SIGSALY

The system entered service in 1943 under the cover name X-System and was known in practice as the Green Hornet, after the buzzing the Germans heard when they intercepted it. The voice first went through a vocoder of Homer Dudley's design: instead of sending the waveform it sent measurements of how much energy sat in each of ten frequency bands, plus the pitch. Those values were quantised to six levels. Speech had thereby been turned into numbers — the first practical pulse-code modulation of voice, seven years before PCM became ordinary in telephone networks.

The encryption was a genuine one-time pad. The key was thermal noise from mercury-vapour rectifier tubes, recorded onto gramophone records in two identical pressings, one for each end. The records were played in lockstep, held in time by the WWV time signal, and each key value was added modulo six to the speech value. One record lasted twelve minutes and was destroyed afterwards. Without the physical disc there was nothing to analyse — and nobody has ever shown the system was broken.

Each terminal weighed over fifty tons, drew thirty kilowatts and needed its own air conditioning; twelve were built, from a Pentagon basement to Guam. Alan Turing reviewed the design during a Bell Labs visit over the winter of 1942–43. The system was retired in 1946 and stayed classified until 1976, after which its list of firsts could be written into telecommunications history retroactively — thirty years too late to influence any of it.