Editing Alan Turing (section)

===Bombe===
Within weeks of arriving at Bletchley Park,<ref name="Copeland2006p378" /> Turing had specified an electromechanical machine called the [[bombe]], which could break Enigma more effectively than the Polish ''[[bomba (cryptography)|bomba kryptologiczna]]'', 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.<ref>{{Cite book |last= Welchman |first= Gordon |author-link= Gordon Welchman |orig-year= 1982 |year= 1997 |title= The Hut Six story: Breaking the Enigma codes |page= 81 |location= Cleobury Mortimer, England |publisher= M&M Baldwin |isbn= 978-0-947712-34-1 }}</ref>

[[File:Bombe-rebuild.jpg|thumbnail|right|A working replica of a [[bombe]] now at [[The National Museum of Computing]] on Bletchley Park]]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 (cryptanalysis)|crib]]'': a fragment of probable [[plaintext]]. For each possible setting of the rotors (which had on the order of 10<sup>19</sup> states, or 10<sup>22</sup> states for the four-rotor U-boat variant),<ref>Jack Good in "The Men Who Cracked Enigma", 2003: with his caveat: "if my memory is correct".</ref> the bombe performed a chain of logical deductions based on the crib, implemented [[Electromechanics|electromechanically]].<ref>{{cite web |url=https://www.tnmoc.org/bombe |title=The Turing-Welchman Bombe |website=The National Museum of Computing |access-date=18 March 2021 |archive-date=24 January 2022 |archive-url=https://web.archive.org/web/20220124200142/https://www.tnmoc.org/bombe |url-status=live }}</ref>

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. A contradiction would occur when an enciphered letter would be turned back into the same plaintext letter, which was impossible with the Enigma. The first bombe was installed on 18 March 1940.<ref>{{Harvnb|Oakley|2006|p=40/03B}}</ref>

==== Action This Day ====
{{main|Action This Day (memo)}}

By late 1941, Turing and his fellow cryptanalysts [[Gordon Welchman]], [[Conel Hugh O'Donel Alexander|Hugh Alexander]] and [[Stuart Milner-Barry]] were frustrated. Building on the [[Biuro Szyfrów#Gift to allies|work of the Poles]], they had set up a good working system for decrypting Enigma signals, but their limited staff and bombes meant they could not translate all the signals. In the summer, they had considerable success, and shipping losses had fallen to under 100,000 tons a month; however, 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 had failed.<ref name=":0" />

On 28 October they wrote directly to [[Winston Churchill]] explaining their difficulties, with Turing as the first named. 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 forces.<ref name=":0">{{Harvnb|Hodges|1983|p=218}}</ref> As [[Andrew Hodges]], biographer of Turing, later wrote, "This letter had an electric effect."<ref name="Hodges 1983 221">{{Harvnb|Hodges|1983|p=221}}</ref> Churchill wrote a memo to [[Hastings Ismay, 1st Baron Ismay|General Ismay]], which read: "ACTION THIS DAY. Make sure they have all they want on extreme priority and report to me that this has been done." On 18 November, the chief of the secret service reported that every possible measure was being taken.<ref name="Hodges 1983 221"/> The cryptographers at Bletchley Park did not know of the Prime Minister's response, but as Milner-Barry recalled, "All that we did notice was that almost from that day the rough ways began miraculously to be made smooth."<ref>Copeland, ''The Essential Turing'', [http://www.maths.ed.ac.uk/~aar/turingletter.pdf pp. 336–337] {{Webarchive|url=https://web.archive.org/web/20150218142127/http://www.maths.ed.ac.uk/~aar/turingletter.pdf |date=18 February 2015 }}.</ref> More than two hundred bombes were in operation by the end of the war.<ref name=codebreaker>{{cite web | last1 = Copeland | first1 = Jack | last2 = Proudfoot | first2 = Diane | author-link = Jack Copeland | title = Alan Turing, Codebreaker and Computer Pioneer | url = http://www.alanturing.net/turing_archive/pages/Reference%20Articles/codebreaker.html | publisher = alanturing.net | date = May 2004 | access-date = 27 July 2007 | archive-url = https://web.archive.org/web/20070709065520/http://www.alanturing.net/turing_archive/pages/Reference%20Articles/codebreaker.html | archive-date = 9 July 2007 | url-status = live }}</ref>