hop mechanism. See
Section 14.3 for more information about securing the channel between
the UAC and the authentication service.
When a UAC sends a request, it MUST accurately populate the From
header field with a value corresponding to an identity that it
believes it is authorized to claim. In a request, it MUST set the
URI portion of its From header to match a SIP, SIPS, or TEL URI AoR
that it is authorized to use in the domain (including anonymous URIs,
as described in RFC 3323 [RFC3323]).
Note that this document defines a number of new 4xx response codes.
If user agents support these response codes, they will be able to
respond intelligently to Identity-based error conditions.
The UAC MUST also be capable of sending requests, including mid-call
requests, through an 'outbound' proxy (the authentication service).
The best way to accomplish this is using pre-loaded Route headers and
loose routing. For a given domain, if an entity that can instantiate
the authentication service role is not in the path of dialog-forming
requests, identity for mid-dialog requests in the backwards direction
cannot be provided.
As a recipient of a request, a user agent that can verify signed
identities should also support an appropriate user interface to
render the validity of identity to a user. User agent
implementations SHOULD differentiate signed From header field values
from unsigned From header field values when rendering to an end-user
the identity of the sender of a request.
9. Considerations for Proxy Servers
Peterson, et al. Expires January 16, 2014 [Page 18]
Internet-Draft SIP Identity July 2013
Domain policy may require proxy servers to inspect and verify the
identity provided in SIP requests. A proxy server may wish to
ascertain the identity of the sender of the message to provide spam
prevention or call control services. Even if a proxy server does not
act as an verification service, it MAY validate the Identity header
before it makes a forwarding decision for a request. Compliant proxy
servers MUST NOT remove or modify an existing Identity or Identity-
Info header in a request.
10. Header Syntax
This document specifies three SIP headers: Identity, Identity-
Reliance and Identity- Info. Each of these headers can appear only
once in a SIP request; Identity-Reliance is OPTIONAL, while Identity
and Identity-Info are REQUIRED for securing requests with this
specification. The grammar for these three headers is (following the
ABNF [6] in RFC 3261 [1]):
Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT
Identity-Reliance = "Identity-Reliance" HCOLON signed-identity-reliance-digest
signed-identity-reliance-digest = LDQUOT 32LHEX RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-info-extension
ident-info-alg = "alg" EQUAL token
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
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 policy for authentication
services. To create the contents of the signed-identity-digest, the
following element of a SIP message MUST be placed in a bit-exact
string:
Peterson, et al. Expires January 16, 2014 [Page 19]
Internet-Draft SIP Identity July 2013
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.
[TBD: Explore alternatives to including the whole body for INVITE
requests]
The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character:
First, the identity. If the user part of the AoR in the From
header field of the request contains a telephone number, then the
canonicalization of that number goes into the first slot (see
Section 12). Otherwise, the first slot contains the AoR of the UA
sending the message, or addr-spec of the From header field.
Second, the target. If the user part of the AoR in the To header
field of the request contains a telephone number, then the
canonicalization of that number goes into the second slot (see
Section 12). Otherwise, the second slot contains the addr-spec
component of the To header field, which is the AoR to which the
request is being sent.
Third, the request method.
Fourth, the Date header field, with exactly one space each for
each SP and the weekday and month items case set as shown in BNF
in RFC 3261 [RFC3261]. RFC 3261 specifies that the BNF for
weekday and month is a choice amongst a set of tokens. The RFC
2234 [RFC2234] rules for the BNF specify that tokens are case
sensitive. However, when used to construct the canonical string
defined here, the first letter of each week and month MUST be
capitalized, and the remaining two letters must be lowercase.
This matches the capitalization provided in the definition of each
token. All requests that use the Identity mechanism MUST contain
a Date header.
Fifth, the Identity-Reliance header field value, if there is an
Identity-Reliance field in the request. If the message has no
body, or no Identity-Reliance header, then the fifth slot will be
empty, and the final "|" will not be followed by any additional
characters.
Peterson, et al. Expires January 16, 2014 [Page 20]
Internet-Draft SIP Identity July 2013
[TBD: Should there be a special case for security parameters that
would appear in SDP?]
