: Many original retail keys can still be redeemed in the Steam client under "Activate a Product on Steam". Redeeming a single Half-Life key often grants you the Half-Life Platinum Collection , which includes Opposing Force , Blue Shift , and Team Fortress Classic .

Today, that same key is a relic. If it hasn't been burned onto a Steam account by now, its value is primarily nostalgic. However, for the dedicated retro gamer, successfully activating a 25-digit key from a 27-year-old cardboard box is a ritual—a digital handshake with gaming history.

In many era-specific validation algorithms (similar to the Luhn algorithm used in credit cards), a portion of the key is used to mathematically validate the remaining portion. If the Half-Life key uses, for example, 5 digits as a checksum function of the other 20, the entropy is reduced, but the likelihood of a user typing a "valid but wrong" key decreases.