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What a cute observation!

If there's any way for the client to influence the input, it may be prone to DoS attacks: By my calculations, with a million random attempts, you would expect to find a cycle of length at least 435, which is over 13x the average. (Mind you, multiplying the number of attempts by 10 only adds about 72.5 to the expected cycle length, and probably no one has the patience to try more than 100 billion or so attempts.)




The properties of the permutation are dependent upon the encryption key, so a client being able to select malicious inputs to get long cycles implies either that the client knows the AES key, or that the client has broken AES.

In any case, as I mentioned in a sibling comment, with 3 AES encryptions one can construct a 122-bit balanced Feistel cipher with a constant amount of work.




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