For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 14 and
[RFC3893]. The precise formulation of this digest-string is,
therefore (following the ABNF[RFC4234] in RFC 3261 [RFC3261]):
digest-string = addr-spec / tn-spec "|" addr-spec / tn-spec "|"
Method "|" SIP-date "|" [ signed-identity-reliance-digest ]
For the definition of 'tn-spec' see Section 12.
After the digest-string or reliance-digest-string is formed, each
MUST be hashed and signed with the certificate of authority over the
identity. The hashing and signing algorithm is specified by the
'alg' parameter of the Identity-Info header (see below for more
information on Identity-Info header parameters). This document
defines only one value for the 'alg' parameter: 'rsa-sha1'; further
values MUST be defined in a Standards Track RFC, see Section 14.7 for
more information. All implementations of this specification MUST
support 'rsa-sha1'. When the 'rsa-sha1' algorithm is specified in
the 'alg' parameter of Identity-Info, the hash and signature MUST be
generated as follows: compute the results of signing this string with
sha1WithRSAEncryption as described in RFC 3370 [RFC3370] and base64
encode the results as specified in RFC 3548 [RFC3548]. A 1024-bit or
longer RSA key MUST be used. The result of the digest-string hash is
placed in the Identity header field; the optional reliance-digest-
string hash goes in the Identity-Reliance header. For detailed
examples of the usage of this algorithm, see Section 11.
The 'absoluteURI' portion of the Identity-Info header MUST contain a
URI which dereferences to a resource containing the certificate of
the authentication service. All implementations of this
specification MUST support the use of HTTP and HTTPS URIs in the
Identity-Info header. Such HTTP and HTTPS URIs MUST follow the
conventions of RFC 2585 [RFC2585], and for those URIs the indicated
resource MUST be of the form 'application/pkix-cert' described in
that specification. Note that this introduces key lifecycle
management concerns; were a domain to change the key available at the
Identity-Info URI before a verifier evaluates a request signed by an
authentication service, this would cause obvious verifier failures.
When a rollover occurs, authentication services SHOULD thus provide
new Identity-Info URIs for each new certificate, and SHOULD continue
to make older key acquisition URIs available for a duration longer
than the plausible lifetime of a SIP message (an hour would most
likely suffice).
Peterson, et al. Expires January 16, 2014 [Page 21]
Internet-Draft SIP Identity July 2013
The Identity-Info header field MUST contain an 'alg' parameter. No
other parameters are defined for the Identity-Info header in this
document. Future Standards Track RFCs may define additional
Identity-Info header parameters.
This document adds the following entries to Table 2 of RFC 3261
[RFC3261] (this repeats the registrations of RFC4474):
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Info R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Reliance R a o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Note, in the table above, that this mechanism does not protect the
CANCEL method. The CANCEL method cannot be challenged, because it is
hop-by-hop, and accordingly authentication service behavior for
CANCEL would be significantly limited. The Identity and Identity-
Info header MUST NOT appear in CANCEL. Note as well that the use of
Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility
reasons.
Peterson, et al. Expires January 16, 2014 [Page 22]
Internet-Draft SIP Identity July 2013
11. Compliance Tests and Examples
[TBD: Need to fix examples for RFC4474bis]
The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementers are advised
to verify their compliance with the specification against the
following criteria:
Implementations of the authentication service role MUST generate
identical base64 identity strings to the ones shown in the
Identity headers in these examples when presented with the source
message and utilizing the appropriate supplied private key for the
domain in question.
Implementations of the verifier role MUST correctly validate the
given messages containing the Identity header when utilizing the
supplied certificates (with the caveat about self-signed
certificates below).
Note that the following examples use self-signed certificates, rather
than certificates issued by a recognized certificate authority. The
use of self-signed certificates for this mechanism is NOT
RECOMMENDED, and it appears here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD
generate appropriate warnings about the use of self-signed
certificates. Also, the example certificates in this section have
placed their domain name subject in the subjectAltName field; in
practice, certificate authorities may place domain names in other
locations in the certificate (see Section 14.4 for more information).
Note that all examples in this section use the 'rsa-sha1' algorithm.
Bit-exact reference files for these messages and their various
transformations are supplied in Appendix B.
