Format-preserving encryption

In cryptography, format-preserving encryption (FPE), refers to encrypting in such a way that the output (the ciphertext) is in the same format as the input (the plaintext). The meaning of "format" varies. Typically only finite sets of characters are used; numeric, alphabetic or alphanumeric. For example:

• Encrypting a 16-digit credit card number so that the ciphertext is another 16-digit number.
• Encrypting an English word so that the ciphertext is another English word.
• Encrypting an n-bit number so that the ciphertext is another n-bit number (this is the definition of an n-bit block cipher).

For such finite domains, and for the purposes of the discussion below, the cipher is equivalent to a permutation of N integers {0, ... , N−1} where N is the size of the domain.