draft-ietf-sip-identity-02.txt   draft-ietf-sip-identity-03.txt 
SIP WG J. Peterson SIP WG J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Expires: November 15, 2004 C. Jennings Expires: March 30, 2005 C. Jennings
Cisco Systems Cisco Systems
May 17, 2004 September 29, 2004
Enhancements for Authenticated Identity Management in the Session Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP) Initiation Protocol (SIP)
draft-ietf-sip-identity-02 draft-ietf-sip-identity-03
Status of this Memo Status of this Memo
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all provisions of Section 10 of RFC2026. patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice Copyright Notice
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Abstract Abstract
The existing security mechanisms in the Session Initiation Protocol The existing security mechanisms in the Session Initiation Protocol
are inadequate for cryptographically assuring the identity of the end are inadequate for cryptographically assuring the identity of the end
users that originate SIP requests and responses, especially in an users that originate SIP requests, especially in an interdomain
interdomain context. This document recommends practices and context. This document recommends practices and conventions for
conventions for identifying end users in SIP messages, and proposes a identifying end users in SIP messages, and proposes a way to
way to distribute cryptographically secure authenticated identities. distribute cryptographically-secure authenticated identities.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Overview of Operations . . . . . . . . . . . . . . . . . . . . 6 5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
6. User Agent Behavior: Sending Messages . . . . . . . . . . . . 7 6. Authentication Service Behavior . . . . . . . . . . . . . . 7
7. Authentication Service Behavior . . . . . . . . . . . . . . . 7 7. Verifying Identity . . . . . . . . . . . . . . . . . . . . . 9
7.1 UAs acting as an Authentication service . . . . . . . . . . . 9 8. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 10
8. Verifying Identity . . . . . . . . . . . . . . . . . . . . . . 9 9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . 10
9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . . 10 10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 11
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12 11. Compliance Tests and Examples . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13 11.1 Identity-Info with a Singlepart MIME body . . . . . . . 14
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 11.2 Identity for a Request with no MIME body or Contact . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17 12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 19
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
Normative References . . . . . . . . . . . . . . . . . . . . . 16 14. Security Considerations . . . . . . . . . . . . . . . . . . 21
Informative References . . . . . . . . . . . . . . . . . . . . 16 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25
B. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 17 15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 25
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 19 15.2 Response Code . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 26
B. Bit-exact archive of example messages . . . . . . . . . . . 27
B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 27
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
16.1 Normative References . . . . . . . . . . . . . . . . . . . 25
16.2 Informative References . . . . . . . . . . . . . . . . . . 26
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 32
1. Introduction 1. Introduction
This document provides enhancements to the existing mechanisms for This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol authenticated identity management in the Session Initiation Protocol
(SIP [1]). An identity, for the purposes of this document, is (SIP [1]). An identity, for the purposes of this document, is
defined as a canonical SIP address-of-record URI employed to reach a defined as a canonical SIP address-of-record URI employed to reach a
user (such as 'sip:alice@atlanta.com'). user (such as 'sip:alice@atlanta.example.com').
RFC3261 enumerates a number of places within a SIP request that a RFC3261 enumerates a number of places within a SIP request that a
user can express an identity for themselves, notably the user- user can express an identity for themselves, notably the
populated From header field. However, the recipient of a SIP request user-populated From header field. However, the recipient of a SIP
has no way to verify that the From header field has been populated request has no way to verify that the From header field has been
accurately, in the absence of some sort of cryptographic populated accurately, in the absence of some sort of cryptographic
authentication mechanism. authentication mechanism.
RFC3261 specifies a number of security mechanisms that can be RFC3261 specifies a number of security mechanisms that can be
employed by SIP UAs, including Digest, TLS and S/MIME employed by SIP UAs, including Digest, TLS and S/MIME
(implementations may support other security schemes as well). (implementations may support other security schemes as well).
However, few SIP user agents today support the end-user certificates However, few SIP user agents today support the end-user certificates
necessary to authenticate themselves via TLS or S/MIME, and necessary to authenticate themselves via TLS or S/MIME, and
furthermore Digest authentication is limited by the fact that the furthermore Digest authentication is limited by the fact that the
originator and destination must share a pre-arranged secret. It is originator and destination must share a pre-arranged secret. It is
desirable for SIP user agents to be able to send requests to desirable for SIP user agents to be able to send requests to
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In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC2119 [2] and indicate requirement levels for described in RFC2119 [2] and indicate requirement levels for
compliant SIP implementations. compliant SIP implementations.
3. Background 3. Background
All RFC3261-compliant SIP user agents support a means of All RFC3261-compliant SIP user agents support a means of
authenticating themselves to a SIP registrar - commonly with a shared authenticating themselves to a SIP registrar, commonly with a shared
secret (Digest authentication, which MUST be supported by SIP user secret; Digest authentication, which MUST be supported by SIP user
agents, is typically used for this purpose). Registration allows a agents, is typically used for this purpose. Registration allows a
user agent to express that it is the proper entity to which requests user agent to express that it is the proper entity to which requests
should be sent for a particular address-of-record SIP URI. should be sent for a particular address-of-record SIP URI (e.g.,
'sip:alice@atlanta.example.com').
Coincidentally, the address-of-record URI of a SIP user is also the The address-of-record URI used for registration is also the URI with
URI with which a user commonly populates the From header of requests which a UA commonly populates the From header of requests in order to
- in other words, the address-of-record is an identity. So in this provide their 'return address' identity to recipients. If you can
context, users already have a means of providing their identity, prove you are eligible to register in a domain under a particular
which makes good sense: since the contents of a From header field are address-of-record, you are proving that you are capable of
essentially a 'return address' for SIP requests, being able to prove legitimately receiving requests for that address-of-record, and
that you are eligible to receive requests for that 'return address' accordingly, when you place that address-of-record in the From header
should be identical to proving that you are authorized to assert this field of a SIP request other than a registration (like an INVITE),
identity. 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', you are also authorized to assert that
'return address' in your From header field.
However, the credentials with which a user agent proves their In the context of registration, users already have a means of proving
identity to a registrar cannot be validated by a user agent or proxy their identity to a registrar. However, the credentials with which a
server outside your local domain - these credentials are currently user agent proves their identity to a registrar cannot be validated
only useful for registration. For the purposes of determining by just any user agent or proxy server - these credentials are only
whether or not the 'return address' of a request can legitimately be shared between the user agent and their domain administrator. For
asserted as the identity of the user, SIP entities in other domains the purposes of determining whether or not the 'return address' of a
require an assurance that the sender of a message is capable of request can legitimately be asserted in the From header field of a
authenticating themselves to a registrar in their own domain. request, SIP entities that are not operated by the domain
administrator require an assurance that the sender of a message is
capable of authenticating themselves to a registrar in their own
domain.
Ideally, then, SIP user agents should have some way of proving to Ideally, then, SIP user agents should have some way of proving to
recipients of SIP messages that their local domain has authenticated recipients of SIP requests that their local domain has authenticated
them. In the absence of end-user certificates in user agents, it is them. In the absence of end-user certificates in user agents, it is
possible to implement a mediated authentication architecture for SIP possible to implement a mediated authentication architecture for SIP
in which requests are sent to a server in the user's local domain in which requests are sent to a server in the user's local domain
which authenticates such requests (using the same practices by which which authenticates such requests (using the same practices by which
the domain would authenticate REGISTER requests). Once a message has the domain would authenticate REGISTER requests). Once a message has
been authenticated, the local domain then needs some way to been authenticated, the local domain then needs some way to
communicate to other SIP entities the sending user has been communicate to other SIP entities that the sending user has been
authenticated. This draft addresses how that imprimatur of authenticated. This draft addresses how that imprimatur of
authentication can be shared. authentication can be shared.