11.1. Identity-Info with a Singlepart MIME body
Consider the following private key and certificate pair assigned to
'atlanta.example.com' (rendered in OpenSSL format).
-----BEGIN RSA PRIVATE KEY-----
MIICXQIBAAKBgQDPPMBtHVoPkXV+Z6jq1LsgfTELVWpy2BVUffJMPH06LL0cJSQO
aIeVzIojzWtpauB7IylZKlAjB5f429tRuoUiedCwMLKblWAqZt6eHWpCNZJ7lONc
IEwnmh2nAccKk83Lp/VH3tgAS/43DQoX2sndnYh+g8522Pzwg7EGWspzzwIDAQAB
AoGBAK0W3tnEFD7AjVQAnJNXDtx59Aa1Vu2JEXe6oi+OrkFysJjbZJwsLmKtrgtt
PXOU8t2mZpi0wK4hX4tZhntiwGKkUPC3h9Bjp+GerifP341RMyMO+6fPgjqOzUDw
+rPjjMpwD7AkcEcqDgbTrZnWv/QnCSaaF3xkUGfFkLx5OKcRAkEA7UxnsE8XaT30
Peterson, et al. Expires January 16, 2014 [Page 23]
Internet-Draft SIP Identity July 2013
tP/UUc51gNk2KGKgxQQTHopBcew9yfeCRFhvdL7jpaGatEi5iZwGGQQDVOVHUN1H
0YLpHQjRowJBAN+R2bvA/Nimq464ZgnelEDPqaEAZWaD3kOfhS9+vL7oqES+u5E0
J7kXb7ZkiSVUg9XU/8PxMKx/DAz0dUmOL+UCQH8C9ETUMI2uEbqHbBdVUGNk364C
DFcndSxVh+34KqJdjiYSx6VPPv26X9m7S0OydTkSgs3/4ooPxo8HaMqXm80CQB+r
xbB3UlpOohcBwFK9mTrlMB6Cs9ql66KgwnlL9ukEhHHYozGatdXeoBCyhUsogdSU
6/aSAFcvWEGtj7/vyJECQQCCS1lKgEXoNQPqONalvYhyyMZRXFLdD4gbwRPK1uXK
Ypk3CkfFzOyfjeLcGPxXzq2qzuHzGTDxZ9PAepwX4RSk
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
A user of atlanta.example.com, Alice, wants to send an INVITE to
bob@biloxi.example.org. She therefore creates the following INVITE
request, which she forwards to the atlanta.example.org proxy server
that instantiates the authentication service role:
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 147
v=0
Peterson, et al. Expires January 16, 2014 [Page 24]
Internet-Draft SIP Identity July 2013
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
When the authentication service receives the INVITE, it authenticates
Alice by sending a 407 response. As a result, Alice adds an
Authorization header to her request, and resends to the
atlanta.example.com authentication service. Now that the service is
sure of Alice's identity, it calculates an Identity header for the
request. The canonical string over which the identity signature will
be generated is the following (note that the first line wraps because
of RFC editorial conventions):
sip:alice@atlanta.example.com|sip:bob@biloxi.example.org|
INVITE|Thu, 21 Feb 2002 13:02:03 GMT|
The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is the following:
ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=
Accordingly, the atlanta.example.com authentication service will
create an Identity header containing that base64 signature string
(175 bytes). It will also add an HTTPS URL where its certificate is
made available. With those two headers added, the message looks like
the following:
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Identity:
"ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
Peterson, et al. Expires January 16, 2014 [Page 25]
Internet-Draft SIP Identity July 2013
Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
atlanta.example.com then forwards the request normally. When Bob
receives the request, if he does not already know the certificate of
atlanta.example.com, he dereferences the URL in the Identity-Info
header to acquire the certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP
request. Using this canonical string, the signed digest in the
Identity header, and the certificate discovered by dereferencing the
Identity-Info header, Bob can verify that the given set of headers
and the message body have not been modified.
11.2. Identity for a Request with No MIME Body or Contact
Consider the following private key and certificate pair assigned to
"biloxi.example.org".
