SIGSALY — The Encrypted Speech System
Twelve rooms, 50 tons, two synchronized phonograph records — the system that kept Churchill's phone calls secret from Hitler.
Why This Matters
SIGSALY was the world's first secure voice communication system and the first operational digital voice to be transmitted over radio. Bell Laboratories engineers — including Claude Shannon, who used his SIGSALY work as the foundation for his 1945 classified paper "A Mathematical Theory of Cryptography" (later the basis of information theory) — designed a system that digitized speech using 6-level pulse-code modulation, encrypted each sample with a random digit from a vinyl phonograph record, and transmitted the encrypted digital signal. An identical phonograph record, synchronized within milliseconds, decrypted at the receiving end. Because the noise levels on the phonograph records were genuinely random, the system had the same theoretical security as a written one-time pad.
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 Germany's Forschungsamt intercept service. 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 converted speech to digital form using 6-level quantization at 50 samples per second — the first operational PCM voice digitization in history. Each 2-bit sample was added modulo 6 to a random digit from a specially recorded vinyl phonograph record. The encrypted samples were transmitted as frequency-shift keying on shortwave radio. At the receiver, an identical phonograph record played in exact synchronization and the subtraction was performed, recovering the original PCM stream. The recovered PCM was then converted back to analog speech through a vocoder (voice coder) — also a SIGSALY invention.
Claude Shannon worked on SIGSALY at Bell Labs from 1941. His classified 1945 paper "A Mathematical Theory of Cryptography" — which proved that the one-time pad is the only provably perfect cipher — was directly motivated by SIGSALY's design. Shannon's 1948 paper "A Mathematical Theory of Communication," the foundation of information theory, used the same mathematical framework. The world's greatest theoretical contribution to communications security came directly from the problem of keeping Churchill's phone calls private.
Twelve SIGSALY terminals were installed worldwide: the Pentagon, 10 Downing Street, SHAEF headquarters, Algiers (for Eisenhower), Brisbane (MacArthur), Manila, Guam, and others. Each terminal weighed approximately 50 tons and occupied 12 large rooms. The matched phonograph records were physically carried to both terminals and played exactly once before being destroyed. When Churchill called Roosevelt, the records at both ends were started simultaneously; if synchronization drifted, the call could not continue. The record pairs were produced at a single Bell Labs facility and shipped under armed guard.
| Development | Bell Laboratories (Claude Shannon, et al.), 1941–1943 |
| Security | Theoretically unbreakable — voice one-time pad |
| Machine size | 50 tons; 40+ racks of vacuum tubes; 12 rooms |
| Synchronization | Matched vinyl phonograph records as key material |
| Lines installed | Washington D.C., London, Paris, Algiers, Brisbane, Guam, SHAPE HQ |
| Modern Lesson | The first practical secure voice communication predates digital age by 50 years |