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Frequency analysis is a very effective way to break substitution ciphers. However, with the methods I've seen, a lot of the work requires guesswork and intuition of a human, so it would be interesting to design a method without this.

In general, if I list out the frequencies of the letters in the cipher text, the first guess to make is that the frequencies line up with those in normal English (that the most common letter in the ciphertext corresponds to e, second most common to t, etc). But what is the second guess to make? What is the third? What is the nth? And how many would I need to make to be reasonably sure I've tried the right one?

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  • $\begingroup$ It is easy to write some programs to test the first candidates and see the result. If the text is long and similar to standard English, then breaking is not hard. Note: years ago I wrote such a program that lost in history. $\endgroup$
    – kelalaka
    Jan 2, 2021 at 21:56
  • $\begingroup$ Could you elaborate on what the first candidates are? Like which swaps should I make first? $\endgroup$ Jan 3, 2021 at 3:35
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    $\begingroup$ Select 3 or more top frequency on ciphertext and the real frequency than permute and check, that is. $\endgroup$
    – kelalaka
    Jan 3, 2021 at 10:01

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