Network Working Group J. Peterson
Request for Comments: 3893 NeuStar
Category: Standards Track September 2004
Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
RFC 3261 introduces the concept of adding an S/MIME body to a Session
Initiation Protocol (SIP) request or response in order to provide
reference integrity over its headers. This document provides a more
specific mechanism to derive integrity and authentication properties
from an 'authenticated identity body', a digitally-signed SIP
message, or message fragment. A standard format for such bodies
(known as Authenticated Identity Bodies, or AIBs) is given in this
document. Some considerations for the processing of AIBs by
recipients of SIP messages with such bodies are also given.
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RFC 3893 SIP Authenticated Identity Body (AIB) FormatSeptember 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation. . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 11
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
14. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Section 23.4 of RFC 3261 [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 the tunneled 'message/sip'
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@example.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 de-referenced
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
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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 correlate a signature with a request. RFC 3261
therefore recommends copying all of the headers from the request into
a signed MIME body; however, SIP messages can be large, and many of
the headers in a SIP message would not be relevant in determining the
identity of the sender or assuring reference integrity with the
request, and moreover some headers may change in transit for
perfectly valid reasons. Thus, this large tunneled 'message/sip'
body will almost necessarily be at variance with the headers in a
request when it is received by the UAS, and the burden in on the UAS
to determine which header changes were legitimate, and which were
security violations. 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 and correlated with the
request. 'message/sipfrag' [4] provides a way for a subset of SIP
headers to be included in a MIME body; the Authenticated Identity
Body (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.
This document proposes that the AIB format should be used instead of
the existing tunneled 'message/sip' mechanism described in RFC 3261,
section 23.4, in order to provide the identity of the caller; if
integrity over other, unrelated headers is required, then the
'message/sip' mechanism should be used.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [2].
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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/sipfrag' - for more
information on constructing sipfrags, including examples, see [4].
This MIME body MUST have a Content-Disposition [3] 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,
Call-ID, and Contact; they SHOULD also contain the To and CSeq
header. The security properties of these headers, and circumstances
in which they should be used, are described in Section 10. 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@example.com>
To: Bob <sip:bob@example.net>
Contact: <sip:alice@pc33.example.com>
Date: Thu, 21 Feb 2002 13:02:03 GMT
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Unsigned AIBs MUST be treated by any recipients according to the
rules set out in Section 7 for AIBs that do not validate. After the
AIB has been signed, it SHOULD be added to 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@example.net SIP/2.0
Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@example.net>
From: Alice <sip:alice@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.example.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 example.com
s=Session SDP
c=IN IP4 pc33.example.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@example.com>
To: Bob <sip:bob@example.net>
Contact: <sip:alice@pc33.example.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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RFC 3893 SIP Authenticated Identity Body (AIB) FormatSeptember 2004
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 with a third party before an INVITE is sent,
and in which the identity of the third party needs to be carried in
the subsequent INVITE. The details of addressing identity in such
contexts are outside the scope of this document. At a high level, it
is possible that identity information for a third party might be
carried in a supplemental AIB. The presence of a supplemental AIB
within a message would not preclude the appearance of a 'regular' AIB
as specified in this document.
Example cases in which supplemental AIBs might appear include:
The use of the REFER [5] 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.
Third-party call control (3PCC [6]) 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) would not correspond with that of the request
in by the subsequent INVITE, nor would the To or From. In both the
REFER case and the 3PCC case, the Call-ID and CSeq cannot be used to
guarantee reference integrity, and it is therefore much harder to
correlate an AIB to a subsequent INVITE request.
Thus, in these cases some other headers might be used to provide
reference integrity between the headers in a supplemental AIB with
the headers of a 3PCC or REFER-generated INVITE, but this usage is
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outside of the scope of this document. In order for AIBs to be used
in these third-party contexts, further specification work is required
to determine which additional headers, if any, need to be included in
an AIB in a specific third-party case, and how to differentiate the
primary AIB in a message from a third-party AIB.
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, Date, and
Contact header fields are REQUIRED.
Some non-INVITE requests, however, may have different identity
requirements. New SIP methods or extensions that leverage AIB
security MUST 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 MUST
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 re-
targeting 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 MUST
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 domain portion of the URI in the From
header field of the request (for non-INVITE requests, other headers
MAY be subject to this comparison). The two should correspond
exactly; if they do not, the user agent MUST report this condition to
its user before proceeding. User agents MAY distinguish between
plausibly minor variations (the difference between 'example.com' and
'sip.example.com') and major variations ('example.com' vs.
