I stumbled across a question while studying about MAC,
I was thinking if the attacker knows the key and the MAC and he wants to change the message to send to the victim, then how many blocks can he change with whatever he wants?
I think that he is unable to change the last plaintext block as the MAC will depend on it and to ensure that he has to generate a fixed cipher text for second last plaintext block so that he can force the last plain text to be exactly what he needs. So I guess he can modify all blocks freely except the last two.
The second option which I was thinking is that as the attacker can modify the plaintext, what if leaves the plaintext of the last block exactly same?
I believe that though MAC depends on all blocks, but as its value is only decided by the final plain text, we can keep the final plain text as the original message.
Which interpretation is correct? Or am I completely wrong?