One of the giveaways is that many transmissions ended with “Heil Hitler”, so that helped identify some sort of pattern. Another was that letters would never be encoded as themselves, which reduced the number of possible combinations. See https://www.scienceabc.com/innovation/cracking-the-uncrackable-how-did-alan-turing-and-his-team-crack-the-enigma-code.html.
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