SIP WG J. Peterson
Internet-Draft NeuStar
Expires: August 17, 2005 C. Jennings
Cisco Systems
February 16, 2005
Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)
draft-ietf-sip-identity-04
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
The existing security mechanisms in the Session Initiation Protocol
are inadequate for cryptographically assuring the identity of the end
users that originate SIP requests, especially in an interdomain
context. This document recommends practices and conventions for
identifying end users in SIP messages, and proposes a way to
distribute cryptographically-secure authenticated identities.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
6. Authentication Service Behavior . . . . . . . . . . . . . . 7
6.1 Identity within a Dialog and Retargeting . . . . . . . . . 9
7. Verifying Identity . . . . . . . . . . . . . . . . . . . . . 11
8. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 12
9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . 12
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 13
11. Compliance Tests and Examples . . . . . . . . . . . . . . . 15
11.1 Identity-Info with a Singlepart MIME body . . . . . . . 16
11.2 Identity for a Request with no MIME body or Contact . . 18
12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 21
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
14. Security Considerations . . . . . . . . . . . . . . . . . . 23
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27
15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 27
15.2 428 'Use Identity Header' Response Code . . . . . . . . 27
15.3 436 'Bad Identity-Info' Response Code . . . . . . . . . 28
15.4 437 'Unsupported Certificate' Response Code . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 30
B. Bit-exact archive of example messages . . . . . . . . . . . 30
B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 31
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
16.1 Normative References . . . . . . . . . . . . . . . . . . . 28
16.2 Informative References . . . . . . . . . . . . . . . . . . 29
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . 35
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1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP [1]). An identity, for the purposes of this document, is
defined as a canonical SIP address-of-record URI employed to reach a
user (such as 'sip:alice@atlanta.example.com').
RFC3261 stipulates several places within a SIP request that 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
accurately, in the absence of some sort of cryptographic
authentication mechanism.
RFC3261 specifies a number of security mechanisms that can be
employed by SIP UAs, including Digest, 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 TLS or S/MIME, and
furthermore Digest authentication is limited by the fact that the
originator and destination must share a pre-arranged 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 their displayed Caller-ID is accurate.
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 RFC2119 [2] and indicate requirement levels for
compliant SIP implementations.
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 which 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 begin sending presence notifications. In both of these cases,
attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of
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a SIP request, the From header field, can be populated arbitrarily be
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 in which authorization policies cannot be
circumvented by impersonation.
All RFC3261-compliant SIP user agents support a means of
authenticating themselves to a SIP registrar, commonly with a shared
secret; Digest authentication, which MUST be supported by SIP user
agents, is typically used for this purpose. Registration allows a
user agent to express that it is an appropriate entity to which
requests should be sent for a particular address-of-record SIP URI
(e.g., 'sip:alice@atlanta.example.com').
By the definition of identity used in this document, registration is
a proof of the identity of the user to a registrar. However, the
credentials with which a user agent proves their 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 address-of-record URI used for registration is also the URI with
which a UA commonly populates the From header of requests in order to
provide a 'return address' identity to recipients. The identity
mechanism specified in this document derives from the following
principle: if you can prove you are eligible to register in a domain
under a particular address-of-record, you are proving that you are
capable of legitimately receiving requests for that
address-of-record, and accordingly, when you place that
address-of-record 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.
Ideally, then, SIP user agents should have some way of proving to
recipients of SIP requests that their local domain 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 to a server in the user's local domain, which
authenticates such requests (using 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
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communicate to other SIP entities that the sending user has been
authenticated and their use of the From header field has been
authorized. This draft addresses how that imprimatur of
authentication can be shared.
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 [8]. 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
context which 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
act as an authentication service. To maximize end-to-end security,
it is obviously preferable for end users to hold their own
certificates; 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 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
certificates 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
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instantiate the authentication service role.
