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Traditionally book ciphers work by replacing words in the Plaintext of a message with the location of words from a book. In this mode, book ciphers are more properly called Code s. This can have problems as if a word appears in the plaintext that doesn't appear in the book then it can't be encoded. An alternative approach which gets around this problem is to replace individual letters rather than words, in which case the book cipher is properly a cipher — specifically, a Homophonic Substitution Cipher . However, if needed often, this has the side effect of creating a larger ciphertext (typically 4 to 6 digits being required to encipher each letter or syllable). DICTIONARY Another approach is to use a dictionary as the codebook. This guarantees that nearly all words will be found, and also makes it much easier to find a word when encoding. This approach was used by unless the message is Superenciphered . BIBLE CIPHER Bibles have various translations (i.e. King James, New International Standard, Etc.), word for word each is exact over many printings. However, many versions stemming from the King James version change the text by adding to or taking away from what was written. Since the Bible is a widely available book, it makes a convenient key. SECURITY Essentially, the code version of a "book cipher" is just like any other code, but one in which the trouble of preparing and distributing the Codebook has been eliminated by using an existing text. However this means that as well as being attacked by all the means employed against other codes, partial solutions may help the Cryptanalyst to guess other codewords, or even to completely break the code by identifying the key text. If used carefully, the cipher version is probably much stronger, because it acts as a homophonic cipher with an extremely large number of equivalents. However, this is at the cost of a very large ciphertext expansion. In the electronic era, both types are likely to fall easily to a sophisticated opponent, who may have available a large Digital Library which can be used to Brute-force Search many millions of possible key texts. EXAMPLES
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