public function verifyWebhookSignature() {

        $timestamp = $this->request->input('timestamp');
        $token     = $this->request->input('token');
        $signature = $this->request->input('signature');


        // The Mailgun config param is an array, so grab the full array
        $configMailgun = config('services.mailgun');

        $hmac = hash_hmac('sha256', $timestamp. $token, $configMailgun['secret']);

        if(function_exists('hash_equals')) {

            // hash_equals is constant time, but will not be introduced until PHP 5.6
            return hash_equals($hmac, $signature);
        }

        return ($hmac == $signature);
    }