X9。124-2-2018定义了格式保留加密-计数器模式（FPCM）的要求。FPCM方法使用相同的字符集，并使用NIST SP38B中定义的计数器模式（CTR）等效物，将特定长度和字符集的数据字符串加密为相同长度的密文。在必须加密固定格式数据（如主帐号（PAN）或社会安全号码）的情况下，保留格式的加密非常有用，但需要限制对现有通信协议、数据库模式或应用程序代码的更改。FPCM是实现格式保留加密的一种特别简单高效的机制，它与CTR有许多共同的优点和挑战。

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.