I'm using Firebase to authenticate the users on my application but, since the app is very early stage, I would like to restrict the login (or registration) to only users that have a specific code.
It looks like there's no option like this and I was wondering if there's any solution that doesn't involve a back-end.
Right now I'm using a specific code in the database that the user has to enter while logging in. If that code is not correct you can't login. The problem is the function (obviously) is executed on the front-end so a person with the right knowledge could easily modify the code and still access without token.
Is there a more robust solution?