By Dan Girard


Image "Turing Bombe Menu - Rasch Message"
Bombe Menu
In a 1995 letter to the editors of the journal Cryptologia1,  Ralph Erskine included a transcription of a page from a Royal Navy HF/DF operator’s report that he had found in the U.K. National Archives. The HF/DF report had been appended to a report dated 25th November, 1942, by the Commanding Officer of the Royal Navy destroyer H.M.S. Hurricane of Escort Group B1, concerning their just-completed escort of Convoy HX 215. The page gave the texts of three radio messages that the HF/DF operator had intercepted during that crossing, on the 19th and 21st November. It was evident that the three intercepts, judging by their formats and by the circumstances of their interception, were almost certainly German Navy messages, probably sent to or from U-boats, and enciphered with the four-wheel Model M4 Enigma machine.

I began trying to break these messages about ten years ago; but in 2006 Stefan Krah’s distributed-computing  M4 Project made the first two breaks, on the second and third of the intercepts. Since then, I’ve continued trying to break the remaining message, until some recent help from Michael Hörenberg finally enabled me to do it. I’ve already given the solution in a previous article; this article will explain the method used for the break, and also give some additional background and analysis.

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1Ralph Erskine, letter to the editor, Cryptologia Oct. 1996 Vol. XX, No.4