One document matched: draft-jennings-dispatch-rfc4474bis-00.xml
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<rfc category="std" docName="draft-jennings-dispatch-rfc4474bis-00"
ipr="pre5378Trust200902">
<front>
<title abbrev="SIP Identity">Authenticated Identity Management in the
Session Initiation Protocol (SIP)</title>
<author fullname="Jon Peterson" initials="J." surname="Peterson">
<organization>NeuStar</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<email>jon.peterson@neustar.biz</email>
</address>
</author>
<author fullname="Cullen Jennings" initials="C." surname="Jennings">
<organization>Cisco</organization>
<address>
<postal>
<street>400 3rd Avenue SW, Suite 350</street>
<city>Calgary</city>
<region>AB</region>
<code>T2P 4H2</code>
<country>Canada</country>
</postal>
<email>fluffy@iii.ca</email>
</address>
</author>
<author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
<organization>RTFM, Inc.</organization>
<address>
<postal>
<street>2064 Edgewood Drive</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94303</code>
<country>USA</country>
</postal>
<phone>+1 650 678 2350</phone>
<email>ekr@rtfm.com</email>
</address>
</author>
<date day="29" month="May" year="2013" />
<area>RAI</area>
<abstract>
<t>The existing 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 messages. It does so by defining two new SIP header
fields, Identity, for conveying a signature used for validating the
identity, and Identity-Info, for conveying a reference to the
certificate of the signer.</t>
</abstract>
</front>
<middle>
<section anchor="sec-preamble" title="Preamble">
<t>The sip-identity drafts that lead to <xref target="RFC4474">RFC
4474</xref> were first published in 2002. Since that point many things
have changed that impact that design. <list style="symbols">
<t>The DNS root has been signed.</t>
<t> SPAM continues to be a problem.</t>
<t>A clearer understanding has evolved of the use of B2BUAs including
standardization such as the STRAW WG.</t>
<t>Multipart MIME has failed as a SIP extension mechanism.</t>
<t>Widespread identity providers such as Facebook have emerged.</t>
<t>Techniques for non-carrier entities to verify phone numbers and then
use them for addressing (such as Apple's iMessage) have been shown to
be commercially feasible.</t>
<t>Substantial portions of commercial, government, and personal voice
communications rely on SIP at some stage in the communications.</t>
<t>The cost of operating large databases has fallen and outsourced
versions of these databases have become cheaply available. </t>
<t>Extensive experience and user research has improved our
understanding of how to present security information to users.</t>
<t>The world is in the a huge transition to mobile devices. Even the
most limited modern mobile devices have user interface and
computational capabilities that greatly exceed a 2002-era SIP
phone.</t>
</list></t>
<t>The authors believe that the confluence of changing technology, the
evolution of mobile devices and internet, and a political will to change
make this the right time to consider an expansion of the scope of 4474
to solve the following problems: <list style="symbols">
<t>Assert strong identity for domain scoped names such as
alice@example.com</t>
<t>Assert strong identity for E.164 numbers such as +1 408
555-1212</t>
<t>Provide organization attributes such as "this is a Bank".</t>
<t>Work for calls crossing even the most adverse networks such as
the PSTN.</t>
<t>Provide reliable information about who is calling before the call
is answered to help stop SPAM.</t>
<t>Provide reliable information about who you are talking to.</t>
<t>Work with evolving non SIP based communications systems such as
WebRTC.</t>
</list></t>
<t>We believe it is possible to solve all of these in a way that is
commercially viable, deployable, and provides a delightful user
experience.</t>
<t>The core problem in a global identity system with delegated names is
understanding who is authorized to make assertions about a given name.
The proposal is to solve that problem with a two pronged approach.</t>
<t>First have responsibility for a number delegated down from the root
in a series of delegation sub delegation towards the user. For example,
the North American number operator may assign a portion of the +1 space
to a service provider. That service provider may assign a sub space to a
company and that company may assign a number to a user. At each level of
delegation, cryptographic credentials could be provided that allow the
user to prove the space was delegated to them given some common trust
root. This approach is referred to as "delegation" and effectively works
from the top down.</t>
<t>The other prong to solving the problem is called "claims" and works
via a bottom up approach. The end user of a number basically claims it
and some trusted system validates this claim. The validation may be as
simple as sending a SMS to the number or more complicated such as the
VIPR system.</t>
<t>The delegation approach creates an easier user experience but is
harder to deploy from a business incentive point of view so our approach
is to do both and work down from the top and up from the bottom with a
meet in the middle approach to coverage of the full name space.</t>
<t>User agents that have a credential (whether of the
delegation or claim variety) for a name can create
two types of assertions: replay assertions and reliance assertions.
These two assertion are signed over a different set of the call
information and protect against different types of attacks. Some
networks might modify the signaling in ways that impact one of these
assertions but not the other.</t>
<t>The assertions are passed from the caller to the callee for
verification. This can be done by either passing the assertion along with
the signaling, or alternatively passing it through a web based Call Detail
Service (CDS) where the caller saves the assertion on the Call Detail
Service and the callee retrieves it from the Call Detail Service
service. There are some call signaling environments, such as when a call
passes through the PSTN, where it is not possible to transfer the
assertion in the call signaling path. The Call Detail Service is in place
to make things work in this environment thought some privacy information
around who is calling who is reveled to the Call Detail Service service.
An outline for this design is described in <xref
target="I-D.rescorla-callerid-fallback" />.</t>
</section>
<section anchor="sec-prop" title="Scope of Proposed Changes">
<!--
<t>To summarize the work proposed and changes from RFC 4474. <list
style="numbers">
<t>Add a delegation approach for E.164 numbers.</t>
<t>Add a claims approach.</t>
<t>Provide two types of assertions instead of one.</t>
<t>Add an out of band rendezvous service to pass assertions.</t>
</list></t>
-->
<section anchor="sec-prop-id-info"
title="Changes to the Identity-Info Header">
<t>Currently, RFC4474 restricts the subject of the certificate to a
domain name, and accordingly the RFC4474 Identity-Info header contains a
URI which designates a certificate whose subject (more precisely,
subjectAltName) must correspond to the domain of the URI in the From
header field value of the SIP request. Per the analysis in <xref
target="I-D.peterson-secure-origin-ps" />, we propose to relax this to
allow designating an alternative authority for telephone numbers, when
telephone numbers appear in the From header field value.</t>
<t>These changes will allow the Identity-Info URI to point to the
certificate with authority over the calling telephone number. A
verification service will therefore authorize a SIP request when the
telephone number in the From header field value agrees with the subject
of the certificate. Verification services must of course trust the
certificate authority that issued the certificate in question. To
implement this change to the Identity-Info header, we must allow for two
possibilities for the conveyance of a telephone number in a request:
appearing within a tel URI or appearing as the user portion of a SIP
URI. Therefore, we must prescribe verification service behind in the
case where the From header field value URI contains a telephone user
part followed by a domain -- which should the verification service
expect to find in a certificate?</t>
<t>There are also a few other potential changes within the scope of a
revision to the Identity-Info header. We might consider implementing
enough flexibility in the URI to allow a model more like the IdP model
described in <xref target="I-D.rescorla-rtcweb-generic-idp" />; this could be
useful as RTCWeb sees increasing deployment. We also should consider any
implications of the signing of the DNSSEC root and the DANE
specifications to the existing Identity-Info uses with domain name. At a
high level, it is not expected that the proposed changes will radically
alter the semantics of Identity-Info.</t>
<t>Although deployment of RFC4474 to date has been essentially
non-existent, we will during this revision process consider any
realistic backwards compatibility concerns.</t>
</section>
<section anchor="sec-prop-id-header"
title="Changes to the Identity Header">
<t>Per the analysis in <xref
target="I-D.peterson-secure-origin-ps" />, we propose
to change the signature mechanism that RFC44474 specified for the
Identity header: in particular, to replace this signature mechanism with
one that is more likely to survive end-to-end in SIP networks where
intermediaries act as back-to-back user agents rather than proxy
servers.</t>
<t>To accomplish this, we propose creating two distinct signatures
within SIP requests: a replay assurance and a reliance assurance. The
replay assurance prevents impersonation attacks by providing a signature
over the From header field value and certain other headers which will
allow a verification service to detect a cut-and-paste attack. The
reliance assurance protects against men-in-the-middle unilaterally
changing other parameters of the call: these include the target of
future requests (Contact header field) and the entirely of the SDP,
including the target IP address and ports which, if unprotected, can
allow a man-in-the-middle to impersonate an intended
listener. Verification services behavior would change to allow them to
decide, based on their configuration in a deployment environment,
whether the replay assurance alone can realistically survive network
transit, or if the reliance assurance should be available.</t>
<t>There are a number of ways to implement this change to the signature
in the Identity header field. One possibility is to design two
new headers, which we might call "Identity-Reliance" and "Identity-Replay"
with the reliance signature being over a canonical representation of the
reliance field and then the Identity-Replay header covering the
From header field value, other headers needed for
replay protection, and well as the contents of the Identity-Reliance
header. It might also be possible to preserve the existing
Identity header as the reliance header.
