I've often heard/read that AES key sizes 256 & 192 would be weaker than 128 or not stronger as expected from the size increase, but I've never seen a proof. How does one proof the strength of a Rijndael key size?
If I understand correctly, the main effect of having the key size different from the block size is on the key expansion. The iterative algorithm is moving along the columns, but the key size determines the number of columns. Later, this expansion is used in the
addRoundKey() by taking sets that fit with the number of columns due to the block size.
I've been trying to understand, assuming this issue would be correct, from where this would come from.