SIP WG J. Peterson
Internet-Draft NeuStar
Expires: December 27, 2003 June 28, 2003
SIP Authenticated Identity Body (AIB) Format
draft-ietf-sip-authid-body-02
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
RFC3261 introduces the concept of adding an S/MIME body to a SIP
request or response in order to provide reference integrity over its
headers. This document provides a more specific mechanism to derive
integrity and authentication properties from an 'authenticated
identity body', a digitally-signed SIP message or message fragment.
A standard format for such bodies (known as Authenticated Identity
Bodies, or AIBs) is given in this document. Some considerations for
the processing of AIBs by recipients of SIP messages with such bodies
are also given.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. AIB Format . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Example of a Request with AIB . . . . . . . . . . . . . . . . 5
4. AIBs for Identifying Third-Parties . . . . . . . . . . . . . . 6
5. Identity in non-INVITE Requests . . . . . . . . . . . . . . . 7
6. Identity in Responses . . . . . . . . . . . . . . . . . . . . 7
7. Receiving an AIB . . . . . . . . . . . . . . . . . . . . . . . 7
8. Encryption of Identity . . . . . . . . . . . . . . . . . . . . 8
9. Example of Encryption . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . . 10
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Section 23.4 of RFC3261 [1] describes an integrity mechanism that
relies on signing tunneled 'message/sip' MIME bodies within SIP
requests. The purpose of this mechanism is to replicate the headers
of a SIP request within a body carried in that request in order to
provide a digital signature over these headers. The signature on
this body also provides authentication.
The core requirement that motivates this mechanism is the problem of
providing a cryptographically verifiable identity within a SIP
request. The baseline SIP protocol allows a user agent to express
the identity of its user in any of a number of headers. The primary
place for identity information asserted by the sender of a request is
the From header. The From header field contains a URI (like
'sip:alice@atlanta.com') and an optional display-name (like "Alice")
that identifies the originator of the request. A user may have many
identities that are used in different contexts.
Typically, this URI is an address-of-record that can be dereferenced
in order to contact the originator of the request; specifically, it
is usually the same address-of-record under which a user registers
their devices in order to receive incoming requests. This address-
of-record is assigned and maintained by the administrator of the SIP
service in the domain identified by the host portion of the address-
of-record. However, the From field of a request can usually be set
arbitrarily by the user of a SIP user agent; the From header of a
message provides no internal assurance that the originating user can
legitimately claim the given identity. Nevertheless, many SIP user
agents will obligingly display the contents of the From field as the
identity of the originator of a received request (as a sort of caller
identification function), much as email implementations display the
From field as the sender's identity
In order to provide the recipient of a SIP message with greater
assurance of the identity of the sender, a cryptographic signature
can be provided over the headers of the SIP request, which allows the
signer to assert a verifiable identity. Unfortunately, a signature
over the From header alone is insufficient because it could be cut-
and-pasted into a replay or forwarding attack, and more headers are
therefore needed to correlated a signature with a request. RFC3261
therefore recommends copying all of the headers from the request into
a signed MIME body; however, SIP messages can also be large, and many
of the headers in a SIP message would not be relevant to determining
the identity of the sender or assuring reference integrity with the
request, and moreover some headers may change in transit. It is
therefore desirable to find a happy medium - to provide a way of
signing just enough headers that the identity of the sender can be
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ascertained and correlated with the request. 'message/sipfrag' [3]
provides a way for a subset of SIP headers to be included in a MIME
body; the AIB format described in Section 2 is based on 'message/
sipfrag'.
For reasons of end-to-end privacy, it may also be desirable to
encrypt AIBs; procedures for this encryption are given in Section 8.
2. AIB Format
As a way of sharing authenticated identity among parties in the
network, a special type of MIME body format, the Authenticated
Identity Body (AIB) format, is defined in this section. AIBs allow a
party in a SIP transaction to cryptographically sign the headers that
assert the identity of the originator of a message, and provide some
other headers necessary for reference integrity.
