Book Cipher

The Book cipher encryption consists of indexing a text by numbering from 1 to n each word. A letter (or a word) is coded by the number of the word beginning with that same letter (or the whole word).

Choose a well-defined text or book in order to have a precise word list and to facilitate decoding.

Example: Using the Declaration of Independence of the United States of America. To encode DCODE, therefore, the words DISSOLVE,COURSE,ONE,DECENT,EVENTS (with D,C,O,D,E as initials) can be taken (with their respective indexes 15,4,12,52,7) to describe a coded message.

Book cipher decryption consists in retrieving the word corresponding to the number and extracting the latter or only its first letter.

Example: For 221,132,136,305 the words are BY,OF,OF,KING or (take the first letters) BOOK.

Any sequence of 1 to n consecutive letters (or digits) (not interrupted by any other character) is considered as a word.

Example: ALICE'S ADVENTURES IN WONDERLAND contains 5 words: ALICE,S,ADVENTURES,IN,WONDERLAND

The message usually consists of several numbers, from a few tens to sometimes a thousand for the big books (but it is rare because it requires to count up to 1000 words of the book).

Without knowing the dictionary used it is impossible to decode this cipher.

Beale has proposed three cryptograms, one of which (the second one, called the Beale cipher) is coded by this principle. It uses precisely as a dictionary the declaration of independence of the United States of America.

The first traces of the book cipher are datet from the invention of printing.