An Opportunistic Approach for Secure Real-time Transport Protocol (OSRTP)

Opportunistic Security (OS) 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 is secured derivatives, cleartext is represented by an RTP media stream which is negotiated with the RTP/AVP (Audio Video Profile) or the RTP/AVPF profile , whereas, comprehensive protection is represented by a Secure RTP stream, negotiated with a secure profile, such as SAVP or SAVPF . 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 (capneg) . However, these approaches are complex and have very limited deployment in communication systems. Other key management protocols for SRTP have been developed which by design use OS, such as ZRTP . This approach for OSRTP is based on where it was called "best effort SRTP". 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 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 .

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 which are attempting to transition to fully secure communications. New applications and new deployments will not use OSRTP.

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 when, and only when, they appear in all capitals, as shown here.

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 oportunistic 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, 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 , that is the "comprehensive protection" for media mode.

To indicate support for OSRTP in an SDP offer, the offerer uses the RTP/AVP profile or the RTP/AVPF profile 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 , then an a=fingerprint attribute will be present, or If the offerer supports SDP Security Descriptions key agreement , then an a=crypto attribute will be present, or If the offerer supports ZRTP key agreement , then an a=zrtp-hash attribute will be present.

To accept OSRTP, an answerer receiving an offer indicating support for OSRTP generates an SDP answer containing SRTP keying attributes which 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.

If the offerer of OSRTP receives an SDP answer which 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.

When an offerer generates a subsequent SDP offer it should do so following the principles of 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 .

The security considerations of 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 . For example: For DTLS-SRTP key agreement , an authenticated signaling channel does not need to be used with OSRTP if it is not available. For SDP Security Descriptions key agreement , 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 , 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 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 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 , OSRTP is used in cases where support for encryption by the other party is not known in advance, and 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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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.