There are however several similar alternatives we might
consider, and some analysis will be required to identify the best
option.</t>
<t>In order to preserve critical security parameters even in adverse
network conditions, the replay assurance integrity protection must
always cover security parameters of the SDP required to negotiate
media-level security. There may be other exception cases, or
extensibility mechanisms, worth considering here. In cases where the
From header field value of a SIP request contains a SIP URI with a
telephone number user part, we will also consider replay assurance
canonicalizations that do not cover the domain portion of the URI.</t>
<t>We will furthermore give due consideration to changes in SIP
architecture and deployment since the publication of RFC4474, including
the ongoing work in the STRAW working group.</t>
<t>As with Identity-Info, any necessary consideration will be given to
backwards compatibility of the Identity header.</t>
</section>
</section>
<section anchor="sec-1" title="Introduction">
<t>This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP, <xref target="RFC3261">RFC 3261</xref>). An identity, for the
purposes of this document, is defined as a SIP URI, commonly a canonical
address-of-record (AoR) employed to reach a user (such as
'sip:alice@atlanta.example.com').</t>
<t><xref target="RFC3261">RFC 3261</xref> 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.</t>
<t><xref target="RFC3261">RFC 3261</xref> specifies a number of security
mechanisms that can be employed by SIP user agents (UAs), including
Digest, Transport Layer Security (TLS), and S/MIME (implementations may
support other security schemes as well). However, few SIP user agents
today support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and furthermore Digest
authentication is limited by the fact that the originator and
destination must share a prearranged secret. It is desirable for SIP
user agents to be able to send requests to destinations with which they
have no previous association -- just as in the telephone network today,
one can receive a call from someone with whom one has no previous
association, and still have a reasonable assurance that the person's
displayed Caller-ID is accurate. A cryptographic approach, like the one
described in this document, can probably provide a much stronger and
less spoofable assurance of identity than the telephone network provides
today.</t>
</section>
<section anchor="sec-2" title="Terminology">
<t>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 <xref target="RFC2119">RFC 2119</xref> and <xref target="RFC6919">RFC
6919</xref>.</t>
</section>
<section anchor="sec-3" title="Background">
<t>The usage of many SIP applications and services is governed by
authorization policies. These policies may be automated, or they may be
applied manually by humans. An example of the latter would be an
Internet telephone application that displays the Caller-ID of a caller,
which a human may review before answering a call. An example of the
former would be a presence service that compares the identity</t>
<t>of potential subscribers to a whitelist before determining whether it
should accept or reject the subscription. In both of these cases,
attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of a
SIP request, the From header field, can be populated arbitrarily by the
controller of a user agent, impersonation is very simple today. The
mechanism described in this document aspires to provide a strong
identity system for SIP in which authorization policies cannot be
circumvented by impersonation.</t>
<t>All <xref target="RFC3261">RFC 3261</xref> 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').</t>
<t>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 its identity to a registrar
cannot be validated by just any user agent or proxy server -- these
credentials are only shared between the user agent and their domain
administrator. So this shared secret does not immediately help a user to
authenticate to a wide range of recipients. Recipients require a means
of determining whether or not the 'return address' identity of a
non-REGISTER request (i.e., the From header field value) has
legitimately been asserted.</t>
<t>The AoR URI used for registration is also the URI with which a UA
commonly populates the From header field of requests in order to provide
a 'return address' identity to recipients. From an authorization
perspective, if you can prove you are eligible to register in a domain
under a particular AoR, you can prove you can legitimately receive
requests for that AoR, and accordingly, when you place that AoR in the
From header field of a SIP request other than a registration (like an
INVITE), you are providing a 'return address' where you can legitimately
be reached. In other words, if you are authorized to receive requests
for that 'return address', logically, it follows that you are also
authorized to assert that 'return address' in your From header field.
This is of course only one manner in which a domain might determine how
a particular user is authorized to populate the From header field; as an
aside, for other sorts of URIs in the From (like anonymous URIs), other
authorization policies would apply.</t>
<t>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 communicate to
other SIP entities that the sending user has been authenticated and its
use of the From header field has been authorized. This document
addresses how that imprimatur of authentication can be shared.</t>
<t><xref target="RFC3261">RFC 3261</xref> 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.</t>
<t>One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity header)
is given in [12]. However, this solution allows only hop- by-hop trust
between intermediaries, not end-to-end cryptographic authentication, and
it assumes a managed network of nodes with strict mutual trust
relationships, an assumption that is incompatible with widespread
Internet deployment.</t>
<t>Accordingly, this document specifies a means of sharing a
cryptographic assurance of end-user SIP identity in an interdomain or
intradomain context that is based on the concept of an 'authentication
service' and a new SIP header, the Identity header. Note that the scope
of this document is limited to providing this identity assurance for SIP
requests; solving this problem for SIP responses is more complicated and
is a subject for future work.</t>
<t>This specification allows either a user agent or a proxy server to
provide identity services and to verify identities. To maximize
end-to-end security, it is obviously preferable for end-users to acquire
their own certificates and corresponding private keys; if they do, they
can act as an authentication service. However, end- user certificates
may be neither practical nor affordable, given the difficulties of
establishing a Public Key Infrastructure (PKI) that extends to
end-users, and moreover, given the potentially large number of SIP user
agents (phones, PCs, laptops, PDAs, gaming devices) that may be employed
by a single user. In such environments, synchronizing keying material
across multiple devices may be very complex and requires quite a good
deal of additional endpoint behavior. Managing several certificates for
the various devices is also quite problematic and unpopular with users.
Accordingly, in the initial use of this mechanism, it is likely that
intermediaries will instantiate the authentication service role.</t>
</section>
<section anchor="sec-4" title="Overview of Operations">
<t>This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document.</t>
<t>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.</t>
<t>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.</t>
<t>The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized to
assert the identity that is populated in the From header field. This
value may be Alice's AoR, or it may be some other value that the policy
of the proxy server permits her to use. 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.</t>
<t>The proxy, as the holder of the private key of its domain, is
asserting that the originator of this request has been authenticated and
that she is authorized to claim the identity (the SIP address-
of-record) that appears in the From header field. The proxy also inserts
a companion header field, Identity-Info, that tells Bob how to acquire
its certificate, if he doesn't already have it.</t>
<t>When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validate that the domain
indicated by the host portion of the AoR 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).</t>
</section>
<section anchor="sec-5" title="Authentication Service Behavior">
<t>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. Intermediaries that instantiate this role
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 have 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 an entity that acts as a redirect server, but that is
left as a topic for future work.</t>
<t>SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present (see <xref
target="sec-syntax"></xref> for information on how the Date header field
assists verifiers). Similarly, authentication services MUST add a
Content- Length header field to SIP requests if one is not already
present; this can help verifiers to double-check that they are hashing
exactly as many bytes of message-body as the authentication service when
they verify the message.</t>
<t>Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP
request:</t>
<t>Step 1:</t>
<t>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,
the authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is
responsible (following the procedures in <xref target="RFC3261">RFC
3261</xref>, 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, the policy of the authentication service determines
whether or not it is responsible for this identity; see <xref
target="sec-identity-tel"></xref> 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.</t>
<t>Step 2:</t>
<t>The authentication service MUST determine whether or not the sender
of the request is authorized to claim the identity given in the identity
field. In order to do so, the authentication service MUST authenticate
the sender of the message. Some possible ways in which this
authentication might be performed include: <list>
<t>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 <xref target="RFC3261">RFC 3261</xref>, 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.</t>
<t>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 preferably by providing a password that
unlocks said private key.</t>
</list></t>
<t>Authorization of the use of a particular username in 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 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 that do not correspond to the user-portion of SIP
From headers, or a user might manage multiple accounts in the same
administrative domain. In this latter case, a domain might maintain a
mapping between the values in the 'username' parameter of Proxy-
Authorization and a set of one or more SIP URIs that might legitimately
be asserted for that 'username'. For example, the username can
correspond to the 'private identity' as defined in Third Generation
Partnership Project (3GPP), in which case the From header field can
contain any one of the public identities associated with this private
identity. In this instance, another policy might be as follows: the URI
in the From header field MUST correspond exactly to one of the mapped
URIs associated with the 'username' given in the Proxy-Authorization
header. 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.</t>
<t>Note that this check is performed on the addr-spec in the From header
field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display- name
portion of the From header field (e.g., 'Alice Atlanta'). Authentication
services MAY check and validate the display-name as well, and compare it
to a list of acceptable display-names that may be used by the sender; if
the display-name does not meet policy constraints, the authentication
service MUST return a 403 response code. The reason phrase should
indicate the nature of the problem; for example, "Inappropriate Display
Name". However, the display-name is not always present, and in many
environments the requisite operational procedures for display-name
validation may not exist. For more information, see <xref
target="sec-display-name"></xref>.</t>
<t>Step 3:</t>
<t>The authentication service SHOULD ensure that any preexisting Date
header in the request is accurate. Local policy can dictate precisely
how accurate the Date must be; a RECOMMENDED maximum discrepancy of ten
minutes will ensure that the request is unlikely to upset any verifiers.
