SIP WG J. Peterson
Internet-Draft NeuStar
Expires: April 28, 2003 October 28, 2002
SIP Authenticated Identity Body (AIB) Format
draft-ietf-sip-authid-body-00
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Copyright Notice
Copyright (C) The Internet Society (2002). 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 . . . . . . . . . . . . . . . . 4
4. Identity in Responses . . . . . . . . . . . . . . . . . . . . 6
5. Receiving an AIB . . . . . . . . . . . . . . . . . . . . . . . 6
6. Encryption of Identity . . . . . . . . . . . . . . . . . . . . 7
7. Example of Encryption . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10
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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 the request in order to
provide a digital signature over these headers.
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 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 (using the SIP REGISTER method) 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 (which may have any of a
number of relationships with the end user). 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 this 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-ID' function).
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. 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. 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 ascertained. 'message/
sipfrag' [3] allows a subset of SIP headers to be included in a MIME
body; the AIB format described in Section 2 is based on 'message/
sipfrag'.
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For reasons of end-to-end privacy, it may also be desirable to
encrypt AIBs; procedures for this encryption are given in Section 6.
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.
AIBs using the 'message/sipfrag' MIME type MUST contain the following
headers: From, Date and Call-ID; they SHOULD also contain the To,
Contact 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 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 it to any existing MIME bodies in
the request (such as SDP), if necessary by transitioning the
outermost MIME body to a 'multipart/mixed' format.
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
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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
Content-Disposition: attachment; filename=smime.p7s;
handling=required
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
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--boundary42--
--unique-boundary-1--
4. 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 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.
5. 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 the From header field of the request. 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.
When the originating user agent of a request receives a response
containing an AIB, it SHOULD compare the identity in the To header
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.
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6. Encryption of Identity
Many SIP entities that support the use of S/MIME for signatures will
also support S/MIME encryption, as described in RFC3261 Section
23.4.3. Encryption of a body prevents any parties other those that
hold the decryption key from inspecting the body.
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 AIBF 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).
7. 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 encrypted.
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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--
8. Security Considerations
This document recommends the inclusion of the Contact, CSeq and To
headers in AIBs when 'message/sipfrag' is used. If these headers are
omitted, some important security properties of AIB are lost. For
example, the Contact header determines how new requests in a dialog
are routed. If an attacker were to modify the Contact header of a
SIP request in transit, and that header were not protected by the
AIBF, then new requests might not return to the originator of the
request.
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9. 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", draft-ietf-
sip-sipfrag-00 (work in progress), September 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/
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Full Copyright Statement
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