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Internet Engineering Task Force (IETF)                       A. JohnstonRequest for Comments: 8643                          Villanova UniversityCategory: Informational                                         B. AbobaISSN: 2070-1721                                                Microsoft                                                               A. Hutton                                                                    Atos                                                               R. Jesske                                                        Deutsche Telekom                                                                T. Stach                                                            Unaffiliated                                                             August 2019An Opportunistic Approach for Secure Real-time Transport Protocol(OSRTP)Abstract   Opportunistic Secure Real-time Transport Protocol (OSRTP) is an   implementation of the Opportunistic Security mechanism, as defined inRFC 7435, applied to the Real-time Transport Protocol (RTP).  OSRTP   allows encrypted media to be used in environments where support for   encryption is not known in advance and is not required.  OSRTP does   not require Session Description Protocol (SDP) extensions or features   and is fully backwards compatible with existing implementations using   encrypted and authenticated media and implementations that do not   encrypt or authenticate media packets.  OSRTP is not specific to any   key management technique for Secure RTP (SRTP).  OSRTP is a   transitional approach useful for migrating existing deployments of   real-time communications to a fully encrypted and authenticated   state.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are candidates for any level of Internet   Standard; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8643.Johnston, et al.              Informational                     [Page 1]

RFC 8643                          OSRTP                      August 2019Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Applicability Statement . . . . . . . . . . . . . . . . .32.  Requirements Language . . . . . . . . . . . . . . . . . . . .33.  SDP Offer/Answer Considerations . . . . . . . . . . . . . . .33.1.  Generating the Initial OSRTP Offer  . . . . . . . . . . .43.2.  Generating the Answer . . . . . . . . . . . . . . . . . .43.3.  Offerer Processing the Answer . . . . . . . . . . . . . .43.4.  Modifying the Session . . . . . . . . . . . . . . . . . .54.  Security Considerations . . . . . . . . . . . . . . . . . . .55.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .66.  References  . . . . . . . . . . . . . . . . . . . . . . . . .66.1.  Normative References  . . . . . . . . . . . . . . . . . .66.2.  Informative References  . . . . . . . . . . . . . . . . .7   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .7   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .81.  Introduction   Opportunistic Security (OS) [RFC7435] is an approach to security that   defines a third mode for security between "cleartext" and   "comprehensive protection" that allows encryption and authentication   of media to be used if supported but will not result in failures if   it is not supported.  In the context of the transport of secure media   streams using RTP and its secured derivatives, cleartext is   represented by an RTP [RFC3550] media stream that is negotiated with   the RTP/AVP (Audio-Visual Profile) [RFC3551] or the RTP/AVPF profile   [RFC4585], whereas comprehensive protection is represented by a   Secure RTP [RFC3711] stream negotiated with a secure profile, such as   SAVP or SAVPF [RFC5124].  OSRTP allows SRTP to be negotiated with the   RTP/AVP profile, with fallback to RTP if SRTP is not supported.Johnston, et al.              Informational                     [Page 2]

RFC 8643                          OSRTP                      August 2019   There have been some extensions to SDP to allow profiles to be   negotiated, such as SDP Capabilities Negotiation (SDPCapNeg)   [RFC5939].  However, these approaches are complex and have very   limited deployment in communication systems.  Other key management   protocols for SRTP that have been developed, such as ZRTP [RFC6189],   use OS by design.  This approach for OSRTP is based on [Kaplan06]   where it was called "best effort SRTP".  [Kaplan06] has a full   discussion of the motivation and requirements for opportunistic   secure media.   OSRTP uses the presence of SRTP keying-related attributes in an SDP   offer to indicate support for opportunistic secure media.  The   presence of SRTP keying-related attributes in the SDP answer   indicates that the other party also supports OSRTP and that encrypted   and authenticated media will be used.  OSRTP requires no additional   extensions to SDP or new attributes and is defined independently of   the key agreement mechanism used.  OSRTP is only usable when media is   negotiated using the Offer/Answer protocol [RFC3264].1.1.  Applicability Statement   OSRTP is a transitional approach that provides a migration path from   unencrypted communication (RTP) to fully encrypted communication   (SRTP).  It is only to be used in existing deployments that are   attempting to transition to fully secure communications.  New   applications and new deployments will not use OSRTP.2.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.3.  SDP Offer/Answer Considerations   This section defines the SDP offer/answer considerations for   opportunistic security.   The procedures are for a specific "m=" section describing RTP-based   media.  If an SDP offer or answer contains multiple such "m="   sections, the procedures are applied to each "m=" section   individually.   "Initial OSRTP offer" refers to the offer in which opportunistic   security is offered for an "m=" section for the first time within an   SDP session.Johnston, et al.              Informational                     [Page 3]

