private function existCertificate()
    {
        if (isset($this->config['certificate'])) {
            $certificate = $this->config['certificate'];
            if (!file_exists($certificate)) {
                $this->messageNoExistCertificate();
                return false;
            }

            return true;
        }

        $this->messageNoExistCertificate();
        return false;
    }