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Does there exist a block cipher which provides security against ciphertext only attacks, which has an unconditional security proof guaranteeing this is true? I know the existence of a provably secure block cipher with unconditional proof is not yet known, but I am unsure if there exists a block cipher that is provably secure against just ciphertext only attacks.

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    $\begingroup$ ciphertext only attack doesn't provide information to test except if you know some knowledge about the plaintext beforehand like it is ASCII. In which case in a text, one know some bits are zero. The ciphertext only attack of Siegenthaler actually used this fact to execute a correlation attack on combined LFSRs. $\endgroup$ – kelalaka May 30 at 15:35
  • $\begingroup$ Also, a block cipher is a primitive, like AES. The COA, CPA, CCA, CCA2, CCA3 are considered with a mode of operation. Which mode do you consider? $\endgroup$ – kelalaka May 30 at 17:42
  • $\begingroup$ @kelalaka any mode of operation, I am really just wondering about existence $\endgroup$ – GEG May 30 at 18:41
  • $\begingroup$ Actually I'm not even sure whether there is a formal model capturing what a "ciphertext-only" attack means beyond the standard models of semantic security or IND-EAV security where you only see 1 ciphertext. I think that if you can see an arbitrary amount of ciphertext (even when not knowing the underlying messages), perfect security (aka an advantage of 0) is probably impossible similar to the standard perfect secrecy argument. $\endgroup$ – SEJPM May 31 at 11:34

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