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Secure speech system

SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest level Allied communications. It 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 Army Signal Corps names (e. g., SIGABA).

At the time of its inception, long distance telephone communications used the "A 3" voice scrambler developed by Western Electric. It worked on the voice inversion principle. 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 not to be very secure 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 led 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 demonstrated to the US Army. The Army was impressed and awarded Bell Labs a contract for two systems in 1942. SIGSALY went into service in 1943 and remained in service until 1946.