private function _verifySignature(ValidatorState $state, Certificate $cert)
    {
        try {
            $valid = $cert->verify($state->workingPublicKey(), $this->_crypto);
        } catch (\RuntimeException $e) {
            throw new PathValidationException(
                "Failed to verify signature: " . $e->getMessage(), 0, $e);
        }
        if (!$valid) {
            throw new PathValidationException(
                "Certificate signature doesn't match.");
        }
    }