SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: April 19, 2004 NTT Corporation
October 20, 2003
Requirements for End-to-middle Security for the Session Initiation
Protocol (SIP)
draft-ietf-sipping-end2middle-security-reqs-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
A SIP UA does not always trust all proxy servers in a request path to
decide whether to inspect the message bodies and/or headers contained
in a message. The UA might want to protect the message bodies and/or
headers from proxy servers excluding the particular proxy that
provides some features based on reading them. This situation
requires a mechanism for securing information passed between the UA
and an intermediary proxy, also called "end-to-middle security",
which can work with end-to-end security. This document defines a set
of requirements for a mechanism to achieve end-to-middle security.
Conventions used in this document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems with the Existing Situations . . . . . . . . . . . . 5
3. Requirements for a Solution . . . . . . . . . . . . . . . . . 7
3.1 Requirements from UA's Perspective . . . . . . . . . . . . . . 7
3.2 Requirements from Proxy's Perspective . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
The Session Initiation Protocol (SIP) [2] supports hop-by-hop
security using TLS [3] and end-to-end security using S/MIME [4].
This assumes that a SIP UA trusts all proxy servers in a request path
to decide whether or not to inspect the message bodies contained in a
message.
However, there is a model where trusted and partially-trusted proxy
servers are mixed along a message path. The partially-trusted proxy
servers are only trusted in terms of the SIP routing. Hop-by-hop
confidentiality services using TLS are not suitable for this model.
End-to-end confidentiality services using S/MIME are also not
suitable when the intermediaries provide features based on reading
the message bodies and/or headers. This problem is described in
Section 23 of [2].
One example of such features is firewall traversal. A firewall
entity that supports SIP protocol or a midcom [5] agent co-located
with a proxy server controls a firewall based on certain SDP
attributes in a SIP transaction.
Another example is transcoding [6]. A transcoder related to a SIP
proxy transfers coding based on certain SDP attributes in a SIP
transaction or transfers text-to-speech based on a message body in
the MESSAGE [7] method.
A third example is the archiving of instant messaging traffic, where
the archiving function co-located with a proxy server logs the
message bodies in the MESSAGE method. This feature is deployed for
financial or health care applications.
In these cases, a UA might want to protect the message bodies and/or
headers from proxy servers excluding the particular proxy that
provides these features. Conversely, a proxy might want to view the
message bodies and/or headers to provide these features. Such a proxy
is not always the first hop for the UA. These situations require
security between the UA and the intermediary proxy for the message
bodies and/or message headers. We call this "end-to-middle security".
End-to-middle security consists of authentication, message integrity,
and message confidentiality. As for authentication, HTTP digest
authentication described in [2] is used for user-to-proxy and
proxy-to-user authentication. The authenticating proxy is not limited
to the first hop for the UA. Thus, HTTP digest authentication can be
used for end-to-middle security. Digital signatures in a Public Key
Infrastructure, that is S/MIME CMS [8] SignedData body with
certificate, can also be used for authentication. As for message
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integrity, S/MIME CMS SignedData body can be used. S/MIME CMS
SignedData body is created with the original data and the
originator's private key, and anyone can verify the integrity using
the originator's public key and the certificate. Thus, S/MIME CMS
SignedData body can be used for end-to-middle security at the same
time as end-to-end security. However, proxy servers usually transfer
SIP messages without interpreting the S/MIME bodies.
This document mainly discusses requirements for the message
confidentiality and integrity of end-to-middle security. Proposed
mechanisms are discussed in [9].
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2. Problems with the Existing Situations
We describe here examples of models in which trusted and
partially-trusted proxy servers are mixed along a message path. These
situations demonstrate the reasons for requiring end-to-middle
security.
The following example is that User#1 does not know the features or
security policy on Proxy #1. User#1 sends an INVITE request including
encrypted SDP for end-to-end security as shown in Figure 1. Proxy #1
may reject the request because of the impossibility of offering a
firewall traversal feature. Or Proxy#1 may drop the encrypted data
based on a security policy that prevents the sending of unknown data.
Thus, there is a problem of discovering an intermediary's feature or
security policy that may conflict with end-to-end confidentiality.
Home network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+
User#1------| | C |-----| * |-----| * |-----| C |-- User#2
| +-----+ +-----+ | +-----+ +-----+
| UA#1 Proxy#1 | Proxy#2 UA#2
+---------------------+
C: Content that UA#1 allows the entity to inspect
*: Content that UA#1 prevent the entity from inspecting
Figure 1: Deployment example#1
In the second example, Proxy server#1 (Proxy#1) is the home proxy
server of User#1 using UA#1. User#1 communicates with User#2 through
Proxy#1 and Proxy#2 as shown in Figure 2. UA#1 already knows the
public key certificate of Proxy#1, and it allows Proxy#1 to inspect
the message bodies in a request for some purpose. However, User#1
does not know whether Proxy#2 is trustworthy, and thus wants to
protect the message bodies in the request. Thus, there is the
problem of granting a trusted intermediary permission to inspect
message bodies while preserving their confidentiality with respect to
other intermediaries.
Even if UA#1's request message authorizes a selected proxy (Proxy#1)
to see the message body, UA#1 is unable to authorize the same proxy
to see the message body in the response from UA#2. Thus, there is the
problem of designating and sharing a key that can be reused as a CEK
for bidirectional exchanges of S/MIME-secured messages within SIP.
