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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; if a word appears in the plaintext but not in the book, it cannot be encoded. An alternative approach which gets around this problem is to replace individual letters rather than words. One such method, used in the second Beale cipher, substitutes the first letter of a word in the book with that word's position. In this case, the book cipher is properly a cipher — specifically, a homophonic substitution cipher. However, if used often, this technique has the side effect of creating a larger ciphertext (typically 4 to 6 digits being required to encipher each letter or syllable).
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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 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 (indicating 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.
Bible Cipher
The Bible is a widely available book that is almost always printed with chapter and verse markings making it easy to find a specific string of text within it, making it particularly useful for this purpose.
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
- A 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.
- In the Sherlock Holmes story, The Valley of Fear, the detective decrypts a message enciphered with a book cipher by deducing which book had been used as a key text.
- Graham Greene often has his protagonists use book codes. In The Human Factor, several books are used, and an edition of Charles Lamb's Tales from Shakespeare is used in Our Man in Havana.
- A book cipher plays an important role in the TV version of Sharpe's Sword. The key text is Voltaire's Candide.
- In the 2004 film National Treasure, an Ottendorf cipher is discovered on the back of the U. S. Declaration of Independence, using the "Silence Dogood" letters as the key text.
- The protagonists of the Matthew Reilly novel The Six Sacred Stones used a book cipher to send confidential messages to each other. The key text was the Harry Potter books, but the messages were sent via a Lord of the Rings forum to make the key text harder to identify.
- A book cipher was used in Runningflame’s 2008 Online Treasure Hunt; the key was the epilogue of one of the Warriors books by Erin Hunter.[1]
See also
References
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