Internet DRAFT - draft-jennings-stir-rfc4474bis
draft-jennings-stir-rfc4474bis
Network Working Group J. Peterson
Internet-Draft NeuStar
Intended status: Standards Track C. Jennings
Expires: August 18, 2014 Cisco
E. Rescorla
RTFM, Inc.
February 14, 2014
Authenticated Identity Management in the Session Initiation Protocol
(SIP)
draft-jennings-stir-rfc4474bis-01.txt
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 August 18, 2014.
Copyright Notice
Copyright (c) 2014 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
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(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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of Operations . . . . . . . . . . . . . . . . . . . 5
4. Signature Generation and Validation . . . . . . . . . . . . . 6
4.1. Authentication Service Behavior . . . . . . . . . . . . . 6
4.1.1. Intermediary Authentication Services . . . . . . . . 9
4.2. Verifier Behavior . . . . . . . . . . . . . . . . . . . . 10
4.3. Identity within a Dialog and Retargeting . . . . . . . . 11
5. Credentials . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Credential Use by the Authentication Service . . . . . . 12
5.2. Credential Use by the Verification Service . . . . . . . 13
5.3. Handling Identity-Info URIs . . . . . . . . . . . . . . . 14
5.4. Credential Systems . . . . . . . . . . . . . . . . . . . 14
6. Identity Types . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Telephone Numbers . . . . . . . . . . . . . . . . . . . . 15
6.2. Usernames with Domain Names . . . . . . . . . . . . . . . 16
7. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
10.1. Handling of digest-string Elements . . . . . . . . . . . 22
10.2. Securing the Connection to the Authentication Service . 24
10.3. Authorization and Transitional Strategies . . . . . . . 25
10.4. Display-Names and Identity . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11.1. Header Field Names . . . . . . . . . . . . . . . . . . . 27
11.2. 428 'Use Identity Header' Response Code . . . . . . . . 27
11.3. 436 'Bad Identity-Info' Response Code . . . . . . . . . 27
11.4. 437 'Unsupported Credential' Response Code . . . . . . . 28
11.5. 438 'Invalid Identity Header' Response Code . . . . . . 28
11.6. Identity-Info Parameters . . . . . . . . . . . . . . . . 28
11.7. Identity-Info Algorithm Parameter Values . . . . . . . . 28
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
13. Changes from RFC4474 . . . . . . . . . . . . . . . . . . . . 29
14. Informative References . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP, RFC 3261 [1]). 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 [1] 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 [1] 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 calling party number (and/or 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.
2. 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 calling party number
(and/or Caller-ID) of a caller, which a human may review to make a
policy decision before answering a call. An example of the former
would be a voicemail service that compares the identity of the caller
to a whitelist before determining whether it should allow the caller
access to recorded messages. 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
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controller of a user agent, impersonation is very simple today. The
mechanism described in this document provides a strong identity
system for SIP requests in which authorization policies cannot be
circumvented by impersonation.
This document proposes an authentication architecture for SIP in
which requests are processed by a logical authentication service that
may be implemented as part of a user agent or as a proxy server.
Once a message has been authenticated, the service then adds new
cryptographic information to requests to communicate to other SIP
entities that the sending user has been authenticated and its use of
the From header field has been authorized.
But authorized by whom? Identities are issued to users by
authorities. When a new user becomes associated with example.com,
the administrator of the SIP service for that domain will issue them
an identity in that namespace, such as alice@example.com. Alice may
then send REGISTER requests to example.com that make her user agents
eligible to receive requests for sip:alice@example.com. In some
cases, Alice may be the owner of the domain herself, and may issue
herself identities as she chooses. But ultimately, it is the
controller of the SIP service at example.com that must be responsible
authorizing the use of names in the example.com domain. Therefore,
the credentials needed to prove this authorization must ultimately
derive from the domain owner: either a user agent gives requests to
the domain name owner in order for them to be signed by the domain
owner's credentials, or the user agent must possess credentials that
prove in some fashion that the domain owner has given the user agent
the right to a name.
The situation is however more complicated for telephone numbers.
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 currently reflected on the Internet.
