|
Banburismus was a cryptanalytic process developed by Alan Turing at Bletchley Park in England during the Second World War. It was used by Bletchley Park's Hut 8 to help break German ''Kriegsmarine'' (naval) messages enciphered on Enigma machines. The process used sequential conditional probability to infer information about the likely settings of the Enigma machine.〔Although this method is frequently stated to be an example of Bayesian inference, Donald Gilles has argued (), that the process is not really Bayesian, but rather Popperian.〕 It gave rise to Turing's invention of the ''ban'' as a measure of the weight of evidence in favour of a hypothesis. This concept was later applied in Turingery and all the other methods used for breaking the Lorenz cipher. The aim of Banburismus was to reduce the time required of the electromechanical Bombe machines by identifying the most likely right-hand and middle wheels of the Enigma.〔Mahon (1945) p. 17〕 Hut 8 performed the procedure continuously for two years, stopping only in 1943 when sufficient bombe time became readily available.〔Mahon (1945) p. 95〕 Banburismus was a development of the "clock method" invented by the Polish cryptanalyst Jerzy Różycki.〔Good (1993) p. 155〕 Hugh Alexander was regarded as the best of the Banburists. He and I. J. Good considered the process more an intellectual game than a job. It was "not easy enough to be trivial, but not difficult enough to cause a nervous breakdown".〔Good (1993) p. 157〕 ==History== In the first few months after arriving at Bletchley Park in September 1939, Alan Turing correctly deduced that the message-settings of Kriegsmarine Enigma signals were enciphered on a common ''Grundstellung'' (starting position of the rotors), and were then super-enciphered with a bigram and a trigram lookup table. These trigram tables were in a book called the ''Kenngruppenbuch (K book)''. However, without the bigram tables, Hut 8 were unable to start attacking the traffic.〔Alexander (''c.'' 1945) p. 94〕 A breakthrough was achieved after the ''Narvik pinch'' in which the disguised armed trawler ''Polares'', which was on its way to Narvik in Norway, was seized by HMS Griffin in the North Sea on 26 April 1940. The Germans did not have time to destroy all their cryptographic documents, and the captured material revealed the precise form of the indicating system, supplied the plugboard connections and ''Grundstellung'' for April 23 and 24 and the operators' log, which gave a long stretch of paired plaintext and enciphered message for the 25th and 26th.〔Mahon (1945) p. 22〕 The bigram tables themselves were not part of the capture, but Hut 8 were able to use the settings-lists to read retrospectively, all the Kriegsmarine traffic that had been intercepted from 22 to 27 April. This allowed them do a partial reconstruction of the bigram tables and start the first attempt to use Banburismus to attack Kriegsmarine traffic, from 30 April onwards. Eligible days were those where at least 200 messages were received and for which the partial bigram-tables deciphered the indicators. The first day to be broken was 8 May 1940, thereafter celebrated as "Foss's Day" in honour of Hugh Foss, the cryptanalyst who achieved the feat. This task took until November that year, by which time the intelligence was very out of date, but it did show that Banburismus could work. It also allowed much more of the bigram tables to be reconstructed, which in turn allowed April 14 and June 26 to be broken. However, the Kriegsmarine had changed the bigram tables on 1 July.〔Mahon (1945) p. 26〕 By the end of 1940, much of the theory of the Banburismus scoring system had been worked out. The ''First Lofoten pinch'' from the trawler ''Krebs'' on 3 March 1941 provided the complete keys for February - but no bigram tables or ''K book''. The consequent decrypts allowed the statistical scoring system to be refined so that Banburismus could become the standard procedure against Kriegsmarine Enigma until mid-1943.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Banburismus」の詳細全文を読む スポンサード リンク
|