derive.ex

def derive_key(%Private{chain_code: chain_code} = private_key, index)
     when normal?(index) do
  chain_code
  |> compressed_hmac_sha512(private_key, index)
  |> private_derivation(private_key)
end

def derive_key(%Private{key: key, chain_code: chain_code} = private_key, index)
     when hardened?(index) do
  chain_code
  |> Crypto.hmac_sha512(<<0::8, key::binary, index::32>>)
  |> private_derivation(private_key)
end

def derive_key(%Public{chain_code: chain_code} = public_key, index)
     when normal?(index) do
  chain_code
  |> compressed_hmac_sha512(public_key, index)
  |> elliptic_curve_point_addition(public_key)
end

def derive_key(%Public{}, index)
     when hardened?(index),
     do: raise("Cannot derive Public Hardened Child!")