scope:

The American National Standard (ANS) X9.124, Format Preserving Encryption, defines a collection of methods for encrypting data strings so that the length and the character set for the ciphertext is the same as those of the plaintext. This is called format-preserving encryption (FPE). These methods are useful in situations where fixed-format data, such as Primary Account Numbers (PANs) or Social Security Numbers (SSNs), must be encrypted, but there is a requirement to limit changes to existing communication protocols, database schemata or application code.

Part 1 of this standard (Reference 1) includes a set of definitions common to all FPE techniques, a security model for FPE block cipher techniques, and a description of the pseudocode language used in defining the mode specified in this document.

This document, part 5 of the Standard, defines requirements for using the AES or TDEA block cipher to perform FPE using a format-preserving Feistel-based mode known as FF3.1, which is part of the FFX family. The FFX method is a family of format-preserving block ciphers. FFX stands for Format-preserving Feistel-based where the X reflects a specific algorithm.