private function getSignature($base, $key)
    {
        switch ($this->sigMethod) {
            case 'PLAINTEXT':
                $signature = $key;
                break;
            case 'HMAC-SHA1':
                $signature = base64_encode(hash_hmac('sha1', $base, $key, true));
                break;
        }
        
        return $signature;
    }