I would start with an algorithm first: Define which one you'll need to use for your data. ASP.NET has a lot more flexibility than an app store, so start with app store encryption reading on MSDN, then work that back to ASP.NET.
Next, build a console app (instead of jumping right to ASP.NET) that takes something encrypted, and build a sample Windows Store app that decrypts it. Ensure that very simple process works well before you move to actual ASP.net and a more complicated
store. If you're good at ASP.NET, and specifically debugging web code - you may jump right to ASP.NET and skip the console application.
Start small, focus on good encryption, then incorporate them into a good app on both platforms.
Darin R.