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIC1jCCAj+gAwIBAgIBADANBgkqhkiG9w0BAQUFADBXMQswCQYDVQQGEwJVUzEL
MAkGA1UECAwCTVMxDzANBgNVBAcMBkJpbG94aTENMAsGA1UECgwESUVURjEbMBkG
Peterson, et al. Expires January 16, 2014 [Page 26]
Internet-Draft SIP Identity July 2013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-----END CERTIFICATE-----
Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
at the end of the dialog initiated in the previous example. He
therefore creates the following BYE request, which he forwards to the
'biloxi.example.org' proxy server that instantiates the
authentication service role:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Content-Length: 0
When the authentication service receives the BYE, it authenticates
Bob by sending a 407 response. As a result, Bob adds an
Authorization header to his request, and resends to the
biloxi.example.org authentication service. Now that the service is
sure of Bob's identity, it prepares to calculate an Identity header
for the request. Note that this request does not have a Date header
field. Accordingly, the biloxi.example.org will add a Date header to
the request before calculating the identity signature. If the
Content-Length header were not present, the authentication service
would add it as well. The baseline message is thus:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
Peterson, et al. Expires January 16, 2014 [Page 27]
Internet-Draft SIP Identity July 2013
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Content-Length: 0
[TBD: Fix example.] Also note that this request contains no Contact
header field. Accordingly, biloxi.example.org will place no value in
the canonical string for the addr-spec of the Contact address. Also
note that there is no message body, and accordingly, the signature
string will terminate, in this case, with two vertical bars. The
canonical string over which the identity signature will be generated
is the following (note that the first line wraps because of RFC
editorial conventions):
sip:bob@biloxi.example.org|sip:alice@atlanta.example.com|
a84b4c76e66710|231 BYE|Thu, 21 Feb 2002 14:19:51 GMT||
The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above for biloxi.example.org, with base64 encoding, is the
following:
sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=
Accordingly, the biloxi.example.org authentication service will
create an Identity header containing that base64 signature string.
It will also add an HTTPS URL where its certificate is made
available. With those two headers added, the message looks like the
following:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Identity:
"sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="
Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
Content-Length: 0
Peterson, et al. Expires January 16, 2014 [Page 28]
Internet-Draft SIP Identity July 2013
biloxi.example.org then forwards the request normally.
12. Identity and Telephone Numbers
Since many SIP applications provide a Voice over IP (VoIP) service,
telephone numbers are commonly used as identities in SIP deployments.
In order for telephone numbers to be used with the mechanism
described in this document, authentication services must enroll with
a certificate authority that issues certificates for telephone
numbers or telephone number ranges, and verification services must
trust the certificate authority employed by the authentication
service that signs a request.
Given the existence of such authorities, authentication and
verification services must furthermore identify when a request should
be signed by an authority for a telephone number, and when it should
be signed by an authority for a domain. Telephone numbers most
commonly appear in SIP requests in the username portion of a SIP URI
(e.g., 'sip:+17005551008@chicago.example.com;user=phone'). The user
part of that URI conforms to the syntax of the TEL URI scheme (RFC
3966 [RFC3966]). It is also possible for a TEL URI to appear in the
SIP To or From header field outside the context of a SIP or SIPS URI
(e.g., 'tel:+17005551008'). In both of these cases, it's clear that
the signer must have authority over the telephone number, not the
domain name of the SIP URI. It is also possible, however, for
requests to contain a URI like 'sip:7005551000@chicago.example.com'.
It may be non-trivial for a service to ascertain in this case whether
the URI contains a telephone number or not.
To address this problem, the authentication service and verification
service both must perform the following canonicalization procedure on
any SIP URI they inspect which contains a wholly numeric user part.
[TBD: the algorithm] If the result of this procedure forms a complete
telephone number, that number is used for the purpose of creating and
signing the digest-string at the authentication service and
verification service. If the result 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 14.4.
This specification assumes that UACs will have an appropriate means
to discover an authentication service that can sign with a telephone
number certificate corresponding to the UAC's telephone number. Most
likely, this information will simply be provisioned in UACs.
Certificates that prove authority over telephone numbers should
contain the telephone number, or number range, in the [TBD] field of
the certificate. Verification services must compare the
Peterson, et al. Expires January 16, 2014 [Page 29]
Internet-Draft SIP Identity July 2013
canonicalized telephone number to the contents of the [TBD] field in
order to establish that the proper authority has signed the request.
