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Use of ZRTP in Jingle RTP Sessions This specification defines a Jingle application type for negotiating one or more sessions that use the Real-time Transport Protocol (RTP) to exchange media such as voice or video. The application type includes a straightforward mapping to Session Description Protocol (SDP) for interworking with SIP media endpoints. &LEGALNOTICE; 0262 Experimental Standards Track Standards Council XMPP Core XEP-0166 XEP-0167 RFC 6189 jingle-zrtp jingle &stpeter; 0.3 2011-04-12 psa

Updated reference to reflect publication of RFC 6189; clarified that inclusion of the hash is optional.

0.2 2010-08-09 psa

In harmony with the ZRTP spec, moved communication of <zrtp-hash/> element from Jingle session-info message to jingle-invite, specifically as a child of the <encryption/> element from XEP-0167; increased protocol version from zero to one.

0.1 2009-02-25 psa

Initial published version.

0.0.2 2009-02-24 psa

Defined dedicated namespace for the zrtp-hash element to improve service discovery and handling of session-info message; clarified protocol flow and security considerations.

0.0.1 2009-02-17 psa

First draft, copied from XEP-0167.

&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 &rfc6189;, 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). Inclusion of this information is OPTIONAL in both SIP/SDP and Jingle.

The SDP format is shown below.

a=zrtp-hash:zrtp-version zrtp-hash-value

An example follows.

a=zrtp-hash:1.10 fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df

This SDP attribute can be translated into Jingle as a <zrtp-hash/> element qualified by the 'urn:xmpp:jingle:apps:rtp:zrtp:1' namespace, as shown below.

zrtp-hash-value ]]>

An example follows.

fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df ]]>

The <zrtp-hash/> element is sent as a child of the <encryption/> element defined in &xep0167;.

If the Jingle initiator wishes to use ZRTP, it includes the <zrtp-hash/> element in its session invitation (where it hashes over its own Hello message as described in the ZRTP specification).

fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df ]]>

If the receiving party wishes to proceed with ZRTP negotiation, it also includes the <zrtp-hash/> element in its session-accept message (where it hashes over its own Hello message as described in the ZRTP specification).

badfbe66ff87fe135750377509b09b0babd1c3ec25fa4314565e2bf7ccc30299 ]]>

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:1":

]]> ]]>

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 RFC 6189.

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;.

Thanks to Werner Dittmann and Emil Ivov for their implementation feedback.

This specification defines the following XML namespace:

  • urn:xmpp:jingle:apps:rtp:zrtp:1

Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespace to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.

&NSVER;
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