If the Date header contains a time different by more than ten minutes
from the current time noted by the authentication service, the
authentication service SHOULD reject the request. This behavior is not
mandatory because a user agent client (UAC) could only exploit the Date
header in order to cause a request to fail verification; the Identity
header is not intended to provide a source of non-repudiation or a
perfect record of when messages are processed. Finally, the
authentication service MUST verify that the Date header falls within the
validity period of its certificate. For more information on the security
properties associated with the Date header field value, see <xref
target="sec-syntax"></xref>.</t>
<t>Step 4:</t>
<t>The authentication service MUST form the identity signature and add
an Identity header to the request containing this signature. After the
Identity header has been added to the request, the authentication
service MUST also add an Identity-Info header. The Identity-Info header
contains a URI from which its certificate can be acquired. Details on
the generation of both of these headers are provided in <xref
target="sec-syntax"></xref>.</t>
<t>Finally, the authentication service MUST forward the message
normally.</t>
<section anchor="sec-5.1"
title="Identity within a Dialog and Retargeting">
<t>Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting supplants the
original target URI with one that corresponds to a different user, a
user that is not authorized to register under the original target URI.
By this definition, retargeting does not include translation of the
Request-URI to a contact address of an endpoint that has registered
under the original target URI, for example.</t>
<t>When a dialog-forming request is retargeted, this can cause a few
wrinkles for the Identity mechanism when it is applied to requests
sent in the backwards direction within a dialog. This section provides
some non-normative considerations related to this case.</t>
<t>When a request is retargeted, it may reach a SIP endpoint whose
user is not identified by the URI designated in the To header field
value. The value in the To header field of a dialog-forming request is
used as the From header field of requests sent in the backwards
direction during the dialog, and is accordingly the header that would
be signed by an authentication service for requests sent in the
backwards direction. In retargeting cases, if the URI in the From
header does not identify the sender of the request in the backwards
direction, then clearly it would be inappropriate to provide an
Identity signature over that From header. As specified above, if the
authentication service is not responsible for the domain in the From
header field of the request, it MUST NOT add an Identity header to the
request, and it should process/forward the request normally.</t>
<t>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.</t>
<t>For further information on the problems of response identity and
the potential solution spaces, see [15].</t>
</section>
</section>
<section anchor="sec-verifier-behavior" title="Verifier Behavior">
<t>This document introduces a new logical role for SIP entities called a
server. When a verifier receives a SIP message containing an Identity
header, it may inspect the signature to verify the identity of the
sender of the message. Typically, the results of a verification are
provided as input to an authorization process that is outside the scope
of this document. If an Identity header is not present in a request, and
one is required by local policy (for example, based on a
per-sending-domain policy, or a per-sending-user policy), then a 428
'Use Identity Header' response MUST be sent.</t>
<t>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.</t>
<t>Step 1:</t>
<t>The verifier MUST 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 cached in this manner
should be indexed by the URI given in the Identity- Info header field
value.</t>
<t>Provided that the domain certificate used to sign this message is not
previously known to the verifier, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header, unless they have
some more efficient implementation-specific way of acquiring
certificates 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 verifier processes this certificate in the usual
ways, including checking that it has not expired, that the chain is
valid back to a trusted certification authority (CA), and that it does
not appear on revocation lists. Once the certificate is acquired, it
MUST be validated following the procedures in <xref target="RFC3280">RFC
3280</xref>. If the certificate cannot be validated (it is self-signed
and untrusted, or signed by an untrusted or unknown certificate
authority, expired, or revoked), the verifier MUST send a 437
'Unsupported Certificate' response.</t>
<t>Step 2:</t>
<t>The verifier MUST follow the process described in <xref
target="sec-security-subordination"></xref> to determine if the signer
is authoritative for the URI in the From header field.</t>
<t>Step 3:</t>
<t>The verifier MUST verify the signature in the Identity header field,
following the procedures for generating the hashed digest-string
described in <xref target="sec-syntax"></xref>. 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.</t>
<t>Step 4:</t>
<t>The verifier MUST validate the Date, Contact, and Call-ID headers in
the manner described in <xref target="sec-security-digest"></xref>;
recipients that wish to verify Identity signatures MUST support all of
the operations described there. It must furthermore ensure that the
value of the Date header falls within the validity period of the
certificate whose corresponding private key was used to sign the
Identity header.</t>
</section>
<section anchor="sec-7" title="Considerations for User Agent">
<t>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 <xref
target="sec-secure-connect-auth-serv"></xref> for more information about
securing the channel between the UAC and the authentication service.</t>
<t>When a UAC sends a request, it MUST accurately populate the From
header field with a value corresponding to an identity that it believes
it is authorized to claim. In a request, it MUST set the URI portion of
its From header to match a SIP, SIPS, or TEL URI AoR that it is
authorized to use in the domain (including anonymous URIs, as described
in <xref target="RFC3323">RFC 3323</xref>). In general, UACs SHOULD NOT
use the TEL URI form in the From header field (see <xref
target="sec-identity-tel"></xref>).</t>
<t>Note that this document defines a number of new 4xx response codes.
If user agents support these response codes, they will be able to
respond intelligently to Identity-based error conditions.</t>
<t>The UAC MUST also be capable of sending requests, including mid-call
requests, through an 'outbound' proxy (the authentication service). The
best way to accomplish this is using pre-loaded Route headers and loose
routing. For a given domain, if an entity that can instantiate the
authentication service role is not in the path of dialog-forming
requests, identity for mid-dialog requests in the backwards direction
cannot be provided.</t>
<t>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.</t>
</section>
<section anchor="sec-8" title="Considerations for Proxy Servers">
<t>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 validate the Identity header before it
makes a forwarding decision for a request. Proxy servers MUST NOT remove
or modify an existing Identity or Identity-Info header in a request.</t>
</section>
<section anchor="sec-syntax" title="Header Syntax">
<t>This document specifies two new SIP headers: Identity and Identity-
Info. Each of these headers can appear only once in a SIP message. The
grammar for these two headers is (following the ABNF [6] in RFC 3261
[1]):</t>
<figure>
<artwork><![CDATA[
Identity = "Identity" HCOLON signed-identity-digest
signed-identity-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
]]></artwork>
</figure>
<t>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: <list>
<t>The AoR of the UA sending the message, or addr-spec of the From
header field (referred to occasionally here as the 'identity
field').</t>
<t>The addr-spec component of the To header field, which is the AoR
to which the request is being sent.</t>
<t>The callid from Call-Id header field.</t>
<t>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 linear whitespace (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.</t>
<t>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 <xref
target="RFC3261">RFC 3261</xref>. RFC 3261 specifies that the BNF
for weekday and month is a choice amongst a set of tokens. The <xref
target="RFC2234">RFC 2234</xref> 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.</t>
<t>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 "|"
characters in the canonical string).</t>
<t>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. If the message has no body, then message-body will be empty,
and the final "|" will not be followed by any additional
characters.</t>
</list></t>
<t>For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see <xref
target="sec-security-considerations"></xref> and <xref
target="RFC3893"></xref>. The precise formulation of this digest-string
is, therefore (following the ABNF<xref target="RFC4234"></xref> in <xref
target="RFC3261">RFC 3261</xref>):</t>
<figure>
<artwork><![CDATA[
digest-string = addr-spec "|" addr-spec "|" callid "|"
1*DIGIT SP Method "|" SIP-date "|" [ addr-spec ] "|"
message-body
]]></artwork>
</figure>
<t>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.</t>
<t>After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain. The hashing and signing algorithm is
specified by the 'alg' parameter of the Identity-Info header (see below
for more information on Identity-Info header parameters). This document
defines only one value for the 'alg' parameter: 'rsa-sha1'; further
values MUST be defined in a Standards Track RFC, see Section 14.7 for
more information. All implementations of this specification MUST support
'rsa-sha1'. When the 'rsa-sha1' algorithm is specified in the 'alg'
parameter of Identity-Info, the hash and signature MUST be generated as
follows: compute the results of signing this string with
sha1WithRSAEncryption as described in <xref target="RFC3370">RFC
3370</xref> and base64 encode the results as specified in <xref
target="RFC3548">RFC 3548</xref>. A 1024-bit or longer RSA key MUST be
used. The result is placed in the Identity header field. For detailed
examples of the usage of this algorithm, see <xref
target="sec-test-examples"></xref>.</t>
<t>The 'absoluteURI' portion of the Identity-Info header MUST contain a
URI which dereferences to a resource containing the certificate of the
authentication service. All implementations of this specification MUST
support the use of HTTP and HTTPS URIs in the Identity-Info header. Such
HTTP and HTTPS URIs MUST follow the conventions of <xref
target="RFC2585">RFC 2585</xref>, and for those URIs the indicated
resource MUST be of the form 'application/pkix-cert' described in that
specification. Note that this introduces key lifecycle management
concerns; were a domain to change the key available at the Identity-Info
URI before a verifier evaluates a request signed by an authentication
service, this would cause obvious verifier failures. When a rollover
occurs, authentication services SHOULD thus provide new Identity-Info
URIs for each new certificate, and SHOULD continue to make older key
acquisition URIs available for a duration longer than the plausible
lifetime of a SIP message (an hour would most likely suffice).</t>
<t>The Identity-Info header field MUST contain an 'alg' parameter. No
other parameters are defined for the Identity-Info header in this
document. Future Standards Track RFCs may define additional
Identity-Info header parameters.</t>
<t>This document adds the following entries to Table 2 of <xref
target="RFC3261">RFC 3261</xref>:</t>
<figure>
<artwork><![CDATA[
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
]]></artwork>
</figure>
<t>Note, in the table above, that this mechanism does not protect the
CANCEL method. The CANCEL method cannot be challenged, because it is
hop-by-hop, and accordingly authentication service behavior for CANCEL
would be significantly limited. Note as well that the REGISTER method
uses Contact header fields in very unusual ways that complicate its
applicability to this mechanism, and the use of Identity with REGISTER
is consequently a subject for future study, although it is left as
optional here for forward-compatibility reasons. The Identity and
Identity-Info header MUST NOT appear in CANCEL.</t>
</section>
<section anchor="sec-test-examples" title="Compliance Tests and Examples">
<t>The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementers are advised to
verify their compliance with the specification against the following
criteria: <list>
<t>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.</t>
<t>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).</t>
</list> Note that the following examples use self-signed certificates,
rather than certificates issued by a recognized certificate authority.
