Network Working Group D. Wing Internet-Draft Cisco Systems Intended status: Standards Track F. Audet Expires: August 21, 2007 Nortel S. Fries Siemens AG H. Tschofenig Siemens Networks GmbH & Co KG February 17, 2007 Disclosing Secure RTP (SRTP) Session Keys with a SIP Event Package draft-wing-sipping-srtp-key-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 21, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Many Secure RTP (SRTP) key exchange mechanisms do not disclose the SRTP session keys to intermediate SIP proxies. However, these key exchange mechanisms cannot be used In environments where transcoding, Wing, et al. Expires August 21, 2007 [Page 1] Internet-Draft SRTP Event Package February 2007 monitoring, or call recording are needed. This document specifies a secure mechanism for a cooperating endpoint to disclose its SRTP master keys to an authorized party. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Learning Name and Certificate of ESC . . . . . . . . . . . 5 3.2. Sending SRTP Session Keys to ESC . . . . . . . . . . . . . 5 4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5.1. Incorrect ESC . . . . . . . . . . . . . . . . . . . . . . 6 5.2. Risks of Sharing SRTP Session Key . . . . . . . . . . . . 7 5.3. Disclosure Flag . . . . . . . . . . . . . . . . . . . . . 7 5.4. Integrity and encryption of keying information . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 8.2. Informational References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Wing, et al. Expires August 21, 2007 [Page 2] Internet-Draft SRTP Event Package February 2007 1. Introduction This document addresses 2 difficulties with end-to-end encryption of RTP (SRTP [RFC3711]): transcoding and media recording. When peering with other networks, different codecs are sometimes necessary (transcoding a surround-sound codec for transmission over a highly- compressed bandwidth-constrained network, for example). In some environments (e.g., stock brokerages and banks) regulations and business needs require recording calls with coworkers or with customers. In many environments, quality problems such as echo can only be diagnosed by listening to the call (analyzing SRTP headers is not sufficient). With an RTP stream, transcoding is accomplished by modifying SDP to offer a different codec through a transcoding device [RFC4117], and call recording or monitoring can be accomplished with an Ethernet sniffer listening for SIP and its associated RTP, with a media relay, or with a Session Border Controller. However, when media is encrypted end-to-end[I-D.wing-rtpsec-keying-eval], these existing techniques fail because they are unable to decrypt the media packets. When a media session is encrypted with SRTP, there are three techniques to decrypt the media for monitoring or call recording: 1. the endpoint establishes a separate media stream to the recording device, with a separate SRTP key, and sends the (mixed) media to the recording device. The disadvantages of this technique include doubling bandwidth requirements, loss of media recording facility doesn't cause loss of call (as is required in some environments). A significant advantage of this technique, however, is that it's secure: a malicious media recording device cannot inject media to the connected party on behalf of the endpoint. Depending on the application requirements it may be necessary to establish a reliable connection to the recording device to cope with possible packet loss on the unreliable link, typically used for media transport. 2. the endpoint relays media through a device which forks a separate media stream to the recording device. This technique is often employed by Session Border Controllers, and could also be employed by TURN servers. 3. Network sniffing devices are used to listen to the SRTP traffic and correlate SRTP with SIP (with cooperation of call signaling devices, if the call signaling is encrypted). This document describes cases (2) and (3) where a cooperating endpoints publishes its SRTP master keys to an authorized party using Wing, et al. Expires August 21, 2007 [Page 3] Internet-Draft SRTP Event Package February 2007 the SIP Event State Publication Extension [RFC3903]. The mechanism described in this paper allows secure disclosure of SRTP session keys to authorized parties so that an endpoints media stream can be transcoded or decrypted, as needed by that environment. 2. Terminology The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The following terminology is taken directly from SIP Event State Publication Extension [RFC3903]: Event Publication Agent (EPA): The User Agent Client (UAC) that issues PUBLISH requests to publish event state. Event State Compositor (ESC): The User Agent Server (UAS) that processes PUBLISH requests, and is responsible for compositing event state into a complete, composite event state of a resource. Publication: The act of an EPA sending a PUBLISH request to an ESC to publish event state. 