[TBD: This would refer to an external specification, most likely]
In the longer term, it is possible that some directory or other
discovery mechanism may provide a way to determine which
administrative domain is responsible for a telephone number, and this
may aid in the signing and verification of SIP identities that
contain telephone numbers. This is a subject for future work.
13. Privacy Considerations
The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in RFC 3323
[RFC3323]. A SIP proxy server can act both as a privacy service and
as an authentication service. Since a user agent can provide any
From header field value that the authentication service is willing to
authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity
header is the same for private URIs as it is for any other sort of
URIs.
Note, however, that for using anonymous SIP URIs, an authentication
service must possess a certificate corresponding to the host portion
of the addr-spec of the From header field of the request;
accordingly, using domains like 'anonymous.invalid' will not be
possible for privacy services that also act as authentication
services. The assurance offered by the usage of anonymous URIs with
a valid domain portion is "this is a known user in my domain that I
have authenticated, but I am keeping its identity private". The use
of the domain 'anonymous.invalid' entails that no corresponding
authority for the domain can exist, and as a consequence,
authentication service functions are meaningless.
Peterson, et al. Expires January 16, 2014 [Page 30]
Internet-Draft SIP Identity July 2013
RFC 3325 [RFC3325] defines the "id" priv-value token, which is
specific to the P-Asserted-Identity header. The sort of assertion
provided by the P-Asserted-Identity header is very different from the
Identity header presented in this document. It contains additional
information about the sender of a message that may go beyond what
appears in the From header field; P-Asserted-Identity holds a
definitive identity for the sender that is somehow known to a closed
network of intermediaries that presumably the network will use this
identity for billing or security purposes. The danger of this
network-specific information leaking outside of the closed network
motivated the "id" priv-value token. The "id" priv-value token has
no implications for the Identity header, and privacy services MUST
NOT remove the Identity header when a priv-value of "id" appears in a
Privacy header.
Finally, note that unlike RFC 3325 [RFC3325], the mechanism described
in this specification adds no information to SIP requests that has
privacy implications.
14. Security Considerations
14.1. Handling of digest-string Elements
This document describes a mechanism that provides a signature over
the Date header field, and either the whole or part of the To and
From header fields of SIP requests, as well as optional protections
for the message body. While a signature over the From header field
would be sufficient to secure a URI alone, the additional headers
provide replay protection and reference integrity necessary to make
sure that the Identity header will not be used in cut-and-paste
attacks. In general, the considerations related to the security of
these headers are the same as those given in RFC 3261 [RFC3261] for
including headers in tunneled 'message/sip' MIME bodies (see
Section 23 in particular). The following section details the
individual security properties obtained by including each of these
header fields within the signature; collectively, this set of header
fields provides the necessary properties to prevent impersonation.
Peterson, et al. Expires January 16, 2014 [Page 31]
Internet-Draft SIP Identity July 2013
The From header field indicates the identity of the sender of the
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 RFC
3261 [RFC3261], Section 23.4.2. Implementations of this
specification MUST NOT deem valid a request with an outdated Date
header field (the RECOMMENDED interval is that the Date header must
indicate a time within 3600 seconds of the receipt of a message).
The result of this is that if an Identity header is replayed within
the Date interval, verifiers will recognize that it is invalid; if an
Identity header is 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
attacker and transformed into a MESSAGE request without changing any
fields covered by the Identity header, and moreover requests within a
certain transaction could be replayed in potentially confusing or
malicious ways.
RFC4474 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 against the simple unauthorized claiming
of a From header field value.
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
Peterson, et al. Expires January 16, 2014 [Page 32]
Internet-Draft SIP Identity July 2013
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.
This mechanism also provides an optional 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. While this is not strictly
necessary to prevent the impersonation attacks, there is little
purpose in establishing the identity of the user that originated a
SIP request if this assurance is not coupled with a comparable
assurance over the contents of the message. There are furthermore
some baiting attacks (where the attacker receives a request from the
target and reoriginates it to a third party) that might not be
prevented by only a signature over the From, To and Date, but could
be prevented by securing 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.
