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The result of the cipher should be indistinguishable from random, but can it happen that cipher produces the ciphertext that is equal to the plaintext as it also lies in the corresponding space? I guess it's just the probability is extremely low, probably equal to guessing the clear text on the first attempt, but just out of curiosity: is it theoretically possible and/or happened in real life?

The same goes to other types of cipher and e.g. hash function on small inputs.

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  • $\begingroup$ The question differs but the accepted answer should also answer your question. Let me know if you disagree. $\endgroup$ – otus Aug 31 '15 at 7:45
  • $\begingroup$ That's a bit hard to find that question by this query :) $\endgroup$ – Ilya Chernomordik Aug 31 '15 at 7:51
  • $\begingroup$ Good point, I'll edit it to be more search friendly. $\endgroup$ – otus Aug 31 '15 at 7:53
  • $\begingroup$ I did not quite find/understand the answer to my question there... $\endgroup$ – Ilya Chernomordik Aug 31 '15 at 7:53
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    $\begingroup$ "The probability that a random permutation has no fixed points is 1/e≈0.37" so they are likely for a given key. This question deals more with real life. Please edit the question to ask specifically what is missing or you don't understand. $\endgroup$ – otus Aug 31 '15 at 7:58

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