RFC3261 already describes an architecture very similar to this in RFC3261 already describes an architecture very similar to this in
Section 26.3.2.2, in which a user agent authenticates itself to a 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 local proxy server which in turn authenticates itself to a remote
proxy server via mutual TLS, creating a two-link chain of transitive proxy server via mutual TLS, creating a two-link chain of transitive
authentication between the originator and the remote domain. While authentication between the originator and the remote domain. While
this works well in some architectures, there are a few respects in this works well in some architectures, there are a few respects in
which this is impractical. For one, transitive trust in inherently which this is impractical. For one, transitive trust is inherently
weaker than an assertion that can be validated end-to-end. It is weaker than an assertion that can be validated end-to-end. It is
possible for SIP requests to cross multiple intermediaries in possible for SIP requests to cross multiple intermediaries in
separate administrative domains, in which case transitive trust separate administrative domains, in which case transitive trust
becomes even less compelling. It also requires intermediaries to act becomes even less compelling. It also requires intermediaries to act
as proxies, rather than redirecting requests to their destinations as proxies, rather than redirecting requests to their destinations
(redirection lightens loads on SIP intermediaries). (redirection lightens loads on SIP intermediaries).
One solution to this problem is to use 'trusted' SIP intermediaries One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP 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. A mechanism for doing so (with the P-Asserted-Identity
header) is given in [6]. However, this solution allows only hop-by- header) is given in [8]. However, this solution allows only
hop trust between intermediaries, not end-to-end cryptographic hop-by-hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict authentication, and it assumes a managed network of nodes with strict
mutual trust relationships, an assumption that is incompatible with mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment. widespread Internet deployment.
Accordingly, a new tactic is required for sharing a cryptographic Accordingly, this document specifies a means of sharing a
assurance of end-user identity in an intradomain context. cryptographic assurance of end-user SIP identity in an interdomain
Furthermore, this new mechanism must work for both SIP requests and context based on the concept of an 'authentication service' and a new
responses. However, there is an additional wrinkle specific to SIP header, the Identity header. Note that the scope of this
providing identity in a response. While the original address-of- document is limited to providing this identity assurance for SIP
record to which a request is sent is stored in the To header field of requests; solving this problem for SIP responses is more complicated,
the request, it is possible, due to retargeting at intermediaries, it and is a subject for future work.
is possible that the request will be forwarded to an entity that has
a different AoR (i.e. identity). Since the To header is not changed This specification allows either a user agent or a proxy server to
in responses to a SIP request, the UAC has no way of discovering that act as an authentication service. To maximize end-to-end security,
new AoR. This is generally known as the "response identity" or it is obviously preferable for end users to hold their own
"connected party" problem. certificates; if they do, they can act as an authentication service.
However, end-user certificates may be neither practical nor
affordable, given the difficulties of establishing a PKI that extends
to end users, and moreover, given the potentially large number of SIP
user agents (phones, PCs, laptops, PDAs, gaming devices) that may be
employed by a single user. In such environments, synchronizing
certificates across multiple devices may be very complex, and
requires quite a good deal of additional endpoint behavior. Managing
several certificates for the various devices is also quite
problematic and unpopular with users. Accordingly, in the initial
use of this mechanism, it is likely that intermediaries will
instantiate the authentication service role.
4. Requirements 4. Requirements
This draft addresses the following requirements: This draft addresses the following requirements:
o The mechanism must allow a UAC to provide a strong cryptographic
The mechanism must allow a UAC to provide a strong cryptographic identity assurance in a request that can be verified by a proxy
identity assurance to the UAS in a request. server or UAS.
o User agents that receive identity assurances must be able to
The mechanism must allow a UAS to provide a strong cryptographic
identity assurance to the UAC in a response.
User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup. validate these assurances without performing any network lookup.
o User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to requests.
o Proxy servers that hold certificates on behalf of their domain
must be capable of adding this identity assurance to requests; a
UAC is not required to support the Identity header in order for
identity to be added to a request in this fashion.
Proxy servers must be capable of adding this identity assurance to o The mechanism must prevent replay of the identity assurance by an
requests or responses.
The mechanism must prevent replay of the identity assurance by an
attacker. attacker.
o The mechanism must be capable of protecting the integrity of SIP
The mechanism must be capable of protecting the integrity of SIP
message bodies (to ensure that media offers and answers are linked message bodies (to ensure that media offers and answers are linked
to the signaling identity). to the signaling identity).
o It must be possible for a user to have multiple AoRs (i.e.
It must be possible for a user to have multiple AoRs (i.e.
accounts or aliases) under which it is known at a domain, and for accounts or aliases) under which it is known at a domain, and for
the UAC to assert one identity while authenticating itself as the UAC to assert one identity while authenticating itself as
another, related, identity, as permitted by the local policy of another, related, identity, as permitted by the local policy of
the domain. the domain.
o It must be possible, in cases where a request has been retargeted
It must be possible, in cases where a request has been retargeted
to a different AoR than the one designated in the To header field, to a different AoR than the one designated in the To header field,
for the UAC to ascertain the AoR to which the request has been for the UAC to ascertain the AoR to which the request has been
sent. sent.
5. Overview of Operations 5. Overview of Operations
This section provides an informative (non-normative) high-level This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document. overview of the mechanisms described in this document.
Imagine the case where Alice, who has the home proxy of example.com Imagine the case where Alice, who has the home proxy of example.com
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field of the request. She then sends an INVITE over TLS to an field of the request. She then sends an INVITE over TLS to an
authentication service proxy for her domain. authentication service proxy for her domain.
The authentication service authenticates Alice (possibly by sending a The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized Digest authentication challenge) and validates that she is authorized
to populate the value of the From header field (which may be Alice's to populate the value of the From header field (which may be Alice's
AoR, or it may be some other value that the policy of the proxy 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 server permits her to use). It then computes a hash over some
particular headers, including the From header field and the bodies in particular headers, including the From header field and the bodies in
the message. This hash is signed with the certificate for the domain the message. This hash is signed with the certificate for the domain
(example.com, in Alice's case) and inserted in a header field (the (example.com, in Alice's case) and inserted in a new header field in
new Identity header) in the SIP message. The proxy, as the holder the SIP message, the 'Identity' header.
the private key of its domain, is asserting that the originator of
this request has been authenticated and that she is authorized to
claim the identity (the SIP address-of-record) which appears in the
From header field. The proxy also inserts a companion header field
that tells Bob how to acquire its certificate, if he doesn't already
have it.
When Bob returns a response to the INVITE (such as a 200 OK), a
similar set of steps happen. Bob's home proxy asserts his identity
in the response. In this instance, the proxy has to insert the
header directly into the request - redirection of responses is not
possible. When Alice receives the response, she verifies Bob's
identity.
If Alice's request for Bob were retargeted, one of two things might
happened. If it were retargeted to a domain that was also the
responsibility of Bob's home proxy (for example, retargeted from
sip:bob@example.com to sip:carol@example.com), then the request would
proceed normally and receive an Identity. If Bob's home proxy would
retarget the request to some other domain (e.g.
sip:bob@example.ORG), then his home proxy would redirect the request
rather than proxying it, and Alice would send a new request that
could receive a response with an Identity from the new domain.
6. User Agent Behavior: Sending Messages The proxy, as the holder the private key of its domain, is asserting
that the originator of this request has been authenticated and that
she is authorized to claim the identity (the SIP address-of-record)
which appears in the From header field. The proxy also inserts a
companion header field that tells Bob how to acquire its certificate,
if he doesn't already have it.
This mechanism requires one important change to existing user agent When Bob's domain receives the request, it verifies the signature
behavior for sending requests and responses: user agents using this provided in the Identity header, and thus can authenticate that the
mechanism to send requests or responses MUST support TLS; moreover, domain indicated by the host portion of the AoR in the From header
they MUST be capable of establishing a persistent TLS connection with field authenticated the user, and permitted them to assert that From
a proxy server that acts as an authentication service. Additionally, header field value.
there are several practices that should be highlighted in the context
of this identity solution.
When a UAC sends a request, it MUST accurately populate the header 6. Authentication Service Behavior
field that asserts its identity (for a SIP request, this is the From
header field). In a request it MUST set the URI portion of its From
header to match a SIP, SIPS or TEL URI AoR under which the UAC can
register (including anonymous URIs, as described in RFC 3323 [3]).
The UAC MUST also be capable of sending requests through an
'outbound' proxy (the authentication service), and of course MUST
support the Digest authentication mechanism described in RFC3261.
Because this mechanism does not provide integrity protection for the
first hop to the authentication service, the UAC MUST send requests
to an authentication service only over a TLS connection.