Peterson, et al. Expires January 16, 2014 [Page 33]
Internet-Draft SIP Identity July 2013
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. However, 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.
14.2. Display-Names and Identity
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
identity of the caller; there is a significant precedent in email
user interfaces for this practice. As such, it might seem that the
lack of a signature over the display-name is a significant omission.
However, there are several important senses in which a signature over
the display-name does not prevent impersonation. In the first place,
a particular display-name, like "Jon Peterson", is not unique in the
world; many users in different administrative domains might
legitimately claim that name. Furthermore, enrollment practices for
SIP-based services might have a difficult time discerning the
legitimate display-name for a user; it is safe to assume that
impersonators will be capable of creating SIP accounts with arbitrary
display-names. The same situation prevails in email today. Note
that an impersonator who attempted to replay a message with an
Identity header, changing only the display-name in the From header
field, would be detected by the other replay protection mechanisms
described in Section 14.1.
Of course, an authentication service can enforce policies about the
display-name even if the display-name is not signed. The exact
mechanics for creating and operationalizing such policies is outside
the scope of this document. The effect of this policy would not be
to prevent impersonation of a particular unique identifier like a SIP
URI (since display-names are not unique identifiers), but to allow a
domain to manage the claims made by its users. If such policies are
enforced, users would not be free to claim any display-name of their
choosing. In the absence of a signature, man-in-the-middle attackers
could conceivably alter the display-names in a request with impunity.
Note that the scope of this specification is impersonation attacks,
Peterson, et al. Expires January 16, 2014 [Page 34]
Internet-Draft SIP Identity July 2013
however, and that a man-in-the-middle might also strip the Identity
and Identity-Info headers from a message.
There are many environments in which policies regarding the display-
name aren't feasible. Distributing bit-exact and internationalizable
display-names to end-users as part of the enrollment or registration
process would require mechanisms that are not explored in this
document. In the absence of policy enforcement regarding domain
names, there are conceivably attacks that an adversary could mount
against SIP systems that rely too heavily on the display-name in
their user interface, but this argues for intelligent interface
design, not changes to the mechanisms. Relying on a non-unique
identifier for identity would ultimately result in a weak mechanism.
14.3. Securing the Connection to the Authentication Service
The assurance provided by this mechanism is strongest when a user
agent forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are
twofold:
If a user does not receive a certificate from the authentication
service over this TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a
mechanism such as Digest), then it is possible that a rogue server
is attempting to pose as an authentication service for a domain
that it does not control, possibly in an attempt to collect shared
secrets for that domain. A similar practice could be used for
telephone numbers, though the application of certificates for
telephone numbers to TLS is left as a matter for future study.
Without TLS, the various header field values and the body of the
request will not have integrity protection when the request
arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message
arbitrarily.
Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is possible that applications relying on
the presence of the Identity header could leverage this integrity
protection, especially body integrity, for services other than replay
protection.
Peterson, et al. Expires January 16, 2014 [Page 35]
Internet-Draft SIP Identity July 2013
Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS are not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association
with the authentication service is too weak.
14.4. Domain Names and Subordination
When a verifier processes a request containing an Identity-Info
header with a domain signature, it must compare the domain portion of
the URI in the From header field of the request with the domain name
that is the subject of the certificate acquired from the Identity-
Info header. While it might seem that this should be a
straightforward process, it is complicated by two deployment
realities. In the first place, certificates have varying ways of
describing their subjects, and may indeed have multiple subjects,
especially in 'virtual hosting' cases where multiple domains are
managed by a single application. Secondly, some SIP services may
delegate SIP functions to a subordinate domain and utilize the
procedures in RFC 3263 [RFC3263] that allow requests for, say,
'example.com' to be routed to 'sip.example.com'. As a result, a user
with the AoR 'sip:jon@example.com' may process requests through a
host like 'sip.example.com', and it may be that latter host that acts
as an authentication service.
To meet the second of these problems, a domain that deploys an
authentication service on a subordinate host MUST be willing to
supply that host with the private keying material associated with a
certificate whose subject is a domain name that corresponds to the
domain portion of the AoRs that the domain distributes to users.
