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).