SIGSALY – The Unbreakable Secure Speech System Used in WWII by the Allies

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. 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. For example, if the voice amplitude value was 3 and the random value was 5, then the subtraction would work as follows:

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)”.

Source: en.wikipedia.org