Note that this corresponds to the comparable case of routing inbound
SIP requests to a domain. When the NAPTR and SRV procedures of RFC
3263 are used to direct requests to a domain name other than the
domain in the original Request-URI (e.g., for 'sip:jon@example.com',
the corresponding SRV records point to the service
'sip1.example.org'), the client expects that the certificate passed
back in any TLS exchange with that host will correspond exactly with
the domain of the original Request-URI, not the domain name of the
host. Consequently, in order to make inbound routing to such SIP
services work, a domain administrator must similarly be willing to
share the domain's private key with the service. This design
Peterson, et al. Expires January 16, 2014 [Page 36]
Internet-Draft SIP Identity July 2013
decision was made to compensate for the insecurity of the DNS, and it
makes certain potential approaches to DNS-based 'virtual hosting'
unsecurable for SIP in environments where domain administrators are
unwilling to share keys with hosting services.
A verifier MUST evaluate the correspondence between the user's
identity and the signing certificate by following the procedures
defined in RFC 2818 [RFC2818], Section 3.1. While RFC 2818 [RFC2818]
deals with the use of HTTP in TLS, the procedures described are
applicable to verifying identity if one 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 request with an Identity header.
Because the domain certificates that can be used by authentication
services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and
user agents will be able to support the root certificates of all
certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but
it is anticipated that operational experience will create de facto
standards for authentication services. Some federations of service
providers, for example, might only trust certificates that have been
provided by a certificate authority operated by the federation. It
is strongly RECOMMENDED that self-signed domain certificates should
not be trusted by verifiers, unless some previous key exchange has
justified such trust.
[TBD: DANE?]
For further information on certificate security and practices, see
RFC 3280 [RFC3280]. The Security Considerations of RFC 3280
[RFC3280] are applicable to this document.
14.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the domain that issues the
assurance. Relying on an Identity header generated by a remote
administrative domain assumes that the issuing domain used its
administrative practices to authenticate its users. However, it is
possible that some domains will implement policies that effectively
make users unaccountable (e.g., ones that accept unauthenticated
registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a
Peterson, et al. Expires January 16, 2014 [Page 37]
Internet-Draft SIP Identity July 2013
verifier to distinguish "good" from "bad" domains by inspecting a SIP
request, it is expected that further work in authorization practices
could be built on top of this identity solution; without such an
identity solution, many promising approaches to authorization policy
are impossible. That much said, it is RECOMMENDED that
authentication services based on proxy servers employ strong
authentication practices such as token-based identifiers.
One cannot expect the Identity and Identity-Info headers to be
supported by every SIP entity overnight. This leaves the verifier in
a compromising position; when it receives a request from a given SIP
user, how can it know whether or not the sender's domain supports
Identity? In the absence of ubiquitous support for identity, some
transitional strategies are necessary.
A verifier could remember when it receives a request from a domain
that uses Identity, and in the future, view messages received from
that domain without Identity headers with skepticism.
A verifier could query the domain through some sort of callback
system to determine whether or not it is running an authentication
service. There are a number of potential ways in which this could
be implemented; use of the SIP OPTIONS method is one possibility.
This is left as a subject for future work.
In the long term, some sort of identity mechanism, either the one
documented in this specification or a successor, must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected by verifiers.
Finally, it is worth noting that the presence or absence of the
Identity headers cannot be the sole factor in making an authorization
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
request. Authorization policies are outside the scope of this
specification, but this specification advises any future
authorization work not to assume that messages with valid Identity
headers are always good.
15. IANA Considerations
This document requests changes to the header and response-code sub-
registries of the SIP parameters IANA registry, and requests the
creation of two new registries for parameters for the Identity-Info
header.
