'SIGSALY' was an Allied maximum-security speech communications system that was otherwise known as the 'X System', 'Project X', 'Ciphony I' and 'Green Hornet' (1943/46).

'SIGSALY' pioneered a number of digital communications concepts, including the first transmission of speech using pulse-code modulation.

The name 'SIGSALY' was not an acronym, but a cover name that resembled an acronym: the SIG part was common in US Army Signal Corps names. The prototype was called 'Green Hornet' after the radio show The Green Hornet as it sounded like a buzzing hornet, resembling the show’s theme tune, to anyone trying to eavesdrop on the conversation.

At the time of the inception of 'SIGSALY', long-distance telephone communications were broadcast using the A-3 voice scrambler developed by Western Electric. The Germans had a listening station on the Dutch coast which could intercept and break A-3 traffic.

Although telephone scramblers were used by both sides in World War II, they were known to be very insecure in general, and both sides often cracked the scrambled conversations of the other. Inspection of the audio spectrum using a spectrum analyzer often provided significant clues to the scrambling technique. The insecurity of most telephone scrambler schemes thus paved the way to the development of a more secure scrambler, based on the one-time pad principle.

A prototype was developed at Bell Telephone Laboratories, under the direction of A. B. Clark, assisted by British mathematician Alan Turing, and was demonstrated to the US Army. This service was highly impressed and awarded Bell Telephone Laboratories a contract for an initial pair of systems in 1942. 'SIGSALY' went into service during 1943 and remained in service until 1946.