private function addAuthenticationHeaders(Request $request) {
        $client = $request->getClient();

        // Get a GMT/UTC timestamp
        $timestamp = gmdate('Y-m-d\TH:i:s\Z');

        // Build the data to base the hash on
        $data = $request->getMethod() . '|' .
                $request->getUrl() . '|' .
                $client->getConfig('publicKey') . '|' .
                $timestamp;

        // Generate signature
        $signature = hash_hmac('sha256', $data, $client->getConfig('privateKey'));

        // Add relevant request headers (overwriting once that might already exist)
        $request->setHeader('X-Imbo-Authenticate-Signature', $signature);
        $request->setHeader('X-Imbo-Authenticate-Timestamp', $timestamp);
    }