Telephone numbers do not share the domain-scope 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 is the identity of the user. As with SIP URIs, the
necessary credentials to prove authority for a name might reside
either in the endpoint or at some intermediary.
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 authentication service adding a SIP header, the
Identity header. In order to assist in the validation of this
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assurance, this specification also describes an Identity-Info header
that can be used by the recipient of a request to recover the
credentials of the signer. Note that the scope of this document is
limited to providing this identity assurance for SIP requests;
solving this problem for SIP responses is outside the scope of this
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 credentials; if they do, their user agents can act as an
authentication service. However, end-user credentials may be neither
practical nor affordable, 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 credentials for the various devices could also be burdensome.
This trade-off needs to be understood by implementers of this
specification.
3. 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 assert the identity that is populated in the From header field.
This value may be Alice's AoR, or in other cases it may be some
different value that the proxy server has authority over, such as a
telephone number. It then computes a hash over some particular
headers, including the From header field (and optionally the body) of
the message. This hash is signed with the appropriate credential
(example.com, in the sip:alice@example.com case) and inserted in a
new header field in the SIP message, the 'Identity' header.
The proxy, as the holder of the private key for its domain, is
asserting that the originator of this request has been authenticated
and that she is authorized to claim the identity (the SIP address-
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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 keying material necessary to validate its credentials, 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).
4. Signature Generation and Validation
4.1. Authentication Service Behavior
This document specifies a role for SIP entities called an
authentication service. The authentication service role can be
instantiated by an intermediary such as a proxy server or a user
agent. Any entity that instantiates the authentication service role
MUST possess the private key of one or more credentials that can be
used to sign for a domain or a telephone number (see Section 5.1).
Intermediaries that instantiate this role MUST be capable of
authenticating one or more SIP users who 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 corresponding credentials, 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 7 for information on how the Date header field assists
verifiers).
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
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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 [1], 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 6.1 for more
information. If the authentication service is not authoritative for
the identity in question, it SHOULD process and forward the request
normally, but it MUST NOT following the steps below to add an
Identity header; see below for more information on authentication
service handling of an existing Identity header. [where?]
Step 2:
The authentication service MUST then 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 [1], 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 credential, 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, see Section 5.1 for more information.
Note that this check is performed only 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').
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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 10.4.
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 credential. For
more information on the security properties associated with the Date
header field value, see Section 7.
[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
environments where body security of INVITE transactions is necessary.
Details on the generation of this header is provided in Section 7.
If the authentication service is adding an Identity-Reliance header,
it MUST also add a Content-Length header field to SIP requests if one
is not already present; this can help verifiers to double-check that
they are hashing exactly as many bytes of message-body as the
authentication service when they verify the message.
Step 5:
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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 credential can be acquired; see
Section 5.3 for more on credential acquisition. Details on the
syntax of both of these headers are provided in Section 7.
Finally, the authentication service MUST forward the message
normally.
4.1.1. Intermediary Authentication Services
In cases where a user agent does not possess its own credentials to
sign an Identity header, the user agent can send its request through
an intermediary that will provide a signed Identity header based on
the contents of the request. This requires, among other things, that
intermediaries have some means of authenticating the user agents
sending requests.
All RFC 3261 [1] compliant user agents support Digest authentication,
which utilizes a shared secret, as a means for authenticating
themselves to a SIP registrar. Registration allows a user agent to
express that it is an appropriate entity to which requests should be
sent for a particular SIP AoR URI (e.g.,
'sip:alice@atlanta.example.com'). For such SIP URIs, by the
definition of identity used in this document, registration proves the
identity of the user to a registrar. Similar checks might be
performed for telephone numbers as identities. This is of course
only one manner in which a domain might determine how a particular
user is authorized to populate the From header field; as an aside,
for other sorts of URIs in the From (like anonymous URIs), other
authorization policies would apply.
RFC 3261 [1] already describes an intermediary architecture very
similar to the one proposed in this document in Section 26.3.2.2, in
which a user agent authenticates itself to a local proxy server,
which in turn authenticates itself to a remote proxy server via
mutual TLS, creating a two-link chain of transitive authentication
between the originator and the remote domain. While this works well
in some architectures, there are a few respects in which this is
impractical. For one, transitive trust is inherently weaker than an
assertion that can be validated end-to-end. It is possible for SIP
requests to cross multiple intermediaries in separate administrative
domains, in which case transitive trust becomes even less compelling.