Additionally, in order to provide identity for responses, user agents
MUST form a persistent TLS connection to a proxy server when a
REGISTER is sent.
Since a UAS cannot send a response that does not replicate the This document defines a new role for SIP entities called an
contents of the To and From header fields in the corresponding authentication service. The authentication service role can be
request, UAS response-sending behavior is unchanged. Again, because instantiated by a proxy server, redirect server or a user agent. Any
this mechanism does not provide integrity protection for the first entity that instantiates the authentication service role MUST possess
hop of the response path, the UAS SHOULD send responses only over a the private key of a domain certificate, and MUST be capable of
TLS connection. authenticating one or more SIP users that can register in that
domain. Commonly, this role will be instantiated by a proxy server
or redirect server, since these entities are more likely to have a
static hostname, hold a corresponding certificate, and access to SIP
registrar capabilities that allow them to authenticate users in their
domain.
7. Authentication Service Behavior SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present.
Similarly, authentication services MUST add a Content-Length header
field to SIP requests if one is not already present; this can help
the verifier to double-check that they are hashing exactly as many
bytes of message-body as the authentication service when they verify
the message.
The authentication service authenticates the identity of the message The authentication service authenticates the identity of the message
sender and validates that the identity given in the message can sender and validates that the identity given in the message can
legitimately be asserted by the sender. Then it computes a signature legitimately be asserted by the sender. Then it computes a signature
over the canonical form of several headers and all the bodies, and over the canonical form of several headers and all the bodies, and
inserts this signature into the message. inserts this signature into the message.
First, an authentication service MUST extract the identity of the First, an authentication service MUST extract the identity of the
sender. For requests, it inspects the From header field; for sender from the request. The authentication service takes this value
responses, the To header field (henceforth the result of this from the From header field; this AoR will be referred to here as the
inspection will be referred to as the "identity field). If the 'identity field'. If the identity field contains a SIP or SIPS URI,
identity field contains a SIP or SIPS URI, the authentication service the authentication service MUST extract the hostname portion of the
MUST extract the hostname portion of the URI in that header field, identity field and compare it to the domain(s) for which it is
and compare this to the domain(s) for which it is responsible. If responsible. If the identity field uses the TEL URI scheme, the
the identity field uses the TEL URI scheme, the policy of the policy of the authentication service determines whether or not it is
authentication service determines whether or not it is responsible responsible for this identity; see Section 12 for more information.
for this identity. Some example policies are described in [TODO].
If the authentication service is not responsible for the identity in If the authentication service is not responsible for the identity in
question, it MAY handle the request as a normal proxy server; see question, it MAY handle the request normally, but it MUST NOT add an
below for more information on authentication service handling of an Identity header; see below for more information on authentication
existing Identity header. service handling of an existing Identity header.
Second, the authentication service must determine whether or not the Second, the authentication service needs to determine whether or not
sender of the request is authorized to claim the identity given in the sender of the request is authorized to claim the identity given
the identity field. In order to do so, the authentication service in the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include: which this authentication might be performed include:
o If the authentication service is instantiated by a SIP
For requests, challenging the request with a 407 response code intermediary (proxy or redirect server), it may challenge the
using the Digest authentication scheme (or viewing a Proxy- request with a 407 response code using the Digest authentication
Authentication header sent in the request which was sent in scheme (or viewing a Proxy-Authentication header sent in the
anticipation of a challenge using cached credentials, as described request which was sent in anticipation of a challenge using cached
in RFC 3261 Section 22.3) credentials, as described in RFC 3261 Section 22.3).
o If the authentication service is instantiated by a SIP user agent,
For requests and responses that are sent over a persistent TLS a user agent can be said to authenticate its user on the grounds
connection, relying on some prior authentication that was that the user can provision the user agent with the private key of
performed at the formation of the connection (most likely, the the domain, or by preferably by providing a password that unlocks
authentication service previously challenged a REGISTER request said private key.
sent after the TLS connection was formed, or possibly a prior
challenged INVITE that was sent over the TLS connection)
Authorization of the assertion of a particular username in the From Authorization of the assertion of a particular username in the From
header field of a SIP message is a matter of local policy for the header field of a SIP message is a matter of local policy for the
authorization service which depends greatly on the manner in which authorization service which depends greatly on the manner in which
authentication is performed. A RECOMMENDED policy is as follows: the authentication is performed. A RECOMMENDED policy is as follows: the
username asserted during Digest authentication MUST correspond username asserted during Digest authentication MUST correspond
exactly to the username in the From header field of the SIP message. exactly to the username in the From header field of the SIP message.
However, there are many cases in which a user might manage multiple However, there are many cases in which a user might manage multiple
accounts in the same administrative domain. Accordingly, provided accounts in the same administrative domain. Accordingly, provided
the authentication service is aware of the relationships between the authentication service is aware of the relationships between
these accounts, it might allow a user providing credentials for one these accounts, it might allow a user providing credentials for one
account to assert a username associated with another account account to assert a username associated with another account
controlled by the user name. Furthermore, if the AoR asserted in the controlled by the user name. Furthermore, if the AoR asserted in the
From header field is anonymous (per RFC3323), then the proxy should From header field is anonymous (per RFC3323 [3]), then the proxy
authenticate that the user is any valid user in the domain and insert should authenticate that the user is a valid user in the domain and
the signature over the From header field as usual. insert the signature over the From header field as usual.
Third, the authentication service MUST form the identity signature, Note that this check is performed on the addr-spec in the From header
as described in Section 10, and add an Identity header to the request field (e.g., the URI of the sender, like
containing this signature. After the Identity header has been added 'sip:alice@atlanta.example.com'); it does not convert the
to the request, the authentication service MUST also add an Identity- display-name portion of the From header field (e.g., 'Alice
Info header. The Identity-Info header contains a URI from which its Atlanta'). Some SIP user agents that receive requests render the
certificate can be acquired. Details are provided in section Section display-name of the caller as the identity of the caller. However,
10. there are many environments in which legislating the display-name
isn't feasible, judging from experience with email, where users
frequent make slight textual changes to their display-names.
Ultimately, there is more value in focusing on the SIP address of the
sender (which has some meaning in the network and provides a chain of
accountability) than trying to constrain how the display-name is set.
As such, authentication services MAY check the display-name as well,
and compare it to a list of acceptable display-names that may be used
by the sender; if the display-name does not meet policy constraints,
the authentication service MUST return a 403 'Inappropriate
Display-Name' response code. However, in many environments this will
not make sense. For more information on rendering identity in a user
interface, see Section 8.
Third, the authentication service MUST form the identity signature
and add an Identity header to the request containing this signature.
After the Identity header has been added to the request, the
authentication service MUST also add an Identity-Info header. The
Identity-Info header contains a URI from which its certificate can be
acquired. Details are provided in section Section 10.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
7.1 UAs acting as an Authentication service 7. Verifying Identity
There are some instances in which a user agent may hold the private
key of the domain Certificate for its address-of-record. In these
cases, the UA MAY perform the services, and add the headers, that the
authentication service would normally add.
8. Verifying Identity
When a user agent or proxy server receives a SIP message containing When a user agent or proxy server receives a SIP message containing
an Identity header, it can inspect the signature to verify the an Identity header, it can inspect the signature to verify the
identity of the sender of the message. If an Identity header is not identity of the sender of the message. If an Identity header is not
present in a request, and one is required by local policy, then a 428 present in a request, and one is required by local policy (for
Use Identity response is sent. If an Identity header is not present example, based on a global policy, a per-sending-domain policy, or a
in a response, and one is required by local policy, then the per-sending-user policy), then a 428 'Use Identity Header' response
recipient of the response MUST communicate this lapse to its user, MUST be sent.
and MAY immediately terminate any created dialog or ignore
transactions, as policy dictates.
In order to verify the identity of the sender of a message, the user In order to verify the identity of the sender of a message, the user
agent or proxy server MUST first acquire the certificate for the agent or proxy server MUST first acquire the certificate for the
signing domain. Implementations supporting this specification should signing domain. Implementations supporting this specification should
have some means of retaining domain certificates (in accordance with have some means of retaining domain certificates (in accordance with
normal practices for certificate lifetimes and revocation) in order normal practices for certificate lifetimes and revocation) in order
to prevent themselves from needlessly downloading the same to prevent themselves from needlessly downloading the same
certificate every time a request from the same domain is received. certificate every time a request from the same domain is received.
