SIGSALY — The Encrypted Speech System
About 50 tons of equipment and matched key records — a pioneering system for secure Allied voice communication.
Why This Matters
SIGSALY was a pioneering secure digital voice system developed at Bell Laboratories. A vocoder represented speech by changing parameters, which were quantised and combined with matched key material. The one-time-pad principle required the random key values to remain secret and never be reused.
By mid-1942, both Roosevelt and Churchill knew that AT&T's A-3 scrambler system — which they had been using for transatlantic phone calls — had been broken by a Deutsche Reichspost research station. German intelligence was reading their phone calls. Bell Labs was commissioned to build a replacement at the highest urgency. The resulting SIGSALY system was operational by July 1943, just in time for the buildup to Operation Overlord planning.
SIGSALY quantised vocoder parameters, including information about the speech spectrum and pitch, into discrete levels. The commonly quoted 50-per-second rate concerns these parameters, not direct sampling of an audio waveform at 50 Hz. Modular addition and subtraction with synchronised key records concealed and recovered the parameter stream.
Shannon’s wartime cryptographic work led to the published 1949 paper Communication Theory of Secrecy Systems. It formalised perfect secrecy and analysed key requirements. It did not prove that the one-time pad was the only possible perfectly secret cipher; his separate 1948 communication-theory paper should not be conflated with it.
A network of twelve SIGSALY terminals served high-level Allied communications. The number twelve refers to terminals, not rooms per installation. Each terminal needed substantial equipment and power, and the communicating ends needed matching, synchronised key records.
| Development | Bell Laboratories (Claude Shannon, et al.), 1941–1943 |
| Trust | Historical Only |
| Machine size | About 50 tons; roughly 40 racks of equipment per terminal |
| Synchronization | Matched vinyl phonograph records as key material |
| Lines installed | Washington D.C., London, Paris, Algiers, Brisbane, Guam, SHAEF HQ |
| Modern Lesson | An early operational use of digital speech parameters and one-time keys |