This specification assumes that UACs will have an appropriate means
to discover an authentication service that can sign with a credential
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corresponding to the UAC's identity. Most likely, this information
will simply be provisioned in UACs.
One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity
header) is given in RFC 3325 [9]. However, this solution allows only
hop- by-hop trust between intermediaries, not end-to-end
cryptographic authentication, and it assumes a managed network of
nodes with strict mutual trust relationships, an assumption that is
incompatible with widespread Internet deployment.
4.2. Verifier Behavior
This document specifies a logical role for SIP entities called a
verification service, or verifier. When a verifier receives a SIP
message containing an Identity header, it inspects 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:
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 6.1. 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 6.2 to
determine if the signer is authoritative for the URI in the From
header field.
Step 2:
The verifier must first ensure that it possesses the proper keying
material to validate the signature in the Identity header field. See
Section 5.2 for more information on these procedures.
Step 3:
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The verifier MUST verify the signature in the Identity header field,
following the procedures for generating the hashed digest-string
described in Section 7. 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 7. 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 10.1; 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 credential used to sign the Identity header.
4.3. Identity within a Dialog and Retargeting
The mechanism is this document provides a signature over the URI in
the To header field value. The recipient of a request must compare
that value to their own identity in order to determine whether or not
the identity information in this call might have been replayed.
Retargeting, however, complicates this evaluation.
Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting supplants the
original target URI with one that corresponds to a different user,
potentially 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.
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.
Moreover, the value in the To header field of a dialog-forming
request is used as the From header field of requests sent in the
backwards direction during the dialog, and is accordingly the header
that would be signed by an authentication service for requests sent
in the backwards direction. But in retargeting cases, if the URI in
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the From header does not identify the sender of the request in the
backwards direction, then clearly it would be inappropriate to
provide an Identity signature over that From header. As specified
above, if the authentication service is not responsible for the
domain in the From header field of the request, it MUST NOT add an
Identity header to the request, and it should process/forward the
request normally.
Any means of anticipating retargeting, and so on, is outside the
scope of this document, and likely to have equal applicability to
response identity as it does to requests in the backwards direction
within a dialog. Consequently, no special guidance is given for
implementers here regarding the 'connected party' problem;
authentication service behavior is unchanged if retargeting has
occurred for a dialog-forming request. Ultimately, the
authentication service provides an Identity header for requests in
the backwards dialog when the user is authorized to assert the
identity given in the From header field, and if they are not, an
Identity header is not provided.
For further information on the problems of response identity see
[17].
5. Credentials
5.1. Credential Use by the Authentication Service
In order to act as an authentication service, a SIP entity must have
access to the private keying material of one or more credentials that
cover URIs, domain names or telephone numbers. These credentials may
represent authority over only a single name (such as
alice@example.com), an entire domain (such as example.com), or
potentially a set of domains. Similarly, a credential may represent
authority over a single telephone number or a range of telephone
numbers. The way that the scope of a credential is expressed is
specific to the credential mechanism.
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
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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
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.
5.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a
way to acquire and retain credentials for authorities over particular
URIs, domain names and/or telephone numbers. The Identity-Info
header (as described in the next section) is supported by all
verification service implementations to create a baseline means of
credential acquisition. Provided that the credential used to sign a
message is not previously known to the verifier, SIP entities SHOULD
discover this credential 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.
Verification service implementations supporting this specification
SHOULD have some means of retaining credentials (in accordance with
normal practices for credential lifetimes and revocation) in order to
prevent themselves from needlessly downloading the same credential
every time a request from the same identity is received. Credentials
cached in this manner max be indexed in accordance with local policy:
for example, by their scope, or the URI given in the Identity-Info
header field value.
[TBD: Should we add some kind of hash or similar indication to the
Identity-Info header to make it easier for verifiers to ascertain
that they already possess a credential without dereferencing the
URI?]
