&xep0167; recommends the use of the Secure Real-time Transport Protocol (SRTP) for end-to-end encryption of RTP sessions negotiated using &xep0166;. An alternative approach to end-to-end encryption of RTP traffic is provided by &zrtp;, developed by Phil Zimmermann, the inventor of "Pretty Good Privacy" (PGP). Although negotiation of ZRTP mainly occurs in the media channel rather than the signalling channel, the ZRTP specification defines one SDP attribute called "zrtp-hash" (this communicates the ZRTP version supported as well as a hash of the Hello message).

The SDP format is shown below.

An example follows.

This SDP attribute can be translated into Jingle as a <zrtp-hash/> element, as shown below.

An example follows.

If either party to a Jingle RTP session wishes to use ZRTP, the party SHOULD send a Jingle session-info message that includes a <zrtp-hash/> element qualified by the 'urn:xmpp:jingle:apps:rtp:zrtp:0' namespace, as shown in the following example.

In accordance with XEP-0166, if the receiving party does not understand the payload of the session-info message then it MUST return a &feature; error.

However, if the receiving party also supports and wishes to use ZRTP, it too SHOULD send a session-info message containing a zrtp-hash element.

If an entity supports the zrtp-hash session-info message, it MUST advertise that fact in its responses to &xep0030; information ("disco#info") requests by returning a feature of "urn:xmpp:jingle:apps:rtp:zrtp:0":

In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

Security considerations for ZRTP itself are provided in draft-zimmermann-avt-zrtp.

XMPP stanzas such as Jingle session-info messages and service discovery exchanges are not encrypted or signed. As a result, it is possible for an attacker to intercept these stanzas and modify them, thus convincing one party that the other party does not support ZRTP and therefore denying the parties an opportunity to use ZRTP. However, because the zrtp-hash is mostly advisory, the parties could still use ZRTP even if the signalling channel is compromised.

This document requires no interaction with &IANA;.