The American National Institute for Standards in Technology (NIST) is considering proposals for several modes of operation for Format Preserving Encryption (FPE). The idea behind FPE is quite simple: A plaintext should encipher to a ciphertext with exactly the same format and length. The classic example would be a credit card number, in which case the 18 decimal digit plaintext should encrypt to an 18 decimal digit ciphertext. This is clearly very convenient.…