4. Requirements
This draft addresses the following requirements:
o The mechanism must allow a UAC to provide a strong cryptographic
identity assurance in a request that can be verified by a proxy
server or UAS.
o User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup.
o User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to requests.
o 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.
o The mechanism must prevent replay of the identity assurance by an
attacker.
o 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).
o It must be possible for a user to have multiple AoRs (i.e.
accounts or aliases) under which it is known at 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.
o It must be possible, in cases where a request has been retargeted
to a different AoR than the one designated in the To header field,
for the UAC to ascertain the AoR to which the request has been
sent.
5. Overview of Operations
This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document.
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 populate the value of the From header field (which may be Alice's
AoR, or it may be some other value that the policy of the proxy
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server permits her to use). It then computes a hash over some
particular headers, including the From header field and the bodies in
the message. This hash is signed with the certificate for the domain
(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 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)
which appears in the From header field. The proxy also inserts a
companion header field 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 authenticate that the
domain indicated by the host portion of the AoR in the From header
field authenticated the user, and permitted them to assert that From
header field value.
6. Authentication Service Behavior
This document defines a new 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 domain certificate, and MUST be capable of authenticating
one or more SIP users that can register in that domain. 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 access to SIP registrar capabilities
that allow them to authenticate users in their domain. It is also
possible that the authentication service role might be instantiated
by 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.
Similarly, authentication services MUST add a Content-Length header
field to SIP requests if one is not already present; this can help
the verifier to double-check that they are hashing exactly as many
bytes of message-body as the authentication service when they verify
the message.
The authentication service authenticates the identity of the message
sender and validates that the identity given in the message can
legitimately be asserted by the sender. Then it computes a signature
over the canonical form of several headers and all the bodies, and
inserts this signature into the message.
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First, an 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 SIPS URI,
the authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is
responsible. If the identity field uses the TEL URI scheme, the
policy of the authentication service determines whether or not it is
responsible for this identity; 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.
Second, the authentication service needs to 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:
o 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 Section 22.3).
o 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 domain, or by preferably by providing a password that unlocks
said private key.
Authorization of the use of a particular username in the From header
field is a matter of local policy for the authorization service, one
which depends greatly on the manner in which authentication is
performed. For example, 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 which 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 which might
legitimately be asserted for that 'username'. 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. Various
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exceptions to such policies might arise for cases like anonymity; if
the AoR asserted in the From header field is anonymous (per RFC3323
[3]), then the 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'). Some SIP user agents that receive requests render the
display-name of the caller as the identity of the caller. However,
there are many environments in which legislating the display-name
isn't feasible, judging from experience with email, where users
frequent make slight textual changes to their display-names.
Ultimately, there is more value in focusing on the SIP address of the
sender (which has some meaning in the network and provides a chain of
accountability) than trying to constrain how the display-name is set.
As such, authentication services MAY check 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 'Inappropriate
Display-Name' response code. However, in many environments this will
not make sense. For more information on rendering identity in a user
interface, see Section 8.
Third, 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 are provided in section Section 10.
Finally, the authentication service MUST forward the message
normally.
6.1 Identity within a Dialog and Retargeting
Retargeting, the alteration by intermediaries of the Request-URI of a
SIP request, 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
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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 should process/forward the request normally.
If there were a means in backwards-direction requests to signify a
'connected party', an identity of the unanticipated user whose SIP
endpoint was reached by the dialog-forming request, it isn't clear
that it would actually be beneficial to provide a corresponding
Identity header signature over that information. The Identity header
is designed to prevent impersonation-based attacks, and it is very
unclear how and why an attacker might attempt to impersonate an
unanticipated third party in a backwards-direction request within an
existing dialog. That is, it's unclear how the caller's potential
authorization policies would be any more successful at thwarting
impersonation if new requests in the backwards direction came from an
assured unanticipated third-party instead of an unassured
unanticipated third-party. Thwarting impersonation is, ultimately,
the purpose of this Identity mechanism, and it must be left to other
mechanisms to solve other security problems for SIP.
The mechanism in this draft cannot aid in determining whether or not
the unanticipated party is an appropriate target of this request and,
accordingly, solving this problem is outside the scope of this draft.
