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
July 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
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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
USA
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