def to_payload
      version.htb << [depth].pack('C') << parent_fingerprint.htb <<
          [number].pack('N') << chain_code << [0x00].pack('C') << key.priv_key.htb
    end
def to_payload
      version.htb << [depth].pack('C') << parent_fingerprint.htb <<
          [number].pack('N') << chain_code << [0x00].pack('C') << key.priv_key.htb
    end
serialize extended private key