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Substitution cipher

A substitution cipher is a cipher that replaces each plaintext symbol for another ciphertext symbol. The receiver decodes using the inverse substitution. A substitution alphabet is the extended list of ciphertext symbols. Examples are Caesar ciphers (such as ROT13) and the atbash cipher.

Modern Feistel ciphers such as DES and Rijndael are similar in principle to substitution ciphers. They treat each 64-bit or 128-bit block of the plaintext as a symbol and perform several rounds of substitutions and transpositions on the bits in the block to approximate a general block-to-block substitution.

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