SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: June 8, 2005 NTT Corporation
December 8, 2004
Requirements for End-to-Middle Security for the Session Initiation
Protocol (SIP)
draft-ietf-sipping-e2m-sec-reqs-05
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Copyright (C) The Internet Society (2004).
Abstract
A SIP User Agent (UA) does not always trust all intermediaries in its
request path to inspect its message bodies and/or headers contained
in its message. The UA might want to protect the message bodies
and/or headers from intermediaries except those that provide services
based on its content. This situation requires a mechanism called
"end-to-middle security" to secure the information passed between the
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UA and intermediaries, which does not interfere 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
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. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Examples of Scenarios . . . . . . . . . . . . . . . . . . . . 3
2.2 Service Examples . . . . . . . . . . . . . . . . . . . . . . . 5
3. Scope of End-to-Middle Security . . . . . . . . . . . . . . . 7
4. Requirements for a Solution . . . . . . . . . . . . . . . . . 7
4.1 General Requirements . . . . . . . . . . . . . . . . . . . . . 7
4.2 Requirements for End-to-Middle Confidentiality . . . . . . . . 8
4.3 Requirements for End-to-Middle Integrity . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
The Session Initiation Protocol (SIP) [2] supports hop-by-hop
security using Transport Layer Security (TLS) [3] and end-to-end
security using Secure MIME (S/MIME) [4]. These security mechanisms
assume that a SIP UA trusts all proxy servers along its request path
to inspect the message bodies contained in the message, or a SIP UA
does not trust any proxy servers to do so.
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 to provide SIP routing, but these proxy
servers are not trusted by users to inspect its data except routing
headers. A hop-by-hop confidentiality service using TLS is not
suitable for this model. An end-to-end confidentiality service using
S/MIME is also not suitable when the intermediaries provide services
based on reading the message bodies and/or headers. This problem is
described in Section 23 of [2].
In some cases, a UA might want to protect its message bodies and/or
headers from proxy servers along its request path except from those
that provide services based on reading its message bodies and/or
headers. Conversely, a proxy server might want to view the message
bodies and/or headers to sufficiently provide these services. Such
proxy servers are not always the first hop from the UA. This
situation requires a security mechanism to secure message bodies
and/or headers between the UA and the proxy servers, yet disclosing
information to those that need it. We call this "end-to-middle
security".
2. Use Cases
2.1 Examples of Scenarios
We describe here examples of scenarios in which trusted and
partially-trusted proxy servers both exist in a message path. These
situations demonstrate the reasons why end-to-middle security is
required.
In the following example, User #1 does not know the security policies
or services provided by Proxy server #1 (Proxy#1). User #1 sends a
MESSAGE [5] request including S/MIME-encrypted message content for
end-to-end security as shown in Figure 1, while Proxy #1 rejects the
request base on its strict security policy that prohibits the
forwarding of unknown data.
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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 prevents the entity from inspecting
Figure 1: Deployment example #1
In the second example, Proxy server #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. Although User #1 already knows
Proxy #1's security policy which requires the inspection of the
content of the MESSAGE request, User #1 does not know whether Proxy
#2 is trustworthy, and thus wants to protect the message bodies in
the request. To accomplish this, UA #1 will need to be able to grant
a trusted intermediary (Proxy #1) to inspect message bodies, while
preserving their confidentiality from other intermediaries (Proxy
#2).
Even if UA #1's request message authorizes Proxy #1 to inspect the
message bodies, UA #1 is unable to authorize the same proxy server to
inspect the message bodies in subsequent MESSAGE requests from UA #2.
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 (potentially insecure) 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 performs access control based on the destination
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addresses of calls. User #1 only trusts Proxy A to route requests,
not to inspect the message bodies the requests contain as shown in
Figure 3. 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 also exist here.
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
2.2 Service Examples
We describe here several services that require end-to-middle
security.
2.2.1 Logging Services for Instant Messages
Logging Services are provided by the archiving function, which is
located in the proxy server, that logs the message content exchanged
between UAs. The archiving function could be located at the
originator network and/or the destination network. When the content
of an instant message contains private information, UACs (UA Clients)
encrypt the content for the UASs (UA Servers). The archiving
function needs a way to log the content in a message body in
bidirectional MESSAGE requests in such a way that the data is
decipherable. The archiving function also needs a way to verify the
data integrity of the content before logging.
This service might be deployed in financial or health care service
provider's networks, where archiving communication is required by
their security policies, as well as other networks.
2.2.2 Non-emergency Call Routing Based on the Location Object
The Location Object [6] includes private information as well as
routing information for appropriate proxy servers. Some proxy
servers have the capability to provide location-based routing. When
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UAs want to employ location-based routing in non-emergency
situations, the UAs need to connect to the proxy servers with such a
capability and disclose the location object contained in the message
body of the INVITE request, while protecting it from other proxy
servers along the request path.