If, however, it were possible for the sender of the dialog-forming
request to anticipate that retargeting had occurred, and to gain some
kind of assurance of the new target of the request before any
requests in the backwards direction were received, this would open up
some new approaches to authorization policy.
Any such means of anticipating retargeting and so on is outside the
scope of this document, and likely to have equal applicability to
response identity as it does to requests in the backwards direction
within a dialog. Consequently, no special guidandance 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.
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7. Verifying Identity
When a user agent or proxy server receives a SIP message containing
an Identity header, it may inspect the signature to verify the
identity of the sender of the message. If an Identity header is not
present in a request, and one is required by local policy (for
example, based on a global policy, 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, the user
agent or proxy server MUST first acquire the certificate for the
signing domain. Implementations supporting this specification should
have some means of retaining domain 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 domain is received.
Certificates retained in this manner should be indexed by the URI
given in the Identity-Info header field value.
Provided that the domain certificate used to sign this message is not
previously known to the recipient, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header, unless they
have some more efficient implementation-specific way of acquiring
certificates for that domain. If the URI scheme in the Identity-Info
header cannot be dereferenced, then a 436 'Bad Identity-Info'
response MUST be returned. The client processes this certificate in
the usual ways, including checking that it has not expired, that the
chain is valid back to a trusted CA, and that it does not appear on
revocation lists. Once the certificate is acquired, it MUST be
validated. If the certificate cannot be validated (it is self-signed
and untrusted, or signed by an untrusted or unknown certificate
authority), the verifier MUST send a 437 'Unsupported Certificate'
response.
Subsequently, the recipient MUST verify the signature in the Identity
header, and compare the identity of the signer (the subjectAltName of
the certificate) with the domain portion of the URI in the From
header field of the request as described in Section 14.
Additionally, the Date, Contact and Call-ID headers MUST be analyzed
in the manner described in Section 14; recipients that wish to verify
Identity signatures MUST support all of the operations described
there.
If a verifier determines that the signature on the message does not
correspond to the text of the message, then a 428 'Invalid Identity
Header' response MUST be returned.
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Once the identity of the sender of a request has been ascertained,
various policies MAY be used to make authorization decisions about
accepting communications and the like. Such policies are outside the
scope of this document.
8. User Agent Behavior
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 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 header
field that asserts its identity (for a SIP request, this is the From
header field). In a request it MUST set the URI portion of its From
header to match a SIP, SIPS or TEL URI AoR under which the UAC can
register (including anonymous URIs, as described in RFC 3323 [3]).
In general, UACs SHOULD NOT use the TEL URI form in the From header
field (see Section 12).
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. UAC implementations MUST provide a way of
provisioning pre-loaded Route headers in order for this mechanism to
work for mid-call requests in the backwards direction of a dialog.
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. Proxy Server Behavior
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 authentication service, it MAY verify the existence of an
Identity before it makes a forwarding decision for a request. Proxy
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servers MUST NOT remove or modify an existing Identity or
Identity-Info header in a request.
For the purposes of identifying mid-dialog requests, proxy servers
that instantiate the authentication service role MUST Record-Route
themselves in dialog-forming requests.
10. Header Syntax
This document specifies two new SIP headers: Identity and
Identity-Info. Each of these headers can appear only once in a SIP
message.
Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info
ident-info = LAQUOT absoluteURI RAQUOT
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 placed in a bit-exact string in the order specified
here, separated by a colon:
o The AoR of the UA sending the message, or the 'identity field'.
