14.6.5 ZRTP

ZRTP [10] is a self-contained media path keying protocol invented by Phil Zimmermann, the inventor of “pretty good privacy” or PGP encryption protocol [36]. ZRTP does not require an offer/answer exchange using the SAVP profile, and instead does discovery and negotiation in the media path. Early versions of ZRTP were actually RTP header extensions. However, the current version is a separate protocol that is multiplexed over the same transport address as the resulting media stream. ZRTP performs a Diffie-Hellman key agreement to derive the SRTP master secret. ZRTP uses a short authentication string (SAS) rendered to each UA, and compared by human users, to authenticate the Diffie-Hellman exchange and protect against a MitM attack. As a result, ZRTP does not require even an integrity protected signaling channel. Also, ZRTP does not require the same signaling protocol be used between the endpoints. For example, endpoints that support SIP, H.323, and Jingle without any extensions can all negotiate secure media sessions if the endpoints all support ZRTP. The ZRTP key agreement exchange is shown in Figure 14.9.

Figure 14.9 ZRTP key agreement.