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)
     ...

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