3. Operation For transcoding, RTP packets must be sent from and received by a device which performs the transcoding. When the media is encrypted, this device must be capable of decrypting the media, performing the transcoding function, and re-encrypting the media. ISSUE-1: should we consider providing some or all of the SIP headers, as well? Some recording functions will need to know the identity of the remote party. This information could be gleaned from the SIP proxies, though, and starts to fall outside the intended scope of this document. ISSUE-2: The authors have been considering use of MIKEY [RFC3830], but MIKEY may not be used off the shelf. Certain changes to the state machine may have to be made (RFC3830 describes the TGK transport rather than SRTP master key transport). Wing, et al. Expires August 21, 2007 [Page 4] Internet-Draft SRTP Event Package February 2007 3.1. Learning Name and Certificate of ESC The endpoint will be configured with the AOR of its ESC (e.g., "transcoder@example.com"). If S/MIME is used to send the SRTP master key to the ESC, the endpoint is additionally configured with the certificate of its ESC. This configuration is outside the scope of this document, but some examples are CLI, [I-D.ietf-sipping-config-framework], or [I-D.ietf-sip-certs]. 3.2. Sending SRTP Session Keys to ESC SDP is used to describe the media session to the ESC. However, the existing Security Descriptions [RFC4568] only describes the master key and parameters of the SRTP packets being sent -- it does not describe the master key (and parameters) of the SRTP being received, or the SSRC being transmitted. For transcoding and media recording, both the sending key and receiving key are needed and in some cases the SSRC is needed. Thus, we hereby extend the existing crypto attribute to indicate the SSRC. We also create a new SDP attribute, "rcrypto", which is identical to the existing "crypto" attribute, except that it describes the receiving keys and their SSRCs. For example: a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 SSRC=1899 a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 inline:AmO4q1OVAHNiYRj6HmS3JFWNCFqSpTqHWKKIN1Mw|2^20|1:32 SSRC=3289 a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 inline:Hw3JFWNCFqSpTqNiYRj6HmSWKMHAmO4q1KIN1OVA|2^20|1:32 SSRC=4893 Figure 1: Example SDP The full SDP, including the keying information, is then sent to the ESC. The keying information MUST be encrypted and integrity protected. Existing mechanisms such as S/MIME [RFC3261] and SIPS [I-D.ietf-sip-sips] MAY be used to achieve this goal, or other mechanisms may be defined. Wing, et al. Expires August 21, 2007 [Page 5] Internet-Draft SRTP Event Package February 2007 [[ ISSUE-3: if a endpoint is receiving multiple incoming streams from multiple endpoints, it will have negotiated different keys with each of them, and all of that traffic is coming to the same transport address on the endpoint. Thus, we need a way to describe the different keys we're using to/from different transport addresses. One solution is to indicate the remote transport address. Indicating the remote SSRC is insufficient for this task, as several SRTP keying mechanisms do not include SSRC in their signaling (DTLS-SRTP, ZRTP, Security Descriptions). For example, if there were two remote peers with different keys, we could signal it like this: a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 192.0.2.1:5678 SSRC=1899 SSRC=3892 a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 inline:AmO4q1OVAHNiYRj6HmS3JFWNCFqSpTqHWKKIN1Mw|2^20|1:32 192.0.2.1:5678 SSRC=3289 SSRC=2813 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:GdUJShpX1ZLEw6UzF3WSJjNzB4d1BINUAv+PSdFc|2^20|1:32 192.0.2.222:2893 a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 inline:6UzF3IN1ZLEwAv+PSdFcWUGdUJShpXSJjNzB4d1B|2^20|1:32 192.0.2.222:2893 Figure 2: Strawman solution ]] 4. Grammar [[Grammar will be provided in a subsequent version of this document.]] 5. Security Considerations 5.1. Incorrect ESC Insertion of the incorrect public key of the SRTP ESC will result in disclosure of the SRTP session key to an unauthorized party. Thus, the UA's configuration MUST be protected to prevent such misconfiguration. If the configuration or the ESC's certificate are obtained over the network [I-D.ietf-sipping-config-framework], [I-D.ietf-sip-certs], the certificate MUST be suitably authenticated and integrity protected. Wing, et al. Expires August 21, 2007 [Page 6] Internet-Draft SRTP Event Package February 2007 5.2. Risks of Sharing SRTP Session Key A party authorized to obtain the SRTP session key can listen to the media stream and could inject data into the media stream as if it were either party. The alternatives are worse: disclose the device's private key to the transcoder or media recording device, or abandon using secure SRTP key exchange in environments that require media transcoding or media recording. As we wish to promote the use of secure SRTP key exchange mechanisms, disclosure of the SRTP session key appears the least of these evils. 5.3. Disclosure Flag Secure SRTP key exchange techniques which implement this specification SHOULD provide a "disclosure flag", similar to that first proposed in Appendix B of [I-D.zimmermann-avt-zrtp]. 