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Alan Turing did not figure out the Enigma machine, Marian Rejewski did.
The critical parts of the Enigma machine were its rotors and the reflector. The interwiring of the rotors (and of course the number and which rotors were used) and the reflector controlled the enciphering of the Enigma machine.
The Polish Cipher Bureau became aware of a new German code in 1926 and set to work on it.
Without having access to an Enigma machine and only having access to enciphered messages, Rejewski was able to deduce the wiring of the rotors and the reflector; this was a huge intellectual accomplishment that is unfortunately little known today.
Thanks to Rejewski, Poland was able to read Enigma enciphered messages from 1932 to the outbreak of World War II.
The Polish Cipher Bureau provided all its information on the Enigma machine – a reconstruction of the Enigma machine, details on decryption techniques and “bombe” decryption machines – to French and British intelligence services in July 1939. Both France and Britain had made no headway on the Enigma based ciphers up to that point.
The cryptanalysts of the Cipher Bureau, including Rejewski, escaped to France at the outbreak of the war and continued their work on Enigma. With the fall of France, Rejewski fled again making a roundabout escape to Britain.
On arriving in Britain, Rejewski was inducted into the Polish Army and was set to work on low level German ciphers. He and other Polish cryptographers were not allowed to work on Enigma at Bletchley Park, a senseless waste of their talents.
So what contribution to breaking Enigma did Alan Turing make?
Knowing all the internal details of Enigma does not mean any given message can be readily decrypted. If the initial settings of the rotors and plug board are not known, there were 100,000,000,000,000,000,000,000 possibilities of initial settings to check.
Polish cryptanalysts had invented techniques and machines – “bombes” – to automate the search for the initial settings but these had proved increasing inefficient as Germany increased the complexity of the Enigma machine.
Turing’s great contribution to breaking Enigma was the design of an improved “bombe” to search for the daily Enigma settings.
Unlike the mechanical Polish bombe, the Turing-Welchman bombe, developed by Turing and Gordon Welchman, was electro-mechanical and far faster than the mechanical bombes.
The Turing-Welchman bombes, followed by many improvements in design and speed, reduced the time to decrypt Enigma messages to hours instead of days or weeks making Enigma decrypts much more valuable.
Turing also devised a number of statistical tests to reduce the number of initial settings checked by bombes, thus making Enigma decryption more efficient.
Alan Turing has become something of an icon for his war time decryption contributions and his subsequent persecution by British authorities and has overshadowed the contributions of many others, including Marian Rejewski. The stories of the Polish cryptographers are no less compelling.
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