For a request, this is the addr-spec from the From header field.
o The addr-spec component of the To header field, which is the AoR
to which the request is being sent.
o The callid from Call-Id header field.
o The digit (1*DIGIT) and method (method) portions from CSeq header
field, separated by a single space (ABNF SP, or %x20). Note that
the CSeq header field allows LWS rather than SP to separate the
digit and method portions, and thus the CSeq header field may need
to be transformed in order to be canonicalized. The
authentication service MUST strip leading zeros from the 'digit'
portion of the Cseq before generating the digest-string.
o 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 3261. The
first letter is upper case and the rest of the letters are lower
case. All requests that use the Identity mechanism MUST contain a
Date header.
o The addr-spec component of the Contact header field value. If the
request does not contain a Contact header, this field MUST be
empty (i.e., there will be no whitespace between the fourth and
fifth colons in the canonical string).
o The body content of the message with the bits exactly as they are
in the Message (in the ABNF for SIP, the message-body). Note that
the message-body does NOT include the CRLF separating the SIP
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headers from the message-body, but does include everything that
follows that CRLF. If the message has no body, then message-body
will be empty, and the final colon will not be followed by any
additional characters.
For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 14 and [5].
The precise formulation of this digest-string is, therefore
(following the ABNF [6] in RFC3261):
digest-string = addr-spec ":" addr-spec ":" callid ":" 1*DIGIT SP method ":"
SIP-Date ":" [ addr-spec ] ":" message-body
Note again that the first addr-spec MUST be taken from the From
header field value, the second addr-spec MUST be taken from the To
header field value, and the third addr-spec MUST be taken from the
Contact header field value, provided the Contact header is present in
the request.
After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain, as follows: compute the results of
signing this string with sha1WithRSAEncryption as described in RFC
3370 and base64 encode the results as specified in RFC 3548. Put the
result in the Identity header.
Note on this choice: Assuming a 1024 bit RSA key, the raw signature
will result in about 170 octets of base64 encoded data (without
base64, as an aside, it would be about 130 bytes). For comparison's
sake, a typical HTTP Digest Authorization header (such as those used
in RFC3261) with no cnonce is around 180 octets. From a speed point
of view, a 2.8GHz Intel processor does somewhere in the range of 250
RSA 1024 bits signs per second or 1200 RSA 512 bits signs; verifies
are roughly 10 times faster. Hardware accelerator cards are
available that speed this up.
The Identity-Info header MUST contain either an HTTPS URI or a SIPS
URI. If it contains an HTTPS URI, the URI must dereference to a
resource that contains a single MIME body containing the certificate
of the authentication service. If it is a SIPS URI, then the
authentication service intends for a user agent that wishes to fetch
the certificate to form a TLS connection to that URI, acquire the
certificate during normal TLS negotiation, and close the connection.
This document adds the following entries to Table 2 of [1]:
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Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity R a 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 -
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
Note, in the table above, that this mechanism does not protect the
REGISTER method or 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
REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to this mechanism. Accordingly, the
Identity and Identity-Info header MUST NOT appear in REGISTER or
CANCEL.
11. Compliance Tests and Examples
The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementations MUST
verify their compliance with these examples, i.e.:
o 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.
o 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 appear here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD
generated appropriate warnings about the use of self-signed
certificates.
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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'.
-----BEGIN RSA PRIVATE KEY-----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-----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:
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INVITE sip:bob@biloxi.exmple.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
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, in 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:a84b4c76e66710:314159 INVITE:Thu, 21 Feb 2002 13:02:03 GMT:alice@pc33.atlanta.example.com: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
The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is the following:
CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
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
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like:
INVITE sip:bob@biloxi.exmple.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: "CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2s
N3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGC
lsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI="
Identity-Info: https://atlanta.example.com/cert
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 de-references the URL 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 de-referencing 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".
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-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----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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
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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 calcuating 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
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
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 colons. 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:
A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=
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:
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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: "A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlr
ZOuwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9
BfxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM="
Identity-Info: https://biloxi.example.org/cert
Content-Length: 0
biloxi.example.org then forwards the request normally.
12. Identity and the TEL URI Scheme
Since many SIP applications provide a VoIP service, telephone numbers
are commonly used as identities in SIP deployments. In the majority
of cases, this is not problematic for the identity mechanism
described in this document. Telephone numbers commonly appear in the
username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com'). That username conforms to
the syntax of the TEL URI scheme (RFC2806bis [9]). For this sort of
SIP address-of-record, chicago.example.com is the appropriate
signatory.
