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Wikipedia: EnigmaWikipedia: Enigma
The heart of the machine consists of several connected rotors. A rotor is a flat round disk having 26 contacts on each side, arranged in a circular manner. Every contact on one side of the disk is wired to a contact on the other side in a particular way. Each rotor in an Enigma was wired differently than the others, and the German military models used different rotor wirings than the commercial models. The output contacts of one rotor are connected to the input contacts of the next rotor, but this connection is not rigid and allows for the rotors to rotate. Three rotors are connected in this manner; the third rotor is connected to a reflector which is hard wired to feed outputs of the third rotor back into different contacts of the third rotor, thence back to the first rotor, but by a different route.
The enigma has a typewriter?-like keyboard and 26 letter labeled lamps. If a key is pressed, current from a battery passes through the rotors, through the reflector and back through the rotors to light a lamp; the letter of the lamp that lights up is the encryption of the typed character. Another effect of pressing a key is that the first rotor rotates by one position. If it has completed one full rotation (26 positions), the second rotor will advance by one position. After the second rotor has advanced 26 times, the third rotor advances one position. Some Enigma models cause a step motion of the next rotor after other than a single full rotation of the preceding rotor. These models were not commonly used.
The machine is symmetrical in the sense that decryption? works in the same way as encryption?: type in the encrypted text and the sequence of lit lamps will correspond to the plain text. This only works if the decrypting machine has the same configuration as the encrypting machine (rotor sequence, wiring, ring settings, and initial positions); these changed regularly (daily and later much more often) and were specified in key schedules distributed to Enigma users. Furthermore, the encrypting operator would randomly choose a three letter 'rotor setting' for each message. These were sent as part of the message and the receiving operator was required to set his Enigma machine accordingly. How this was done changed througout the War; the first techniques used were a considerable help to Polish and later Allied cryptanalysts?.
The efforts to break the German code began in 1929 when the Poles intercepted an Enigma machine being shipped from Berlin and mistakenly not protected in diplomatic baggage. When the Germans began using modified Enigmas a few years later, the Poles attempted to 'break the system' by finding the wirings of the rotors in use and by finding a way to recover the settings used for particular messages. A young Polish mathematician, [Marian Rejewski]?, made one of the most signficant breakthroughs in cryptanalytic history by using fundamental mathematical and statistical techniques to find a way to do both. The Poles were able to build equivalents to the then current German version of the Enigma, and using them, to decrypt a large portion of German traffic for much of the 1930s. They received some assistance from the French, who had an agent in Berlin who had access to Enigma key schedules, manuals, etc. Rejewski's cryptanalytic breakthrough did not, however, depend on that information. However in 1939 the Germans increased the complexity of their Enigmas and the Poles, realizing time was against them, gave copies of all their achievements to the British. Other sources claim that in 1938 a Polish mechanic employed in a German factory producing Enigma machines took notes of the components before being repatriated and, with the help of the British and French secret services, constructed a wooden mockup of the machine. In 1939, the Poles decided to share their work and passed to the French and the British some of their ersatz 'Enigmas', information on Rejeski's breakthrough, and on the other techniques they had developed. The information was shipped to France in diplomatic baggage and the British share on to Bletchley Park in England.
With this massive boost the British began to work on the codes themselves. Early in 1939 Britain's secret service set up its [Code and Cypher School]? at Bletchley Park, 50 miles (80 km) north of London, for the purpose of intercepting the Germans' Enigma signals and controlling the distribution of the resultant secret information. There, British mathematicians and cryptographers?, among them Alan Turing, conquered the problems of many of the German Enigma variations and found means of cracking the ciphers. The information so produced was eventually termed 'Ultra'. Strict rules were established to restrict the number of people who knew about the existence of the Ultra information and to ensure that no actions would alert the Axis? powers that the Allies possessed knowledge of their plans.