ZRTP was developed by Phil Zimmermann, with help from Bryce Wilcox-O'Hearn, Colin Plumb, Jon Callas and Alan Johnston and was submitted to the Internet Engineering Task Force (IETF) by Zimmermann, Callas and Johnston on March 5, 2006 and published on April 11, 2011 as RFC 6189.
ZRTP can be used with any signaling protocol, including SIP, H.323, Jingle, and distributed hash table systems.
ZRTP is independent of the signaling layer, because all its key negotiations occur via the RTP media stream.
ZRTP/S, a ZRTP protocol extension, can run on any kind of legacy telephony networks including GSM, UMTS, ISDN, PSTN, SATCOM, UHF/VHF radio, because it is a narrow-band bitstream-oriented protocol and performs all key negotiations inside the bitstream between two endpoints.
To carry out authentication, this SAS value is read aloud to the communication partner over the voice connection.
ZRTP provides a second layer of authentication against a MitM attack, based on a form of key continuity.