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Security by obscurity
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.
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Codebooks on this website
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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.
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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 a codebook.
Nevertheless it is often treated as such, as they both contain
classified cipher material.
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© Crypto Museum. Created: Friday 04 September 2015. Last changed: Monday, 12 February 2024 - 14:35 CET.
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