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I have a question. If you receive a DES encrypted message for which you would NOT recognize the plaintext if it were decrypted, is there any way to potentially determine the key if all keys are likely? What about for AES?

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  • $\begingroup$ Depends... With authenticated modes of encryption it is easy to confirm the key and DES keys could be recovered (AES keyspace is too large). Even with e.g. CBC and a padded message it could be possible, if message length is known (7-8 byte DES padding is probably unique to a single key). $\endgroup$ – otus Oct 18 '15 at 18:32
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No. For any key $k$ your ciphertext $c$ has a corresponding plaintext $p$ (since block ciphers are families of bijections). Hence, since you can't (in)validate plaintexts, you can't discard any possible key.

Or, in other words: If someone claimed the right key was a certain $k$, you had absolutely no way to ever prove them wrong.

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  • $\begingroup$ Padding is missing from this answer. Usually it's considered part of the algorithm, if just in cryptographic API's. Otus also mentioned in the comment below the question. $\endgroup$ – Maarten Bodewes Dec 11 '16 at 23:19

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