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Part of an encrypted message would be the subject of inspired guesswork. For instance, U boats sent weather reports, and these had to take similar formats to make sense. Attempts were made to find evidence of such things in the encrypted material, the process was called filtering. Success meant that some kind of pattern was emerging and the number of possible meanings of the encrypted material was reduced. The bombe would be set to compute a series of remaining possibilities, matching rotor settings on an Enigma machine. (Photo Sept 2009).