Enigma

An electromechanical cipher machine, patented by Arthur Scherbius in 1918 and adopted by the German armed forces in the 1930s. Every keypress advanced at least one rotor, so the same letter encrypted differently each time. Counting the plugboard, there were more possible key settings than could be tried in a human lifetime.

Poland's Biuro Szyfrów broke early versions as far back as 1932 and handed their work to the British in 1939. At Bletchley Park, Alan Turing built the bombe, a machine that mechanically ruled out impossible settings. The decisive flaw was the reflector: it meant no letter could ever encrypt to itself, which combined with predictable messages such as daily weather reports gave the attack the known plaintext it needed.