RFC 6189 -- ZRTP: Media Path Key Agreement for Unicast Secure RTP
ZRTP is a protocol for negotiating cryptographic keys for end-to-end secure VoIP telephony sessions.
The ZRTP protocol specification is available as RFC 6189 on the IETF site.
Here is RFC 6189 nicely rendered in easily navigated hypertext formats:
RFC 6189bis - The "Director's Cut", with bonus material
Even better than the original.
Since RFC 6189 was published, we've gained deployment experience in the real world. So, we've added some clarifications and refinements. The result is not an official IETF RFC, but implementors will find this "Director's Cut" more useful.
Implementations of RFC 6189
Open source implementations of ZRTP available in C++ and Java.
Here are some links to download open source libraries of the ZRTP protocol. They can be used to develop ZRTP VoIP client apps.