public function setSignatureMethod($method)
    {
        $method = strtoupper($method);
        
        switch ($method) {
            case 'PLAINTEXT':
            case 'HMAC-SHA1':
                $this->sigMethod = $method;
                break;
            default:
                throw new \Dropbox\Exception('Unsupported signature method ' . $method);
        }
    }