Your Cryptography

BOOK CIPHER [INTRODUCTION]

[INTRODUCTION]
A book cipher is a cipher in which the key is some aspect of a book or other piece of text; books being common and widely available in modern times, users of book ciphers take the position that the details of the key are sufficiently well hidden from attackers in practice. It is typically 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 the book being used. In this mode, book ciphers are more properly called codes.
[CHOOSING A RIGHT KEY]
The main strength of a book cipher is the key. The sender and receiver of encoded messages can agree to use any book or other publication available to both of them as the key to their cipher. Someone intercepting the message and attempting to decode it, unless they are a skilled cryptographer, must somehow identify the key from a huge number of possibilities available. In the context of
espionage, a book cipher has a considerable advantage for a spy in enemy territory.
[WIDELY USED PUBLICATIONS AS KEY]
- 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.
- 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; the widespread availability of concordances can ease the encoding process as well.
[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, as well as being attacked by all the usual means employed against other codes or ciphers, partial solutions may help the cryptanalyst to guess other codewords, or even to break the code completely 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.
[SAMPLES OF USAGE]
- A famous use of a book cipher is in the Beale ciphers, of which document no. 2 uses a (variant printing of) the United States Declaration of Independence as the key text.
- Book ciphers have consistently been used throughout the Cicada 3301 mystery.
-In Sherlock Holmes-
(*) In The Valley of Fear, Sherlock Holmes decrypts a message enciphered with a book cipher by deducing which book had been used as a key text.
(*) In the episode "The Blind Banker" of the BBC series Sherlock, Sherlock Holmes searches for a book that is the key to a cipher being used by Chinese Tong smugglers to communicate with their agents and with each other through graffiti messages.
[HOW TO]
(1) Choose a book.
For a book code, you will make the code by giving the location of words in a book. If you want to increase the chance that any word you need will be in the book, use a book like a dictionary or a large travel guide. You want the range of words that the book uses to be wide and cover many topics.
(2) Change the words of your message into numbers. Take the first word of your message and find that word somewhere in the book. Now, take the page number or chapter number, line number, and word number. Write them altogether to stand in for your word. Do this for each word. You can also do this for phrases, if your book can accommodate what you want to say.
So, for example a word on page 105, fifth line down, 12th word would become 105512, 1055.12, or something similar.
(3) Translate the message.
Give your encoded message to your friend. They'll need to use an identical copy of the same book to translate the message back.