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Home network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+
User#1------| | C |-----| C |-----| * |-----| C |-- User#2
| +-----+ +-----+ | +-----+ +-----+
| UA#1 Proxy#1 | Proxy#2 UA#2
+---------------------+
C: Content that UA#1 needs to disclose
*: Content that UA#1 needs to protect
Figure 2: Deployment example#2
In the third example, User#1 connects UA#1 to a proxy server in a
visited network, e.g. a hotspot service or a roaming service. Since
User#1 wants to utilize certain home network services, UA#1 connects
to a home proxy server, Proxy#1. However, UA#1 must connect to
Proxy#1 via the proxy server of the visited network (Proxy A),
because User#1 must follow the policy of that network. Proxy A may
perform access control based on the destination addresses of calls.
As shown in Figure 3, User#1 trusts Proxy A to route requests, but
not to inspect the message bodies they contain. User#1 trusts Proxy#1
both to route requests and to inspect the message bodies for some
purpose.
The same problems as in the second example exist.
Visited network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+ +-----+
User#1 -- | | C |-----| * |-----| C |-----| * |-----| C |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| UA#1 Proxy A | Proxy#1 Proxy#2 UA#2
+---------------------+
C: Content that UA#1 needs to disclose
*: Content that UA#1 needs to protect
Figure 3: Deployment example#3
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3. Requirements for a Solution
We describe here requirements for a solution. The requirements are
mainly applied for the phase of a dialog creation or sending MESSAGE
method.
3.1 Requirements from UA's Perspective
1. The solution MUST work even with SIP end-to-end encryption for
confidentiality service enabled.
2. It SHOULD work even with SIP end-to-end integrity service
enabled.
3. It SHOULD have little impact on the way of a UA handles messages
with S/MIME bodies.
4. It SHOULD allow a UA to discover which proxy needs to view some
data in a request/response for a certain feature.
This requirement is for the case that the UA does not know the
proxy or domain that provides the feature in advance.
5. It SHOULD allow a UA to discover what data in a request/response
the proxy needs to view in order to provide the feature.
This requirement is for the above case.
6. It MUST allow a UA to request selected proxy servers to view
selected message bodies. The request itself SHOULD be secure.
7. It SHOULD allow a UA to request the UA on the opposite-side to
impose the same type of data on the same proxy server. The
request itself SHOULD be secure.
It is not appropriate for the UA on the opposite-side to have
knowledge of the public key certificate of the proxy server on
the originating network. This last requirement can be modified
into the following:
+ The solution SHOULD allow a UA to request the opposite-side
UA to reuse a content-encryption-key in subsequent messages
during a dialog.
+ It SHOULD allow a UA to request a selected proxy server to
keep a content-encryption-key in a message during a dialog.
The requests themselves SHOULD be secure.
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8. It MAY allow a UA to notify the opposite-side UA which proxy
needs to view some data in a request/response for the services.
9. It MAY allow a UA to notify the opposite-side UA what data the
proxy is permitted to view in a request/response for the
services.
These last two requirements might be applied for a
registration phase.
3.2 Requirements from Proxy's Perspective
1. It SHOULD have no impact on proxy servers that do not provide
features based on S/MIME bodies in terms of handling the existing
SIP headers.
2. It SHOULD have little impact on standardized mechanism of proxy
servers that provide features based on S/MIME bodies.
When a proxy server receives an S/MIME message, it should be
able to quickly and easily determine the need to investigate
the S/MIME body. This last requirement can be modified into
the following:
+ It SHOULD allow proxy servers to quickly and easily
determine whether to handle S/MIME bodies and, if so, how
and which ones.
3. It SHOULD allow a proxy to notify a UA its own security policy
for a request/response.
4. It SHOULD allow a proxy to notify a UA what data in a request/
response is needed in order to provide a feature.
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4. Security Considerations
This documents presents requirements including security viewpoints in
Section 3.
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5. IANA Considerations
This document requires no additional considerations.
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6. Acknowledgments
Thanks to Rohan Mahy and Cullen Jennings for their initial support of
this concept, and to Jon Peterson, Gonzalo Camarillo, and Sean Olson
for their helpful comments.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] 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.
[3] Allen, C. and T. Dierks, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1992.
[5] Srisuresh, P., Kuthan, J., Rosenberg, J., Brim, S., Molitor, A.
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[6] Camarillo, G., "Framework for trasnscoding with the Session
Initiation Protocol",
draft-camarillo-sipping-transc-framework-00.txt (work in
progress), August 2003.
[7] Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[8] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[9] Ono, K. and S. Tachimoto, "End-to-middle security in the
Session Initiation Protocol(SIP)",
draft-ono-sipping-end2middle-security-00 (work in progress),
June 2003.
[10] Rosenberg, J., "Requirements for Session Policy for the Session
Initiation Protocol (SIP)",
draft-ietf-sipping-session-policy-req-00 (work in progress),
June 2003.
[11] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
Keys", RFC 3185, October 2001.
[12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[13] Crocker, D. and P. Overell, "Augmented BNF for Syntax
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Specifications: ABNF", RFC 2234, November 1997.
Authors' Addresses
Kumiko Ono
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: ono.kumiko@lab.ntt.co.jp
Shinya Tachimoto
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: tachimoto.shinya@lab.ntt.co.jp
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