The block should be complete and the final
        signature from the publishing validator (this validator) needs to
        be added.

def _sign_block(self, block):
        """ The block should be complete and the final
        signature from the publishing validator (this validator) needs to
        be added.
        """
        block_header = block.block_header
        header_bytes = block_header.SerializeToString()
        signature = self._identity_signer.sign(header_bytes)
        block.set_signature(signature)
        return block