before the ciphertext can be decrypted. If we can securely transmit the pad, why not simply transmit the plaintext by the same means and do away with the encryption?
Below, we'll see an historical example, where it actually did make sense to use a one‐time pad—in spite of its limitations. However, for modern high data‐rate systems, a one‐time pad cipher would generally be impractical.
Why is it that the one‐time pad can only be used once? Suppose we have two plaintext messages
and we see that the key has disappeared from the problem. In this case, the ciphertext does yield some information about the underlying plaintext. Another way to see this is to consider an exhaustive key search. If the pad is only used once, then the attacker has no way to know whether the guessed key is correct or not. But if two messages are in depth, for the correct key, both putative plaintexts must make sense. This provides the attacker with a means to distinguish the correct key from incorrect guesses. The problem only gets worse (or better, from a cryptanalyst's perspective) the more times the key is reused.
Let's consider an example of one‐time pad encryptions that are in depth. Using the same bit encoding as in Table 2.1, suppose we have
and both are encrypted with the same key
and
If Trudy the cryptanalyst knows that the messages are in depth, she immediately sees that the second and fourth letters of
and she can then use this
Since this
2.3.6 Codebook Cipher
A classic codebook cipher is, literally, a dictionary‐like book containing (plaintext) words and their corresponding (ciphertext) codewords. To encrypt a word, the cipher clerk would simply look it up in the codebook and replace it with the corresponding codeword. Decryption, using the inverse codebook, is equally straightforward. Below, we briefly discuss the Zimmermann Telegram, which is surely the most infamous use of a codebook cipher in history.
The security of a classic codebook cipher depends primarily on the physical security of the book itself. That is, the book must be protected from capture by the enemy. In addition, statistical attacks analogous to those used to break a simple substitution cipher apply to codebooks, although the amount of data required is much larger. The reason that a statistical attack on a codebook is more difficult is due to the fact that the size
