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Network Working Group K. Ono
Request for Comments: 4189 S. Tachimoto
Category: Informational NTT Corporation
October 2005
Requirements for End-to-Middle Security for
the Session Initiation Protocol (SIP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
A Session Initiation Protocol (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 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.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. Use Cases .......................................................2
2.1. Examples of Scenarios ......................................2
2.2. Service Examples ...........................................4
3. Scope of End-to-Middle Security .................................6
4. Requirements for a Solution .....................................6
4.1. General Requirements .......................................6
4.2. Requirements for End-to-Middle Confidentiality .............7
4.3. Requirements for End-to-Middle Integrity ...................7
5. Security Considerations .........................................8
6. Acknowledgments .................................................9
7. References ......................................................9
7.1. Normative References .......................................9
7.2. Informative References .....................................9
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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]. Use of TLS assumes that a
SIP UA trusts all proxy servers along its request path to inspect the
message bodies contained in the message, and use of S/MIME assumes
that a SIP UA does not trust any proxy servers to do so.
However, there is a model in which 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
the 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, while disclosing
information to those that need it. We call this "end-to-middle
security".
1.1. 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].
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.
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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 based on its strict security policy that prohibits the
forwarding of unknown data.
Home network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+
User #1-----| | C |-----| [C] |-----| [C] |-----| C |-----User #2
| +-----+ +-----+ | +-----+ +-----+
| UA #1 Proxy #1 | Proxy #2 UA #2
+---------------------+
C: Content that UA #1 allows the entity to inspect
[C]: 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] |-----| C |----- User #2
| +-----+ +-----+ | +-----+ +-----+
| UA #1 Proxy #1 | Proxy #2 UA #2
+---------------------+
C: Content that UA #1 needs to disclose
[C]: Content that UA #1 needs to protect
Figure 2: Deployment example #2
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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
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 the requests and to
inspect the message bodies.
The same problems as in the second example also exist here.
Visited network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+ +-----+
User #1 -- | | C |-----| [C] |-----| C |-----| [C] |-----| C |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| UA #1 Proxy A | Proxy #1 Proxy #2 UA #2
+---------------------+
C: Content that UA #1 needs to disclose
[C]: 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 UASes (UA Servers). The archiving
function needs 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 networks, health care
service provider's networks, as well as other networks in which
archiving communication is required by their security policies.
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2.2.2. Non-emergency Call Routing Based on the Location Object
The Location Object [6] includes a person's geographical location
information that is privacy-sensitive. Some proxy servers will have
the ability to provide routing based on the geographical location
information. When 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 geographical location
information 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 data integrity by
the proxy servers before location-based routing is applied.
Sometimes the UACs want to send the Location Object to the UASes.
This is another good example that presents the need for UACs to
simultaneously send secure data to a proxy server and to the UASes.
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 for identifying 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 adjacently and/or non-adjacently 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) [9]. A firewall entity that supports the SIP
protocol, or a midcom [10] agent co-located with a proxy server,
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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 server operating as a midcom agent. Therefore,
there is a need for a proxy server to be able to decrypt the SDP, as
well as to verify the integrity of the SDP.
When the SDP includes key parameters for Secure RTP (SRTP) [11], 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. Providing data integrity
requires authenticating peer who creates the data. 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 [12], or an AIB.
As for data integrity, the CMS SignedData body can be used for
verification of the data integrity and authentication of the signer
by any entities. The CMS SignedData body can be used for end-to-
middle security and end-to-end security simultaneously. However, a
proxy server generally does not verify the data integrity using the
CMS SignedData body, and there is no way for a UA to request the
proxy server to verify the message. 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 request.
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 NOT have an impact on proxy servers that do not
provide services based on S/MIME-secured 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 a proxy server, 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 a proxy server to view
specific message bodies. The request itself SHOULD be
secure; namely it SHOULD be authenticated for the UA and
verified for the data integrity.
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; namely it SHOULD be
authenticated for the UA and verified for the data
integrity.
This requirement is necessary when a provider
operating the proxy server allows its security
policies to be revealed to the provider serving the
recipient UA.
4.3. Requirements for End-to-Middle Integrity
This section enumerates the requirements for the end-to-middle
integrity. Verifying the data integrity requires checking that the
data is created by the authenticated user and not forged by a
malicious user. Therefore, verification of the data integrity
requires the user authentication.
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REQ-INT-1: The solution SHOULD work even when the SIP end-to-end
authentication and integrity services are enabled.
REQ-INT-2: It SHOULD allow a UA to request a proxy server to verify
specific message bodies and authenticate the user. The
request itself SHOULD be secure; namely it SHOULD be
authenticated for the UA and verified for the data
integrity.
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; namely it SHOULD be
authenticated for the UA and verified for the data
integrity.
This requirement is necessary when a provider operating
the proxy server allows its security policies to be
revealed to the provider serving the recipient UA.
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 any requirements for authentication, verifying the
data integrity requires peer authentication. Also, peer
authentication 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. A threat model is where a malicious user
sends many complicated-MIME-structure messages to a proxy server,
containing user authentication data obtained by eavesdropping.
Another threat model is where a malicious proxy server sends many
complicated-MIME-structure messages to a proxy server, containing the
source IP address and the Via header of an adjacent proxy server.
These attacks will slow down the overall performance of target proxy
servers.
To prevent these attacks, user and server authentication mechanisms
need to be protected against replay attacks, or the user and server
authentication always need to be executed simultaneously with
protection of data integrity. In order to prevent these attacks, the
following requirements should be met.
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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.
o It MUST support mutual authentication, data confidentiality, and
data integrity protection between proxy servers.
o It SHOULD support protection against a replay attack for server
authentication.
o It SHOULD simultaneously support server authentication and data
integrity protection.
These last three requirements are met by TLS.
6. Acknowledgments
The authors would like to thank to Rohan Mahy and Cullen Jennings for
their initial support of this concept, and to Jon Peterson, Gonzalo
Camarillo, Sean Olson, Mark Baugher, Mary Barnes, and others for
their reviews and constructive comments.
7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, 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.
7.2. Informative References
[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
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[4] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004.
[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] Peterson, J., "A Presence-based GEOPRIV Location Object Format",
RFC 4119, October 2005.
[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] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[10] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[11] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[12] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
July 2004.
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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, kumiko@cs.columbia.edu
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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