RFC 8643                          OSRTP                      August 2019   It is important to note that OSRTP makes no changes to and has no   effect on media sessions in which the offer contains a secure profile   of RTP, such as SAVP or SAVPF.  As discussed in [RFC7435], that is   the "comprehensive protection" for media mode.3.1.  Generating the Initial OSRTP Offer   To indicate support for OSRTP in an SDP offer, the offerer uses the   RTP/AVP profile [RFC3551] or the RTP/AVPF profile [RFC4585] but   includes SRTP keying attributes.  OSRTP is not specific to any key   management technique for SRTP, and multiple key management techniques   can be included on the SDP offer.  For example:      If the offerer supports DTLS-SRTP key agreement [RFC5763], then an      "a=fingerprint" attribute will be present.  Or:      If the offerer supports SDP Security Descriptions key agreement      [RFC4568], then an "a=crypto" attribute will be present.  Or:      If the offerer supports ZRTP key agreement [RFC6189], then an      "a=zrtp-hash" attribute will be present.3.2.  Generating the Answer   To accept OSRTP, an answerer receiving an offer indicating support   for OSRTP generates an SDP answer containing SRTP keying attributes   that match one of the keying methods in the offer.  The answer MUST   NOT contain attributes from more than one keying method, even if the   offer contained multiple keying method attributes.  The selected SRTP   key management approach is followed, and SRTP media is used for this   session.  If the SRTP key management fails for any reason, the media   session MUST fail.  To decline OSRTP, the answerer generates an SDP   answer omitting SRTP keying attributes, and the media session   proceeds with RTP with no encryption or authentication used.3.3.  Offerer Processing the Answer   If the offerer of OSRTP receives an SDP answer that does not contain   SRTP keying attributes, then the media session proceeds with RTP.  If   the SDP answer contains SRTP keying attributes, then the associated   SRTP key management approach is followed and SRTP media is used for   this session.  If the SRTP key management fails, the media session   MUST fail.Johnston, et al.              Informational                     [Page 4]

RFC 8643                          OSRTP                      August 20193.4.  Modifying the Session   When an offerer generates a subsequent SDP offer, it should do so   following the principles of [RFC6337], meaning that the decision to   create the new SDP offer should not be influenced by what was   previously negotiated.  For example, if a previous OSRTP offer did   not result in SRTP being established, the offerer may try again and   generate a new OSRTP offer as specified inSection 3.1.4.  Security Considerations   The security considerations of [RFC4568] apply to OSRTP, as well as   the security considerations of the particular SRTP key agreement   approach used.  However, the authentication requirements of a   particular SRTP key agreement approach are relaxed when that key   agreement is used with OSRTP, which is consistent with the   Opportunistic Security approach described in [RFC7435].  For example:      For DTLS-SRTP key agreement [RFC5763], an authenticated signaling      channel does not need to be used with OSRTP if it is not      available.      For SDP Security Descriptions key agreement [RFC4568], an      authenticated signaling channel does not need to be used with      OSRTP if it is not available, although an encrypted signaling      channel MUST still be used.      For ZRTP key agreement [RFC6189], the security considerations are      unchanged, since ZRTP does not rely on the security of the      signaling channel.   While OSRTP does not require authentication of the key agreement   mechanism, it does need to avoid exposing SRTP keys to eavesdroppers,   since this could enable passive attacks against SRTP.Section 8.3 of   [RFC4568] requires that any messages that contain SRTP keys be   encrypted, and further says that encryption SHOULD provide end-to-end   confidentiality protection if intermediaries that could inspect the   SDP message are present.  At the time of this writing, it is   understood that the requirement in [RFC4568] for end-to-end   confidentiality protection is commonly ignored.  Therefore, if OSRTP   is used with SDP Security Descriptions, any such intermediaries   (e.g., SIP proxies) must be assumed to have access to the SRTP keys.   As discussed in [RFC7435], OSRTP is used in cases where support for   encryption by the other party is not known in advance and is not   required.  For cases where it is known that the other party supports   SRTP or SRTP needs to be used, OSRTP MUST NOT be used.  Instead, a   secure profile of RTP is used in the offer.Johnston, et al.              Informational                     [Page 5]