Certificates retained in this manner should be indexed by the URI Certificates retained in this manner should be indexed by the URI
given in the Identity-Info header field value. given in the Identity-Info header field value.
Provided that the domain certificate used to sign this message is Provided that the domain certificate used to sign this message is not
unknown, SIP entities discover this certificate by dereferencing the previously known to the recipient, SIP entities SHOULD discover this
Identity-Info header. The client processes this certificate in the certificate by dereferencing the Identity-Info header, unless they
usual ways including checking that it has not expired, that the chain have some more efficient implementation-specific way of acquiring
is valid back to a trusted CA, and that it does not appear on certificates for that domain. The client processes this certificate
revocation lists. in the usual ways, including checking that it has not expired, that
the chain is valid back to a trusted CA, and that it does not appear
on revocation lists. Once the certificate is acquired, it MUST be
validated.
Subsequently, the recipient MUST verify the signature in the Identity Subsequently, the recipient MUST verify the signature in the Identity
header, and compare the identity of the signer (the subjectAltName of header, and compare the identity of the signer (the subjectAltName of
the certificate) with the domain portion of the URI in the From the certificate) with the domain portion of the URI in the From
header field of the request as described in Section 11. header field of the request as described in Section 14.
Additionally, the Date, Contact and Call-ID headers MUST be analyzed Additionally, the Date, Contact and Call-ID headers MUST be analyzed
in the manner described in Section 11; recipients that wish to verify in the manner described in Section 14; recipients that wish to verify
Identity signatures MUST support all of the operations described Identity signatures MUST support all of the operations described
there. Any discrepancies or violations MUST be reported to the user. there. Any discrepancies or violations MUST be reported to the user.
When the originating user agent of a request receives a response If a verifier determines that the signature on the message does not
containing an Authenticated Identity Body (AIB, see [4]), it SHOULD correspond to the text of the message, then a 428 'Invalid Identity
compare the identity in the From header field of the AIB of the Header' response MUST be returned.
response with the original value of the To header field in the
request. If these represent different identities, the user agent
SHOULD render the identity in the AIB of the response to its user.
Note that a discrepancy in these identity fields is not necessary an
indication of a security breach; normal retargeting may simply have
directed the request to a different final destination. Implementers
therefore may consider it unnecessary to alert the user of a security
violation in this case.
9. Proxy Server Behavior
In most respects, a proxy server behaves normally when it receives a Once the identity of the sender of a request has been ascertained,
SIP request or response containing an Identity header. This various policies MAY be used to make authorization decisions about
mechanism is fully backwards-compatible with existing RFC3261 proxy accepting communications and the like. Such policies are outside the
behavior. However, if a proxy intends to act as an authentication scope of this document.
service for responses to requests it receives, it must exhibit some
additional behavior to ensure that retargeting requests are handled
properly. Essentially, a proxy server MUST NOT provide an Identity
header for a request that it retargets to a different administrative
domain. It is the responsibility of that administrative domain to
provide its own identity assertion, if it can. However, proxying the
request to a remote domain where identity services may be provided
has its own problems - the originator of the request has no way to
know whether the request was legitimately retargeted, or if any
responses it receives from the new domain are spoofed or otherwise
illegitimate. It is thus much more secure for the proxy server to
redirect in cases where it might otherwise retarget.
If a proxy server intends to act as an authentication service for a 8. User Agent Behavior
response to a SIP request that it is forwarding, it MUST do ALL of
the following:
Ascertain if it is responsible for the domain indicated in the This mechanism requires one important change to existing user agent
Request-URI field of the request. If not, it MUST forward the requirements for sending requests: user agents using this mechanism
request normally. If so, it must then: to send requests to an authentication service MUST support TLS.
Because this mechanism does not provide integrity protection for the
first hop to the authentication service, the UAC MUST send requests
to an authentication service only over a TLS connection.
Determine the route set of targets to which this request might be When a UAC sends a request, it MUST accurately populate the header
forwarded. From that target list, the proxy must determine which field that asserts its identity (for a SIP request, this is the From
contact addresses are associated with persistent TLS connections header field). In a request it MUST set the URI portion of its From
that have been established to the proxy server. It places all header to match a SIP, SIPS or TEL URI AoR under which the UAC can
such targets (if any) into a primary route set for the call, and register (including anonymous URIs, as described in RFC 3323 [3]).
places remaining targets into a secondary route set for the call. In general, UACs SHOULD NOT use the TEL URI form in the From header
It performs this operations irrespective of any qvalues associated field (see Section 12).
with the contact addresses.
The proxy then MUST follow normal administrative policies for The UAC MUST also be capable of sending requests, including mid-call
forwarding the request to any targets in the primary route set requests, through an 'outbound' proxy (the authentication service).
(which may involve qvalue calculations or any other behaviors The best way to accomplish this is using pre-loaded Route headers and
described in RFC3261). Before the proxy forwards any responses to loose routing. UAC implementations MUST provide a way of
this request upstream, the proxy server MUST act as an provisioning pre-loaded Route headers in order for this mechanism to
authentication service (as described in Section 7), adding an work for mid-call requests in the backwards direction of a dialog.
Identity and Identity-Info header.
If there are no appropriate responses from the primary route set As a recipient of a request, a user agent that can verify signed
for the proxy server to forward upstream, it moves on to the identities should also support an appropriate user interface to
secondary route set (essentially, the proxy server forks render the validity of identity to a user. User agent
sequentially, exploring the primary route set as one cluster, and implementations SHOULD differentiate signed From header field values
then moves on to the secondary set). The proxy server is unable from unsigned From header field values when rendering to an end user
to act as an authentication service for those contact addresses. the identity of the sender of a request.
Accordingly, the proxy server MUST NOT explore these route targets
itself; instead, it MUST redirect the request with a 3xx class
response containing the contact addresses that constitute the
secondary route set.
In order to build the primary route set for the call, the location 9. Proxy Server Behavior
service associated with the domain of the proxy server MUST implement
additional intelligence to determine which contact addresses are
associated with a persistent TLS connection - this is used to
determine when the server should act as a proxy and when it should
redirect. When the SIP registrar receives a REGISTER request over a
persistent TLS connection, it MUST compare any contact addresses
appearing in Contact header fields to the topmost Via header field in
the REGISTER request. If the host portion of a contact address
matches the hostname given in the topmost Via header field, then that
contact address is said to be "associated" with the persistent TLS
connection over which the REGISTER was received. Location services
must mark or flag these contact addresses accordingly, and remember
the identity that the user provided when they were authenticated
during registration. Only these contact addresses are added to the
primary route set by a proxy server that wishes to act as an
authentication service for responses.
Additionally, domain policy may require proxy servers to inspect and Domain policy may require proxy servers to inspect and verify the
verify the identity provided in SIP requests. The proxy server may identity provided in SIP requests. A proxy server may wish to
wish to ascertain the identity of the sender of the message to ascertain the identity of the sender of the message to provide spam
provide spam prevention or call control services. Even if a proxy prevention or call control services. Even if a proxy server does not
server does not act as an authentication service, it MAY verify the act as an authentication service, it MAY verify the existence of an
signature present in an Identity header before it makes a forwarding Identity before it makes a forwarding decision for a request. Proxy
decision for a request. Proxy servers MUST NOT remove or modify the servers MUST NOT remove or modify an existing Identity or
Identity or Identity-Info headers. Identity-Info header in a request.
10. Header Syntax 10. Header Syntax
This document specifies two new SIP headers: Identity and Identity- This document specifies two new SIP headers: Identity and
Info. Each of these headers can appear only once in a SIP message. Identity-Info. Each of these headers can appear only once in a SIP
message.
Identity = "Identity" HCOLON signed-identity-digest Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT signed-identity-digest = LDQUOT 32LHEX RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info Identity-Info = "Identity-Info" HCOLON ident-info
Ident-info = LAQUOT absoluteURI RAQUOT ident-info = LAQUOT absoluteURI RAQUOT
To create the contents of the signed-identity-digest, the following
elements of a SIP message MUST placed in the string in the order
specified here, separated by a colon:
The AoR of the UA sending the message, or the 'identity field'.