The use of self-signed certificates for this mechanism is NOT
RECOMMENDED, and it appears here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD
generate appropriate warnings about the use of self-signed certificates.
Also, the example certificates in this section have placed their domain
name subject in the subjectAltName field; in practice, certificate
authorities may place domain names in other locations in the certificate
(see <xref target="sec-security-subordination"></xref> for more
information).</t>
<t>Note that all examples in this section use the 'rsa-sha1'
algorithm.</t>
<t>Bit-exact reference files for these messages and their various
transformations are supplied in Appendix B.</t>
<section anchor="sec-10.1"
title="Identity-Info with a Singlepart MIME body">
<t>Consider the following private key and certificate pair assigned to
'atlanta.example.com' (rendered in OpenSSL format).</t>
<figure>
<artwork><![CDATA[
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
]]></artwork>
</figure>
<figure>
<artwork><![CDATA[
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
]]></artwork>
</figure>
<t>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:</t>
<figure>
<artwork><![CDATA[
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 147
v=0
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
]]></artwork>
</figure>
<t>When the authentication service receives the INVITE, it
authenticates Alice by sending a 407 response. As a result, Alice adds
an Authorization header to her request, and resends to the
atlanta.example.com authentication service. Now that the service is
sure of Alice's identity, it calculates an Identity header for the
request. The canonical string over which the identity signature will
be generated is the following (note that the first line wraps because
of RFC editorial conventions):</t>
<figure>
<artwork><![CDATA[
sip:alice@atlanta.example.com|sip:bob@biloxi.example.org|
a84b4c76e66710|314159 INVITE|Thu, 21 Feb 2002 13:02:03 GMT|
sip: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
]]></artwork>
</figure>
<t>The resulting signature (sha1WithRsaEncryption) using the private
RSA key given above, with base64 encoding, is the following:</t>
<figure>
<artwork><![CDATA[
ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=
]]></artwork>
</figure>
<t>Accordingly, the atlanta.example.com authentication service will
create an Identity header containing that base64 signature string (175
bytes). It will also add an HTTPS URL where its certificate is made
available. With those two headers added, the message looks like the
following:</t>
<figure>
<artwork><![CDATA[
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Identity:
"ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
]]></artwork>
</figure>
<t>atlanta.example.com then forwards the request normally. When Bob
receives the request, if he does not already know the certificate of
atlanta.example.com, he dereferences the URL in the Identity-Info
header to acquire the certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP
request. Using this canonical string, the signed digest in the
Identity header, and the certificate discovered by dereferencing
the</t>
<t>Identity-Info header, Bob can verify that the given set of headers
and the message body have not been modified.</t>
</section>
<section anchor="sec-10.2"
title="Identity for a Request with No MIME Body or Contact">
<t>Consider the following private key and certificate pair assigned to
"biloxi.example.org".</t>
<figure>
<artwork><![CDATA[
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
]]></artwork>
</figure>
<figure>
<artwork><![CDATA[
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
]]></artwork>
</figure>
<t>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:</t>
<figure>
<artwork><![CDATA[
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
]]></artwork>
</figure>
<t>When the authentication service receives the BYE, it authenticates
Bob by sending a 407 response. As a result, Bob adds an Authorization
header to his request, and resends to the biloxi.example.org
authentication service. Now that the service is sure of Bob's
identity, it prepares to calculate an Identity header for the request.
Note that this request does not have a Date header field. Accordingly,
the biloxi.example.org will add a Date header to the request before
calculating the identity signature. If the Content-Length header were
not present, the authentication service would add it as well. The
baseline message is thus:</t>
<figure>
<artwork><![CDATA[
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
]]></artwork>
</figure>
<t>Also note that this request contains no Contact header field.
Accordingly, biloxi.example.org will place no value in the canonical
string for the addr-spec of the Contact address. Also note that there
is no message body, and accordingly, the signature string will
terminate, in this case, with two vertical bars. The canonical string
over which the identity signature will be generated is the following
(note that the first line wraps because of RFC editorial
conventions):</t>
<t>sip:bob@biloxi.example.org|sip:alice@atlanta.example.com|
a84b4c76e66710|231 BYE|Thu, 21 Feb 2002 14:19:51 GMT||</t>
<t>The resulting signature (sha1WithRsaEncryption) using the private
RSA key given above for biloxi.example.org, with base64 encoding, is
the following:</t>
<t>sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=</t>
<t>Accordingly, the biloxi.example.org authentication service will
create an Identity header containing that base64 signature string. It
will also add an HTTPS URL where its certificate is made available.
With those two headers added, the message looks like the
following:</t>
<figure>
<artwork><![CDATA[
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Identity:
"sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="
Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
Content-Length: 0
]]></artwork>
</figure>
<t>biloxi.example.org then forwards the request normally.</t>
</section>
</section>
<section anchor="sec-identity-tel" title="Identity and the TEL URI Scheme">
<t>Since many SIP applications provide a Voice over IP (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;user=phone'). That username
conforms to the syntax of the TEL URI scheme (<xref target="RFC3966">RFC
3966</xref>). For this sort of SIP address-of-record,
chicago.example.com is the appropriate signatory.</t>
<t>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
SHOULD put telephone numbers in the From header field into SIP or SIPS
URIs whenever possible.</t>
<t>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. This is permitted in this
specification strictly for forward compatibility purposes. In the
longer-term, it is possible that ENUM [14] 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.</t>
</section>
<section anchor="sec-12" title="Privacy Considerations">
<t>The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in <xref
target="RFC3323">RFC 3323</xref>. 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.</t>
<t>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 '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.</t>
<t>The "header" level of privacy described in <xref target="RFC3323">RFC
3323</xref> requests that a privacy service 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.</t>
<t><xref target="RFC3325">RFC 3325</xref> 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.</t>
<t>Finally, note that unlike <xref target="RFC3325">RFC 3325</xref>, the
mechanism described in this specification adds no information to SIP
requests that has privacy implications.</t>
</section>
<section anchor="sec-security-considerations"
title="Security Considerations">
<section anchor="sec-security-digest"
title="Handling of digest-string Elements">
<t>This document describes a mechanism that provides a signature over
the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
requests. 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 <xref target="RFC3261">RFC 3261</xref>
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.</t>
<t>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
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.</t>
<t>The Date and Contact headers provide reference integrity and replay
protection, as described in <xref target="RFC3261">RFC 3261</xref>,
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). Because a SIP-compliant
UA never generates the same Call-ID twice, verifiers can use the
Call-ID to recognize cut-and-paste attacks; the Call-ID serves as a
nonce. The result of this is that if an Identity header is replayed
within the Date interval, verifiers will recognize that it is invalid
because of a Call-ID duplication; if an Identity header is replayed
after the Date interval, verifiers 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.</t>
<t>The Contact header field is included to tie the Identity header to
a particular user agent 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 its own request
(reusing the From, To, Contact, Date, and Call-ID fields that appear
in the original message), the attacker 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.</t>
<t>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.</t>
<t>This mechanism also provides a signature over the bodies of SIP
requests. The most important reason for doing so is to protect Session
Description Protocol (SDP) bodies carried in SIP requests. 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 media descriptors. 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
replayer or man-in-the-middle will modify SDP, 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. Note that <xref target="RFC3261">RFC
3261</xref>, Section 16.6 states that SIP proxy servers "MUST NOT add
to, modify, or remove the message body."</t>
<t>In the end analysis, 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. 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 SIP users can prove definitively 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.</t>
<t>One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. However, it is important
to note that preventing men-in-the-middle is not the primary impetus
for this mechanism. Moreover, changing the 'alg'</t>
<t>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.</t>
</section>
<section anchor="sec-display-name" title="Display-Names and Identity">
<t>As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email user
interfaces for this practice. As such, it might seem that the lack of
a signature over the display-name is a significant omission.</t>
<t>However, there are several important senses in which a signature
over the display-name does not prevent impersonation. In the first
place, a particular display-name, like "Jon Peterson", is not unique
in the world; many users in different administrative domains might
legitimately claim that name. Furthermore, enrollment practices for
SIP-based services might have a difficult time discerning the
legitimate display-name for a user; it is safe to assume that
impersonators will be capable of creating SIP accounts with arbitrary
display-names. The same situation prevails in email today. Note that
an impersonator who attempted to replay a message with an Identity
header, changing only the display-name in the From header field, would
be detected by the other replay protection mechanisms described in
<xref target="sec-security-digest"></xref>.</t>
<t>Of course, an authentication service can enforce policies about the
display-name even if the display-name is not signed. The exact
mechanics for creating and operationalizing such policies is outside
the scope of this document. The effect of this policy would not be to
prevent impersonation of a particular unique identifier like a SIP URI
(since display-names are not unique identifiers), but to allow a
domain to manage the claims made by its users. If such policies are
enforced, users would not be free to claim any display-name of their
choosing. In the absence of a signature, man-in-the-middle attackers
could conceivably alter the display-names in a request with impunity.
