function queryPrivate($method, array $request = array())
    {
        if(!isset($request['nonce'])) {
            // generate a 64 bit nonce using a timestamp at microsecond resolution
            // string functions are used to avoid problems on 32 bit systems
            $nonce = explode(' ', microtime());
            $request['nonce'] = $nonce[1] . str_pad(substr($nonce[0], 2, 6), 6, '0');
        }

        // build the POST data string
        $postdata = http_build_query($request, '', '&');

        // set API key and sign the message
        $path = '/' . $this->version . '/private/' . $method;
        $sign = hash_hmac('sha512', $path . hash('sha256', $request['nonce'] . $postdata, true), base64_decode($this->secret), true);
        $headers = array(
            'API-Key: ' . $this->key,
            'API-Sign: ' . base64_encode($sign)
        );

        // make request
        curl_setopt($this->curl, CURLOPT_URL, $this->url . $path);
        curl_setopt($this->curl, CURLOPT_POSTFIELDS, $postdata);
        curl_setopt($this->curl, CURLOPT_HTTPHEADER, $headers);
        $result = curl_exec($this->curl);
        if($result===false)
            throw new \Exception('CURL error: ' . curl_error($this->curl));

        // decode results
        $result = json_decode($result, true);
        if(!is_array($result))
            throw new \Exception('JSON decode error');

        return $result;
    }