For a request, this is the addr-spec from the From header field;
for responses, the addr-spec of the To header field. This needs
to be in lower case and to be represented as a SIP URI.
The callid from Call-Id header field.
The Date header field, with exactly one space each for each SP and The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a
SIP message MUST placed in a bit-exact string in the order specified
here, separated by a colon:
o The AoR of the UA sending the message, or the 'identity field'.
For a request, this is the addr-spec from the From header field.
o The addr-spec component of the To header field, which is the AoR
to which the request is being sent.
o The callid from Call-Id header field.
o The digit (1*DIGIT) and method (method) portions from CSeq header
field, separated by a single space (ABNF SP, or %x20). Note that
the CSeq header field allows LWS rather than SP to separate the
digit and method portions, and thus the CSeq header field may need
to be transformed in order to be canonicalized.
o The Date header field, with exactly one space each for each SP and
the weekday and month items case set as shown in BNF in 3261. The the weekday and month items case set as shown in BNF in 3261. The
first letter is upper case and the rest of the letters are lower first letter is upper case and the rest of the letters are lower
case. case. All requests that use the Identity mechanism MUST contain a
Date header.
The addr-spec component of the Contact header field value. o The addr-spec component of the Contact header field value. If the
request does not contain a Contact header, this field MUST be
The body content of the message with the bits exactly as they are empty (i.e., there will be no whitespace between the fourth and
in the message. fifth colons in the canonical string).
o The body content of the message with the bits exactly as they are
in the Message (in the ABNF for SIP, the message-body). Note that
the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that
follows that CRLF. If the message has no body, then message-body
will be empty, and the final colon will not be followed by any
additional characters.
For more information on the security properties of these headers, and For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see [4]. The precise why their inclusion mitigates replay attacks, see Section 14 and [5].
formulation of this digest-string is, therefore: The precise formulation of this digest-string is, therefore
(following the ABNF [6] in RFC3261):
digest-string = addr-spec HCOLON callid HCOLON SIP-Date digest-string = addr-spec ":" addr-spec ":" callid ":" 1*DIGIT SP method ":"
HCOLON addr-spec HCOLON message-body SIP-Date ":" [ addr-spec ] ":" message-body
Note again that the first addr-spec MUST be taken from the From Note again that the first addr-spec MUST be taken from the From
header field value, and the second addr-spec from the Contact header header field value, the second addr-spec MUST be taken from the To
field value. header field value, and the third addr-spec MUST be taken from the
Contact header field value, provided the Contact header is present in
the request.
After the digest-string is formed, it MUST be hashed and signed with After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain, as follows: compute the results of the certificate for the domain, as follows: compute the results of
signing this string with sha1WithRSAEncryption as described in RFC signing this string with sha1WithRSAEncryption as described in RFC
3370 and base64 encode the results as specified in RFC 3548. Put the 3370 and base64 encode the results as specified in RFC 3548. Put the
result in the Identity header. result in the Identity header.
Note on this choice: Assuming a 1024 bit RSA key, the raw signature Note on this choice: Assuming a 1024 bit RSA key, the raw signature
will result in about 170 octets of base64 encoded data. For will result in about 170 octets of base64 encoded data (without
comparison's sake, a typical HTTP Digest Authorization header (such base64, as an aside, it would be about 130 bytes). For comparison's
as those used in RFC3261) with no cnonce is around 180 octets. From sake, a typical HTTP Digest Authorization header (such as those used
a speed point of view, a 2.8GHz Intel processor does somewhere in the in RFC3261) with no cnonce is around 180 octets. From a speed point
range of 250 RSA 1024 bits signs per second or 1200 RSA 512 bits of view, a 2.8GHz Intel processor does somewhere in the range of 250
signs; verifies are roughly 10 times faster. Hardware accelerator RSA 1024 bits signs per second or 1200 RSA 512 bits signs; verifies
cards are available that speed this up. are roughly 10 times faster. Hardware accelerator cards are
available that speed this up.
The Identity-Info header MUST contain either an HTTPS URI or a SIPS The Identity-Info header MUST contain either an HTTPS URI or a SIPS
URI. If it contains an HTTPS URI, the URI must dereference to a URI. If it contains an HTTPS URI, the URI must dereference to a
resource that contains a single MIME body containing the certificate resource that contains a single MIME body containing the certificate
of the authentication service. If it is a SIPS URI, then the of the authentication service. If it is a SIPS URI, then the
authentication service intends for a user agent that wishes to fetch authentication service intends for a user agent that wishes to fetch
the certificate to form a TLS connection to that URI, acquire the the certificate to form a TLS connection to that URI, acquire the
certificate during normal TLS negotiation, and close the connection. certificate during normal TLS negotiation, and close the connection.
This document adds the following entries to Table 2 of [1]: This document adds the following entries to Table 2 of [1]:
Header field where proxy ACK BYE CAN INV OPT REG Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- --- ------------ ----- ----- --- --- --- --- --- ---
Identity a - o - o o - Identity R a o o - o o -
SUB NOT REF INF UPD PRA SUB NOT REF INF UPD PRA
--- --- --- --- --- --- --- --- --- --- --- ---
o o o - - - o o o o o o
Identity-Info a - o - o o -
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Info R a o o - o o -
SUB NOT REF INF UPD PRA SUB NOT REF INF UPD PRA
--- --- --- --- --- --- --- --- --- --- --- ---
o o o - - - o o o o o o
11. Security Considerations Note, in the table above, that this mechanism does not protect the
REGISTER method or the CANCEL method. The CANCEL method cannot be
challenged, because it is hop-by-hop, and accordingly authentication
service behavior for CANCEL would be significantly limited. The
REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to this mechanism. Accordingly, the
Identity and Identity-Info header MUST NOT appear in REGISTER or
CANCEL.
11. Compliance Tests and Examples
The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementations MUST
verify their compliance with these examples, i.e.:
o Implementations of the authentication service role MUST generate
identical base64 identity strings to the ones shown in the
Identity headers in these examples when presented with the source
message and utilizing the appropriate supplied private key for the
domain in question.
o Implementations of the verifier role MUST correctly validate the
given messages containing the Identity header when utilizing the
supplied certificates (with the caveat about self-signed
certificates below).
Note that the following examples use self-signed certificates, rather
than certificates issued by a recognized certificate authority. The
use of self-signed certificates for this mechanism is NOT
RECOMMENDED, and appear here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD
generated appropriate warnings about the use of self-signed
certificates.
Bit-exact reference files for these messages and their various
transformations are supplied in Appendix B.
11.1 Identity-Info with a Singlepart MIME body
Consider the following private key and certificate pair assigned to
'atlanta.example.com'.
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
A user of atlanta.example.com, Alice, wants to send an INVITE to
bob@biloxi.example.org. She therefore creates the following INVITE
request, which she forwards to the atlanta.example.org proxy server
that instantiates the authentication service role:
INVITE sip:bob@biloxi.exmple.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
When the authentication service receives the INVITE, in authenticates
Alice by sending a 407 response. As a result, Alice adds an
Authorization header to her request, and resends to the
atlanta.example.com authentication service. Now that the service is
sure of Alice's identity, it calculates an Identity header for the
request. The canonical string over which the identity signature will
be generated is the following (note that the first line wraps because
of RFC editorial conventions):
sip:alice@atlanta.example.com:sip:bob@biloxi.example.org:a84b4c76e66710:314159 INVITE:Thu, 21 Feb 2002 13:02:03 GMT:alice@pc33.atlanta.example.com:v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is the following:
CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
Accordingly, the atlanta.example.com authentication service will
create an Identity header containing that base64 signature string
(175 bytes). It will also add an HTTPS URL where its certificate is
made available. With those two headers added, the message looks
like:
INVITE sip:bob@biloxi.exmple.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
Identity-Info: https://atlanta.example.com/cert
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
atlanta.example.com then forwards the request normally. When Bob
receives the request, if he does not already know the certificate of
atlanta.example.com, he de-references the URL the Identity-Info
header to acquire the certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP
request. Using this canonical string, the signed digest in the
Identity header, and the certificate discovered by de-referencing the
Identity-Info header, Bob can verify that the given set of headers
and the message body have not been modified.
