In case multiple stream ciphers exist, I'm refering to this specific instance in which you generate a key that is just as long as the msg, M, as a function of a nonce and a smaller key K.
My textbook classifies this as computational secure. But why is that?
I would say that it was unconditionally secure since assuming the adversary is able to find a long key O_2 that when XOR'ed with the ciphertext produces a sensible M="sensible text", the adversary still has no clue whether that was the original message or not (it could have been the case the sender's actual msg was pure garbage).