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5.3. Handling Identity-Info URIs
An Identity-Info header MUST contain a URI which dereferences to a
resource which contains the public key components of the credential
used by the authentication service to sign a request. Much as is the
case with the trust anchor(s) required for deployments of this
specification, it is essential that a URI in the Identity-Info header
be dereferencable by any entity that could plausibly receive the
request. For common cases, this means that the URI must be
dereferencable by any entity on the public Internet. In constrained
deployment environments, a service private to the environment might
be used instead.
Beyond providing a means of accessing credentials for an identity,
the Identity-Info header further services a means of differentiating
which particular credential was used to sign a request, when there
are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service
role for example.com, and a user agent belonging to Alice has
acquired a credential for alice@example.com. Either would be
eligible to sign a SIP request from alice@example.com. Verification
services however need a means to differentiate which one performed
the signature. The Identity-Info header performs that function.
5.4. Credential Systems
This document makes no specific recommendation for the use of any
credential system. Today, there are two primary credential systems
in place for proving ownership of domain names: certificates (e.g.,
X.509 v3, see [8]) and the domain name system itself (e.g., DANE, see
[10]). It is envisioned that either could be used in the SIP
context: an Identity-Info header could for example give an HTTP URL
of the form 'application/pkix-cert' pointing to a certificate
(following the conventions of [3]). The Identity-Info headers may
use the DNS URL scheme (see [11]( to indicate keys in the DNS.
While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential
system based on certificates is given in draft-peterson-stir-
certificates [TBD - fix after submitting]. One based on the domain
name system is given in [18].
In order for a credential system to work with this mechanism, its
specification must detail:
which URIs schemes the credential will use in the Identity-Info
header, and any special procedures required to dereference the
URIs
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how the verifier can learn the scope of the credential.
any special procedures required to extract keying material from
the resources designated by the URI
any algorithms that would appear in the Identity-Info "alg"
parameter other than 'rsa-sha256.' Note that per the IANA
Considerations of this document (Section 11.7), new algorithms can
only be specified by Standards Action.
SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted, or that deployments restrict the use
of this mechanism to environments where the reliance on particular
trust anchors is assured by business arrangements or similar
constraints.
Note that credential systems must address key lifecycle management
concerns: were a domain to change the credential 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 credential, 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).
[TBD: What will the normative language here be? Support for which
mechanisms?]
6. Identity Types
6.1. 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
an authority that issues credentials for telephone numbers or
telephone number ranges, and verification services must trust the
authority employed by the authentication service that signs a
request. Enrollment procedures and credential management are outside
the scope of this document.
Given the existence of such authorities, authentication and
verification services must identify when a request should be signed
by an authority for a telephone number, and when it should be signed
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by an authority for a domain. Telephone numbers most commonly appear
in SIP header field values in the username portion of a SIP URI
(e.g., 'sip:+17005551008@chicago.example.com;user=phone'). The user
part of that URI conforms to the syntax of the TEL URI scheme (RFC
3966 [5]). 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 canonicalization algorithm - drop the characters, +'s, assess if
its a valid local number (if so, append country code), etc]
[TBD define tn-spec here for ABNF purposes]
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 by both 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 6.2.
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.
6.2. Usernames with Domain Names
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 credential acquired from the Identity-Info
header. While this might seem that this should be a straightforward
process, it is complicated by two deployment realities. In the first
place, credentials 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
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to a subordinate domain and utilize the procedures in RFC 3263 [2]
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
credential 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
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 credential by following the procedures
defined in RFC 2818 [7], Section 3.1. While RFC 2818 [7] deals with
the use of HTTP in TLS and is specific to certificates, 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.
7. 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 [12] in RFC 3261 [1]):
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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 local policy for
authentication services. To create the contents of the signed-
identity-reliance-digest, the following element of a SIP message MUST
be placed in a bit-exact string:
The body content of the message with the bits exactly as they are
in the message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that
the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that
follows that CRLF.
[TBD: Explore alternatives to including the whole body for INVITE
requests; should there be a special case for security parameters that
would appear in SDP?]
The signed-identity-digest is a signed hash of a canonical string
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:
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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 6.1). 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 6.1). 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 the
BNF of RFC 3261 [1]. RFC 3261 specifies that the BNF for weekday
and month is a choice amongst a set of tokens. The RFC 4234 [12]
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.
