Hey!
I plan on implementing a feature that people can get a key (like physically printed on paper) that they can enter in my game to unlock a certain weapon.
Each code should only be useable once and then be ‚blocked‘.
Do you have any idea of how to accomplish something like this? 
Thanks in advance! 
If it’s all on the client, you could store a list of keys and whether they’re unlocked or not. I guess that is kind of obvious.
If there’s a server involved, then the same idea, but stored there?
Please clarify if I missed something in your question.
Thank you for your answer!
So basically, this check if a key is used should be constant… so that 2 people cannot use the same key… I think a server that stores the keys and stores if they have already been used is needed then right? Or wrong? 
Well, for sure… otherwise how would you work it out?
In addition to that, note that anything that’s deployed with your application can be reverse-engineered… no matter whether it’s a trivial comparison with hardcoded values or some smart and flexible algorithm that only accepts a subset of keys out of all permutations.
All of that can be read out and can be “cracked” when it’s part of a client application. So if you’re serious about proper key management, you’ll definitely need some server-driven validation and even that is sometimes not as easy.