If you dont link the data with the steam account or HWID your encryption and key mechanism is worthless.
You can CHEAT in the game itself which will generate nicely more keys for you.
And yes, I know how the public/private key machinc works

The best way is to hardcode ONE public key per game (and dif URL parameter to differ from) and give all customers a different version of the game containing a different public key.
I dont know if steam allows such a thing directly.