Cryptographic History of Work on the German Naval Enigma

these reasons were logically satisfactory they imbued those who held them with a determination that the problem should be solved and it is to the pertinacity and force that, in utterly different ways, both of them showed that success was ultimately due.

11. Turing first got interested in the problem for the quite typical reason that "no one else was doing anything about it and I could have it to myself". He started where the Poles left off and set to work to discover how the indicating system worked using the information provided by the 100 or so messages in the period May 1st - 8th 1937, whose starting positions were known.

12. The successful conclusion of this initial task is described in "Prof's Book", Turing's write-up made in 1940 of Enigma theory at that time. The neatness and clarity of the reasoning makes it worth reading including a summary of his remarks even in a non-technical account.

13. On May 5th he had four messages with indicators and message setting as follows

Indicator. Message setting.

(1) The two EW's as 3rd bigrams in the indicators and the two JM's as 1st bigrams combined with the two V's as 3rd letters of the message setting and the two M's as 1st letters indicate strongly that the first bigram represents the first letter of the window position, the second and third bigrams the second and third letters.

(2) The message setting cannot be obtained merely by replacing the letters in the message setting by their bigram equivalents as in that case the fourth message with bigrams JM FE FE would presumably have the second and third letters of the message setting the same.


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