. Turing-Welchman bombe
A complete and working replica of a bombe at the National Codes Centre at Bletchley Park
Within weeks of arriving at Bletchley Park, Turing had specified an electromechanical machine that could help break Enigma more effectively than the Polish bomba Kryptologictna. from which its name was derived. The bombe, with an enhancement suggested by mathematician Gordon Welchman, became one of the primary tools, and the major automated one, used to attack Enigma-enciphered messages.
Jack Good opined:
Turing ' s most important contribution, I think, was of part of the design of the bombe, the cryptanalytic machine. He had the idea that you could use, in effect, a theorem in logic which sounds to the untrained ear rather absurd; namely that from a contradiction, you can deduce everything.
The bombe searched for possible correct settings used for an Enigma message( i. e. rotor order, rotor settings and plugboard settings), using a suitable crib: a fragment of probable plaintext.
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For each possible setting of the rotors( which had of the order of 10 states, or 10 for the four-rotor U-boat variant), the bombe performed a chain of logical deductions based on the crib, implemented electrically. The bombe detected when a contradiction had occurred, and ruled out that setting, moving on to the next. Most of the possible settings would cause contradictions and be discarded, leaving only a few to be investigated in detail. The first bombe was installed on 18 March 1940.
By the Autumn of 1941, Turing and his fellow cryptanalysts Gordon Welchman, Hugh Alexander, and Stuart Milner-Barry were frustrated. Building on the brilliant work of the Poles, they had set up a good working system for decrypting Enigma signals but they only had a few people and a few bombes so they did not have time to translate all the signals. In the summer they had had considerable success and shipping losses had fallen to under 100,000 tons a month but they were still on a knife-edge. They badly needed more resources to keep abreast of German adjustments. They had tried to get more people and fund more bombes through the proper channels but they were getting nowhere. Finally, breaking all the rules, on 28 October they wrote directly to Churchill spelling out their difficulties. They emphasised how small their need was compared with the vast expenditure
of men and money by the forces and compared with the level of assistance they could offer to the
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