XEP-0262: Use of ZRTP in Jingle RTP Sessions

WARNING: Consideration of this document has been Deferred by the XMPP Standards Foundation. Implementation of the protocol described herein is not recommended.

Table of Contents

1. Protocol

Jingle RTP Sessions [1] recommends the use of the Secure Real-time Transport Protocol (SRTP) for end-to-end encryption of RTP sessions negotiated using Jingle [2]. An alternative approach to end-to-end encryption of RTP traffic is provided by ZRTP [3], 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.

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, as shown below.

<zrtp-hash version='zrtp-version'>zrtp-hash-value</zrtp-hash>
  

An example follows.

<zrtp-hash version='1.10'>fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df</zrtp-hash>
  

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.

Example 1. Initiator sends session-info message with zrtp-hash

<iq from='romeo@montague.lit/orchard'
    id='vna13b9a'
    from='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:0'
          action='session-info'
          initiator='romeo@montague.lit/orchard'
          sid='a73sjjvkla37jfea'>
    <zrtp-hash xmlns='urn:xmpp:jingle:apps:rtp:zrtp:0'
               version='1.10'>fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df</zrtp-hash>
  </jingle>
</iq>
    

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-not-implemented/> error.

Example 2. Receiver returns feature-not-implemented error

<iq from='juliet@capulet.lit/balcony'
    id='ringing1'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='modify'>
    <feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <unsupported-info xmlns='urn:xmpp:jingle:errors:0'/>
  </error>
</iq>
    

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.

2. Determining Support

If an entity supports the zrtp-hash session-info message, it MUST advertise that fact in its responses to Service Discovery [4] information ("disco#info") requests by returning a feature of "urn:xmpp:jingle:apps:rtp:zrtp:0":

Example 3. A disco#info query

<iq type='get' 
    from='calvin@usrobots.lit/lab'
    to='herbie@usrobots.lit/home'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  

Example 4. A disco#info response

<iq type='result' 
    from='herbie@usrobots.lit/home'
    to='calvin@usrobots.lit/lab'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:jingle:apps:rtp:zrtp:0'/>
  </query>
</iq>
  

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 Entity Capabilities [5]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

3. Security Considerations

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.

4. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

5. XMPP Registrar Considerations

6. XML Schemas

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:jingle:apps:rtp:zrtp:0'
    xmlns='urn:xmpp:jingle:apps:rtp:zrtp:0'
    elementFormDefault='qualified'>

  <xs:element name='zrtp-hash'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='xs:string'>
          <xs:attribute name='version' type='xs:string' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

</xs:schema>
  

Appendices

Appendix A: Document Information

Series: XEP
Number: 0262
Publisher: XMPP Standards Foundation
Status: Deferred
Type: Standards Track
Version: 0.1
Last Updated: 2009-02-25
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0166, XEP-0167, draft-zimmermann-avt-zrtp
Supersedes: None
Superseded By: None
Short Name: jingle-zrtp
Source Control: HTML  RSS
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

There exists a special venue for discussion related to the technology described in this document: the <jingle@xmpp.org> mailing list.

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

END