金鑰推衍函數(Key Derivation Function, KDF)

Ref:
https://en.wikipedia.org/wiki/Key_derivation_function
https://en.wikipedia.org/wiki/PBKDF2

The most common use of KDFs is the password hashing approach to password verification, as used by the passwd file or shadow password file.
The non-secret parameters are called "salt" in this context.
Examples of such key derivation functions include KDF1, defined in IEEE Std 1363-2000, and similar functions in ANSI X9.42.

key derivation functions是由一個強健的金鑰被使用在連續的加密所衍伸來而的。
例如：PBKDF2、bcrypt、scrypt

要符合FIPS 140-2安全認證的規範就用PBKDF2-SHA-256或SHA-512，並用多次的迭代來產生密碼的Digest。