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The Enigma Machine was patented in 1919 and was adopted by the German military services from the mid-20s onwards, although the machine and the mode of its use was continually refined so variant models of the machine for different purposes and services proliferated during the war. The function of the machine was simply to substitute an inputted letter in a manner that would make it as hard as possible to work out what the original letter had been. Its basic mechanism can be fairly easily understood. The user takes the first letter of his plain text message and keys it into the keyboard (“G”, for example). Pressing the key completes an electrical circuit and lights up one of the letter lamps on the lamp-board (e.g. "H"): the complexity comes in the passage of the electrical signal. The signal travels first to the plugboard, which has a plug for each letter so if the machine is configured with a cable on the input letter then the signal is diverted (“G” here being substituted for “L”). Then the signal travels up to the rotors. There are three moveable rotor wheels (each of which has 26 positions) on the standard army Engima, together with a reflector on the far left. As the signal passes through each wheel it goes through another substitution and the reflector both enacts another substitution and then sends the signal back through the rotors a second time, so in this case the “L” is transformed seven times before coming as an output which finally lights the lamp at “H”, which is then written down by the operator as the first letter of their enciphered message. Pressing a key also mechanically pushes at least one of the rotor wheels (and possibly more, depending on the configuration) on one step, so if “G” was pressed a second time the signal would reach a different light. Furthermore, not only could the plugboard be configured in different way, and not only could each of the individual wheels begin a message at any one of their 26 settings, but the wheels were themselves removable and could be put in any order or exchanged for different wheels altogether (the standard army Enigma had a choice of five wheels). The sequence of wheels was changed daily. So whilst the machine would always perform the same conversion with the same setting, overall there were a total number of 159 million million million possible daily keys.
The Enigma machine set a challenge that was answered by the remarkable team at Bletchley Park, whose achievements provide one of the most compelling stories of World War II. Breaking Enigma was the work of many, including Polish cryptographers who had already begun to decript Enigma traffic before the war; naval forces who risked their lives capturing Enigma machines and code books; Alan Turing and other mathematicians with their revolutionary models for deciphering; Tommy Flowers and other mechanical geniuses who designed 'Colossus', the world's first programmable computer, at the GPO Research Centre at Dollis Hill in north west London; the hundreds of Wrens who operated the Bombes and, later, Colossus machines and made possible the daily decrypts. Their work saved countless lives and had an enormous impact on the submarine war in the Atlantic, the North African campaign, and the Normandy invasion; the work of Bletchley Park is often said to have shortened the war by two years. Furthermore, by coming to the understanding that to defeat Enigma it was necessary to mechanise much of the work of decryption, they helped to inaugurated the computer age.
Few Enigma machines survived the War intact: the Germans destroyed them as they retreated, and for decades after the war governments around the world kept close control over Enigma technology (indeed two of Turing's wartime papers on cryptography remained classified until 2012). So secure was the system believed to be that some governments, unaware of the work of Bletchley Park, continued to use Enigmas after 1945.
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