Enigma

German enciphering machine of World War II and, by extension, the codes generated by it. The code was cracked by the British in spring 1940 and the Allies gained much useful intelligence as the Germans believed the code unbreakable.

The machine resembled an electric typewriter; operating a key sent a current through three rotors, the starting positions of which could be altered and ‘stepped’ at each keystroke. This meant that pressing the same key would not produce the same enciphered letter, making the resulting message proof against conventional code-breaking techniques.