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. For messages longer than the unicity distance, there is a high probability that they are 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. The interested reader is referred to Stinson’s book, Cryptography: Theory and Practice,Second Edition, 2nd edition for more information on this interesting topic.