Note that the scope of this specification is impersonation attacks,
however, and that a man-in-the-middle might also strip the Identity
and Identity-Info headers from a message.</t>
<t>There are many environments in which policies regarding the
display- name aren't feasible. Distributing bit-exact and
internationalizable display-names to end-users as part of the
enrollment or registration process would require mechanisms that are
not explored in this</t>
<t>document. In the absence of policy enforcement regarding domain
names, there are conceivably attacks that an adversary could mount
against SIP systems that rely too heavily on the display-name in their
user interface, but this argues for intelligent interface design, not
changes to the mechanisms. Relying on a non-unique identifier for
identity would ultimately result in a weak mechanism.</t>
</section>
<section anchor="sec-secure-connect-auth-serv"
title="Securing the Connection to the Authentication Service">
<t>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: <list>
<t>If a user does not receive a certificate from the
authentication service over this TLS connection that corresponds
to the expected domain (especially when the user receives a
challenge via a mechanism such as Digest), then it is possible
that a rogue server is attempting to pose as an authentication
service for a domain that it does not control, possibly in an
attempt to collect shared secrets for that domain.</t>
<t>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.</t>
</list></t>
<t>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.</t>
<t>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.</t>
</section>
<section anchor="sec-security-subordination"
title="Domain Names and Subordination">
<t>When a verifier processes a request containing an Identity-Info
header, it must compare the domain portion of the URI in the From
header field of the request with the domain name that is the subject
of the certificate acquired from the Identity-Info header. While it
might seem that this should be a straightforward process, it is
complicated by two deployment realities. In the first place,
certificates have varying ways of describing their subjects, and may
indeed have multiple subjects, especially in 'virtual hosting' cases
where multiple domains are managed by a single application. Secondly,
some SIP services may delegate SIP functions to a subordinate domain
and utilize the procedures in <xref target="RFC3263">RFC 3263</xref>
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 its requests through a host like
'sip.example.com', and it may be that latter host that acts as an
authentication service.</t>
<t>To meet the second of these problems, a domain that deploys an
authentication service on a subordinate host MUST be willing to supply
that host with the private keying material associated with a
certificate whose subject is a domain name that corresponds to the
domain portion of the AoRs that the domain distributes to users. Note
that this corresponds to the comparable case of routing inbound SIP
requests to a domain. When the NAPTR and SRV procedures of RFC 3263
are used to direct requests to a domain name other than the domain in
the original Request-URI (e.g., for 'sip:jon@example.com', the
corresponding SRV records point to the service 'sip1.example.org'),
the client expects that the certificate passed back in any TLS
exchange with that host will correspond exactly with the domain of the
original Request-URI, not the domain name of the host. Consequently,
in order to make inbound routing to such SIP services work, a domain
administrator must similarly be willing to share the domain's private
key with the service. This design 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.</t>
<t>A verifier MUST evaluate the correspondence between the user's
identity and the signing certificate by following the procedures
defined in <xref target="RFC2818">RFC 2818</xref>, Section 3.1. While
<xref target="RFC2818">RFC 2818</xref> deals with the use of HTTP in
TLS, the procedures described are applicable to verifying identity if
one substitutes the "hostname of the server" in HTTP for the domain
portion of the user's identity in the From header field of a SIP
request with an Identity header.</t>
<t>Because the domain certificates that can be used by authentication
services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and
user agents will be able to support the root certificates of all
certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but it
is anticipated that operational experience will create de facto
standards for authentication services. Some federations of service
providers, for example, might only trust certificates that have been
provided by a certificate authority operated by the federation. It is
strongly RECOMMENDED that self-signed domain certificates should not
be trusted by verifiers, unless some previous key exchange has
justified such trust.</t>
<t>For further information on certificate security and practices, see
<xref target="RFC3280">RFC 3280</xref>. The Security Considerations of
<xref target="RFC3280">RFC 3280</xref> are applicable to this
document.</t>
</section>
<section anchor="sec-13.5"
title="Authorization and Transitional Strategies">
<t>Ultimately, the worth of an assurance provided by an Identity
header is limited by the security practices of the domain that issues
the assurance. Relying on an Identity header generated by a remote
administrative domain assumes that the issuing domain used its
administrative practices to authenticate its users. However, it is
possible that some domains will implement policies that effectively
make users unaccountable (e.g., ones that accept unauthenticated
registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a
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.</t>
<t>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. <list>
<t>A verifier could remember when it receives a request from a
domain that uses Identity, and in the future, view messages
received from that domain without Identity headers with
skepticism.</t>
<t>A verifier could query the domain through some sort of callback
system to determine whether or not it is running an authentication
service. There are a number of potential ways in which this could
be implemented; use of the SIP OPTIONS method is one possibility.
This is left as a subject for future work.</t>
</list></t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section anchor="sec-14" title="IANA Considerations">
<t>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.</t>
<section anchor="sec-14.1" title="Header Field Names">
<t>This document specifies two new SIP headers: Identity and Identity-
Info. Their syntax is given in <xref target="sec-syntax"></xref>.
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</t>
<figure>
<artwork><![CDATA[
Header Name: Identity
Compact Form: y
Header Name: Identity-Info
Compact Form: n
]]></artwork>
</figure>
</section>
<section anchor="sec-14.2"
title="428 'Use Identity Header' Response Code">
<t>This document registers a new SIP response code, which is described
in <xref target="sec-verifier-behavior"></xref>. 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</t>
<figure>
<artwork><![CDATA[
Response Code Number: 428
Default Reason Phrase: Use Identity Header
]]></artwork>
</figure>
</section>
<section anchor="sec-14.3" title="436 'Bad Identity-Info' Response Code">
<t>This document registers a new SIP response code, which is described
in <xref target="sec-verifier-behavior"></xref>. 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</t>
<figure>
<artwork><![CDATA[
Response Code Number: 436
Default Reason Phrase: Bad Identity-Info
]]></artwork>
</figure>
</section>
<section anchor="sec-14.4"
title="437 'Unsupported Certificate' Response Code">
<t>This document registers a new SIP response code, which is described
in <xref target="sec-verifier-behavior"></xref>. 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 has been added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters</t>
<figure>
<artwork><![CDATA[
Response Code Number: 437
Default Reason Phrase: Unsupported Certificate
]]></artwork>
</figure>
</section>
<section anchor="sec-14.5"
title="438 'Invalid Identity Header' Response Code">
<t>This document registers a new SIP response code, which is described
in <xref target="sec-verifier-behavior"></xref>. 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</t>
<figure>
<artwork><![CDATA[
Response Code Number: 438
Default Reason Phrase: Invalid Identity Header
]]></artwork>
</figure>
</section>
<section anchor="sec-14.6" title="Identity-Info Parameters">
<t>The IANA has created a new registry for Identity-Info headers. This
registry is to be prepopulated with a single entry for a parameter
called 'alg', which describes the algorithm used to create the
signature that appears in the Identity header. Registry entries must
contain the name of the parameter and the specification in which the
parameter is defined. New parameters for the Identity-Info header may
be defined only in Standards Track RFCs.</t>
</section>
<section anchor="sec-14.7"
title="Identity-Info Algorithm Parameter Values">
<t>The IANA has created a new registry for Identity-Info 'alg'
parameter values. This registry is to be prepopulated with a single
entry for a value called 'rsa-sha1', which describes the algorithm
used to create the signature that appears in the Identity header.
