The ANSI X9.124 standard 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 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.