Background:
The German Enigma is surely the best known of the WW2 cipher machines used by either side in the conflict. Invented in 1918, it was developed as both a commercial and military encipherment system before and during the war. Enigma is an electro-mechanical device that utilizes a stepping wheel system to 'scramble' a plaintext message to produce ciphertext via polyalphabetic substitution. Potentially, the number of ciphertext alphabets is astronomically large - a fact that led the German military authorities to believe, wrongly as it turned out, in the absolute security of this cipher system.

Enigma - principle:
Enigma's output is a very complex polyalphabetic substitution ciphertext. The 3-wheel Service Enigma consists of a number of components. A message to be enciphered is input from a keyboard - QWERTZUIO layout. The signal leaves the keyboard and passes through a plugboard where, if the plugboard socket contains a connector, its identity is switched in a monoalphabetic substitution. If the particular socket does not contain a plug, the identity of the input character is unchanged. The plugboard substitution is reciprocal - i.e. if A is switched to Z, then Z is switched to A, a weakness that was to be exploited by Allied cryptanalysts.

From the plugboard the signal then passes to the entry stator which passes it to the first of a series of three wheels. Each of these has twenty-six contacts on each of its faces, cross-wired in a random fashion so that the identity of an incoming character is changed three times as it passes through the three wheels, which are in electrical contact, each with its adjacent companion. With each keyboard input the extreme right-hand wheel moves one position - before encipherment takes place. Additionally, once during a complete revolution of each wheel, the wheel to its left steps once.

After passing through all three wheels the signal reaches the reflector which performs two functions - it changes the signal's identity once again and also sends it back, in the reverse direction, through the three wheels to the entry stator. From here it passes back to the plugboard and then to the lampboard where a lamp corresponding to the now enciphered character is illuminated.

Because of the reflector's function in the encipherment process, no plaintext character can ever encipher to itself - another weakness in the system that was exploited to great effect by the Allied cryptanalysts.