Actually you could do this by using products rather than subscriptions.
Create a product with a recurring payment
In the "Benefits" section, click to create a license key
When someone purchases, they will then be given a license key for that purchase. This will show under their purchase within their client area on the front end (Store>Manage Purchases, Click the purchase)
Outside the software, you can then check that license key with the following URL (replacing the relevant parts with your URL), and after enabling the API for this (see the guide below)
https://yourSiteHere.com/applications/nexus/interface/licenses/?check&key=1JR4P-2Y4W-Z2KA-RVWE-FA0J
This will give a json response with the license details if the key is correct. See the following guide for more information