11.2 Identity for a Request with no MIME body or Contact
Consider the following private key and certificate pair assigned to
"biloxi.example.org".
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
at the end of the dialog initiated in the previous example. He
therefore creates the following BYE request which he forwards to the
'biloxi.example.org' proxy server that instantiates the
authentication service role:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Content-Length: 0
When the authentication service receives the BYE, it authenticates
Bob by sending a 407 response. As a result, Bob adds an
Authorization header to his request, and resends to the
biloxi.example.org authentication service. Now that the service is
sure of Bob's identity, it prepares to calculate an Identity header
for the request. Note that this request does not have a Date header
field. Accordingly, the biloxi.example.org will add a Date header to
the request before calcuating the identity signature. If the
Content-Length header were not present, the authentication service
would add it as well. The baseline message is thus:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Content-Length: 0
Also note that this request contains no Contact header field.
Accordingly, biloxi.example.org will place no value in the canonical
string for the addr-spec of the Contact address. Also note that
there is no message body, and accordingly, the signature string will
terminate, in this case, with two colons. The canonical string over
which the identity signature will be generated is the following (note
that the first line wraps because of RFC editorial conventions):
sip:bob@biloxi.example.org:sip:alice@atlanta.example.com:a84b4c76e66710:231 BYE:Thu, 21 Feb 2002 14:19:51 GMT::
The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above for biloxi.example.org, with base64 encoding, is the
following:
A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=
Accordingly, the biloxi.example.org authentication service will
create an Identity header containing that base64 signature string.
It will also add an HTTPS URL where its certificate is made
available. With those two headers added, the message looks like:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Identity: A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=
Identity-Info: https://biloxi.example.org/cert
Content-Length: 0
biloxi.example.org then forwards the request normally.
12. Identity and the TEL URI Scheme
Since many SIP applications provide a VoIP service, telephone numbers
are commonly used as identities in SIP deployments. In the majority
of cases, this is not problematic for the identity mechanism
described in this document. Telephone numbers commonly appear in the
username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com'). That username conforms to
the syntax of the TEL URI scheme (RFC2806bis [9]). For this sort of
SIP address-of-record, chicago.example.com is the appropriate
signatory.
It is also possible for a TEL URI to appear in the SIP To or From
header field outside the context of a SIP or SIPS URI (e.g.,
'tel:+17005551008'). In this case, it is much less clear which
signatory is appropriate for the identity. Fortunately for the
identity mechanism, this form of the TEL URI is more common for the
To header field and Request-URI in SIP than in the From header field,
since the UAC has no option but to provide a TEL URI alone when the
remote domain to which a request is sent is unknown. The local
domain, however, is usually known by the UAC, and accordingly it can
form a proper From header field containing a SIP URI with a username
in TEL URI form. Implementations that intend to send their requests
through an authentication service MUST put telephone numbers in the
From header field into SIP or SIPS URIs, if possible.
If the local domain is unknown to a UAC formulating a request, it
most likely will not be able to locate an authentication service for
its request, and therefore the question of providing identity in
these cases is somewhat moot. However, an authentication service MAY
sign a request containing a TEL URI in the From header field in
accordance with its local policies. Verifiers SHOULD NOT accept
signatures over From header TEL URIs in the absence of some
pre-provisioned relationship with the signing domain that authorizes
this usage of TEL URIs.
The guidance in the paragraph above is largely provided for forward
compatibility. In the longer-term, it is possible that ENUM [10] may
provide a way to determine which administrative domain is responsible
for a telephone number, and this may aid in the signing and
verification of SIP identities that contain telephone numbers. This
is a subject for future work.
13. Privacy Considerations
The identity mechanism presented in this draft is compatible with the
standard SIP practices for privacy described in RFC3323 [3]. A SIP
proxy server can act both as a privacy service and as an
authentication service. Since a user agent can provide any From
header field value which the authentication service is willing to
authorize, there is no reason why private SIP URIs (e.g.,
sip:anonymous@example.com) cannot be signed by an authentication
service. The construction of the Identity header is the same for
private URIs as it is for any other sort of URIs.
Note, however, that an authentication service must possess a
certificate corresponding to the host portion of the addr-spec of the
From header field of any request that it signs; accordingly, using
domains like 'invalid.net' may not be possible for privacy services
that also act as authentication services. The assurance offered by
this combination service is "this is a known user in my domain that I
have authenticated, but I am keeping their identity private".
The "header" level of privacy described in RFC3323 requests that a
privacy service to alter the Contact header field value of a SIP
message. Since the Contact header field is protected by the
signature in an Identity header, privacy services cannot be applied
after authentication services without a resulting integrity
violation.
RFC3325 [8] defines the "id" priv-value token which is specific to
the P-Asserted-Identity header. The sort of assertion provided by
the P-Asserted-Identity header is very different from the Identity
header presented in this document. It contains additional
information about the sender of a message that may go beyond what
appears in the From header field; P-Asserted-Identity holds a
definitive identity for the sender which is somehow known to a closed
network of intermediaries that presumably the network will use this
identity for billing or security purposes. The danger of this
network-specific information leaking outside of the closed network
motivated the "id" priv-value token. The "id" priv-value token has
no implications for the Identity header, and privacy services MUST
NOT remove the Identity header when a priv-value of "id" appears in a
Privacy header.
14. Security Considerations
This document describes a mechanism which provides a signature over This document describes a mechanism which provides a signature over
the Contact, Date, Call-ID, and 'identity fields' (addr-spec of the the Contact, Date, Call-ID, CSeq To, and From header fields of SIP
From header field for requests, and To header field for responses) of messages. While a signature over the From header field would be
SIP messages. While a signature over the identity field alone would sufficient to secure a URI alone, the additional headers provide
be sufficient to secure a URI alone, the additional headers provide
replay protection and reference integrity necessary to make sure that replay protection and reference integrity necessary to make sure that
the Identity header will not be used in cut-and-paste attacks. In the Identity header will not be used in cut-and-paste attacks. In
general, the considerations related to the security of these headers general, the considerations related to the security of these headers
are the same as those given in RFC3261 for including headers in are the same as those given in RFC3261 for including headers in
tunneled 'message/sip' MIME bodies (see Section 23 in particular). tunneled 'message/sip' MIME bodies (see Section 23 in particular).
The identity field indicates the identity of the sender of the The From header field indicates the identity of the sender of the
message. The Date and Contact headers provide reference integrity message, and the SIP address-of-record URI in the From header field
and replay protection, as described in RFC3261 Section 23.4.2. is the identity of a SIP user, for the purposes of this document.
Implementations of this specification MUST NOT consider valid a The To header field provides the identity of the SIP user that this
request with an outdated Date header field (the RECOMMENDED interval request targets. Providing the To header field in the Identity
is that the Date header must indicate a time within 3600 seconds of signature servers two purposes: first, it prevents replay attacks in
the receipt of a message). Implementations MUST also record Call-IDs which an Identity header from legitimate request for one user is
received in valid requests containing an Identity header, and MUST cut-and-pasted into a request for a different user; second, it
remember those Call-IDs for at least the duration of a single Date preserves the starting URI scheme of the request, which helps prevent
interval (i.e. 3600 seconds). Accordingly, if an Identity header is downgrade attacks against the use of SIPS.
