RFC 5869: HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
hkdf
HKDF-Extract(salt, IKM) -> PRK salt 如果不指定，则默认为hash len的全0 string IKM为初始化key PRK = HMAC-Hash(salt, IKM)
HKDF-Expand(PRK, info, L) -> OKM info为相关初始化信息标识 L为目标OKM的长度
N = ceil(L/HashLen)
T = T(1) | T(2) | T(3) | ... | T(N)
OKM = first L octets of T
T(0) = empty string (zero length)
T(1) = HMAC-Hash(PRK, T(0) | info | 0x01)
T(2) = HMAC-Hash(PRK, T(1) | info | 0x02)
...
pbkdf
NIST Special Publication 800-132: Recommendation for Password-Based Key Derivation
