From Academic Kids

Missing image
SIGSALY exhibit at the National Cryptologic Museum

In cryptography, 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; it was just a cover name that looks like an acronym -- the SIG part was common in Army Signal Corps names (eg, SIGABA). 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.



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 by Bell Telephone Laboratories, better known as "Bell Labs", 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 a random noise mask to encrypt voice conversations which had been encoded by a vocoder. The latter was used both to minimize the amount of redundancy (which is high in voice traffic), and also to reduce the amount of information to be encrypted.

The voice conversation was first encoded by the vocoder as:

  • ten low-frequency (less than 25 Hz) signals, giving the amplitude in ten separate frequency bands, which together covered the normal range of speech (250 Hz - 2,950 Hz));
  • 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 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. 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:

<math>3 - 5 \equiv -2 \equiv -2 + 6 \equiv 4\mod 6<math>

— 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:

<math>4 + 5 \equiv 9 \equiv 9 - 6 \equiv 3\mod 6<math>

— 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:

  • a white noise source (used for unvoiced sounds);
  • a signal generator (used for voiced sounds) generating a set of harmonics, with the base frequency conrolled by the pitch signal;
  • a switch, controlled by the voiced/unvoiced signal, to select one of these two as a source;
  • a set of filters (one for each band), all taking as input the same source (the source selected by the switch), along with amplifiers whose gain was controlled by the band amplitude signals.

The noise values used for the encryption key 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 distribution was very strictly controlled (although had one been seized, it would have been of little import, 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 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".


A SIGSALY terminal in 1943.
A SIGSALY terminal in 1943.

The SIGSALY terminal was massive. Consisting of 40 racks of equipment, it weighed over 50 tons, and used about 30 kW 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.

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. 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.


SIGSALY has been credited with a number of "firsts"; this list is taken from (Bennett, 1983):

  1. The first realization of enciphered telephony
  2. The first quantized speech transmission
  3. The first transmission of speech by Pulse code modulation (PCM)
  4. The first use of companded PCM
  5. The first examples of multilevel Frequency shift keying (FSK)
  6. The first useful realization of speech bandwidth compression
  7. The first use of FSK - FDM (Frequency Shift Keying-Frequency Division Multiplex) as a viable transmission method over a fading medium
  8. The first use of a multilevel "eye pattern" to adjust the sampling intervals (a new, and important, instrumentation technique)

See also

Further reading

  • M. D. Fagen (editor), National Service in War and Peace (1925-1975), Volume II of A History of Engineering and Science in the Bell System (Bell Telephone Laboratories, 1978) pp. 296-317


  • William R. Bennett, Fellow, IEEE, "Secret Telephony as a Historical Example of Spread-Spectrum Communications," IEEE Transactions on Communications, Vol. COM-31, No. 1, January 1983, 99.

External links

This article, or an earlier version of it, incorporates material from Greg Goebel's Codes, Ciphers, & Codebreaking (

Template:Cipher machines


Academic Kids Menu

  • Art and Cultures
    • Art (
    • Architecture (
    • Cultures (
    • Music (
    • Musical Instruments (
  • Biographies (
  • Clipart (
  • Geography (
    • Countries of the World (
    • Maps (
    • Flags (
    • Continents (
  • History (
    • Ancient Civilizations (
    • Industrial Revolution (
    • Middle Ages (
    • Prehistory (
    • Renaissance (
    • Timelines (
    • United States (
    • Wars (
    • World History (
  • Human Body (
  • Mathematics (
  • Reference (
  • Science (
    • Animals (
    • Aviation (
    • Dinosaurs (
    • Earth (
    • Inventions (
    • Physical Science (
    • Plants (
    • Scientists (
  • Social Studies (
    • Anthropology (
    • Economics (
    • Government (
    • Religion (
    • Holidays (
  • Space and Astronomy
    • Solar System (
    • Planets (
  • Sports (
  • Timelines (
  • Weather (
  • US States (


  • Home Page (
  • Contact Us (

  • Clip Art (
Personal tools