The Date and Contact headers provide reference integrity and replay
protection, as described in RFC3261 Section 23.4.2. Implementations
of this specification MUST NOT deem valid a request with an outdated
Date header field (the RECOMMENDED interval is that the Date header
must indicate a time within 3600 seconds of the receipt of a
message). Implementations MUST also record Call-IDs received in
valid requests containing an Identity header, and MUST remember those
Call-IDs for at least the duration of a single Date interval (i.e.
commonly 3600 seconds). Accordingly, if an Identity header is
replayed within the Date interval, receivers will recognize that it replayed within the Date interval, receivers will recognize that it
is invalid because of a Call-ID duplication; if an Identity header is is invalid because of a Call-ID duplication; if an Identity header is
replayed after the Date interval, receivers will recognize that it is replayed after the Date interval, receivers will recognize that it is
invalid because the Date is stale. The Contact header field is invalid because the Date is stale. The CSeq header field contains a
included to tie the Identity header to a particular device instance numbered identifier for the transaction, and the name of the method
that generated the request. Were an active attacker to intercept a of the request; without this information, an INVITE request could be
request containing an Identity header, and cut-and-paste the Identity cut-and-pasted by an attacker and transformed into a BYE request
header field into their own request (reusing the identity fields, without changing any fields covered by the Identity header, and
Contact, Date and Call-ID fields that appear in the original moreover requests within a certain transaction could be replayed in
message), they would not be eligible to receive SIP requests from the potentially confusing or malicious ways.
called user agent, since those requests are routed to the URI
identified in the Contact header field. The Contact header field is included to tie the Identity header to a
particular device instance that generated the request. Were an
active attacker to intercept a request containing an Identity header,
and cut-and-paste the Identity header field into their own request
(reusing the From, To, Contact, Date and Call-ID fields that appear
in the original message), they would not be eligible to receive SIP
requests from the called user agent, since those requests are routed
to the URI identified in the Contact header field. However, the
Contact header is only included in dialog-forming requests, so it
does not provide this protection in all cases.
It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert their own in a
cut-and-paste attack, which would cause all responses to the request
to be routed to a host of the attacker's choosing. However, a
signature over the topmost Via header does not prevent attacks of
this nature, since the attacker could leave the topmost Via intact
and merely insert a new Via header field directly after it, which
would cause responses to be routed to the attacker's host "on their
way" to the valid host, which has exactly the same end result.
Although it is possible that an intermediary-based authentication
service could guarantee that no Via hops are inserted between the
sending user agent and the authentication service, it could not
prevent an attacker from adding a Via hop after the authentication
service, and accordingly pre-empting responses. It is necessary for
the proper operation of SIP for subsequent intermediaries to be
capable of inserting such Via header fields, and thus it cannot be
prevented. As such, though it is desirable, securing Via is not
possible through the sort of identity mechanism described in this
document; the best known practice for securing Via is the use of
SIPS.
Note that this mechanism does not provide any protection for the
display-name portion of the From header field, and thus users are
free to use any display-name of their choosing, and attackers could
conceivably alter the display-names in a request with impunity. If
an administrative domain wants to control the display-names selected
by users, they could do so with policies outside the scope of this
document (for example, their authentication service could reject
requests from valid users that contain an improper display-name in
the From header field). While there are conceivably attacks that an
adversary could mount against SIP systems that rely too heavily on
the display-name in their user interface, this argues for intelligent
interface design, not changes to the protocol.
This mechanism also provides a signature over the bodies of SIP This mechanism also provides a signature over the bodies of SIP
requests. The most important reason for doing so is to protect SDP requests. The most important reason for doing so is to protect SDP
bodies carried in SIP requests. There is little purpose in bodies carried in SIP requests. There is little purpose in
establishing the identity of the user agent that provided the establishing the identity of the user agent that originated a SIP
signaling if a man-in-the-middle can change the SDP and direct media request if a man-in-the-middle can change the SDP and direct media to
to an alternate address. Note however that this is not perfect end- an different IP address. Note however that this is not perfect
to-end security. The authentication service itself, when end-to-end security. The authentication service itself, when
instantiated at a intermediary, can change the SDP (and SIP headers, instantiated at a intermediary, could conceivably change the SDP (and
for that matter) before providing a signature. Thus, while this SIP headers, for that matter) before providing a signature. Thus,
mechanism reduces the chance that a man-in-the-middle will interfere while this mechanism reduces the chance that a man-in-the-middle will
with sessions, it does not eliminate it entirely. Since it is interfere with sessions, it does not eliminate it entirely. Since it
assumed that the user trusts their local domain to vouch for their is a foundational assumption of this mechanism that the user trusts
security, they must also trust the service not to violate the their local domain to vouch for their security, they must also trust
integrity of their message bodies without good reason. the service not to violate the integrity of their message without
good reason. Note that RFC3261 16.6 states that SIP proxy servers
"MUST NOT add to, modify, or remove the message body."
Users SHOULD NOT provide credentials to an authentication service to Users SHOULD NOT provide credentials to an authentication service to
which they cannot initiate a direct connection, preferably one which they cannot initiate a direct connection, preferably one
secured by TLS. If a user does not receive a certificate from the secured by TLS. If a user does not receive a certificate from the
authentication service over this TLS that corresponds to the expected authentication service over this TLS connection that corresponds to
domain (especially when they receive a challenge via a mechanism such the expected domain (especially when they receive a challenge via a
as Digest), then it is possible that a rogue server is attempting to mechanism such as Digest), then it is possible that a rogue server is
pose as a authentication service for a domain that it does not attempting to pose as a authentication service for a domain that it
control, possibly in an attempt to collect shared secrets for that does not control, possibly in an attempt to collect shared secrets
domain. If a user cannot connect directly to the desired for that domain. If a user cannot connect directly to the desired
authentication service, the user SHOULD at least use a SIPS URI to authentication service, the user SHOULD at least use a SIPS URI to
ensure that mutual TLS authentication will be used to reach the ensure that mutual TLS authentication will be used to reach the
remote server. remote server.
Relying on an Identity header generated by a remote administrative Ultimately, the worth of an assurance provided by an Identity header
domain assumes that the issuing domain uses some trustworthy practice is limited by the security practices of the domain that issues the
to authenticate its users. However, it is possible that some domains assurance. Relying on an Identity header generated by a remote
will implement policies that effectively make users unaccountable administrative domain assumes that the issuing domain uses some
(such as accepting unauthenticated registrations from arbitrary trustworthy practice to authenticate its users. However, it is
users). The value of an Identity header for such domains is possible that some domains will implement policies that effectively
questionable. make users unaccountable (such as accepting unauthenticated
registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a
verifier to distinguish "good" from "bad" domains by inspecting a SIP
request, it is expected that further work in authorization practices
could be built on top of this identity solution; without such an
identity solution, many promising approaches to authorization policy
are impossible. That much said, it is RECOMMENDED that
authentication services based on proxy servers employ strong
authentication practices such as token-based identifiers.
Since a domain certificate is used by an authentication service Since a domain certificate is used by an authentication service
(rather than individual certificates for each identity), certain (rather than individual certificates for each identity), certain
problems can arise with name subordination. For example, if an problems can arise with name subordination. For example, if an
authentication service holds a common certificate for the hostname authentication service holds a common certificate for the hostname
'sip.atlanta.com', can it legitimately sign a token containing an 'sip.atlanta.example.com', can it legitimately sign a token
identity of 'sip:alice@atlanta.com'? It is difficult for the containing an identity of 'sip:alice@atlanta.example.com'? It is
recipient of a request to ascertain whether or not 'sip.atlanta.com' difficult for the recipient of a request to ascertain whether or not
is authoritative for the 'atlanta.com' domain unless the recipient 'sip.atlanta.example.com' is authoritative for the
has some foreknowledge of the administration of 'atlanta.com'. 'atlanta.example.com' domain unless the recipient has some
Therefore, it is RECOMMENDED that user agent recipients of foreknowledge of the administration of 'atlanta.example.com'.
authentication tokens notify end users if there is ANY discrepancy Therefore, it is RECOMMENDED that UASs receiving signed requests
between the subjectAltName of the signers certificate and the notify end users if there is ANY discrepancy between the
identity within the authentication token. Minor discrepancies MAY be subjectAltName of the signers certificate and the identity within the
characterized as such. Additionally, relying parties MAY follow the authentication token. Minor discrepancies MAY be characterized as a
procedures in RFC3264 to look up on the domain portion of the warning. Additionally, relying parties MAY follow the procedures in
identity in the From header field in the DNS, and compare the SIP RFC3263 [4] to look up in the DNS the domain portion of the identity
services listed for that domain with the subjectAltName of the in the From header field, and compare the SIP services listed for
certificate; this can give the relying party a better sense of the that domain with the subjectAltName of the certificate; this can give
canonical SIP services for that domain. the relying party a better sense of the canonical SIP services for
that domain.
