protected function _checkSignatureAlgoAndKey(
        SignatureAlgorithmIdentifier $sig_algo, AlgorithmIdentifier $key_algo)
    {
        if (!$sig_algo->supportsKeyAlgorithm($key_algo)) {
            throw new \UnexpectedValueException(
                sprintf(
                    'Signature algorithm %s does not support key algorithm %s.',
                    $sig_algo->name(), $key_algo->name()));
        }
    }