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'example.org') when reporting these discrepancies in order to give
the user some idea of how to handle this situation. Analysis and
comparison of the Date, Call-ID, and Contact header fields as
described in Section 10 MUST also be performed. Any discrepancies or
violations MUST be reported to the user.
When the originating user agent of a request receives a response
containing an AIB, it SHOULD compare the identity in the From 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 necessarily an indication of a security breach; normal
re-targeting may simply have directed the request to a different
final destination. Implementors therefore may consider it
unnecessary to alert the user of a security violation in this case.
8. Encryption of Identity
Many SIP entities that support the use of S/MIME for signatures also
support S/MIME encryption, as described in RFC 3261, 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 be encrypted before it is
signed. Implementations MUST still accept AIBs that have been signed
and then encrypted.
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.
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42
Content-Length: 568
Content-Disposition: aib; handling=optional
--boundary42
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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@example.com *
* Call-ID: a84b4c76e66710 *
* Contact: sip:alice@device21.example.com *
* 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
The purpose of an AIB is to provide an identity for the sender of a
SIP message. This identity is held in the From header field of an
AIB. While other headers are also included, they are provided solely
to assist in detection of replays and cut-and-paste attacks leveraged
to impersonate the caller. The contents of the From header field of
a valid AIB are suitable for display as a "Caller ID" for the sender
of the SIP message.
This document mandates the inclusion of the Contact, Date, Call-ID,
and From header fields within an AIB, and recommends the inclusion of
CSeq and To header fields, 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. In
general, the considerations related to the inclusion of various
headers in an AIB are the same as those given in RFC 3261 for
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including headers in tunneled 'message/sip' MIME bodies (see Section
23 in particular).
The From header field indicates the identity of the sender of the
message; were this header to be excluded, the creator of the AIB
essentially would not be asserting an identity at all. The Date and
Contact headers provide reference integrity and replay protection, as
described in RFC 3261, Section 23.4.2. Implementations of this
specification MUST follow the rules for acceptance of the Date header
field in tunneled 'message/sip' requests described in RFC 3261,
Section 23.4.2; this ensures that outdated AIBs will not be replayed
(the suggested 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 AIBs, and MUST remember those
Call-IDs for at least the duration of a single Date interval (i.e.,
3600 seconds). Accordingly, if an AIB is replayed within the Date
interval, receivers will recognize that it is invalid because of a
Call-ID duplication; if an AIB is replayed after the Date interval,
receivers will recognize that it is invalid because the Date is
stale. The Contact header field is included to tie the AIB to a
particular device instance that generated the request. Were an
active attacker to intercept a request containing an AIB, and cut-
and-paste the AIB into their own request (reusing the From, Contact,
Date, and Call-ID fields that appear in the AIB), 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.
The To and CSeq header fields provide properties that are generally
useful, but not for all possible applications of AIBs. If a new AIB
is issued each time a new SIP transaction is initiated in a dialog,
the CSeq header field provides a valuable property (replay protection
for this particular transaction). If, however, one AIB is used for
an entire dialog, subsequent transactions in the dialog would use the
same AIB that appeared in the INVITE transaction. Using a single AIB
for an entire dialog reduces the load on the generator of the AIB.
The To header field usually designates the original URI that the
caller intended to reach, and therefore it may vary from the
Request-URI if re-targeting occurs at some point in the network.
Accordingly, including the To header field in the AIB helps to
identify cut-and-paste attacks in which an AIB sent to a particular
destination is re-used to impersonate the sender to a different
destination. However, the inclusion of the To header field probably
would not make sense for many third-party AIB cases (as described in
Section 4), nor is its inclusion necessary for responses.
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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.
12. References
12.1. 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, June 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The Content-
Disposition Header Field", RFC 2183, August 1997.
[4] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
12.2. Informative References
[5] Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
Mechanism", RFC 3892, September 2004.
[6] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
13. Acknowledgements
The author would like to thank Robert Sparks, Jonathan Rosenberg,
Mary Watson, and Eric Rescorla for their comments. Rohan Mahy also
provided some valuable guidance.
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14. 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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15. Full Copyright Statement
Copyright (C) The Internet Society (2004).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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