A shift, affine, or any other type of substitution cipher is said to be monoalphabetic because a single cipher alphabet is used throughout the entire encryption process. The fact that a single cipher alphabet is used throughout the entire encryption process is what generally makes a substitution cipher easy to break, by frequency analysis, since the distribution of letter frequencies in plaintexts is preserved into ciphertexts. One way to increase security is to change the cipher alphabet one or more times while encrypting a message. Such a cipher, in which more than one cipher alphabet is used, is said to be polyalphabetic.
Example 7.1 Consider the following pair of cipher alphabets.
Using a polyalphabetic ...
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