Registry entries must contain the name of the 'alg' parameter value
and the specification in which the value is described. New values for
the 'alg' parameter may be defined only in Standards Track RFCs.</t>
</section>
<section anchor="sec-apendix-a" title="Acknowledgements">
<t>The authors would like to thank</t>
</section>
<section anchor="sec-apendix-c" title="Original RFC 4474 Requirements">
<t>The following requirements were crafted throughout the development
of the mechanism described in this document. They are preserved here
for historical reasons. <list>
<t>The mechanism must allow a UAC or a proxy server to provide a
strong cryptographic identity assurance in a request that can be
verified by a proxy server or UAS.</t>
<t>User agents that receive identity assurances must be able to
validate these assurances without performing any network
lookup.</t>
<t>User agents that hold certificates on behalf of their user must
be capable of adding this identity assurance to requests.</t>
<t>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.</t>
<t>The mechanism must prevent replay of the identity assurance by
an attacker.</t>
<t>In order to provide full replay protection, the mechanism must
be capable of protecting the integrity of SIP message bodies (to
ensure that media offers and answers are linked to the signaling
identity).</t>
<t>It must be possible for a user to have multiple AoRs (i.e.,
accounts or aliases) that it is authorized to use within a domain,
and for the UAC to assert one identity while authenticating itself
as another, related, identity, as permitted by the local policy of
the domain.</t>
</list></t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="RFC3261">
<front>
<title>SIP: Session Initiation Protocol</title>
<author fullname="J. Rosenberg" initials="J." surname="Rosenberg">
<organization></organization>
</author>
<author fullname="H. Schulzrinne" initials="H."
surname="Schulzrinne">
<organization></organization>
</author>
<author fullname="G. Camarillo" initials="G." surname="Camarillo">
<organization></organization>
</author>
<author fullname="A. Johnston" initials="A." surname="Johnston">
<organization></organization>
</author>
<author fullname="J. Peterson" initials="J." surname="Peterson">
<organization></organization>
</author>
<author fullname="R. Sparks" initials="R." surname="Sparks">
<organization></organization>
</author>
<author fullname="M. Handley" initials="M." surname="Handley">
<organization></organization>
</author>
<author fullname="E. Schooler" initials="E." surname="Schooler">
<organization></organization>
</author>
<date month="June" year="2002" />
<abstract>
<t>This document describes Session Initiation Protocol (SIP), an
application-layer control (signaling) protocol for creating,
modifying, and terminating sessions with one or more participants.
These sessions include Internet telephone calls, multimedia
distribution, and multimedia conferences. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3261" />
<format octets="647976"
target="http://www.rfc-editor.org/rfc/rfc3261.txt" type="TXT" />
</reference>
<reference anchor="RFC2818">
<front>
<title>HTTP Over TLS</title>
<author fullname="E. Rescorla" initials="E." surname="Rescorla">
<organization></organization>
</author>
<date month="May" year="2000" />
<abstract>
<t>This memo describes how to use Transport Layer Security (TLS)
to secure Hypertext Transfer Protocol (HTTP) connections over the
Internet. This memo provides information for the Internet
community.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="2818" />
<format octets="15170"
target="http://www.rfc-editor.org/rfc/rfc2818.txt" type="TXT" />
</reference>
<reference anchor="RFC3280">
<front>
<title>Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile</title>
<author fullname="R. Housley" initials="R." surname="Housley">
<organization></organization>
</author>
<author fullname="W. Polk" initials="W." surname="Polk">
<organization></organization>
</author>
<author fullname="W. Ford" initials="W." surname="Ford">
<organization></organization>
</author>
<author fullname="D. Solo" initials="D." surname="Solo">
<organization></organization>
</author>
<date month="April" year="2002" />
<abstract>
<t>This memo profiles the X.509 v3 certificate and X.509 v2
Certificate Revocation List (CRL) for use in the Internet.
[STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3280" />
<format octets="295556"
target="http://www.rfc-editor.org/rfc/rfc3280.txt" type="TXT" />
</reference>
<reference anchor="RFC3548">
<front>
<title>The Base16, Base32, and Base64 Data Encodings</title>
<author fullname="S. Josefsson" initials="S." surname="Josefsson">
<organization></organization>
</author>
<date month="July" year="2003" />
<abstract>
<t>This document describes the commonly used base 64, base 32, and
base 16 encoding schemes. It also discusses the use of line-feeds
in encoded data, use of padding in encoded data, use of
non-alphabet characters in encoded data, and use of different
encoding alphabets. This memo provides information for the
Internet community.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3548" />
<format octets="26363"
target="http://www.rfc-editor.org/rfc/rfc3548.txt" type="TXT" />
</reference>
<reference anchor="RFC3370">
<front>
<title>Cryptographic Message Syntax (CMS) Algorithms</title>
<author fullname="R. Housley" initials="R." surname="Housley">
<organization></organization>
</author>
<date month="August" year="2002" />
</front>
<seriesInfo name="RFC" value="3370" />
<format octets="51001"
target="http://www.rfc-editor.org/rfc/rfc3370.txt" type="TXT" />
</reference>
<reference anchor="RFC4234">
<front>
<title abbrev="ABNF">Augmented BNF for Syntax Specifications:
ABNF</title>
<author fullname="Dave Crocker" initials="D." role="editor"
surname="Crocker">
<organization>Brandenburg InternetWorking</organization>
<address>
<postal>
<street>675 Spruce Dr.</street>
<city>Sunnyvale</city>
<region>CA</region>
<code>94086</code>
<country>US</country>
</postal>
<phone>+1.408.246.8253</phone>
<email>dcrocker@bbiw.net</email>
</address>
</author>
<author fullname="Paul Overell" initials="P." surname="Overell">
<organization>THUS plc.</organization>
<address>
<postal>
<street>1/2 Berkeley Square,</street>
<street>99 Berkeley Street</street>
<city>Glasgow</city>
<code>G3 7HR</code>
<country>UK</country>
</postal>
<email>paul.overell@thus.net</email>
</address>
</author>
<date month="October" year="2005" />
<keyword>ABNF</keyword>
<keyword>Augmented</keyword>
<keyword>Backus-Naur</keyword>
<keyword>Form</keyword>
<keyword>electronic</keyword>
<keyword>mail</keyword>
<abstract>
<t>Internet technical specifications often need to define a formal
syntax. Over the years, a modified version of Backus-Naur Form
(BNF), called Augmented BNF (ABNF), has been popular among many
Internet specifications. The current specification documents ABNF.
It balances compactness and simplicity, with reasonable
representational power. The differences between standard BNF and
ABNF involve naming rules, repetition, alternatives, order-
independence, and value ranges. This specification also supplies
additional rule definitions and encoding for a core lexical
analyzer of the type common to several Internet
specifications.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="4234" />
<format octets="26351"
target="http://www.rfc-editor.org/rfc/rfc4234.txt" type="TXT" />
<format octets="52301"
target="http://xml.resource.org/public/rfc/html/rfc4234.html"
type="HTML" />
<format octets="37285"
target="http://xml.resource.org/public/rfc/xml/rfc4234.xml"
type="XML" />
</reference>
<reference anchor="RFC3893">
<front>
<title>Session Initiation Protocol (SIP) Authenticated Identity Body
(AIB) Format</title>
<author fullname="J. Peterson" initials="J." surname="Peterson">
<organization></organization>
</author>
<date month="September" year="2004" />
<abstract>
<t>RFC 3261 introduces the concept of adding an S/MIME body to a
Session Initiation Protocol (SIP) request or response in order to
provide reference integrity over its headers. This document
provides a more specific mechanism to derive integrity and
authentication properties from an 'authenticated identity body', a
digitally-signed SIP message, or message fragment. A standard
format for such bodies (known as Authenticated Identity Bodies, or
AIBs) is given in this document. Some considerations for the
processing of AIBs by recipients of SIP messages with such bodies
are also given. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3893" />
<format octets="28500"
target="http://www.rfc-editor.org/rfc/rfc3893.txt" type="TXT" />
</reference>
<reference anchor="RFC3323">
<front>
<title>A Privacy Mechanism for the Session Initiation Protocol
(SIP)</title>
<author fullname="J. Peterson" initials="J." surname="Peterson">
<organization></organization>
</author>
<date month="November" year="2002" />
</front>
<seriesInfo name="RFC" value="3323" />
<format octets="54116"
target="http://www.rfc-editor.org/rfc/rfc3323.txt" type="TXT" />
</reference>
<reference anchor="RFC2119">
<front>
<title abbrev="RFC Key Words">Key words for use in RFCs to Indicate
Requirement Levels</title>
<author fullname="Scott Bradner" initials="S." surname="Bradner">
<organization>Harvard University</organization>
<address>
<postal>
<street>1350 Mass. Ave.</street>
<street>Cambridge</street>
<street>MA 02138</street>
</postal>
<phone>- +1 617 495 3864</phone>
<email>sob@harvard.edu</email>
</address>
</author>
<date month="March" year="1997" />
<area>General</area>
<keyword>keyword</keyword>
<abstract>
<t>In many standards track documents several words are used to
signify the requirements in the specification. These words are
often capitalized. This document defines these words as they
should be interpreted in IETF documents. Authors who follow these
guidelines should incorporate this phrase near the beginning of
their document: <list>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in RFC 2119.</t>
</list></t>
<t>Note that the force of these words is modified by the
requirement level of the document in which they are used.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="14" />
<seriesInfo name="RFC" value="2119" />
<format octets="4723"
target="http://www.rfc-editor.org/rfc/rfc2119.txt" type="TXT" />
<format octets="17970"
target="http://xml.resource.org/public/rfc/html/rfc2119.html"
type="HTML" />
<format octets="5777"
target="http://xml.resource.org/public/rfc/xml/rfc2119.xml"
type="XML" />
</reference>
</references>
<references title="Informative References">
<reference anchor="RFC4474">
<front>
<title>Enhancements for Authenticated Identity Management in the
Session Initiation Protocol (SIP)</title>
<author fullname="J. Peterson" initials="J." surname="Peterson">
<organization></organization>
</author>
<author fullname="C. Jennings" initials="C." surname="Jennings">
<organization></organization>
</author>
<date month="August" year="2006" />
<abstract>
<t>The existing 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 messages. It does so by
defining two new SIP header fields, Identity, for conveying a
signature used for validating the identity, and Identity-Info, for
conveying a reference to the certificate of the signer.
[STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="4474" />
<format octets="104952"
target="http://www.rfc-editor.org/rfc/rfc4474.txt" type="TXT" />
</reference>
<reference anchor="RFC3325">
<front>
<title>Private Extensions to the Session Initiation Protocol (SIP)
for Asserted Identity within Trusted Networks</title>
<author fullname="C. Jennings" initials="C." surname="Jennings">
<organization></organization>
</author>
<author fullname="J. Peterson" initials="J." surname="Peterson">
<organization></organization>
</author>
<author fullname="M. Watson" initials="M." surname="Watson">
<organization></organization>
</author>
<date month="November" year="2002" />
</front>
<seriesInfo name="RFC" value="3325" />
<format octets="36170"
target="http://www.rfc-editor.org/rfc/rfc3325.txt" type="TXT" />
</reference>
<reference anchor="RFC4475">
<front>
<title>Session Initiation Protocol (SIP) Torture Test
Messages</title>
<author fullname="R. Sparks" initials="R." surname="Sparks">
<organization></organization>
</author>
<author fullname="A. Hawrylyshen" initials="A."
surname="Hawrylyshen">
<organization></organization>
</author>
<author fullname="A. Johnston" initials="A." surname="Johnston">
<organization></organization>
</author>
<author fullname="J. Rosenberg" initials="J." surname="Rosenberg">
<organization></organization>
</author>
<author fullname="H. Schulzrinne" initials="H."
surname="Schulzrinne">
<organization></organization>
</author>
<date month="May" year="2006" />
<abstract>
<t>This informational document gives examples of Session
Initiation Protocol (SIP) test messages designed to exercise and
"torture" a SIP implementation. This memo provides information for
the Internet community.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="4475" />
<format octets="93276"
target="http://www.rfc-editor.org/rfc/rfc4475.txt" type="TXT" />
</reference>
<reference anchor="RFC3761">
<front>
<title>The E.164 to Uniform Resource Identifiers (URI) Dynamic
Delegation Discovery System (DDDS) Application (ENUM)</title>
<author fullname="P. Faltstrom" initials="P." surname="Faltstrom">
<organization></organization>
</author>
<author fullname="M. Mealling" initials="M." surname="Mealling">
<organization></organization>
</author>
<date month="April" year="2004" />
<abstract>
<t>This document discusses the use of the Domain Name System (DNS)
for storage of E.164 numbers. More specifically, how DNS can be
used for identifying available services connected to one E.164
number. It specifically obsoletes RFC 2916 to bring it in line
with the Dynamic Delegation Discovery System (DDDS) Application
specification found in the document series specified in RFC 3401.
It is very important to note that it is impossible to read and
understand this document without reading the documents discussed
in RFC 3401. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3761" />
<format octets="41559"
target="http://www.rfc-editor.org/rfc/rfc3761.txt" type="TXT" />
</reference>
<reference anchor="RFC3966">
<front>
<title>The tel URI for Telephone Numbers</title>
<author fullname="H. Schulzrinne" initials="H."
surname="Schulzrinne">
<organization></organization>
</author>
<date month="December" year="2004" />
<abstract>
<t>This document specifies the URI (Uniform Resource Identifier)
scheme "tel". The "tel" URI describes resources identified by
telephone numbers. This document obsoletes RFC 2806.
[STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3966" />
<format octets="40783"
target="http://www.rfc-editor.org/rfc/rfc3966.txt" type="TXT" />
</reference>
<reference anchor="I-D.peterson-sipping-retarget">
<front>
<title>Retargeting and Security in SIP: A Framework and
Requirements</title>
<author fullname="Jon Peterson" initials="J" surname="Peterson">
<organization></organization>
</author>
<date day="15" month="February" year="2005" />
<abstract>
<t>Retargeting, the alteration by intermediaries of the
destination of a Session Initiation Protocol (SIP) request, is a
common practice performed during the routing of a call. Some forms
of retargeting, however, give rise to security problems and
potential functional gaps in SIP. This document provides a general
framework for discussion of the security problems relating to
retargeting, and gives requirements for mechanisms that might
overcome these problems.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-peterson-sipping-retarget-00" />
<format target="http://www.ietf.org/internet-drafts/draft-peterson-sipping-retarget-00.txt"
type="TXT" />
</reference>
<reference anchor="RFC2585">
<front>
<title abbrev="PKIX Operational Protocols: FTP and HTTP">Internet
X.509 Public Key Infrastructure Operational Protocols: FTP and
HTTP</title>
<author fullname="Russell Housley" initials="R." surname="Housley">
<organization>SPYRUS</organization>
<address>
<postal>
<street>381 Elden Street</street>
<street>Suite 1120</street>
<street>Suite 1120</street>
<city>Herndon</city>
<region>VA</region>
<code>20170</code>
<country>US</country>
</postal>
<email>housley@spyrus.com</email>
</address>
</author>
<author fullname="Paul Hoffman" initials="P." surname="Hoffman">
<organization>Internet Mail Consortium</organization>
<address>
<postal>
<street>127 Segre Place</street>
<city>Santa Cruz</city>
<region>CA</region>
<code>95060</code>
<country>US</country>
</postal>
<email>phoffman@imc.org</email>
</address>
</author>
<date month="May" year="1999" />
<abstract>
<t>The protocol conventions described in this document satisfy
some of the operational requirements of the Internet Public Key
Infrastructure (PKI). This document specifies the conventions for
using the File Transfer Protocol (FTP) and the Hypertext Transfer
Protocol (HTTP) to obtain certificates and certificate revocation
lists (CRLs) from PKI repositories. Additional mechanisms
addressing PKIX operational requirements are specified in separate
documents.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="2585" />
<format octets="14813"
target="http://www.rfc-editor.org/rfc/rfc2585.txt" type="TXT" />
</reference>
<reference anchor="RFC3263">
<front>
<title>Session Initiation Protocol (SIP): Locating SIP
Servers</title>
<author fullname="J. Rosenberg" initials="J." surname="Rosenberg">
<organization></organization>
</author>
<author fullname="H. Schulzrinne" initials="H."
