AES works when each plaintext block is arranged in a 4x4 array of bytes called states. State goes through 10 rounds of encryption consisting of 4 operations: bytesub, shiftrow, mix columns and add round key. I know how it all works but in preparation for my exam I have two questions which I don't understand.

How will the algorithm be affected if the addroundkey wasnt used in any round? Also in another scenario where In each round, in the Add Round Key, instead of computing a bitwise XOR between the state and the key, what would happen if we use a bitwise OR?

My theory is that if the addroundkey wasn't used, then attackers can easily reverse the operations because the symmetric key isn't required.


1 Answer 1


You are correct: if the AddRoundKey step is skipped, no key material is ever used, so decryption is trivial.

In case of using a bitwise OR, information would be lost in the encryption process. XOR is reversible, but OR isn't, so no decryption would be possible.


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