If Alice encrypts a file with GnuPG software using the AES-256 mode, and sends the encrypted file over an insecure channel (like E-mail or HTTP) to Bob. Both Alice and Bob know the password, but only them.
1) What is the probability that Bob received a maliciously manipulated file that opens with the same password and is the same byte size,but assuming that only Alice and Bob know the password?
So what I am asking is that how likely it is that an adversary can recreate a similar sized file with let's say a malware in it, which would open with the same password, but them not knowing the password.
I assume here that the AES ciphertext is like a random oracle in theory so the bits would look random thus the attacker would have no way of replacing the plaintext of it with a malwared version, and create an similar sized ciphertext but without knowing the password, other than brute forcing every bit to the same output .
2) I am assuming serious brute forcing would be needed here, and there would be no easier way unless AES256 has some vulnerability. I also assume that there is a linear relationship between the increasing strenght of the password and the decreasing probability of this being possible. Would encrypting with a secret password known between 2 parties provide the same kind of authenticity as a SHA256 hash? In other words would it be just as hard to modify the ciphertext as to break SHA256 in this instance, or is AES256 more malleable?