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Catalogue reference: HW 25/3; extracts from Turing's notes on the Enigma Machine, c.1939-42 (link to an enlarged view)
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Extracts from Turing’s notes on the Enigma Machine, c.1939-42
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(Catalogue reference: HW 25/3) transcript
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Cracking the Enigma code
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A basic flaw in the design of Enigma was that no letter of the alphabet could ever be represented by itself once enciphered - a could never be a, z could never be z. Together with errors in sent messages and the German habit of using standard phrases at the start of each communication, this allowed cryptanalysts to establish informed ‘guesses’. Nevertheless, without a swift mechanical deciphering system, Enigma’s 150,000,000,000,000,000,000 combinations were practically unbreakable.

Alan Turing (1912-54) was the leading mathematician at Bletchley Park. He’d been recruited from Cambridge University where he had created the pioneering Turing Machine, a forerunner of the modern computer able to perform calculations at astonishing speed.

Turing and Gordon Welchman, a fellow Cambridge mathematician, set to work improving a Polish machine, Bomba, which was built before the war to crack earlier versions of Enigma. The new Turing-Welchman Bombe was perfected in 1940 and they began deciphering Luftwaffe communications. Once set in motion, it would search through all possible Enigma rotor variations until the right combination was found. The complex Enigma methods used in German Naval communications were cracked in 1941, then again in 1943 (after the Navy had introduced extra components to the system).
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Enigma Colossus
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