def _can_revoke(revoker, target_primary):
            for self_sig in target_primary.self_signatures:
                for rk in self_sig.revocation_key:
                    if rk.algorithm == revoker.key_algorithm and rk.fingerprint == revoker.fingerprint:
                        return True
            return False

        hash_algo = prefs.pop('hash', None)
        sig_type = None
        if isinstance(target, PGPUID):
            sig_type = SignatureType.CertRevocation

        elif isinstance(target, PGPKey):
            # Check that we are revoking a key we can revoke.
            if target.is_primary:
                if target.fingerprint == self.fingerprint or _can_revoke(self, target):
                    sig_type = SignatureType.KeyRevocation
            else:
                if target in self or _can_revoke(self, target.parent):
                    sig_type = SignatureType.SubkeyRevocation

        else:  # pragma: no cover
            raise TypeError

        if sig_type is None:
            raise PGPError("Can't revoke the given key with this key.")