Speaking of patterns, this reminds me of the seminar work I wrote in school over the Enigma cipher and how to break it. I've always been a bit of a history buff so "how to break Enigma with at most Colossus at your disposal" was a nice topic
It's a really fascinating example of a fairly strong cipher (for the time) that was totally screwed up by incompetent users. The Germans systematically reused codebooks without understanding that this gave a huge edge to the guys at Bletchley Park.
Also, they could be goaded into sending predictable messages -- for example, the Royal Navy would set up mines somewhere and just wait for the U-boats to send/receive a message with the string MINEN in it. The U-boat weather report was also in a known format. You just took your own weather data, used that as plaintext and there you had the day's codes.
And of course, there was the unfortunate "Heil Hitler" as part of many messages...
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