SIGSALY (also known as theX System,Project X,Ciphony I, and theGreen Hornet) was asecure speech system used inWorld War II for the highest-levelAllied communications. It pioneered a number ofdigital communications concepts, including the first transmission of speech usingpulse-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).[1] The prototype was called the "Green Hornet" after the radio showThe Green Hornet, because it sounded like a buzzinghornet, resembling the show's theme tune, to anyone trying to eavesdrop on the conversation.[2]
At the time of its inception, long-distance telephone communications used the "A-3" voicescrambler developed byWestern Electric. It worked on thevoice inversion principle. The Germans had a listening station on the Dutch coast which could intercept and break A-3 traffic.[1]
Although telephonescramblers 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 aspectrum 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 theone-time pad principle.
A prototype was developed atBell Telephone Laboratories, under the direction of A. B. Clark, assisted by British mathematicianAlan Turing,[1][3] 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.
SIGSALY used arandomnoise mask toencrypt voice conversations which had been encoded by avocoder. 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.[2]
The voice encoding used the fact that speech varies fairly slowly as the components of the throat move. The system extracts information about the voice signal 50 times a second (every 20 milliseconds).[4]
Next, each signal wassampled for its amplitude once every 20 milliseconds.[4] For the band amplitude signals, the amplitude converted into one of six amplitude levels, with values from 0 through 5. The amplitude levels were on a nonlinear scale, with the steps between levels wide at high amplitudes and narrower at low amplitudes. This scheme, known as "companding" or "compressing-expanding", exploits the fact that the fidelity of voice signals is more sensitive to low amplitudes than to high amplitudes. The pitch signal, which required greater sensitivity, was encoded by a pair of six-level values (one coarse, and one fine), giving thirty-six levels in all.
Acryptographic key, consisting of a series of random values from the same set of six levels, was subtracted from each sampled voice amplitude value to encrypt them before transmission. The subtraction was performed usingmodular arithmetic: a "wraparound" fashion, meaning that if there was a negative result, it was added to six to give a positive result. For example, if the voice amplitude value was 3 and the random value was 5, then the subtraction would work as follows:
— giving a value of 4.
The sampled value was then transmitted, with each sample level transmitted on one of six corresponding frequencies in a frequency band, a scheme known as "frequency-shift keying (FSK)". The receiving SIGSALY read the frequency values, converted them into samples, and added the key values back to them to decrypt them. The addition was also performed in a "modulo" fashion, with six subtracted from any value over five. To match the example above, if the receiving SIGSALY got a sample value of 4 with a matching random value of 5, then the addition would be as follows:
— which gives the correct value of 3.
To convert the samples back into a voice waveform, they were first turned back into the dozen low-frequency vocoded signals. An inversion of the vocoder process was employed, which included:
The noise values used for the encryption key were originally produced by largemercury-vapor rectifyingvacuum tubes and stored on aphonograph record. The record was then duplicated, with the records being distributed to SIGSALY systems on both ends of a conversation. The records served as the SIGSALYone-time pad, and distribution was very strictly controlled (although if one had been seized, it would have been of little importance, since only one pair of each was ever produced). For testing and setup purposes, a pseudo-random number generating system made out of relays, known as the "threshing machine", was used.
The records were played onturntables, but since the timing – theclock synchronization – between the two SIGSALY terminals had to be precise, the turntables were by no means just ordinary record-players. The rotation rate of the turntables was carefully controlled, and the records were started at highly specific times, based on precision time-of-day clock standards. Since each record only provided 12 minutes ofkey, each SIGSALY had two turntables, with a second record "queued up" while the first was "playing".
The SIGSALY terminal was massive, consisting of 40 racks of equipment. It weighed over 50 tons, and used about 30kW of power, necessitating an air-conditioned room to hold it. Too big and cumbersome for general use, it was only used for the highest level of voice communications.[5]
A dozen SIGSALY terminal installations were eventually set up all over the world. The first was installed in thePentagon building rather than the White House, which had anextension line, as the USPresident Franklin Roosevelt knew of the British Prime MinisterWinston Churchill's insistence that he be able to call at any time of the day or night. The second was installed 60 metres (200 ft) below street level in the basement ofSelfridges department store onOxford Street,London, close to theUS Embassy onGrosvenor Square. The first conference took place on 15 July 1943, and it was used by both GeneralDwight D. Eisenhower as the commander ofSHAEF, and Churchill, before extensions were installed to the Embassy,10 Downing Street and theCabinet War Rooms.[1] One was installed in a ship and followed GeneralDouglas MacArthur during hisSouth Pacific campaigns. In total during WW2, the system supported about 3,000 high-level telephone conferences.
The installation and maintenance of all SIGSALY machines was undertaken by the specially formed and vetted members of the805th Signal Service Company of theUS Army Signal Corps. The system was cumbersome, but it worked very effectively. When the Allies invaded Germany, an investigative team discovered that the Germans had recorded significant amounts of traffic from the system, but had erroneously concluded that it was a complex telegraphic encoding system.[1][failed verification]
SIGSALY has been credited with a number of "firsts"; this list is taken from (Bennett, 1983):
