This is the API that lets game developers or affiliates retrieve CD keys made available to Steam users so their customers can activate a game installed via the Steam client.

My son lost his cd-key for LoD. We found his disc and the game is loaded on his computer. I would like to get his cd-key in case he ever needs to re-install. So is there a way to get the cd-key ...

No, not really. The CD keys are most likely different for each game. You might be able to pay EA for a new key if you can prove you own the discs, but EA isn't exactly known for their customer ...

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