5.4. Integrity and encryption of keying information The mechanism describe in this specification relies on protecting and encrypting the keying information. There are well known mechanism to achieve that goal. Using SIPS to convey the SRTP key exposes the SRTP master key to all SIP proxies between the Event Publication Agent (ESC, the SIP User Agent) and the Event State Compositor (ESC). S/MIME allows disclosing the SRTP master key to only the ESC. 6. IANA Considerations New SSRC extension of the "crypto" attribute, and the new "rcrypto" attribute will be registered here. Wing, et al. Expires August 21, 2007 [Page 7] Internet-Draft SRTP Event Package February 2007 7. Examples This is an example showing a SIPS AOR for the ESC. This relies on the SIP network providing TLS encryption of the SRTP master keys to the ESC. PUBLISH sips:recorder@example.com SIP/2.0 Via: SIP/2.0/UDP pua.example.com;branch=z9hG4bK652hsge To: From: ;tag=1234wxyz Call-ID: 81818181@pua.example.com CSeq: 1 PUBLISH Max-Forwards: 70 Expires: 3600 Event: srtp Content-Type: application/sdp Content-Length: ... v=0 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com s=- c=IN IP4 192.0.2.101 t=0 0 m=audio 49172 RTP/SAVP 0 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 inline:AmO4q1OVAHNiYRj6HmS3JFWNCFqSpTqHWKI8K1Mw|2^20|1:32 a=rtpmap:0 PCMU/8000 Figure 3: Example with "SIPS:" AOR Wing, et al. Expires August 21, 2007 [Page 8] Internet-Draft SRTP Event Package February 2007 This is an example showing an S/MIME-encrypted transmission to the media recorder's AOR, recorder@example.com. The data enclosed in "*" is encrypted with recorder@example.com's public key. PUBLISH sip:recorder@example.com SIP/2.0 Via: SIP/2.0/UDP pua.example.com;branch=z9hG4bK652hsge To: From: ;tag=1234wxyz Call-ID: 81818181@pua.example.com CSeq: 1 PUBLISH Max-Forwards: 70 Expires: 3600 Event: srtp Content-Type: application/pkcs7-mime;smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: binary Content-ID: 1234@atlanta.example.com Content-Disposition: attachment;filename=smime.p7m;handling=required Content-Length: ... ****************************************************************** * (encryptedContentInfo) * * Content-Type: application/sdp * * Content-Length: ... * * * * v=0 * * o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com* * s=- * * c=IN IP4 192.0.2.101 * * t=0 0 * * m=audio 49172 RTP/SAVP 0 * * a=crypto:1 AES_CM_128_HMAC_SHA1_32 * * inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 * * a=rcrypto:1 AES_CM_128_HMAC_SHA1_32 * * inline:AmO4q1OVAHNiYRj6HmS3JFWNCFqSpTqHWKI8K1Mw|2^20|1:32 * * a=rtpmap:0 PCMU/8000 * * * ****************************************************************** Figure 4: Example with S/MIME-encrypted SDP 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Wing, et al. Expires August 21, 2007 [Page 9] Internet-Draft SRTP Event Package February 2007 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension for Event State Publication", RFC 3903, October 2004. [RFC3261] 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. 8.2. Informational References [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. [I-D.wing-rtpsec-keying-eval] Audet, F. and D. Wing, "Evaluation of SRTP Keying with SIP", draft-wing-rtpsec-keying-eval-02 (work in progress), February 2007. [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, July 2006. [I-D.ietf-sipping-config-framework] Petrie, D. and S. Channabasappa, "A Framework for Session Initiation Protocol User Agent Profile Delivery", draft-ietf-sipping-config-framework-10 (work in progress), January 2007. [I-D.ietf-sip-sips] Audet, F., "Guidelines for the use of the SIPS URI Scheme in the Session Initiation Protocol (SIP)", draft-ietf-sip-sips-01 (work in progress), February 2007. [I-D.ietf-sip-certs] Jennings, C., "Certificate Management Service for The Session Initiation Protocol (SIP)", draft-ietf-sip-certs-02 (work in progress), October 2006. [I-D.zimmermann-avt-zrtp] Zimmermann, P., "ZRTP: Extensions to RTP for Diffie- Hellman Key Agreement for SRTP", draft-zimmermann-avt-zrtp-02 (work in progress), October 2006. Wing, et al. Expires August 21, 2007 [Page 10] Internet-Draft SRTP Event Package February 2007 [RFC4117] Camarillo, G., Burger, E., Schulzrinne, H., and A. van Wijk, "Transcoding Services Invocation in the Session Initiation Protocol (SIP) Using Third Party Call Control (3pcc)", RFC 4117, June 2005. Authors' Addresses Dan Wing Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Email: dwing@cisco.com Francois Audet Nortel 4655 Great America Parkway Santa Clara, CA 95054 USA Email: audet@nortel.com Steffen Fries Siemens AG Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: steffen.fries@siemens.com Hannes Tschofenig Siemens Networks GmbH & Co KG Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: Hannes.Tschofenig@siemens.com Wing, et al. Expires August 21, 2007 [Page 11] Internet-Draft SRTP Event Package February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Wing, et al. Expires August 21, 2007 [Page 12]