Scrambler - Noise

Noise

The first voice scramblers were invented at Bell Labs in the period just before World War II. These sets consisted of electronics that could mix two signals, or alternately "subtract" one signal back out again. The two signals were provided by telephones for one, and a record player for the other. Sets of matching pairs of records were produced containing recordings of noise, which would then be played into the telephone and the mixed signal sent over the wires. The noise would then be subtracted back out at the far end using the matching record, leaving the original voice signal intact. Eavesdroppers would hear only the noisy signal, unable to understand the voice inside.

One of those, used (among other duties) for telephone conversations between Winston Churchill and Franklin D. Roosevelt was intercepted and unscrambled by the Germans. At least one German engineer had worked at Bell Labs before the war and came up with a way to break them. Later versions were sufficiently different that the German team was unable to unscramble them. Early versions were known as "A-3" (from AT&T Corporation). An unrelated device called SIGSALY was used for higher-level voice communications.

The noise was provided on large shellac phonograph records made in pairs, shipped as needed, and destroyed after use. This worked, but was enormously awkward. Just achieving synchronization of the two records proved difficult. Post-war electronics made such systems much easier to work with by creating pseudo-random noise based on a short input tone. In use, the caller would play a tone into the phone, and both scrambler units would then listen to the signal and synchronize to it. This provided limited security, however, as any listener with a basic knowledge of the electronic circuitry could often produce a machine of similar-enough settings to break into the communications.