Find decryption formula with already known encryption formula in block cipher operation mode context

Question

I have this formula for encryption phase of a block cipher mode of operation using AES or DES encoder:

So the question is how can find decryption formula for this. And what is the general workflow to solve problems like this?

Answer 1

There are two XOR operations here, one prior to encryption and one afterwards.

The decryption is the reversal of the encryption operation:

Which means you have to first XOR C1 with the same key (M0) to get the value to decrypt, and then XOR the decrypted value (with C0) to get the original plaintext value.

M1 = D(C1 ⊕ M0, K) ⊕ C0