The Location Object also needs to be verified for integrity before
location-based routing is applied. Sometimes the UAC want to also
send the Location Object to the UASs. This is another good example
of the need for a UAC to simultaneously send secure data to a proxy
server and to the UAS.
2.2.3 User Authentication
2.2.3.1 User Authentication using the AIBs
The Authenticated Identity Bodies (AIBs) [7] is a digitally-signed
data that is used as way to identify users. Proxy servers that need
to authenticate a user verify the signature. When the originator
needs anonymity, the user identity in the AIB is encrypted before
being signed. Proxy servers that authenticate the user need to
decrypt the body in order to view the user identity in the AIB. Such
proxy servers can be located at adjacent and/or non-adjacent to the
UA.
The AIB could be included in all request/response messages. The
proxy server needs to view it in request messages in order to
authenticate users. Another proxy server sometimes needs to view it
in response messages for user authentication.
2.2.3.2 User Authentication in HTTP Digest Authentication
User authentication data for HTTP digest authentication [8] includes
potentially private information, such as a user name. The user
authentication data can be set only in a SIP header of request
messages. This information needs to be transmitted securely to
servers that authenticate users, located either adjacently and/or
non-adjacently to the UA.
2.2.4 Media-related Services
Firewall traversal is an example of services based on media
information in a message body, such as the Session Description
Protocol (SDP). A firewall entity that supports the SIP protocol, or
a midcom [9] agent co-located with a proxy server, controls a
firewall based on the address and port information of media streams
in the SDP offer/answer. The address and port information in the SDP
needs to be transmitted securely to recipient UAs and the proxy
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server operating as a midcom agent. Therefore, there is a need for
proxy server to be able to decrypt the SDP, as well as to verify the
integrity of the SDP.
When the SDPs include key parameters for Secure RTP (SRTP) [10], the
key parameters need to be encrypted only for end-to-end
confidentiality.
3. Scope of End-to-Middle Security
End-to-middle security consists of user authentication, data
integrity, and data confidentiality. However, this document only
describes requirements for data confidentiality and data integrity,
since end-to-middle authentication is covered by existing mechanisms
such as HTTP digest authentication, S/MIME Cryptographic Message
Syntax (CMS) SignedData body [11], or an AIB.
As for data integrity, the CMS SignedData body can be used for
verification of the data integrity by any entities. The CMS
SignedData body could be used for end-to-middle security at the same
time for end-to-end security.
Although a proxy server is able to verify the integrity of the data,
there is no way for UAs to request a selected proxy server to verify
a message with the CMS SignedData body. Therefore some new
mechanisms are needed to achieve data integrity for end-to-middle
security.
This document mainly discusses requirements for data confidentiality
and the integrity of end-to-middle security.
4. Requirements for a Solution
We describe here requirements for a solution. The requirements are
mainly applied during the phase of a dialog creation or sending a
MESSAGE method.
4.1 General Requirements
The following are general requirements for end-to-middle
confidentiality and integrity.
REQ-GEN-1: The solution SHOULD have little impact on the way a UA
handles S/MIME-secured messages.
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REQ-GEN-2: It SHOULD have no impact on proxy servers that do not
provide services based on S/MIME bodies in terms of
handling the existing SIP headers.
REQ-GEN-3: It SHOULD NOT violate the standardized mechanism of proxy
servers in terms of handling message bodies.
REQ-GEN-4: It SHOULD allow a UA to discover security policies of
proxy servers. Security policies imply what data is
needed to disclose and/or verify in a message.
This requirement is necessary when the UA does not know
statically which proxy servers or domains need
disclosing data and/or verification.
4.2 Requirements for End-to-Middle Confidentiality
REQ-CONF-1: The solution MUST allow encrypted data to be shared with
the recipient UA and selected proxy servers, when a UA
wants.
REQ-CONF-2: It MUST NOT violate end-to-end encryption when the
encrypted data does not need to be shared with any proxy
servers.
REQ-CONF-3: It SHOULD allow a UA to request selected proxy servers to
view specific message bodies. The request itself SHOULD
be secure.
REQ-CONF-4: It MAY allow a UA to request that the recipient UA
disclose information to the proxy server, to which the
requesting UA is initially disclosing information. The
request itself SHOULD be secure.
This requirement is not necessary when a provider that
operates the proxy server does not permit revealing
the security policies to a different provider that the
recipient UA belongs to.
4.3 Requirements for End-to-Middle Integrity
REQ-INT-1: The solution SHOULD work even when the SIP end-to-end
integrity service is enabled.
