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The adversary clearly can do that. But if the adversary wins with this strategy, then the scheme in question cannot even be CPA secure and is far away from reaching the goal desired from CCA security. Recall, CCA security requires that even having access to a decryption oracle (for any ciphertext but the challenge ciphertext) does not help the adversary.


This specific hash function is weak; it appears that what this hash function does is pad out the string to be hashed into a 32 byte string, and then take the 8 4-byte substrings, and maps each substring individually into an individual byte. This immediately makes it trivial to find a preimage; start with a random 31 byte preimage (there appears to be a bug ...

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