RFC 8643                          OSRTP                      August 20195.  IANA Considerations   This document has no actions for IANA.6.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model              with Session Description Protocol (SDP)",RFC 3264,              DOI 10.17487/RFC3264, June 2002,              <https://www.rfc-editor.org/info/rfc3264>.   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.              Jacobson, "RTP: A Transport Protocol for Real-Time              Applications", STD 64,RFC 3550, DOI 10.17487/RFC3550,              July 2003, <https://www.rfc-editor.org/info/rfc3550>.   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and              Video Conferences with Minimal Control", STD 65,RFC 3551,              DOI 10.17487/RFC3551, July 2003,              <https://www.rfc-editor.org/info/rfc3551>.   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.              Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC 3711, DOI 10.17487/RFC3711, March 2004,              <https://www.rfc-editor.org/info/rfc3711>.   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session              Description Protocol (SDP) Security Descriptions for Media              Streams",RFC 4568, DOI 10.17487/RFC4568, July 2006,              <https://www.rfc-editor.org/info/rfc4568>.   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,              "Extended RTP Profile for Real-time Transport Control              Protocol (RTCP)-Based Feedback (RTP/AVPF)",RFC 4585,              DOI 10.17487/RFC4585, July 2006,              <https://www.rfc-editor.org/info/rfc4585>.   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for              Real-time Transport Control Protocol (RTCP)-Based Feedback              (RTP/SAVPF)",RFC 5124, DOI 10.17487/RFC5124, February              2008, <https://www.rfc-editor.org/info/rfc5124>.Johnston, et al.              Informational                     [Page 6]

RFC 8643                          OSRTP                      August 2019   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework              for Establishing a Secure Real-time Transport Protocol              (SRTP) Security Context Using Datagram Transport Layer              Security (DTLS)",RFC 5763, DOI 10.17487/RFC5763, May              2010, <https://www.rfc-editor.org/info/rfc5763>.   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:              Media Path Key Agreement for Unicast Secure RTP",RFC 6189, DOI 10.17487/RFC6189, April 2011,              <https://www.rfc-editor.org/info/rfc6189>.   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection              Most of the Time",RFC 7435, DOI 10.17487/RFC7435,              December 2014, <https://www.rfc-editor.org/info/rfc7435>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.6.2.  Informative References   [Kaplan06] Kaplan, H. and F. Audet, "Session Description Protocol              (SDP) Offer/Answer Negotiation For Best-Effort Secure              Real-Time Transport Protocol", Work in Progress,draft-kaplan-mmusic-best-effort-srtp-01, October 2006.   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)              Capability Negotiation",RFC 5939, DOI 10.17487/RFC5939,              September 2010, <https://www.rfc-editor.org/info/rfc5939>.   [RFC6337]  Okumura, S., Sawada, T., and P. Kyzivat, "Session              Initiation Protocol (SIP) Usage of the Offer/Answer              Model",RFC 6337, DOI 10.17487/RFC6337, August 2011,              <https://www.rfc-editor.org/info/rfc6337>.Acknowledgements   This document is dedicated to our friend and colleague Francois Audet   who is greatly missed in our community.  His work on improving   security in SIP and RTP provided the foundation for this work.   Thanks to Eric Rescorla, Martin Thomson, Christer Holmberg, and   Richard Barnes for their comments.Johnston, et al.              Informational                     [Page 7]

RFC 8643                          OSRTP                      August 2019Authors' Addresses   Alan Johnston   Villanova University   Villanova, PA   United States of America   Email: alan.b.johnston@gmail.com   Bernard Aboba   Microsoft   One Microsoft Way   Redmond, WA  98052   United States of America   Email: bernard.aboba@gmail.com   Andrew Hutton   Atos   Mid City Place   London  WC1V 6EA   United Kingdom   Email: andrew.hutton@atos.net   Roland Jesske   Deutsche Telekom   Heinrich-Hertz-Strasse 3-7   Darmstadt  64295   Germany   Email: R.Jesske@telekom.de   Thomas Stach   Unaffiliated   Email: thomass.stach@gmail.comJohnston, et al.              Informational                     [Page 8]