15.1. Header Field Names
Peterson, et al. Expires January 16, 2014 [Page 38]
Internet-Draft SIP Identity July 2013
This document specifies two new SIP headers: Identity and Identity-
Info. Their syntax is given in Section 10. These headers are
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
Compact Form: y
Header Name: Identity-Info
Compact Form: n
15.2. 428 'Use Identity Header' Response Code
This document registers a new SIP response code, which is described
in Section 7. 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
15.3. 436 'Bad Identity-Info' Response Code
This document registers a new SIP response code, which is described
in Section 7. 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
15.4. 437 'Unsupported Certificate' Response Code
This document registers a new SIP response code, which is described
in Section 7. It is used when the verifier cannot validate the
certificate referenced by the URI of the Identity-Info header,
because, for example, the certificate is self-signed, or signed by a
root certificate authority for whom the verifier does not possess a
Peterson, et al. Expires January 16, 2014 [Page 39]
Internet-Draft SIP Identity July 2013
root certificate. 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 Certificate
15.5. 438 'Invalid Identity Header' Response Code
This document registers a new SIP response code, which is described
in Section 7. 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
Default Reason Phrase: Invalid Identity Header
15.6. Identity-Info Parameters
The IANA has created a new registry for Identity-Info headers. This
registry is to be prepopulated with a single entry for a parameter
called 'alg', which describes the algorithm used to create the
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.
15.7. Identity-Info Algorithm Parameter Values
The IANA has created a new registry for Identity-Info 'alg' parameter
values. This registry is to be prepopulated with a single entry for
a value called 'rsa-sha1', which describes the algorithm used to
create the signature that appears in the Identity header. Registry
entries must contain the name of the 'alg' parameter value and the
specification in which the value is described. New values for the
'alg' parameter may be defined only in Standards Track RFCs.
15.8. Acknowledgements
The authors would like to thank
15.9. Original RFC 4474 Requirements
Peterson, et al. Expires January 16, 2014 [Page 40]
Internet-Draft SIP Identity July 2013
The following requirements were crafted throughout the development of
the mechanism described in this document. They are preserved here
for historical reasons.
The mechanism must allow a UAC or a proxy server to provide a
strong cryptographic identity assurance in a request that can be
verified by a proxy server or UAS.
User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup.
User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to requests.
Proxy servers that hold certificates on behalf of their domain
must be capable of adding this identity assurance to requests; a
UAC is not required to support this mechanism in order for an
identity assurance to be added to a request in this fashion.
The mechanism must prevent replay of the identity assurance by an
attacker.
In order to provide full replay protection, the mechanism must be
capable of protecting the integrity of SIP message bodies (to
ensure that media offers and answers are linked to the signaling
identity).
It must be possible for a user to have multiple AoRs (i.e.,
accounts or aliases) that it is authorized to use within a domain,
and for the UAC to assert one identity while authenticating itself
as another, related, identity, as permitted by the local policy of
the domain.
16. References
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
Peterson, et al. Expires January 16, 2014 [Page 41]
Internet-Draft SIP Identity July 2013
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
September 2004.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
16.2. Informative References
[I-D.cooper-iab-secure-origin-00]
Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba,
"Secure Call Origin Identification", draft-cooper-iab-
secure-origin-00 (work in progress), November 2012.
[I-D.peterson-secure-origin-ps]
Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Origin Identification: Problem Statement, Requirements,
and Roadmap", draft-peterson-secure-origin-ps-00 (work in
progress), May 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.rescorla-callerid-fallback]
Rescorla, E., "Secure Caller-ID Fallback Mode", draft-
rescorla-callerid-fallback-00 (work in progress), May
2013.
[I-D.rescorla-rtcweb-generic-idp]
Rescorla, E., "RTCWEB Generic Identity Provider
Interface", draft-rescorla-rtcweb-generic-idp-01 (work in
progress), March 2012.
Peterson, et al. Expires January 16, 2014 [Page 42]
Internet-Draft SIP Identity July 2013
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery
System (DDDS) Application (ENUM)", RFC 3761, April 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP)
Torture Test Messages", RFC 4475, May 2006.
[RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 6919,
April 1 2013.
Authors' Addresses
Jon Peterson
NeuStar
Email: jon.peterson@neustar.biz
Peterson, et al. Expires January 16, 2014 [Page 43]
Internet-Draft SIP Identity July 2013
Cullen Jennings
Cisco
400 3rd Avenue SW, Suite 350
Calgary, AB T2P 4H2
Canada
Email: fluffy@iii.ca
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com
Peterson, et al. Expires January 16, 2014 [Page 44]
