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I know that Vigenère can broken using auto-correlation (as explained at Now my question is: Why does this also work with autokey (tested it using crypttool)?

To prove that I have already put work into this, I have read Simon Singh: “The Code Book”, F.L.Bauer: “Decrypted Secrets”. Though the latter book said in does not work the same way in autokey than it does in Vigenère. Why is there still a maximum of coincidences when the text is shifted by the key length?

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It works with autokey system because the key itself have a bias : in the key is much more E than other letters (this is a natural consequence of using a clear text as key).

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How does that result in a maximum of correlations when shifting by the length of the key? – joz Nov 28 at 16:57

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