An AIB is a MIME body of type 'message/sip' or 'message/sipfrag' (see
[3]). This body MUST have a Content-Disposition disposition-type of
'aib', a new value defined in this document specifically for
authenticated identity bodies. The Content-Disposition header SHOULD
also contain a 'handling' parameter indicating that this MIME body is
optional (i.e. if this mechanism is not supported by the user agent
server, it can still attempt to process the request).
AIBs using the 'message/sipfrag' MIME type MUST contain the following
headers when providing identity for an INVITE request: From, Date and
Contact; they SHOULD also contain the To, Call-ID and CSeq header.
AIBs MAY contain any other headers that help to uniquely identify the
transaction or provide related reference integrity. An example of
the AIB format for an INVITE is:
Content-Type: message/sipfrag
Content-Disposition: aib; handling=optional
From: Alice <sip:alice@atlanta.com>
To: Bob <sip:bob@biloxi.com>
Contact: <sip:alice@pc33.atlanta.com>
Date: Thu, 21 Feb 2002 13:02:03 GMT
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Unsigned AIBs MUST NOT be honored by any recipients. After the AIB
has been signed, it SHOULD be added to any existing MIME bodies in
the request (such as SDP), if necessary by transitioning the
outermost MIME body to a 'multipart/mixed' format.
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3. Example of a Request with AIB
The following shows a full SIP INVITE request with an AIB:
INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.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.com>
Content-Type: multipart/mixed; boundary=unique-boundary-1
--unique-boundary-1
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 here.com
s=Session SDP
c=IN IP4 pc33.atlanta.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
--unique-boundary-1
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42
Content-Length: 608
--boundary42
Content-Type: message/sipfrag
Content-Disposition: aib; handling=optional
From: Alice <sip:alice@atlanta.com>
To: Bob <sip:bob@biloxi.com>
Contact: <sip:alice@pc33.atlanta.com>
Date: Thu, 21 Feb 2002 13:02:03 GMT
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
--boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
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Content-Disposition: attachment; filename=smime.p7s;
handling=required
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary42--
--unique-boundary-1--
4. AIBs for Identifying Third-Parties
There are special-case uses of the INVITE method in which some SIP
messages are exchanged before an INVITE is sent, and the identity of
a party from the prior exchange needs to be carried in the subsequent
INVITE. Such information might be carried in one or more
supplemental AIBs. The presence of these supplemental AIBs does not
preclude the use of a 'regular' AIB as specified in this document to
protect messages in which they appear.
The use of the REFER [4] method, for example, has a requirement for
the recipient of an INVITE to ascertain the identity of the referrer
who caused the INVITE to be sent. In this instance, the From header
of the INVITE would indicate the referee, whereas a separate header
would indicate the referrer.
Third-party call control (3PCC [5]) has an even more complicated
identity problem. A central controller INVITEs one party, gathers
identity information (and session context) from that party, and then
uses this information to INVITE another party. Ideally, the
controller would also have a way to share a cryptographic identity
signature given by the first party INVITEd by the controller to the
second party invited by the controller.
In both of these cases, the Call-ID and CSeq of the original request
(3PCC INVITE or REFER) will not correspond with that of the request
in by the subsequent INVITE, nor would the To and From. In both the
REFER case and the 3PCC case, the Call-ID and CSeq cannot be used to
determine reference integrity, and it is therefore much harder to
correlate an AIB to a subsequent INVITE request. Some other special
headers MAY be used to provide reference integrity between the
headers in an AIB with the headers of a 3PCC or REFER-generated
INVITE, but this usage is outside of the scope of this document.
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5. Identity in non-INVITE Requests
The requirements for populating an AIB in requests within a dialog
generally parallel those of the INVITE: From, Call-ID and Date are
REQUIRED.
Some non-INVITE requests, however, may have different identity
requirements. New methods should identify any special identity
requirements in the Security Considerations of their specification.
