Jingle RTP Sessions (XEP-0167) [1] recommends the use of the Secure Real-time Transport Protocol (SRTP) for end-to-end encryption of RTP sessions negotiated using Jingle (XEP-0166) [2]. An alternative approach to end-to-end encryption of RTP traffic is provided by RFC 6189 [3]. 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.
<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.
<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.
If an entity supports the zrtp-hash session-info message, it MUST advertise that fact in its responses to Service Discovery (XEP-0030) [4] information ("disco#info") requests by returning a feature of "urn:xmpp:jingle:apps:rtp:zrtp:0":
<iq type='get'
from='calvin@usrobots.lit/lab'
to='herbie@usrobots.lit/home'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<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 (XEP-0115) [5]. 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 the Internet Assigned Numbers Authority (IANA) [6].
See the XML Schemas section of this document.
If this specification is advanced to a status of Draft, the XMPP Registrar shall add the following element definition to the schema for the 'urn:xmpp:jingle:apps:rtp:info:1' namespace defined in XEP-0167:
<?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>
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) 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.
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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".
1. XEP-0167: Jingle RTP Sessions <https://xmpp.org/extensions/xep-0167.html>.
2. XEP-0166: Jingle <https://xmpp.org/extensions/xep-0166.html>.
3. RFC 6189: ZRTP: Media Path Key Agreement for Unicast Secure RTP <http://tools.ietf.org/html/rfc6189>.
4. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
5. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.
6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
Defined dedicated namespace for the zrtp-hash element to improve service discovery and handling of session-info message; clarified protocol flow and security considerations.
First draft, copied from XEP-0167.
@report{saint-andre2009n/a,
title = {Use of ZRTP in Jingle RTP Sessions},
author = {Saint-Andre, Peter},
type = {XEP},
number = {xxxx},
version = {0.0.2},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2009-02-17/2009-02-24},
}END