Secure telephone

ZRTP is implemented in, amongst others, Ripcord Networks product SecurePC with up to NSA Suite B compliant Elliptic Curve math libraries.

Scramblers were used to secure voice traffic during World War II (1939-1945), but were often intercepted and decoded due to scrambling's inherent insecurity.

Other products of historical significance are PGPfone and Nautilus (designed as a non-key escrow alternative to Clipper, now officially discontinued, but still available on SourceForge), SpeakFreely, and the security VoIP protocol wrapper Zfone (developed by Phil Zimmermann, who wrote the PGP software).

Scrambling, generally using a form of voice inversion, was available from suppliers of electronic hobbyist kits and is common on FRS radios.

Analog scrambling is still used, as some telecommunications circuits, such as HF links and telephone lines in the developing world, are of very low quality.