VoIP
Some Voice over IP (VoIP) solutions provide for painless encryption of voice traffic when possible. Some versions of the Sipura and Linksys lines of analog telephony adapters (ATA) include a hardware implementation of SRTP with the installation of a certificate from Voxilla, a VoIP information site. When the call is placed an attempt is made to use SRTP, if successful a series of tones are played into the handset, if not the call proceeds without using encryption. Skype and Amicima use only secure connections and Gizmo5 attempts a secure connection between its clients. Phil Zimmermann, Alan Johnston, and Jon Callas have proposed a new VoIP encryption protocol called ZRTP. They have an implementation of it called Zfone whose source and compiled binaries are available.
Read more about this topic: Opportunistic Encryption