The US Army's Sigaba — also known as the ECM, or Electric Cipher Machine, in the Navy — was reputedly the only system used during World War II that remained completely unbroken by an enemy, according to the NSA. ("The Germans never broke it, and the Japanese gave up trying," according to the agency's special coverage of "big machines.")
Although the Sigaba utilized the German Enigma's principle of rotating, removable wired rotor wheels, its stepping motion was far more complicated, appearing random. "Ten of Sigaba's 20 rotors could be taken out and moved to a different position; the rotors could be placed in the machine either 'forward' or 'backward'; and any one or more of the rotors could move with each keystroke," according to the NSA.
Credit: National Security Agency
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