DES/AES invertibility My professor has posted a couple of practice questions that so far I haven't been able to find the answer for and I was hoping you could help.

DES  would  remain  invertible—it  would  still  be  ablockcipher—even if its S-boxes were arbitrarily changed (the number of input and outputbits remaining the same).
AES  would  remain  invertible—it  would  still  be  ablockcipher—even if its S-boxes were arbitrarily changed (the number of input and outputbits remaining the same).

I've been trying to figure out the &quot;how&quot; and &quot;why&quot;, mostly.
DES is a Feistel cipher where the round function doesn't need to be invertible. Therefore, when designing a Feistel cipher you will have invertible and non-invertible S-box options.

In contrast, AES is a substitution-permutation network where the structure is completely different. If AES operations are not invertible, you will not be able to decrypt anything. So, the second claim is wrong.