SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: August 8, 2004 NTT Corporation
Feb 8, 2004
Key reuse in Secure MIME for the Session Initiation Protocol(SIP)
draft-ono-sipping-smime-keyreuse-00
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
SIP uses Secure MIME (S/MIME) Cryptographic Message Syntax (CMS)
EnvelopedData to protect SIP messages for confidentiality. While SIP
can be encrypted with different keying materials for each message, it
usually requires a public key operation for each message and the
computational cost of such operations are relatively expensive. This
draft proposes a method of bidirectional key exchange to reuse keying
materials for S/MIME-secured messages in a dialog and use a symmetric
key mechanism instead of an asymmetric key mechanism such as a public
key operation. The proposed mechanism also achieves the sharing of
keying material among multiple entities in a simple way.
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. Overview of proposed solution . . . . . . . . . . . . . . . . 4
2.1 Preparation for reuse . . . . . . . . . . . . . . . . . . . . 4
2.2 Reuse CEK as KEK . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Lifetime of key reuse . . . . . . . . . . . . . . . . . . . . 5
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 The reused CEK Lifetime in a dialog . . . . . . . . . . . . . 6
3.2 The reused CEK Lifetime when used in the case of a
subsequent message . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
The SIP [2] supports S/MIME [3] CMS [4] EnvelopedData for
confidentiality. The CMS EnvelopedData contains content encrypted
with a content encryption key (CEK) and the CEKs are encrypted with
key encryption keys (KEKs), which are usually public keys of
recipients. The confidential service is currently used for ensuring
end-to-end security, and it is now being considered for use in
end-to-middle security as described in [5]. In SIP, several messages
are transmitted among User Agents (UAs) via proxy servers in a
dialog. While separate keying materials can be used for each
recipient and each message, public key operations and asymmetric key
mechanisms are required for each recipient and each message.
As for end-to-end confidentiality, a User Agent Client (UAC) needs to
send a User Agent Server (UAS) a request with its own public key
certificate (PKC) that is a relatively large amount of data in order
to make sure that the UAC can receive a response properly using the
CMS EnvelopedData. If multiple UAs join a dialog, all UAs need to
send other UAs a request with its own PKC and send other UAs
subsequent messages with multiple KEKs for other UAs. These
operations increase the data size of the initial request by using the
originator's PKC and the number of KEKs in subsequent messages.
As for end-to-middle confidentiality that combines with end-to-end
one, a UAC needs to send a UAS a request with its own PKC and a
selected proxy server's one in order to make sure that the UAC and
the proxy server can receive a response properly using the CMS
EnvelopedData. The UAS also need to create the response explicitly
using the two PKCs. This complicates the specification of
end-to-middle confidentiality.
This draft proposes a method to reuse keying materials for subsequent
messages in SIP. This reuse method is based on [6]. Since the reuse
mechanisms allow UAs to avoid public key operations for each message,
UAs can create CMS EnvelopedData with low computational cost. In
addition, the reuse mechanism also achieves the sharing of keying
materials among multiple entities including proxy servers in a simple
way. It can also reduce the data size of the initial request, the
number of KEKs in subsequent messages, and the complication of the
end-to-middle security's specification.
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2. Overview of proposed solution
This proposed solution has three phases based on [6]. The first
phase is preparing for a CEK to be reused as the KEK in a subsequent
message. The second phase is reusing the KEK derived from a CEK in
subsequent messages, while the CEK is updated for each message. The
third phase is ending the reuse when a KEK is updated or the lifetime
for key reuse ends. The mechanism at the third one needs some
additional considerations for SIP.
2.1 Preparation for reuse
A method of preparation is to include a key identifier of a CEK in
the CMS EnvelopedData in order to reuse the CEK, a symmetric key, as
a KEK of the EnvelopedData in a subsequent message as described in
[6]. A "CEKReference" of "unprotectedAttrs" attributes contains the
key identifier of the symmetric key and the attribute expresses a
sender's preferences to reuse the CEK as the KEK in subsequent
messages.
