private function _generateChecksum($aesKey, $usePadding = false)
    {
        if ($usePadding) {
            $aesKey = $this->pkcs5_pad($aesKey, 16);
        }

        $encrypted = openssl_encrypt($this->_id, 'aes-256-ecb', $aesKey, OPENSSL_RAW_DATA);

        $this->_checksum = base64_encode(substr($encrypted, 0, 8));
    }