For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 10 and [4].
The precise formulation of this digest-string is, therefore
(following the ABNF[12] in RFC 3261 [1]):
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 6.1.
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
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defines only one value for the 'alg' parameter: 'rsa-sha256'; further
values MUST be defined in a Standards Track RFC, see Section 14.7 for
more information. All implementations of this specification MUST
support 'rsa-sha256'. When the 'rsa-sha256' 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 [13] and base64
encode the results as specified in RFC 3548 [14]. A 2048-bit or
longer RSA key MUST be used. The result of the digest-string hash is
placed in the Identity header field; the optional reliance-digest-
string hash goes in the Identity-Reliance header. For detailed
examples of the usage of this algorithm, see Section 8.
The 'absoluteURI' portion of the Identity-Info header MUST contain a
URI; see Section 5.3 for more on choosing how to advertise
credentials through Identity-Info.
This document adds (or amends) the following entries to Table 2 of
RFC 3261 [1] (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
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CANCEL would be significantly limited. The Identity and Identity-
Info header MUST NOT appear in CANCEL. Note as well that the use of
Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility
reasons.
8. Examples
9. Privacy Considerations
The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in RFC 3323 [15]. 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.
RFC 3325 [9] 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.
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Finally, note that unlike RFC 3325 [9], the mechanism described in
this specification adds no information to SIP requests that has
privacy implications.
10. Security Considerations
10.1. 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 replayed 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 [1] 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.
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 [1], 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
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certain transaction could be replayed in potentially confusing or
malicious ways.
RFC4474 originally had protections for the Contact, Call-ID and CSeq.
These are removed from RFC4474bis. The absence of these header
values creates some opportunities for determined attackers to
impersonate based on cut-and-paste attacks; however, the absence of
these headers does not seem impactful to preventing against the
simple unauthorized claiming of a From header field value, which is
the primary scope of the current document.
It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert its own in a cut-
and-paste attack, which would cause all responses to the request to
be routed to a host of the attacker's choosing. However, a signature
over the topmost Via header does not prevent attacks of this nature,
since the attacker could leave the topmost Via intact and merely
insert a new Via header field directly after it, which would cause
responses to be routed to the attacker's host "on their way" to the
valid host, which has exactly the same end result. Although it is
possible that an intermediary-based authentication service could
guarantee that no Via hops are inserted between the sending user
agent and the authentication service, it could not prevent an
attacker from adding a Via hop after the authentication service, and
thereby preempting responses. It is necessary for the proper
operation of SIP for subsequent intermediaries to be capable of
inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS.
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.
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Thus, while this mechanism reduces the chance that a replayer or man-
in-the-middle will modify bodies, it does not eliminate it entirely.
Since it is a foundational assumption of this mechanism that the
users trust their local domain to vouch for their security, they must
also trust the service not to violate the integrity of their message
without good reason.
In the end analysis, the Identity, Identity-Reliance and Identity-
Info headers cannot protect themselves. Any attacker could remove
these headers from a SIP request, and modify the request arbitrarily
afterwards. However, this mechanism is not intended to protect
requests from men-in-the- middle who interfere with SIP messages; it
is intended only to provide a way that the originators of SIP
requests can prove that they are who they claim to be. At best, by
stripping identity information from a request, a man-in-the-middle
could make it impossible to distinguish any illegitimate messages he
would like to send from those messages sent by an authorized user.
However, it requires a considerably greater amount of energy to mount
such an attack than it does to mount trivial impersonations by just
copying someone else's From header field. This mechanism provides a
way that an authorized user can provide a definitive assurance of his
identity that an unauthorized user, an impersonator, cannot.
One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. Once again, it is
important to note that preventing men-in-the-middle is not the
primary impetus for this mechanism. Moreover, changing the 'alg'
would at worst result in some sort of bid-down attack, and at best
cause a failure in the verifier. Note that only one valid 'alg'
parameter is defined in this document and that thus there is
currently no weaker algorithm to which the mechanism can be bid down.
