Editing Book cipher (section)

{{Short description|Encryption and decryption method}}
[[File:Exodus 4 - reference to spiritual possession putting words in mouths.jpg|thumb|The [[King James Version|King James Bible]], a highly available publication suitable for the book cipher.]]
A '''book cipher''' <!--No reliable source on "Ottendorf" cipher found on Google Scholar. Most search results look like they copied *from* Wikipedia. It seems like the only reference is a movie line from Turteltaub, J. (directed by): National Treasure (2004). Screenplay by Jim Kouf, Cormac Wibberley and Marianne Wibberley
https://link.springer.com/chapter/10.1007/978-3-030-85623-6_3
--> is a [[cipher]] in which each word or letter in the [[plaintext]] of a message is replaced by some code that locates it in another text, the [[key (cryptography)|key]].

A simple version of such a cipher would use a specific book as the key, and would replace each word of the plaintext by a number that gives the position where that word occurs in that book. For example, if the chosen key is [[H. G. Wells]]'s novel ''[[The War of the Worlds]]'', the plaintext "''all plans failed, coming back tomorrow''" could be encoded as "''335 219 881, 5600 853 9315''" — since the 335th word of the novel is "''all''", the 219th is "''plans''", etc. 

Instead of the position of the word, sender can also use for each word a triplet indicating page number, line number in the page and word number in the line, avoiding error-prone counting of words from the start of the book. With the Bible, triplet can be chapter number, verse number, word number.

This method requires that the sender and receiver use exactly the same edition of the key book.<ref name=BookCipherIEEE>{{Cite book | last1 = Changda Wang | last2 = Shiguang Ju | chapter = Book Cipher with Infinite Key Space | doi = 10.1109/ISISE.2008.273 | title = 2008 International Symposium on Information Science and Engineering | pages = 456 | year = 2008 | isbn = 978-0-7695-3494-7 | s2cid = 15768123 }}</ref>

This simple version fails if the message uses a word that does not appear in the text. A variant that avoids this problem works with individual letters rather than words. Namely each letter of the plaintext message would be replaced by a number that specifies where that letter occurs in the key book. For example, using the same ''War of the Worlds'' book as the key, the message "no ammo" could be encoded as "12 10 / 50 31 59 34" since the words with those positions in the novel are "'''''n'''ineteenth''", "'''''o'''f''",  "'''''a'''lmost''", "'''''m'''ortal''", "'''''m'''ight''", and "'''''o'''wn''". This method was used in the [[Beale ciphers|second Beale cipher]].  This variant is more properly called a [[substitution cipher]], specifically a [[substitution cipher#Homophonic substitution|homophonic]] one.

Both methods, as described, are quite laborious.  Therefore, in practice, the key has usually been a [[codebook]] created for the purpose: a simple dictionary-like listing of all the words that might be needed to form a message, each with the respective code number(s). This version is called a [[code (cryptography)|code]], and was extensively used from the 15th century up to [[World War II]].