X9.124-2-2018 defines requirements for Format Preserving Encryption - Counter Mode (FPCM). FPCM methods encrypt data strings of a specific length and character set into ciphertext of the same length using the same character set and using the equivalent of Counter Mode (CTR) defined in NIST SP38B. Format Preserving Encryption is useful in situations where fixed-format data, such as Primary Account Numbers (PANs) or Social Security Numbers, must be encrypted, but there is a requirement to limit changes to existing communication protocols, database schemata or application code. FPCM is a particularly simple and efficient mechanism to achieve format preserving encryption, which shares many of the strengths and challenges of CTR.