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 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 8, 2004. Copyright Notice 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 Ono & Tachimoto Expires August 8, 2004 [Page 1] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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 Ono & Tachimoto Expires August 8, 2004 [Page 2] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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. Ono & Tachimoto Expires August 8, 2004 [Page 3] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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 Ono & Tachimoto Expires August 8, 2004 [Page 4] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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. Ono & Tachimoto Expires August 8, 2004 [Page 5] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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) Ono & Tachimoto Expires August 8, 2004 [Page 6] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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 Ono & Tachimoto Expires August 8, 2004 [Page 7] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 4. Security Considerations TBD. Ono & Tachimoto Expires August 8, 2004 [Page 8] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 5. IANA Considerations This document introduces no additional considerations for IANA. Ono & Tachimoto Expires August 8, 2004 [Page 9] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 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 Ono & Tachimoto Expires August 8, 2004 [Page 10] Internet-Draft Key reuse in S/MIME for SIP Feb 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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