REQ-INT-2: It SHOULD allow a UA to request selected proxy servers to
verify specific message bodies. The request itself SHOULD
be secure.
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REQ-INT-3: It SHOULD allow a UA to request the recipient UA to send
the verification data of the same information that the
requesting UA is providing to the proxy server. The
request itself SHOULD be secure.
This requirement is not necessary when a provider that
operates the proxy server does not permit to reveal the
security policies to a different provider that the
recipient UA belongs to.
5. Security Considerations
This document describes the requirements for confidentiality and
integrity between a UA and a proxy server. Although this document
does not cover authentication, it is important in order to prevent
attacks from malicious users and servers.
The end-to-middle security requires additional processing on message
bodies, such as unpacking MIME structure, data decryption, and/or
signature verification to proxy servers. Therefore the proxy servers
that enable end-to-middle security are vulnerable to a
Denial-of-Services attack. There is a threat model where a malicious
user sends many complicated-MIME-structure messages to a proxy
server, containing user authentication data obtained by
eavesdropping. This attack will result in a slow down of the overall
performance of these proxy servers. To prevent this attack, user
authentication mechanism needs protection against replay attack. Or
the user authentication always needs to be executed simultaneously
with protection of data integrity. In order to prevent an attack,
the following requirements should be satisfied.
o The solution MUST support mutual authentication, data
confidentiality and data integrity protection between a UA and a
proxy server.
o It SHOULD support protection against a replay attack for user
authentication.
o It SHOULD simultaneously support user authentication and data
integrity protection.
These last two requirements are met by HTTP Digest
authentication.
6. IANA Considerations
This document requires no additional considerations.
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7. Acknowledgments
Thanks to Rohan Mahy and Cullen Jennings for their initial support of
this concept, and to Jon Peterson, Gonzalo Camarillo, Sean Olson,
Mark Baugher and Mary Barnes for their helpful comments.
8. Changes
[Note to RFC editor. Please remove this entire section when this
draft is published as an RFC.]
o Changes from 04.txt
* Updated references.
* Fixed editorial errors.
o Changes from 03.txt
* Removed some of the text that described an illegal behavior of
a proxy server and the scope of session policies in the
"Examples of Scenarios" section.
* Added notes to describe the requirements met by session
policies in the "Requirements for a Solution" section.
* Added a note to describe the requirements met by an existing
mechanism.
* Changed the last requirements of end-to-middle confidentiality
and integrity from "SHOULD" to "MAY", and added the conditions
of the requirements.
* Categorized references to normative and informative ones.
o Changes from 02.txt
* Changed the text about the use case of SDP-based service in
order to decrease the dependency on session policies
discussion. The title was changed to "media-related service".
* Simplified the "Scope of End-to-Middle Security" section.
* Removed some of the text that described detailed information on
mechanisms in the "Requirements for a Solution" section.
* Closed open issues as follows:
+ Deleted an open issue described in the "General
Requirements" section, since it is no longer an issue. The
issue was concerning the necessity for the proxy server to
notify the UAS after receiving a response, which is not
necessary, because proxy servers' security policies or
services have no dependencies on the information in a
response.
+ Deleted an open issue described in the "Requirements for
End-to-Middle Confidentiality" section, since it is not an
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issue of requirements, but that of a mechanism.
* Changed the last item of the general requirements from
proxy-driven to UA-driven.
* Deleted the text in the requirements that describes the
relation between the requirements and the service examples.
* Added some text in the "Security Consideration" section.
* Many editorial correction.
o "Changes from 01.txt"
* Extracted use cases from the Introduction section, and created
a new section to describe the use cases in more detail. The
use cases are also updated.
* Deleted a few "may" words from the "Problem with Existing
Situations" section to avoid confusion with "MAY" as a key
word.
* Added the relation between the requirements and the service
examples.
* Deleted the redundant requirements for discovery of the
targeted-middle. The requirement is described only in the
"Generic Requirements", not in the "Requirements for
End-to-Middle Confidentiality/Integrity".
* Changed the 4th requirement of end-to-middle confidentiality
from "MUST" to "SHOULD".
* Changed the 3rd requirement of end-to-middle integrity from
"MUST" to "SHOULD".
* Added some text about DoS attack prevention in the "Security
Consideration" section.
o "Changes from 00.txt"
* Reworked the subsections in Section 4 to clarify the
objectives, separating end-to-middle confidentiality and
integrity.
9. References
9.1 Normative 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.
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9.2 Informative References
[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] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[6] Cuellar, J., Morris, J., Mulligan, D., Peterson, J. and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
[7] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004.
[8] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
[9] Srisuresh, P., Kuthan, J., Rosenberg, J., Brim, S., Molitor, A.
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[10] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[11] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
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
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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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