Propagates exception on failure
        :return: byte array to put on the blockchain

def prepare_batch(self):
        """
        Propagates exception on failure
        :return: byte array to put on the blockchain
        """
        
        for cert in self.certificates_to_issue:
            self.certificate_handler.validate_certificate(cert)

        self.merkle_tree.populate(self.get_certificate_generator())
        logging.info('here is the op_return_code data: %s', b2h(self.merkle_tree.get_blockchain_data()))
        return self.merkle_tree.get_blockchain_data()