'alg' has been incorporated into this mechanism for forward-
compatibility reasons in case the current algorithm exhibits
weaknesses, and requires swift replacement, in the future.
10.2. Securing the Connection to the Authentication Service
In the absence of user agent-based authentication services, 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
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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.
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.
10.3. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the authentication service
that issues the assurance. Relying on an Identity header generated
by a remote administrative domain assumes that the issuing domain
uses recommended administrative practices to authenticate its users.
However, it is possible that some authentication services 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 authentication services is
questionable. While there is no magic way for a verifier to
distinguish "good" from "bad" signers by inspecting a SIP request, it
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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.
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
or telephone number that uses Identity, and in the future, view
messages received from that sources without Identity headers with
skepticism.
A verifier could consult some sort of directory that indications
whether a given caller should have a signed identity. There are a
number of potential ways in which this could be implemented. 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.
10.4. 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. Securing the display-name
component of the From header field value is outside the scope of this
document, but may be the subject of future work.
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11. IANA Considerations
[TBD: update for rfc4474bis or remove?]
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.
11.1. Header Field Names
This document specifies three SIP headers: Identity, Identity-
Reliance and Identity- Info. Their syntax is given in Section 7.
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
Header Name: Identity-Reliance
Compact Form:
11.2. 428 'Use Identity Header' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is sent when a verifier receives a SIP request that
lacks an Identity header in order to indicate that the request should
be re-sent with an Identity header. This response code is defined by
the following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
Response Code Number: 428
Default Reason Phrase: Use Identity Header
11.3. 436 'Bad Identity-Info' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the Identity-Info header contains a URI
that cannot be dereferenced by the verifier (either the URI scheme is
unsupported by the verifier, or the resource designated by the URI is
otherwise unavailable). This response code is defined by the
following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
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Response Code Number: 436
Default Reason Phrase: Bad Identity-Info
11.4. 437 'Unsupported Credential' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the verifier cannot validate the
credential referenced by the URI of the Identity-Info header,
because, for example, the credential is self-signed, or signed by an
authority for whom the verifier does not trust. This response code
is defined by the following information, which has been added to the
method and response-code sub-registry under http://www.iana.org/
assignments/sip-parameters
Response Code Number: 437
Default Reason Phrase: Unsupported Credential
11.5. 438 'Invalid Identity Header' Response Code
This document registers a SIP response code, which is described in
Section 4.2. It is used when the verifier receives a message with an
Identity signature that does not correspond to the digest-string
calculated by the verifier. This response code is defined by the
following information, which has been added to the method and
response-code sub-registry under http://www.iana.org/assignments/sip-
parameters
Response Code Number: 438
Default Reason Phrase: Invalid Identity Header
11.6. Identity-Info Parameters
The IANA has created a 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.
11.7. Identity-Info Algorithm Parameter Values
The IANA has created a registry for Identity-Info 'alg' parameter
values. This registry is to be prepopulated with a single entry for
a value called 'rsa-sha256', 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
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specification in which the value is described. New values for the
'alg' parameter may be defined only in Standards Track RFCs.
A previous version of this specification defined the 'rsa-sha1' value
for this registry. That value is hereby deprecated, and should be
removed. It is not believed that any implementations are making use
of this value.
[TBD - consider EC for smaller credential sizes?]
12. Acknowledgments
Lots of people made significant contributions to this document.
13. Changes from RFC4474
Lots of people made significant contributions to this document.
Generalized the credential mechanism; credential enrollment and
acquisition is now outside the scope of this document
Reduced the scope of the Identity signature to remove CSeq, Call-
ID, Contact, and the message body.
Added the Identity-Reliance header
Deprecated 'rsa-sha1' in favor of new baseline signing algorithm
[TBD - more]
14. Informative 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] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.
[3] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[4] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
September 2004.
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[5] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
[6] 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.
[7] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[8] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[9] 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.
[10] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[11] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, May 2006.
[12] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[13] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[14] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[15] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[16] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement", draft-ietf-stir-
problem-statement-03 (work in progress), January 2014.
[17] Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
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[18] Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", draft-kaplan-stir-cider-00
(work in progress), July 2013.
Authors' Addresses
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
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
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