The safest and most reliable way to secure a key is directly through official digital distribution networks.
Would you like the correct steps to redeem an old retail CD key on Steam, or help finding the official store page?
Where you find your key depends on how you acquired the game.
In the late 1990s and early 2000s, digital rights management (DRM) was far less sophisticated. To combat software piracy, developers required a unique CD key—typically printed on a sticker inside the game case or on the back of the manual—to install and play the game. Opposing Force used this system extensively.