Return the binary data to be/which was signed

def _data_to_sign(self) -> bytes:
        """ Return the binary data to be/which was signed """
        packed = pack_reward_proof(
            canonical_identifier=CanonicalIdentifier(
                chain_identifier=self.balance_proof.chain_id,
                token_network_address=self.balance_proof.token_network_address,
                channel_identifier=self.balance_proof.channel_identifier,
            ),
            reward_amount=self.reward_amount,
            nonce=self.balance_proof.nonce,
        )
        return packed