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 this case, it is much less clear which
signatory is appropriate for the identity. Fortunately for the
identity mechanism, this form of the TEL URI is more common for the
To header field and Request-URI in SIP than in the From header field,
since the UAC has no option but to provide a TEL URI alone when the
remote domain to which a request is sent is unknown. The local
domain, however, is usually known by the UAC, and accordingly it can
form a proper From header field containing a SIP URI with a username
in TEL URI form. Implementations that intend to send their requests
through an authentication service MUST put telephone numbers in the
From header field into SIP or SIPS URIs, if possible.
If the local domain is unknown to a UAC formulating a request, it
most likely will not be able to locate an authentication service for
its request, and therefore the question of providing identity in
these cases is somewhat moot. However, an authentication service MAY
sign a request containing a TEL URI in the From header field in
accordance with its local policies. Verifiers SHOULD NOT accept
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signatures over From header TEL URIs in the absence of some
pre-provisioned relationship with the signing domain that authorizes
this usage of TEL URIs.
The guidance in the paragraph above is largely provided for forward
compatibility. In the longer-term, it is possible that ENUM [10] 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 draft is compatible with the
standard SIP practices for privacy described in RFC3323 [3]. 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 which the authentication service is willing to
authorize, there is no reason why private SIP URIs (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 an authentication service must possess a
certificate corresponding to the host portion of the addr-spec of the
From header field of any request that it signs; accordingly, using
domains like 'invalid.net' may not be possible for privacy services
that also act as authentication services. The assurance offered by
this combination service is "this is a known user in my domain that I
have authenticated, but I am keeping their identity private".
The "header" level of privacy described in RFC3323 requests that a
privacy service to alter the Contact header field value of a SIP
message. Since the Contact header field is protected by the
signature in an Identity header, privacy services cannot be applied
after authentication services without a resulting integrity
violation.
RFC3325 [8] 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 which 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
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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.
14. Security Considerations
This document describes a mechanism which provides a signature over
the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
messages. 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 RFC3261 for including headers in
tunneled 'message/sip' MIME bodies (see Section 23 in particular).
The From header field indicates the identity of the sender of the
message, and the SIP address-of-record URI 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 servers two purposes: first, it prevents replay 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 and Contact headers provide reference integrity and replay
protection, as described in 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). Implementations MUST also record Call-IDs received in
valid requests containing an Identity header, and MUST remember those
Call-IDs for at least the duration of a single Date interval (i.e.
commonly 3600 seconds). Accordingly, if an Identity header is
replayed within the Date interval, receivers will recognize that it
is invalid because of a Call-ID duplication; if an Identity header is
replayed after the Date interval, receivers will recognize that it is
invalid because the Date is stale. The CSeq header field contains a
numbered identifier for the transaction, and the name of the method
of the request; without this information, an INVITE request could be
cut-and-pasted by an attacker and transformed into a BYE 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.
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The Contact header field is included to tie the Identity header to a
particular device instance that generated the request. Were an
active attacker to intercept a request containing an Identity header,
and cut-and-paste the Identity header field into their own request
(reusing the From, To, Contact, Date and Call-ID fields that appear
in the original message), they would not be eligible to receive SIP
requests from the called user agent, since those requests are routed
to the URI identified in the Contact header field. However, the
Contact header is only included in dialog-forming requests, so it
does not provide this protection in all cases.
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 their own in a
cut-and-paste attack, which would cause all responses to the request
to be routed to a host of the attacker's choosing. However, a
signature over the topmost Via header does not prevent attacks of
this nature, since the attacker could leave the topmost Via intact
and merely insert a new Via header field directly after it, which
would cause responses to be routed to the attacker's host "on their
way" to the valid host, which has exactly the same end result.
Although it is possible that an intermediary-based authentication
service could guarantee that no Via hops are inserted between the
sending user agent and the authentication service, it could not
prevent an attacker from adding a Via hop after the authentication
service, and accordingly pre-empting 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.
