One common and popular type of cipher for newspaper games and puzzle books is a substitution cipher. In simple substitution ciphers, users agree upon a rearrangement, or permutation, of the alphabet letters, yielding a collection of correspondences to be used for converting plaintext letters into ciphertext letters. This rearrangement of the alphabet letters is often called the cipher alphabet. To say that the cipher alphabet is a permutation means that each possible plaintext letter in the original alphabet is paired with one and only one possible ciphertext letter, and vice versa. With more sophisticated substitution ciphers, messages and cipher alphabets can include numbers, punctuation marks, or mixtures of ...
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