High Quality Content by WIKIPEDIA articles! PGPfone was a secure voice telephony system developed by Philip Zimmermann in 1995. The PGPfone protocol had little in common with Zimmermann`s popular PGP email encryption package, except for the use of the name. It used ephemeral Diffie-Hellman protocol to establish a session key, which was then used to encrypt the stream of voice packets. The two parties compared a short authentication string to detect a Man-in-the-middle attack, which is the most common method of wiretapping secure phones of this type. PGPfone could be used point-to-point (with two modems) over the public switched telephone network, or over the Internet as an early Voice over IP system.