How to implement a secure card collection system?

I’m making a multiplayer card game, but cards have to be unlocked via in-game currency. Players owning many cards that they shouldn’t would be a big problem. How do I implement a system that tracks which cards a player owns? I couldn’t really find any resources for this. This is a PC project that I’m planning to deploy on Steam if that matters.

Thanks in advance!

You could use save game to keep track of the players unlocks very easily, but it surely can be manipulated by anyone who knows how to tinker with the save files. I think a system that is fool proof would need that data stored on a server system using a mysql/php database and get that data when the player connects. Steam has a player achievements system, but I’m not sure if that would apply to your situation.