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Although the word 'code' is loosely used to cover all forms of encryption, strictly speaking it applies only to a special type of encryption. A code, in contrast to a cipher, which makes substitutions at the level of letters or groups of letters, is a system of encryption that swaps each word in a message with, for example, a number between 1 and 10,000. All the possible words and the corresponding numbers are listed in a codebook, a volume that is similar to a dictionary.
The strength of a code is that it is invulnerable to frequency analysis (see The substitution cipher for more about frequency analysis). The weakness of a code is that it requires a great deal of effort to construct, print and distribute a codebook. Therefore a codebook will be used for several months or even years without being changed. During this period the enemy codebreakers will be able to gather clues that enable them to reconstruct the codebook, or they might even be able to steal a copy of the codebook.
How codes work
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