Book Cipher

==A [[!Cipher]] in which the Cipher key is part of a book or other piece of text.[1]== It is also called an Ottendorf cipher (presumably after Major Nicholas Dietrich, Baron de Ottendorf who organised spies in the French and American camps for the British[2]).

The key needs to be a text all parties have access to, such as a particular dictionary or [[!Bible]], but the cipher is can be compromised by a poor choice of text which is easily guessed or, in the case of the dictionary, has recognisable word order.

Strengths

  • Huge number of key possibilities: any [[!book]] may be used and so possibilities are as numerous as the number of books published.
  • Hide in plain sight: no need for an incriminating [[!codebook]] or dedicated device.

Weaknesses

  • Need the same book: Usually requires correspondents to have the same [[!book]] of the same edition. These also need to not look out of place in either locale.
  • Weak text = weak cipher: All parties need to have access to the key text but choose too common a book or one with a predictable word ordering (i.e. a dictionary) and the cipher is more easily compromised.
  • Limited to the words in the book: Unless using a variation where, say, letters are identified instead of whole words (as in the second of the [[!Beale ciphers]]). Also limits choice of book key.
  • Still just a cipher: Can be broken as soon as key book is identified as well as using traditional [[!cryptanalysis]] techniques.
    • ==The 'proper cipher' version, a Substitution cipher (like the [[!Beale ciphers]]), is stronger here because it can use [[!Homophonic substitution]]. However this also expands the [[!Ciphertext]] and takes longer to en/decode.==

Explore

  1. Book cipher wikipedia ↩︎

  2. Book Cipher Decoder ↩︎