Because the domain certificates that can be used by authentication Because the domain certificates that can be used by authentication
services need to assert only the hostname of the authentication services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and certificates for this mechanism. However, not all proxy servers and
user agents will be able support the root certificates of all user agents will be able support the root certificates of all
certificate authorities, and moreover there are some significant certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but particular policies that certificate authorities should follow, but
it is anticipated that operational experience will create de facto it is anticipated that operational experience will create de facto
standards for the purposes of authentication services. Some standards for authentication services. Some federations of service
federations of service providers, for example, might only trust providers, for example, might only trust certificates that have been
certificates that have been provided by a certificate authority provided by a certificate authority operated by the federation.
operated by the federation.
12. IANA Considerations Finally, the Identity and Identity-Info headers cannot protect
themselves. Any attacker could remove these headers from a SIP
request, and modify the request arbitrarily afterwards. Accordingly,
these headers are only truly efficacious if the would-be verifier
knows that they must be included in a request. In the long term,
some sort of identity mechanism along these lines must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected. In the
interim, however, identity reception policies at a domain level or an
address-book level should be used by verifiers to determine whether
or not identity is expected from a particular source of SIP requests.
Those authorization policies are outside the scope of this document.
This document specifies two new SIP headers: Identity and Identity- 15. IANA Considerations
Info. Their syntax is given in Section 10. This document requests
that IANA add these headers to the SIP header registry.
This document also defines a new SIP response code, 428 "Use This document requests changes to the header and response-code
Identity", as described in Section 8. sub-registries of the SIP parameters IANA registry.
Normative References 15.1 Header Field Names
This document specifies two new SIP headers: Identity and
Identity-Info. Their syntax is given in Section 10. These headers
are defined by the following information, which is to be added to the
header sub-registry under
http://www.iana.org/assignments/sip-parameters.
Header Name: Identity
Compact Form: y
Header Name: Identity-Info
Compact Form: (none)
15.2 Response Code
This document registers one new SIP response code which is described
in Section 7. This response codes is defined by the following
information, which is to be added to the method and response-code
sub-registry under http://www.iana.org/assignments/sip-parameters.
Response Code Number: 428
Default Reason Phrase: Use Identity Header
16. References
16.1 Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement [2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997. levels", RFC 2119, March 1997.
[3] Peterson, J., "A Privacy Mechanism for the Session Initiation [3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002. Protocol (SIP)", RFC 3323, November 2002.
[4] Peterson, J., "SIP Authenticated Identity Body (AIB) Format", [4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
draft-ietf-sip-authid-body-01 (work in progress), October 2002. (SIP): Locating SIP Servers", RFC 3263, June 2002.
Informative References [5] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004.
[5] Kohl, J. and C. Neumann, "The Kerberos Network Authentication [6] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997.
16.2 Informative References
[7] Kohl, J. and C. Neumann, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993. Service (V5)", RFC 1510, September 1993.
[6] Jennings, C., Peterson, J. and M. Watson, "Private Extensions to [8] Jennings, C., Peterson, J. and M. Watson, "Private Extensions
the Session Initiation Protocol (SIP) for Asserted Identity to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002. within Trusted Networks", RFC 3325, November 2002.
[7] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, [9] Schulzrinne, H., "The TEL URI for Telephone Numbers",
November 2002. draft-ietf-iptel-rfc2806bis-09 (work in progress), June 2004.
[8] Olson, S., "A Mechanism for Content Indirection in SIP
Messages", draft-ietf-sip-content-indirect-mech-01 (work in
progress), August 2002.
[9] Freed, N., "Definition of the URL MIME External-Body Access- [10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS
Type", RFC 2017, November 1996. Application", RFC 3761, April 2004.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
NeuStar, Inc. NeuStar, Inc.
1800 Sutter St 1800 Sutter St
Suite 570 Suite 570
Concord, CA 94520 Concord, CA 94520
US US
skipping to change at page 17, line 31 skipping to change at page 26, line 46
MS: SJC-21/2 MS: SJC-21/2
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone: +1 408 902-3341 Phone: +1 408 902-3341
EMail: fluffy@cisco.com EMail: fluffy@cisco.com
Appendix A. Acknowledgments Appendix A. Acknowledgments
The authors would like to thank Eric Rescorla, Rohan Mahy, Robert The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson and Patrik Faltstrom for Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
their comments. Johnston and Patrik Faltstrom for their comments. The bit-archive
presented in Appendix B follows the pioneering example of Robert
Sparks' torture-test draft.
Appendix B. Changelog Appendix B. Bit-exact archive of example messages
Changes from draft-ietf-sip-identity-01: The following text block is an encoded, gzip compressed TAR archive
of files that represent the transformations performed on the example
messages discussed in Section 11. It includes for each example:
o (foo).message: the original message
o (foo).canonical: the canonical string constructed from that
message
o (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
o (foo).signed: the RSA-signed SHA1 hash of the canonical string
(binary)
o (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
of the canonical string as it would appear in the request
o (foo).identity: the original message with the Identity and
Identity-Info headers added
Also included in the archive are two public key/certificate pairs,
for atlanta.example.com and biloxi.example.org, respectively,
including:
o (foo).cert: the certificate of the domain
o (foo).privkey: the private key of the domain
o (foo).pubkey: the public key of the domain, extracted from the
cert file for convenience
To recover the compressed archive file intact, the text of this
document may be passed as input to the following Perl script (the
output should be redirected to a file or piped to "tar -xzvf -").
#!/usr/bin/perl
use strict;
my $bdata = "";
use MIME::Base64;
while(<>) {
if (/-- BEGIN MESSAGE ARCHIVE --/ .. /-- END MESSAGE ARCHIVE --/) {
if ( m/^\s*[^\s]+\s*$/) {
$bdata = $bdata . $_;
}
}
}
print decode_base64($bdata);
Alternatively, the base-64 encoded block can be edited by hand to
remove document structure lines and fed as input to any base-64
decoding utility.
B.1 Encoded Reference Files
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-- END MESSAGE ARCHIVE --
Appendix C. Changelog
NOTE TO THE RFC-EDITOR: Please remove this section prior to
publication as an RFC.
Changes from draft-ietf-sip-identity-02:
- Extracted text relating to providing identity in SIP responses;
this text will appear in a separate draft
- Added compliance testing/example section
- Added CSeq to the signature of the Identity header to prevent a
specific cut-and-paste attack; also added addr-spec of the To
header to the signature of the Identity header for similar reasons
- Added text about why neither Via headers nor display-names are
protected by this mechanism
- Added bit-exact reference files for compliance testing
- Added privacy considerations
Changes from draft-ietf-sip-identity-01:
- Completely changed underlying mechanism - instead of using an - Completely changed underlying mechanism - instead of using an
AIB, the mechanism now recommends the use of the Identity header AIB, the mechanism now recommends the use of the Identity header
and Identity-Info header and Identity-Info header
- Numerous other changes resulting from the above - Numerous other changes resulting from the above
- Various other editorial corrections - Various other editorial corrections
Changes from draft-peterson-sip-identity-01: Changes from draft-peterson-sip-identity-01:
- Split off child draft-ietf-sip-authid-body-00 for defining of - Split off child draft-ietf-sip-authid-body-00 for defining of
the AIB the AIB
- Clarified scope in introduction - Clarified scope in introduction
- Removed a lot of text that was redundant with RFC3261 - Removed a lot of text that was redundant with RFC3261
(especially about authentication practices) (especially about authentication practices)
- Added mention of content indirection mechanism for adding token - Added mention of content indirection mechanism for adding token
to requests and responses to requests and responses
- Improved Security Considerations (added piece about credential - Improved Security Considerations (added piece about credential
strength) strength)
Changes from draft-peterson-sip-identity-00: Changes from draft-peterson-sip-identity-00:
- Added a section on authenticated identities in responses - Added a section on authenticated identities in responses
- Removed hostname convention for authentication services - Removed hostname convention for authentication services
- Added text about using 'message/sip' or 'message/sipfrag' in - Added text about using 'message/sip' or 'message/sipfrag' in
authenticated identity bodies, also RECOMMENDED a few more headers authenticated identity bodies, also RECOMMENDED a few more headers
in sipfrags to increase reference integrity in sipfrags to increase reference integrity
- Various other editorial corrections - Various other editorial corrections
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