Enigma was the name of an electromechanical cipher encryption and decryption machine used by the Wehrmacht, the Luftwaffe, the German Navy, the Abwehr (German secret service), and the Schutzstaffel (SS), as well as the German state railway system during World War II. The original basic design had been patented in 1919 by H. A. Koch, a Dutch inventor, and was modified and refined by a German engineer, Arthur Scherbius, in 1923. The German Army and Navy bought all rights to the machine from Scherbius in 1929, and by the outbreak of World War II, all the services mentioned were using various versions of it.

At the time of its invention, development, and use, the Enigma was the most complex encryption-decryption device in use by any nation. The basic Engima machine (and there were a number of more sophisticated variants) resembled a typewriter, but, in addition to a keyboard and type keys, it had a plug board, a light board, and a set of three rotors and half rotors (called "reflectors"). The rotors could be set independently to create a library of 16,900 (26 H 25 H 26) substitution alphabets, so that as long as the message was not longer than 16,900 characters, there would be no repeated use of a substitution alphabet within any given message. Since repetition is the traditional key by which codes are broken, it seemed to the Germans that the Enigma ciphers were inherently unbreakable. Moreover, the Enigma machine added additional complications. The sequence of alphabets used was different if the rotors were started in position ABC, as opposed to ACB; there was a rotating ring on each rotor that could be set in a different position. Additionally, the starting position of each rotor itself was variable. The military version of the Enigma added yet another device, a Stecker, or electric plugboard, by which some key assignments (depending on the model) could be changed. Thus, even the most basic three-wheel Enigma with six plug connections generated 3,283,883,513,796,974, 198,700,882,069,882,752,878,379,955, 261, 095, 623, 685,444,055,315,226,006, 433,616,627,409,666,933, 182,371,154,802,769,920,000, 000,000 coding positions--a staggering number. Of course, complex encryption is useless if it cannot be readily decrypted by the intended recipient. The genius of the Enigma machine was that its complex combination key could be communicated to a recipient by supplying just a few values: what rotors to use, the rotor order, the ring positions (within the rotors), the starting positions of each rotor, and the plugboard settings. The Germans were so confident of the Engima that it was used by every military echelon, from high command to tactical units, including aircraft, tanks, surface ships, and submarines. . .

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