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). The prototype was called the "Green Hornet" after the 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.
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. 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).
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. 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. A cryptographic 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 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:
3 − 5 ≡ − 2 , − 2 + 6 ≡ 4 ( mod 6 ) {\displaystyle 3-5\equiv -2,\ -2+6\equiv 4{\pmod {6}}\,}
— 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:
4 + 5 ≡ 9 , 9 − 6 ≡ 3 ( mod 6 ) {\displaystyle 4+5\equiv 9,\ 9-6\equiv 3{\pmod {6}}\,}
— 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:
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