Note that this mechanism does not provide any protection for the
display-name portion of the From header field, and thus users are
free to use any display-name of their choosing, and attackers could
conceivably alter the display-names in a request with impunity. If
an administrative domain wants to control the display-names selected
by users, they could do so with policies outside the scope of this
document (for example, their authentication service could reject
requests from valid users that contain an improper display-name in
the From header field). While there are conceivably attacks that an
adversary could mount against SIP systems that rely too heavily on
the display-name in their user interface, this argues for intelligent
interface design, not changes to the protocol.
This mechanism also provides a signature over the bodies of SIP
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requests. The most important reason for doing so is to protect SDP
bodies carried in SIP requests. There is little purpose in
establishing the identity of the user agent that originated a SIP
request if a man-in-the-middle can change the SDP and direct media to
an different IP address. Note however that this is not perfect
end-to-end security. The authentication service itself, when
instantiated at a intermediary, could conceivably change the SDP (and
SIP headers, for that matter) before providing a signature. Thus,
while this mechanism reduces the chance that a man-in-the-middle will
interfere with sessions, it does not eliminate it entirely. Since it
is a foundational assumption of this mechanism that the user trusts
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. Note that RFC3261 16.6 states that SIP proxy servers
"MUST NOT add to, modify, or remove the message body."
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 they receive a challenge via a mechanism
such as Digest), then it is possible that a rogue server is
attempting to pose as a authentication service for a domain that
it does not control, possibly in an attempt to collect shared
secrets for that domain.
Without TLS, the various header field values and the body of the
request will not have integrity protection into 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 likely that applications building on
the Identity header could leverage this integrity protection,
especially body integrity, to provide further security services.
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 is not possible, a UAC should use
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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.
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 uses some
trustworthy practice to authenticate its users. However, it is
possible that some domains will implement policies that effectively
make users unaccountable (such as accepting unauthenticated
registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a
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.
Since a domain certificate is used by an authentication service
(rather than individual certificates for each identity), certain
problems can arise with name subordination. For example, if an
authentication service holds a common certificate for the hostname
'sip.atlanta.example.com', can it legitimately sign a token
containing an identity of 'sip:alice@atlanta.example.com'? It is
difficult for the recipient of a request to ascertain whether or not
'sip.atlanta.example.com' is authoritative for the
'atlanta.example.com' domain unless the recipient has some
foreknowledge of the administration of 'atlanta.example.com'.
Therefore, it is RECOMMENDED that UASs receiving signed requests
notify end users if there is ANY discrepancy between the
subjectAltName of the signer's certificate and the host portion of
the identity within the From header field. If the domain name in the
subject of the certificate is subordinate to the domain name in the
identity URI, then verifiers may consider this a minor discrepancy.
Additionally, there are ways that a verifier might leverage the
information about canonical SIP servers within a domain stored in the
DNS (see RFC3263 [4]) to determine whether or not a particular
authentication service is authoritative for a domain; however, this
is a subject for future work.
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
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certificates for this mechanism. However, not all proxy servers and
user agents will be able 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 pre-existing key exchange
has justified such trust.
Finally, the Identity and Identity-Info headers cannot protect
themselves. Any attacker could remove these headers from a SIP
request, and modify the request arbitrarily afterwards. Accordingly,
these headers are only truly efficacious if the would-be verifier
knows that they must be included in a request. In the long term,
some sort of identity mechanism along these lines must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected. In the
interim, however, identity reception policies at a domain level or an
address-book level should be used by verifiers to determine whether
or not identity is expected from a particular source of SIP requests.
Those authorization policies are outside the scope of this document.
15. IANA Considerations
This document requests changes to the header and response-code
sub-registries of the SIP parameters IANA registry.
15.1 Header Field Names
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 is to be 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: (none)
15.2 428 'Use Identity Header' Response Code
This document registers a new SIP response code which is described in
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Section 7. It is used when a verifier received a SIP request that
lacks an Identity header as a response indicating that the request
should be re-sent with an Identity header. This response code is
defined by the following information, which is to be 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 is to be 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
root certificate. This response code is defined by the following
information, which is to be 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
16. References
16.1 Normative References
[1] 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.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
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[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[5] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004.
