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A codebook is a very old and effective method for concealing the contents of a message. Initially, codebooks were not intended for the encryption of messages, but as a simple means to make their distribution more efficient and cost-effective when sending them over telegraph lines or via radio by means of morse code. In most cases, frequently used expressions — and also words and single letters — were replaced by three, four or five-letter codes, making it easier (and cheaper) to send a message over a long distance. The international Q-Codes are a good example of this.

During WWII, codebooks were often used to encrypt a message, sometimes in addition to other cryptographic methods, such as Enigma or hand ciphers. Cracking a codebook is a difficult but not impossible task for a codebreaker. Once a codebook is captured or reconstructed, messages are no longer secret. For this reason, codebooks are often said to provide Security by Obscurity.

Codebooks on this website
Österreichisches Geschäften-Lexikon (Austrian business lexicon) (1793, 1816) Code International Lugagne (1914) Radiokom I and II (1917) Internationales Signalbuch 1931 S.P. 02201 - Signalling instructions of the British Admiralty (1935) The News Boe Code (1937) - Commercial Traffic and Shipping Codes Funkverkehrsheft für die Küstenverteidigung (1943) German Navy bigram tables used during WWII for Enigma traffic
Note that some books do not directly contain codes, but instead offer instructions on how to handle, encode and decode (specific parts of) a message. Such books are procedural rather than operational, but are nevertheless codebooks in the sens of this section of the website. A good example are the S.P. 02201 Signalling Instructions listed above, that were used throughout WWII.

Key material
When using mechanical cipher machines, such as the German Enigma, it was common practice to supply the daily settings of the machine, such as the order of the wheels and the initial position, on a so-called key sheet or key list. In most cases, a key sheet contained the settings for several days — sometimes even a full month — in advance. Please note that a key sheet is not the same as a codebook, although it is often treated as such, as they both contain classified cipher material.

Further information
The New Boe Code book

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© Crypto Museum. Created: Friday 04 September 2015. Last changed: Friday, 23 February 2018 - 21:56 CET.
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