Technical Summary
This document defines ZRTP, a protocol for media path Diffie-Hellman
exchange to agree on a session key and parameters for establishing
Secure Real-time Transport Protocol (SRTP) sessions for VoIP
applications. The ZRTP protocol is media path keying because it is
multiplexed on the same port as RTP and does not require support in
the signaling protocol. ZRTP does not assume a Public Key
Infrastructure (PKI) or require the complexity of certificates in end
devices. For the media session, ZRTP provides confidentiality,
protection against man-in-the-middle (MiTM) attacks, and, in cases
where the signaling protocol provides end-to-end integrity
protection, authentication. ZRTP can utilize a Session Description
Protocol (SDP) attribute to provide discovery and authentication
through the signaling channel. To provide best effort SRTP, ZRTP
utilizes normal RTP/AVP profiles. ZRTP secures media sessions which
include a voice media stream, and can also secure media sessions
which do not include voice by using an optional digital signature.
IETF Discussion Summary
This protocol was proposed as a solution for keying SRTP and received
significant review and discussion while it was being considered. The
IETF chose a different proposal (draft-ietf-avt-dtls-srtp) to publish
as Proposed Standard.
Document Quality
There are multiple implementations of this protocol.
A reference implementation of ZRTP is available as Zfone.