def get_signature_algorithm(self):
        """
        Return the signature algorithm used in the certificate.

        :return: The name of the algorithm.
        :rtype: :py:class:`bytes`

        :raises ValueError: If the signature algorithm is undefined.

        .. versionadded:: 0.13
        """
        algor = _lib.X509_get0_tbs_sigalg(self._x509)
        nid = _lib.OBJ_obj2nid(algor.algorithm)
        if nid == _lib.NID_undef:
            raise ValueError("Undefined signature algorithm")
        return _ffi.string(_lib.OBJ_nid2ln(nid))