Help finding code

Hey everyone, thanks in advance for any help.

I'm building an app for my bar. The idea is users will use this app to track their bourbon tastings. They will log in to the app, select the bourbon they purchased and hand their phone to the bartender. The bartender types in a code or scans a qr code and it marks as tasted on their "bourbon passport". It needs to be a secure system since progress will result in rewards.

I'm looking for some direction on what type of code I should be looking for. I know how to make my app, log in stuff, I just don't know where to start when it comes to a secure purchase tracking system.

If you can help point me in the right direction it would be much appreciated. Thanks