As a prerequisite for a UAC to send a request using the CMS
EnvelopedData to a UAS, the UAC needs to know the public key of the
UAS in order to use this public key as the KEK. The UAC creates a
symmetric key to be used as the CEK. If a UAC needs to reuse the CEK
and/or needs to share the CEK among multiple UASs, it MUST use a
"CEKReference" attribute in a request message to stipulate reuse of
the CEK in subsequent messages and indicate its identifier. When a
UAS needs to reuse the CEK, the UAS MUST use a "CEKReference"
attribute in a response message to request a UAC to reuse the CEK as
the KEK of subsequent messages.
2.2 Reuse CEK as KEK
The following describes the method for KEK reuse, where the KEK is
derived from a CEK. After a UAS receives EnvelopedData that contains
a "CEKReference" attribute in a request message, the UAS creates an
EnvelopedData with the CEK received from the UAC as the KEK and does
not set a "CEKReference" attribute. Not setting a "CEKReference"
attribute indicates that the KEK can be reused as the KEK of
subsequent messages.
When a UAC requests to reuse the CEK, the UAC does not know if a UAS
can support this key reuse mechanism. If the UAS supports this key
reuse mechanism, the UAS SHOULD use a symmetric key received from the
UAC as the KEK to encrypt a new CEK of a response message. The CMS
EnvelopedData that the UAS creates contains a "KEKRecipientInfo" type
of "recipientInfo" attribute. When receiving the response, the UAC
will be able to determine that the UAS supports key reuse and uses
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it. If the UAs support and decide to use this reuse mechanism, the
UAC does not need to send its own PKC. This is because the UAS can
create the CMS EnvelopedData with a new CEK and the KEK derived from
a CEK previously received from the UAC.
If the UAS does not support this reuse mechanism, or for some reason
cannot use it based on a policy, the UAS MUST use the UAC's public
key as a KEK to encrypt a CEK in response. When receiving the
EnvelopedData, the UAC will be able to determine that the UAS did not
accept the request to enable key reuse. The UAC will need to send its
own PKC in a request so that the UAS can create a response with a
valid the CMS EnvelopedData.
Open issue: For end-to-middle security, how does a UA know whether
a proxy server support this key reuse mechanism or not ? One
option is that the proxy server adds a digital signature in a
response when it uses the key reuse mechanism.
2.3 Lifetime of key reuse
The reused CEK is available until the KEK is updated or the maximum
lifetime ends. The originator and recipients SHOULD maintain the
"CEKReference" attribute until the reused CEK is expired.
In [6], the maximum lifetime of the CEK is indicated in a
"CEKMaxDecrypts" attribute in the "unprotectedAttrs" field of
EnvelopedData. If "CEKMaxDecrypts" is missing, or has the value "1",
then each CEK will be reused once as the KEK for the next message.
Generally, reusing the same key many times is weak from a security
viewpoint. When a UA wants to stop reusing the same KEK, the UA can
update the KEK. The UA MUST follow the method of the preparation for
reuse as described above.
In SIP, a UA can know whether a recipient UA receives and reuses the
CEK, when the UA receives a subsequent message. However, a UA does
not always receive a subsequent message to a provisional response and
an ACK request. The UA SHOULD not update the KEK that is derived from
the CEK in such messages even when the "CEKMaxDecrypts" value is one.
That results in the situation that the number in "CEKMaxDecrypts"
does not work correctly in SIP. Therefore, the maximum lifetime of
key reuse in SIP equals to the time until the dialog ends . The
reused CEK is available on several messages until the dialog ends at
the maximum lifetime of key reuse. If the message that indicates the
reuse of the CEK does not create a dialog, the reuse is only
available in a transaction.
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3. Examples
The following examples illustrate the use of the mechanism described
in the previous section.
