Question

If I have an encrypted file that is encrypted with AES CBC, would changing a random byte somewhere in the file cause it so that it would no longer be able to be decrypted?

Is my understanding correct that everything up to the point where the byte was changed would decrypt okay, but from then on afterwards it wouldn’t decrypt?

Answer 1

That is not quite correct. AES encrypts/decrypts data in blocks (128-bit blocks, specifically). Additionally, in CBC mode, the encryption/decryption of the (i+1)th block depends on the
(i)th block.

So if the random byte falls within the ith block (let’s assume for simplicity that the byte doesn’t cross between two blocks), when you go to decrypt the ith block, it will give you the wrong decryption (i.e. a block of 128 bits will be incorrect). Additionally, since the next block was encrypted using the ith block, the (i+1)th block will also decrypt incorrectly (another 128 bits aka 16 bytes). From there, the subsequent blocks will be correct (as will all of the previous blocks).

For more info, I’d read about Modes of Encryption on wikipedia.

One more thing: changing the random byte will likely not prevent decryption from happening – it will just not yield the original plaintext (of course).

Hope that helps!