What was the entity known as SIGSALY?

Electronic systems developed rapidly in World War II, often driven by military necessity. Both the Allies and the Axis powers were acutely aware of weaknesses in communications methods and tried to exploit them, while protecting their own systems. Particularly vulnerable were long-distance phone calls: both sides used “scramblers,” but knew that these could be cracked. For example, the Germans had a listening station on the Dutch coast that could intercept and break phone traffic that used the voice scramblers developed by Western Electric.

The insecurity of most telephone scrambler schemes led to the development of a more secure scrambler, based on the one-time pad principle, and intended for the highest-level Allied communications. This secure speech system was officially known as SIGSALY, and was also sometimes referred to as the X System, Project X, Ciphony I, and the Green Hornet. The SIGSALY project pioneered several digital communications concepts, including the first transmission of speech using pulse-code modulation. In 1942, a prototype was developed by Bell Telephone Laboratories, aided by the mathematician Alan Turing. This was the world’s first voice encryption system and it impressed US Army: SIGSALY went into service in 1943.

The name “SIGSALY” was not an acronym, but just a secure cover name that resembled an acronym. The "Green Hornet" name was used because the calls sounded like a buzzing hornet to anyone trying to eavesdrop on the conversation.