3.1 The reused CEK Lifetime in a dialog
When a UA needs to protect Session Description Protocol (SDP) in a
message for end-to-end confidentiality, the messages that include the
offer/answer procedures use the CMS EnvelopedData. The CEK is reused
in a dialog as illustrated in Figure 1.
UAC -> UAS: INVITE
E-CEK_1(Content), E-pub_key.UAS(CEK_1),CEK_1_id
UAC <- UAS: 200 OK
E-CEK_2(Content), E-CEK_1(CEK_2)
UAC -> UAS: re-INVITE
E-CEK_3(Content), E-CEK_1(CEK_3)
UAC <- UAS: 200 OK
E-CEK_4(Content), E-CEK_1(CEK_4)
Figure 1: Example of key reuse in a dialog for end-to-end
confidentiality
E-CEK_n(Content) : Content encrypted using CEK_n
E-pub_key.x(CEK_n): CEK_n encrypted using x's public key
E-CEK_n(CEK_m) : CEK_m encrypted using CEK_n
CEK_n_id : Key identifier of CEK_n in "CEKReference"
When a UA needs to protect SDP in a message for end-to-middle
confidentiality that combines with end-to-end one, the messages for
the offer/answer procedures use the CMS EnvelopedData. The CEK is
reused in a dialog as illustrated in Figure 2.
UAC -> Proxy: INVITE
E-CEK_1(Content), E-pub_key.UAS(CEK_1), E-pub_key.proxy(CEK_1),CEK_1_id
Proxy -> UAS: INVITE
E-CEK_1(Content), E-pub_key.UAS(CEK_1), E-pub_key.proxy(CEK_1),CEK_1_id
Proxy <- UAS: 200 OK
E-CEK_2(Content), E-CEK_1(CEK_2)
UAC <- Proxy: 200 OK
E-CEK_2(Content), E-CEK_1(CEK_2)
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UAC -> Proxy: re-INVITE
E-CEK_3(Content), E-CEK_1(CEK_3)
Proxy -> UAS: re-INVITE
E-CEK_3(Content), E-CEK_1(CEK_3)
Proxy <- UAS: 200 OK
E-CEK_4(Content), E-CEK_1(CEK_4)
UAC <- Proxy: 200 OK
E-CEK_4(Content), E-CEK_1(CEK_4)
Figure 2: Example of key reuse in a dialog for end-to-middle
confidentiality
3.2 The reused CEK Lifetime when used in the case of a subsequent
message
When a UA needs to protect some SIP headers for end-to-end
confidentiality, all messages in a dialog use the CMS EnvelopedData.
The CEK is reused in a subsequent message as illustrated in Figure 3.
When sending a provisional response like 180, the CEK is updated and
named as CEK_2. CEK_2 may not be received by a UAC. Therefore, when
sending the final response like 200, UAS must use CEK_1 as the KEK
again.
UAC -> UAS: INVITE
E-CEK_1(Content), E-pub_key.UAS(CEK_1),CEK_1_id
UAC <- UAS: 180
E-CEK_2(Content), E-CEK_1(CEK_2),CEK_2_id
UAC <- UAS: 200 OK
E-CEK_3(Content), E-CEK_1(CEK_3),CEK_3_id
UAC -> UAS: ACK
E-CEK_4(Content), E-CEK_3(CEK_4),CEK_4_id
UAC -> UAS: BYE
E-CEK_5(Content), E-CEK_3(CEK_5),CEK_5_id
UAC <- UAS: 200 OK
E-CEK_6(Content), E-CEK_4(CEK_6)
Figure 3: Example of key reuse in a subsequent message
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4. Security Considerations
TBD.
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5. IANA Considerations
This document introduces no additional considerations for IANA.
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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] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1992.
[4] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[5] Ono, K. and S. Tachimoto, "Requirements for end-to-middle
security in the Session Initiation Protocol (SIP)",
draft-ietf-sipping-e2m-sec-reqs-00 (work in progress), October
2003.
[6] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
Keys", RFC 3185, October 2001.
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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