Adding web based video subscription to my ios app

I have an existing one time purchase ios app which has bundled into it a set of learning videos. I also have a learning video annual subscription website. I want to bring in the video subscription website into my app but adding ios in app purchase. Which would mean that user will purchase the in app feature and then be able to view in app the videos from the subscription website. The challenge here is how do we communicate to the web server that a particular user has purchased the in app subscription and create an account on the website for the user. The subscription website currently requires the user to create an account and make payments after which he can see the videos. After inapp purchase we can let the user create an account and let him view the videos. How do we make it fool proof against unauthorized access by someone impersonating as an ipad/iphone user. Any thoughts will be helpful. Thanks