protected function buildCustomAuthToken($prefix, $customerNumber, $timestamp, $apiKey)
    {
        if (empty($customerNumber) || empty($apiKey)) {
            throw new ShopgateLibraryException(
                ShopgateLibraryException::CONFIG_INVALID_VALUE,
                'Shopgate customer number or  API key not set.',
                true,
                false
            );
        }

        return sha1("{$prefix}-{$customerNumber}-{$timestamp}-{$apiKey}");
    }