public function getPrivateKey($caName = null, $caPassword = null)
    {
        // if password is set
        if ($caPassword !== null) {
            return [
                file_get_contents($this->caDataRoot . $caName . DS . static::PRIV_KEY_FILENAME),
                $caPassword
            ];
        }

        return file_get_contents($this->caDataRoot . $caName . DS . static::PRIV_KEY_FILENAME);
    }