Format-preserving encryption

In cryptography, format-preserving encryption (FPE), refers to encrypting in such a way that the output is in the same format as the input. 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.