X9.124 Symmetric Key Cryptography for the Financial Services Industry Format Preserving Encryption is a suite of standards consisting of five parts that defines format-preserving encryption (FPE) methods. Part 1 of this standard 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. Part 5 defines requirements for using the AES or Triple-DES block cipher to perform FPE using a format-preserving Feistel-based mode known as FF3.1. FF3.1 is part of the FFX family of Feistel-based format preserving encryption methods.