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In cryptography, SIGSALY (also Green Hornet) was a telephone scrambler used in World War II for the highest-level Allied communications. The machine also pioneered digital communications, including the first transmission of speech using pulse code modulation.
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 pattern. The insecurity of most telephone scrambler schemes led the US Army to develop a more secure scrambler, based on the one-time pad principle, known as "SIGSALY".
The name SIGSALY was not an acronym; it was just a cover name that looks like an acronym -- the SIG part was common in Army Signal Corps names (eg, SIGABA). A prototype was developed by Bell Telephone Laboratories, better known as "Bell Labs", and demonstrated to the US Army. The prototype was called the "Green Hornet" after the popular radio show The Green Hornet, because it sounded like a buzzing hornet — resembling the show's theme tune — to anyone trying to eavesdrop on the conversation. 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 a random noise mask to encrypt voice conversations. The voice conversation was sampled for its amplitude once every 20 milliseconds, with 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 low amplitudes and narrower at high amplitudes. This scheme, known as "companding" for "compressing-expanding", exploits the fact that the fidelity of voice signals is more sensitive to high amplitudes than to low amplitudes.
A random value from the same set of six levels was subtracted from each sampled voice amplitude value to encrypt them before transmission. The subtraction was performed using modular 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. In any case, 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, added the random values back to them to decrypt them, and converted them into a voice waveform. 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. The noise values were originally produced by large mercury-vapor rectifying vacuum tubes and stored on a phonograph record. The record was then duplicated, with the records being distributed to SIGSALY systems on both ends of a conversation. The records were effectively the SIGSALY "one-time pad", and of course distribution was very strictly controlled.
The records were played on turntables, but since the timing 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 of key, 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 — too big and cumbersome for general use, so it was only used for the highest level of voice communications.
A dozen SIGSALY terminal installations were eventually set up all over the world. One was installed in a ship and followed General Douglas MacArthur during his South Pacific campaigns. It supported about 3,000 high-level telephone conferences. From a modern perspective, the whole scheme seems like a combination of the brilliant and primitive, but it worked very effectively.
SIGSALY has been credited with a number of "firsts"; this list is taken from (Bennett, 1983):
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