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Definition (not exact)

A codebook is a—in general large—list of letter sequences, mostly words or partial words or fixed expressions. In order to enable the encryption of unforeseen texts also the single letters are contained in the list. Each entry in the list is assigned a »code«, a group of symbols of fixed length, typically a five-digit number. Some codes also consisted of more or less random words.

Example from everyday life: the phone book that assigns each phone owner his phone number.


...      on 03917
beat10569     ...  
...      one 35613
buy 42583    ...  
...      price06679
down 53472    ...  
...      then 10247
e 18641    ...  
...      to 22076
million50968     ...  
...      ...  

The plaintext »Beat down price to one million, then buy!« is transformed to the code

    10569 53472 06679 22076 35613 50968 10247 42583
The code is not unique, it also could look like this:
    10569 53472 06679 22076 03917 18641 50968 10247 42583


  1. A codebuch is a useful encryption method only if the complete list is kept secret, that is the list is the key. This is hardly guaranteed. Therefore codes that should be cryptographically secure should be superencrypted. For this purpose even a simple monoalphabetic encryption might suffice.

  2. The decoder uses the inverted list where the codes are sorted alphabetically or numerically. These two lists usually were combined into one book, but not always. This observation may be the historical origin of the idea of one-way encryption (no one can decrypt) or asymmetric encryption (only the owner of the inverse list can decrypt). Today, in the age of electronic information processing, inverting such a list is a child's play.

  3. Early history: »For 450 years, from about 1400 to about 1850, a system that was half a code and half a cipher dominated cryptography. It usually had a separate cipher alphabet with homophones and a codelike list of names, words, and syllables. This list, originally just of names, gave the system its name: nomenclator.« [Kahn, S. xv] The earliest known use of this system is by Gabriele de LAVINDE 1379 who worked for the antipope Clement VII. [Picture: a page from a French nomenclator from 1690.]

  4. The use of a two-part codebook (or nomenclator) seems to go back to Antoine Rossignol, the leading French cryptologist of the 17th Century. Prior to that the nomenclators were constructed in a way that needed only one list: The codes were in alphabetic or numerical order—an immense help for the crypanalyst.

  5. A comprehensive source for Italian cryptography from 14th until 16th Century are two books in German by Aloys MEISTER:
    Die Anfänge der modernen diplomatischen Geheimschrift. Schöningh, Paderborn 1902.
    Die Geheimschrift im Dienste der päpstlichen Kurie. Schöningh, Paderborn 1906.
  6. The idea of superencrypting a code goes back at least to ALBERTI who devised a nomenclator and proposed a polyalphabetic superencryption with the help of his cipher disk.

  7. The main use of codebooks was for diplomatic purposes, but also the military liked it. Thus in World War I codebook + superencryption was still the most used cryptographic method.

  8. Cryptanalysing a code is obvious as soon as the cryptanalyst has the codebook, at least the inverse list. Otherwise he has to undergo the cumbersome task of reconstructing the code piece by piece. The codebook in a certain sense defines a new language the characteristics of which must be explored. The cryptanalyst observes frequencies, repetitions, patterns and uses assumed or known partial plaintext. This task is not significally harder if the code is superencrypted. However superencryption may provide a little additional protection if the enemy captures the codebook.

  9. A frequently used superencryption method is addition modulo 10 (also called »false addition«) of numbers from a more or less random table contained in another book. An additional key then are the page, line, and column numbers from where the summands were taken consecutively. This key could be protected by an extra cipher and appended to the message.

  10. A comprehensive treatment of cryptanalysis of codes is in the books by Kahn and Bauer as well as in US Army Field Manual FM 34-40-2 and in Friedman's Elements of Cryptanalysis. See the references.

Various informations on codebooks for commerce and military can be found in the web:

Author: Klaus Pommerening, 1999-Nov-05; last change: 2014-Jul-16.