private function validateUrl($url)
    {
        $parsed = parse_url($url);
        if (empty($parsed['scheme'])
            || empty($parsed['host'])
            || $parsed['scheme'] !== 'https'
            || substr($url, -4) !== '.pem'
            || !preg_match($this->hostPattern, $parsed['host'])
        ) {
            throw new InvalidSnsMessageException(
                'The certificate is located on an invalid domain.'
            );
        }
    }