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.




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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
  (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 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.




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  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.



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  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.








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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.



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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>.



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  [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.








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Authors' Addresses

  Alan Johnston
  Villanova University
  Villanova, PA
  United States of America

  Email: [email protected]


  Bernard Aboba
  Microsoft
  One Microsoft Way
  Redmond, WA  98052
  United States of America

  Email: [email protected]


  Andrew Hutton
  Atos
  Mid City Place
  London  WC1V 6EA
  United Kingdom

  Email: [email protected]


  Roland Jesske
  Deutsche Telekom
  Heinrich-Hertz-Strasse 3-7
  Darmstadt  64295
  Germany

  Email: [email protected]


  Thomas Stach
  Unaffiliated

  Email: [email protected]










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