6. Identity in Responses
Many of the practices described in the preceding sections can be
applied to responses as well as requests. Note that a new set of
headers must be generated to populate the AIB in a response. The
From header field of the AIB in the response to an INVITE SHOULD
correspond to the address-of-record of the responder, NOT to the From
header field received in the request. The To header field of the
request MUST NOT be included. A new Date header field and Contact
header field should be generated for the AIB in a response. The
Call-ID and CSeq should, however, be copied from the request.
Generally, the To header field of the request will correspond to the
address-of-record of the responder. In some architectures where
redirection is used, however, this need not be the case. Some
recipients of response AIBs may consider it a cause for security
concern if the To header field of the request is not the same as the
address-of-record in the From header field of the AIB in a response.
7. Receiving an AIB
When a user agent receives a request containing an AIB, it should
verify the signature, including validating the certificate of the
signer, and compare the identity of the signer (the subjectAltName)
with, in the INVITE case, the From header field of the request (for
non-INVITE requests, other headers may be used). The two should
correspond exactly; if they do not, the user agent should report this
condition to its user before proceeding. User agents may distinguish
between plausibly minor variations (the difference between
'atlanta.com' and 'sip.atlanta.com') and major variations
('atlanta.com' vs. 'evil.tv') when reporting these discrepancies in
order to give the user some idea of how to handle this situation.
Similar comparison of the Call-ID header is necessary for INVITE
requests. The freshness of the Date header should also be evaluated,
following the guidance in RFC3261.
When the originating user agent of a request receives a response
containing an AIB, it SHOULD compare the identity in the To header
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field of the AIB of the 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.
8. Encryption of Identity
Many SIP entities that support the use of S/MIME for signatures also
support S/MIME encryption, as described in RFC3261 Section 23.4.3.
While encryption of AIBs entails that only the holder of a specific
key can decrypt the body, that single key could be distributed
throughout a network of hosts that exist under common policies. The
security of the AIB is therefore predicated on the secure
distribution of the key. However, for some networks (in which there
are federations of trusted hosts under a common policy), the
widespread distribution of a decryption key could be appropriate.
Some telephone networks, for example, might require this model.
When an AIB is encrypted, the AIB SHOULD always be encrypted before
it is signed. Note that this means that the recipients of the
request, even if they are unable to inspect the AIBF, will still be
able to see who signed that body (although it will not necessarily be
obvious that the body contains an AIB).
9. Example of Encryption
The following is an example of an encrypted and signed AIB (without
any of the preceding SIP headers). In a rendition of this body sent
over the wire, the text wrapped in asterisks would be in ciphertext.
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Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42
Content-Length: 568
--boundary42
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
handling=required
Content-Length: 231
***********************************************************
* Content-Type: message/sipfrag *
* Content-Disposition: aib; handling=optional *
* *
* From: sip:alice@atlanta.com *
* Call-ID: a84b4c76e66710 *
* Date: Thu, 21 Feb 2002 13:02:03 GMT *
***********************************************************
--boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s;
handling=required
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary42--
10. Security Considerations
This document recommends the inclusion of the Call-ID, CSeq and To
headers in AIBs when 'message/sipfrag' is used to represent the
identity of a request's sender. If these headers are omitted, some
important security properties of AIB are lost. For example,
recipients of AIBs might keep a dictionary of received Call-IDs for
some duration of time (perhaps until two hours after the Date header
in the AIB), and compare the Call-ID received in AIBs of new requests
to those in the dictionary in order to detect replays of AIBs.
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Caching CSeqs can also assist in the detection of replays.
11. IANA Considerations
This document defines a new MIME Content-Disposition disposition-type
value of 'aib'. This value is reserved for MIME bodies that contain
an authenticated identity, as described in section Section 2.
Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, May 2002.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[3] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
September 2002.
Informative References
[4] Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07
(work in progress), November 2002.
[5] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
"Best Current Practices for Third-Party Call Control in the
Session Initiation Protocol", draft-ietf-sipping-3pcc-02 (work
in progress), June 2002.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
Appendix A. Acknowledgements
The author would like to thank Robert Sparks, Jonathan Rosenberg, and
Mary Watson for their comments. Rohan Mahy also provided some
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valuable guidance.
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