right|thumb|300px|SIGSALY exhibit at the [[National Cryptologic Museum]]

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).

Development

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 to not 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.

Operation

SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder. The latter was used to minimize the amount of redundancy (which is high in voice traffic), in order to reduce the amount of information to be encrypted.

  • ten channels covering the telephone passband (250 Hz – 2,950 Hz); are rectified and filtered to extract how much energy is in each of these channels.
  • another signal indicating whether the sound is voiced or unvoiced;
  • if voiced, a signal indicating the pitch; this also varied at less than 25 Hz.

Next, each signal was sampled for its amplitude once every 20 milliseconds.

A dozen SIGSALY terminal installations were eventually set up all over the world. The first was installed in the Pentagon building rather than the White House, which had an extension line, as the US President Franklin Roosevelt knew of the British Prime Minister Winston Churchill's insistence that he be able to call at any time of the day or night. The second was installed below street level in the basement of Selfridges department store on Oxford Street, London, close to the US Embassy on Grosvenor Square. The first conference took place on 15 July 1943, and it was used by both General Dwight D. Eisenhower as the commander of SHAEF, and Churchill, before extensions were installed to the Embassy, 10 Downing Street and the Cabinet War Rooms.