Answer ( 1 )

  1. Basically, the bombe stepped through possible settings, eliminating contradictions.

    One of the fascinating weaknesses of the Enigma was that a letter could not be encrypted as itself. The cleartext letter A could be cypher letter B, C, D etc up to Z, but never A. Another was that the plugboard cross-connected up to ten pairs of letters. This vastly increased the number of combinations, but also meant that if you cross-connected, say, W and T, a cypher W would come out as T after the plugboard and a cypher T would come out as W after the plugboard.

    These features could lead to contradictions. The bombe looked for contradictory settings where for instance both a cypher A and a cypher T would come out as a W after the plugboard, and eliminated that setting.

    The trick was to build a menu for the bombe to work on. That required a lot of tricky work by cryptanalysists looking for loops (itself an interesting exercise, which I kinda half-understand). The better the menu, the more contradictions the bombe would find and eliminate.

    The bombe was set according to the menu, and then stepped through the settings one by one. When the bombe found a setting which didn’t have a contradiction, it stopped, thus alerting the operator that there was a candidate. The candidate would be analysed to see if it was a false stop. If it was a false stop, that was another piece of elimination to the menu. If it wasn’t, it was a candidate that would be analysed further.

    The majority of stops were false stops, and most of the candidates as well. So after noting the setting, the bombe would resume its work anyway.

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