surname="Schulzrinne">
<organization></organization>
</author>
<date month="June" year="2002" />
<abstract>
<t>The Session Initiation Protocol (SIP) uses DNS procedures to
allow a client to resolve a SIP Uniform Resource Identifier (URI)
into the IP address, port, and transport protocol of the next hop
to contact. It also uses DNS to allow a server to send a response
to a backup client if the primary client has failed. This document
describes those DNS procedures in detail. [STANDARDS-TRACK]</t>
</abstract>
</front>
<seriesInfo name="RFC" value="3263" />
<format octets="42310"
target="http://www.rfc-editor.org/rfc/rfc3263.txt" type="TXT" />
</reference>
<reference anchor="RFC6919">
<front>
<title>Further Key Words for Use in RFCs to Indicate Requirement
Levels</title>
<author fullname="R. Barnes" initials="R." surname="Barnes">
<organization></organization>
</author>
<author fullname="S. Kent" initials="S." surname="Kent">
<organization></organization>
</author>
<author fullname="E. Rescorla" initials="E." surname="Rescorla">
<organization></organization>
</author>
<date month="April" year="1 2013" />
<abstract>
<t>RFC 2119 defines a standard set of key words for describing
requirements of a specification. Many IETF documents have found
that these words cannot accurately capture the nuanced
requirements of their specification. This document defines
additional key words that can be used to address alternative
requirements scenarios. Authors who follow these guidelines should
incorporate this phrase near the beginning of their
document:</t><t> The key words "MUST (BUT WE KNOW YOU
WON\'T)", "SHOULD CONSIDER", "REALLY SHOULD NOT", "OUGHT TO",
"WOULD PROBABLY", "MAY WISH TO", "COULD", "POSSIBLE", and "MIGHT"
in this document are to be interpreted as described in RFC
6919.</t>
</abstract>
</front>
<seriesInfo name="RFC" value="6919" />
<format octets="11076"
target="http://www.rfc-editor.org/rfc/rfc6919.txt" type="TXT" />
</reference>
<reference anchor="RFC2234">
<front>
<title abbrev="ABNF for Syntax Specifications">Augmented BNF for
Syntax Specifications: ABNF</title>
<author fullname="David H. Crocker" initials="D." role="editor"
surname="Crocker">
<organization>Internet Mail Consortium</organization>
<address>
<postal>
<street>675 Spruce Dr.</street>
<city>Sunnyvale</city>
<region>CA</region>
<code>94086</code>
<country>US</country>
</postal>
<phone>+1 408 246 8253</phone>
<facsimile>+1 408 249 6205</facsimile>
<email>dcrocker@imc.org</email>
</address>
</author>
<author fullname="Paul Overell" initials="P." surname="Overell">
<organization>Demon Internet Ltd.</organization>
<address>
<postal>
<street>Dorking Business Park</street>
<street>Dorking</street>
<city>Surrey</city>
<region>England</region>
<code>RH4 1HN</code>
<country>UK</country>
</postal>
<email>paulo@turnpike.com</email>
</address>
</author>
<date month="November" year="1997" />
<keyword>ABNF</keyword>
<keyword>Augmented</keyword>
<keyword>Backus-Naur</keyword>
<keyword>Form</keyword>
<keyword>electronic</keyword>
<keyword>mail</keyword>
</front>
<seriesInfo name="RFC" value="2234" />
<format octets="24265"
target="http://www.rfc-editor.org/rfc/rfc2234.txt" type="TXT" />
<format octets="42947"
target="http://xml.resource.org/public/rfc/html/rfc2234.html"
type="HTML" />
<format octets="24417"
target="http://xml.resource.org/public/rfc/xml/rfc2234.xml"
type="XML" />
</reference>
<reference anchor="I-D.cooper-iab-secure-origin-00">
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title abbrev="Secure Origin">Secure Call Origin Identification</title>
<!--add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Alissa Cooper" initials="A." surname="Cooper">
<organization>CDT</organization>
<address>
<postal>
<street>1634 Eye St. NW, Suite 1100</street>
<city>Washington</city>
<region>DC</region>
<code>20006</code>
<country>USA</country>
</postal>
<email>acooper@cdt.org</email>
<!-- uri and facsimile elements may also be added -->
</address>
</author>
<author fullname="Hannes Tschofenig" initials="H." surname="Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<email>hannes.tschofenig@gmx.net</email>
</address>
</author>
<author fullname="Jon Peterson" initials="J." surname="Peterson">
<organization>NeuStar</organization>
<address>
<email>jon.peterson@neustar.biz</email>
</address>
</author>
<author fullname="Bernard Aboba" initials="B." surname="Aboba">
<organization>Microsoft</organization>
<address>
<email>bernard.aboba@gmail.com</email>
</address>
</author>
<date day="30" month="November" year="2012"/>
<area>General</area>
<workgroup>Network Working Group</workgroup>
<abstract>
<t>A number of parties have suggested creating mandates such that
networks receiving voice calls would be capable of securely
identifying the call origin. This
document provides insights about the capabilities and limitations of
supporting call origin identification in a secure and privacy-
friendly way in the PSTN and for IP-based real-time communications.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-cooper-iab-secure-origin-00" />
<format target="http://www.ietf.org/internet-drafts/draft-cooper-iab-secure-origin00.txt"
type="TXT" />
</reference>
<reference anchor="I-D.peterson-secure-origin-ps">
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title abbrev="Secure Origin Identification">Secure Origin Identification: Problem Statement, Requirements, and Roadmap</title>
<author initials="J." surname="Peterson" fullname="Jon Peterson">
<organization abbrev="NeuStar, Inc.">NeuStar, Inc.</organization>
<address>
<postal>
<street>1800 Sutter St Suite 570</street>
<city>Concord</city>
<region>CA</region>
<code>94520</code>
<country>US</country>
</postal>
<email>jon.peterson@neustar.biz</email>
</address>
</author>
<author initials="H." surname="Schulzrinne" fullname="Henning Schulzrinne">
<organization>Columbia University</organization>
<address>
<postal>
<street>Department of Computer Science</street>
<street>450 Computer Science Building</street>
<city>New York</city>
<region>NY</region>
<code>10027</code>
<country>US</country>
</postal>
<phone>+1 212 939 7004</phone>
<email>hgs+ecrit@cs.columbia.edu</email>
<uri>http://www.cs.columbia.edu</uri>
</address>
</author>
<author fullname="Hannes Tschofenig" initials="H." surname="Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<postal>
<street>Linnoitustie 6</street>
<city>Espoo</city>
<region></region>
<code>02600</code>
<country>Finland</country>
</postal>
<phone>+358 (50) 4871445</phone>
<email>Hannes.Tschofenig@gmx.net</email>
<uri>http://www.tschofenig.priv.at</uri>
</address>
</author>
<date year="2013" month="May" day="27"/>
<!-- <area>
Security
</area>-->
<keyword>SIP</keyword>
<keyword>XMPP</keyword>
<keyword>Secure Origin Identification</keyword>
<keyword>Communication Security</keyword>
<keyword>RTCWeb</keyword>
<keyword>Problem Statement</keyword>
<keyword>Real-Time Communication</keyword>
<abstract>
<t>Over the past decade, SIP has become a major signaling protocol for voice communications, one which has replaced many traditional telephony deployments. However, interworking SIP with the traditional telephone network has ultimately reduced the security of Caller ID systems. Given the widespread interworking of SIP with the telephone network, the lack of effective standards for identifying the calling party in a SIP session has granted attackers new powers as they impersonate or obscure calling party numbers when orchestrating bulk commercial calling schemes, hacking voicemail boxes or even circumventing multi-factor authentication systems trusted by banks. This document therefore examines the reasons why providing identity for telephone numbers on the Internet has proven so difficult, and shows how changes in the last decade may provide us with new strategies for attaching a secure identity to SIP sessions.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-peterson-secure-origin-ps-00" />
<format target="http://www.ietf.org/internet-drafts/draft-peterson-secure-origin-ps-00.txt"
type="TXT" />
</reference>
<reference anchor='I-D.rescorla-rtcweb-generic-idp'>
<front>
<title>RTCWEB Generic Identity Provider Interface</title>
<author initials='E' surname='Rescorla' fullname='Eric Rescorla'>
<organization />
</author>
<date month='March' day='12' year='2012' />
<abstract><t>Security for RTCWEB communications requires that the communicating endpoints be able to authenticate each other. While authentication may be mediated by the calling service, there are settings in which this is undesirable. This document describes a generic mechanism for leveraging existing identity providers (IdPs) such as BrowserID or OAuth to provide this authentication service.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-rescorla-rtcweb-generic-idp-01' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-rescorla-rtcweb-generic-idp-01.txt' />
</reference>
<reference anchor='I-D.rescorla-callerid-fallback'>
<front>
<title abbrev="Caller-ID Fallback">Secure Caller-ID Fallback Mode</title>
<author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
<organization>RTFM, Inc.</organization>
<address>
<postal>
<street>2064 Edgewood Drive</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94303</code>
<country>USA</country>
</postal>
<phone>+1 650 678 2350</phone>
<email>ekr@rtfm.com</email>
</address>
</author>
<date day="29" month="May" year="2013" />
<area>RAI</area>
<!-- <workgroup>RTCWEB</workgroup> -->
<abstract>
<t>
A major challenge with RFC 4474-style identity assertions has been
that SIP operates in highly mediated and interworked environments.
SIP requests may pass through gateways, policy enforcement devices
or other entities that receive SIP requests and effectively act as user agents,
re-initiating a request. In these circumstances, intermediaries may
recreate the fields protected by the RFC4474 signature, making end-to
end integrity impossible. This document describes a mechanism for two
compliant endpoints to exchange authentication data even in the face of
intermediaries which remove all additional call signaling meta-data.
</t>
</abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-rescorla-callerid-fallback-00' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-rescorla-callerid-fallback-00.txt' />
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 10:11:30 |