Internet Engineering Task Force (IETF)                       A. Johnston
Request for Comments: 8643                          Villanova University
Category: Informational                                         B. Aboba
ISSN: 2070-1721                                                Microsoft
                                                               A. Hutton
                                                                    Atos
                                                               R. Jesske
                                                        Deutsche Telekom
                                                                T. Stach
                                                            Unaffiliated
                                                             August 2019

   An 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 in
   RFC 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; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8643.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability Statement . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  SDP Offer/Answer Considerations . . . . . . . . . . . . . . .   3
     3.1.  Generating the Initial OSRTP Offer  . . . . . . . . . . .   4
     3.2.  Generating the Answer . . . . . . . . . . . . . . . . . .   4
     3.3.  Offerer Processing the Answer . . . . . . . . . . . . . .   4
     3.4.  Modifying the Session . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  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.

   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 in
   BCP 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.

   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.

3.4.  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 in Section 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.

5.  IANA Considerations

   This document has no actions for IANA.

6.  References

6.1.  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>.

   [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 in RFC
              2119 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.

Authors' 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.com