module Kdf = struct
  module XTR = Hkdf.Make (Digestif.SHA512)
  module PRF = Hkdf.Make (Digestif.SHA256)

  let kdf ~xts ~ikm ~ctx ~len =
    let prk = XTR.extract ~salt:xts ikm in
    let okm = PRF.expand ~prk ~info:ctx len in
    okm
end