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A book cipher is a cipher in which the key is the identity of a book or other piece of text. It is generally essential that both correspondents not only have the same book, but the same edition.

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 codes.

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).

Contents 1 Dictionary 2 Security 3 Examples 4 See also 5 References

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 George Scovell for the Duke of Wellington's army in some campaigns of the Peninsular War. In Scovell's method, a codeword would consist of a number (indicting the page of the dictionary), a letter (indicating the column on the page), and finally a number indicating which entry of the column was meant. However, this approach also has a disadvantage: because entries are arranged in alphabetical order, so are the code numbers. This can give strong hints to the cryptanalyst unless the message is superenciphered.

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.

• Perhaps the most famous use of a book cipher is in the Beale ciphers, of which document no. 2 uses a (modified version of) the United States Declaration of Independence as the key text.
• In the American Revolution, Benedict Arnold used a book cipher, sometimes known as the Arnold Cipher, which used Sir William Blackstone's Commentaries on the Laws of England as a key text.
• Simon Singh's book on cryptography The Code Book contained a set of ten "cipher challenges" in supposedly increasing order of difficulty, starting with simple substitution ciphers and ending with a truncated-key (48-bit) version of DES and a 512-bit RSA-encrypted message. A book cipher example was only fifth on the list, but it turned out to be the most difficult of the ten to cryptanalyze.^[1]
• In the Sherlock Holmes story, The Valley of Fear, the detective decrypts a message encyphered with a book cypher by deducing which book had been used as a key text.

• Running key cipher

1. ^ http://simonsingh.com/Cipher_Challenge.html

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