¹There is a whole theory of redundancy of natural language that allows one to calculate a number called the “unicity distance” for a given cryptosystem. If a message is longer than the unicity distance, there is a high probability that it is the only meaningful message with a given ciphertext and hence can be recovered uniquely, as we were able to recover “BUY GUN” from the ciphertext “EXB JXW” in the example. See [Sti06, section 2.6] for more information on this interesting topic.