[6] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997.
16.2 Informative References
[7] Kohl, J. and C. Neumann, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[8] 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.
[9] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966,
December 2004.
[10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS
Application", RFC 3761, April 2004.
Authors' Addresses
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
EMail: fluffy@cisco.com
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Appendix A. Acknowledgments
The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell,
and Aki Niemi for their comments. The bit-archive presented in
Appendix B follows the pioneering example of Robert Sparks'
torture-test draft.
Appendix B. Bit-exact archive of example messages
The following text block is an encoded, gzip compressed TAR archive
of files that represent the transformations performed on the example
messages discussed in Section 11. It includes for each example:
o (foo).message: the original message
o (foo).canonical: the canonical string constructed from that
message
o (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
o (foo).signed: the RSA-signed SHA1 hash of the canonical string
(binary)
o (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
of the canonical string as it would appear in the request
o (foo).identity: the original message with the Identity and
Identity-Info headers added
Also included in the archive are two public key/certificate pairs,
for atlanta.example.com and biloxi.example.org, respectively,
including:
o (foo).cert: the certificate of the domain
o (foo).privkey: the private key of the domain
o (foo).pubkey: the public key of the domain, extracted from the
cert file for convenience
To recover the compressed archive file intact, the text of this
document may be passed as input to the following Perl script (the
output should be redirected to a file or piped to "tar -xzvf -").
#!/usr/bin/perl
use strict;
my $bdata = "";
use MIME::Base64;
while(<>) {
if (/-- BEGIN MESSAGE ARCHIVE --/ .. /-- END MESSAGE ARCHIVE --/) {
if ( m/^\s*[^\s]+\s*$/) {
$bdata = $bdata . $_;
}
}
}
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print decode_base64($bdata);
Alternatively, the base-64 encoded block can be edited by hand to
remove document structure lines and fed as input to any base-64
decoding utility.
B.1 Encoded Reference Files
-- BEGIN MESSAGE ARCHIVE --
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-- END MESSAGE ARCHIVE --
Appendix C. Changelog
NOTE TO THE RFC-EDITOR: Please remove this section prior to
publication as an RFC.
Changes from draft-ietf-sip-identity-03:
- Softened requirement for TLS and direct connections; now
SHOULD-strength, SIPS and Digest auth-int listed as alternatives.
- Added non-normative section about authentication service
behavior for backwards-direction requests within a dialog
- Added support for CID URI in Identity Info
- Added new response codes (436 and 437) corresponding to error
cases for an unsupported URI scheme and an unsupported
certificate, respectively
Changes from draft-ietf-sip-identity-02:
- Extracted text relating to providing identity in SIP responses;
this text will appear in a separate draft
- Added compliance testing/example section
- Added CSeq to the signature of the Identity header to prevent a
specific cut-and-paste attack; also added addr-spec of the To
header to the signature of the Identity header for similar reasons
- Added text about why neither Via headers nor display-names are
protected by this mechanism
- Added bit-exact reference files for compliance testing
- Added privacy considerations
Changes from draft-ietf-sip-identity-01:
- Completely changed underlying mechanism - instead of using an
AIB, the mechanism now recommends the use of the Identity header
and Identity-Info header
- Numerous other changes resulting from the above
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- Various other editorial corrections
Changes from draft-peterson-sip-identity-01:
- Split off child draft-ietf-sip-authid-body-00 for defining of
the AIB
- Clarified scope in introduction
- Removed a lot of text that was redundant with RFC3261
(especially about authentication practices)
- Added mention of content indirection mechanism for adding token
to requests and responses
- Improved Security Considerations (added piece about credential
strength)
Changes from draft-peterson-sip-identity-00:
- Added a section on authenticated identities in responses
- Removed hostname convention for authentication services
- Added text about using 'message/sip' or 'message/sipfrag' in
authenticated identity bodies, also RECOMMENDED a few more headers
in sipfrags to increase reference integrity
- Various other editorial corrections
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