In Chapter II we have seen that the cryptanalysis of a cryptosystem often depends on the structure that is present in most texts. For instance in Table 2.1 we could find the key 19, because BRUTUS was the only word in the table that made sense. This structure in the plaintext remains present in the ciphertext (it is hoped in a hidden form). If the extra information arising from this structure exceeds our uncertainty about the key, one may be able to determine the ciphertext!
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