SIGSALY was an unbreakable voice scrambler that relied on early digital techniques, pretty interesting: https://spectrum.ieee.org/geek-life/hands-on/rebuilding-

SIGSALY was an unbreakable voice scrambler that relied on early digital techniques, pretty interesting: https://spectrum.ieee.org/geek-life/hands-on/rebuilding-a-piece-of-the-first-digital-voice-scrambler https://spectrum.ieee.org/geek-life/hands-on/rebuilding-a-pi... IIRC, it was unbreakable because it amounted to a one-time-pad that was shared between the transmitter and receiver.