Editing Block cipher (section)

===Format-preserving encryption===
{{Main|Format-preserving encryption}}
Block ciphers traditionally work over a binary [[Alphabet (computer science)|alphabet]]. That is, both the input and the output are binary strings, consisting of ''n'' zeroes and ones. In some situations, however, one may wish to have a block cipher that works over some other alphabet; for example, encrypting 16-digit credit card numbers in such a way that the ciphertext is also a 16-digit number might facilitate adding an encryption layer to legacy software. This is an example of ''format-preserving encryption''. More generally, format-preserving encryption requires a keyed permutation on some finite [[Formal language|language]]. This makes format-preserving encryption schemes a natural generalization of (tweakable) block ciphers. In contrast, traditional encryption schemes, such as CBC, are not permutations because the same plaintext can encrypt multiple different ciphertexts, even when using a fixed key.