Book Cipher Decoder

This is a complete guide to book ciphers (also called book codes) and the tools you need to decode them. A book cipher consists of numbers and a book or text that is used to translate the numbers to words or letters.

• Book Cipher Tool
• What is a Book Cipher?
• Examples

What is a Book Cipher?

In a book cipher, a message is translated into numbers using a specific book, dictionary or other text. The numbering system can vary, but typically it is based on page numbers, line numbers, word numbers or character numbers. The plaintext is translated letter by letter, or word by word, into numbers that represent each letter or word. The book or text therefore acts as an encryption key. It is required that both the sender and the receiver of a message use exactly the same book or text as key.

For example, let's use the Verse of the Rings (from Lord of the rings) as our key:

Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for Mortal Men doomed to die,
One for the Dark Lord on his dark throne,
In the Land of Mordor where the Shadows lie,
One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.

Let's say our translation uses row numbers and word numbers. Then this book code:

6:10 8:2 4:4 3:4

would be translated as:

Row number	Word number	Word
6	10	FIND
8	2	THE
4	4	DARK
3	4	MEN

Alternatively, instead of whole words, the book cipher could use just the first letter of each word. The example code would then translate to FTDM. The advantage of translating letter by letter is that you can encode many more different words.

A book cipher is an example of a homophonic substitution cipher, since the same word or letter can be encoded in different ways. For example, the word THE could have been translated into 1:4, 2:3 or any of the other places where it has been used. There is no need in a book cipher to keep using the same location for a word or letter. On the contrary, it is more secure to encode it in different ways.

Ottendorf cipher

An Ottendorf cipher is a book cipher consisting of three parts. Usually in one of these formats:

• page number - word number - letter number
• line number - word number - letter number

The Ottendorf cipher is presumably named after Major Nicholas Dietrich, Baron de Ottendorf who worked for the British, organising spies in the French and American camps.

Choosing a Book

The most important things when using a book cipher is the choice of book. The sender and receiver have to agree beforehand on exactly which book to use, even which exact edition. A spy operating in enemy territory would probably choose a book that would draw as little attention as possible if seen in their home. It is also an advantage if the book isn't too widely available, so that a cryptanalyst likely wouldn't possess it.

Examples

Book ciphers have been used frequently both for real secrecy as well as in popular culture for entertainment.

• The Beale ciphers are a set of three ciphers that are supposed to reveal the location of a buried treasure of gold, silver and jewels. Only the second cipher has been solved. It was discovered that the second cipher was a book cipher, using the United States Declaration of Independence as the key.
• The mysterious Cicada 3301 challenges have frequently used book ciphers. The clues as to which books were used have been disclosed through riddles and hidden codes. One example of the book used is Agrippa (A Book of the Dead).
• In the Sherlock Holmes story, The Valley of Fear, Sherlock manages to decrypt a book cipher by find out which book was used.
• In the 2004 film National Treasure, by Walt Disney, the treasure hunter and cryptologist Benjamin Frankling Gates discovers a book cipher written with invisible ink on the back of the US Declaration of Independence. The cipher key is the Silence Dogood letters written by Benjamin Franklin.