While the theoretical key space is vast, the practical key space is significantly smaller due to the need for efficient validation.
If you have an original 13-digit retail key, you can still attempt to Activate a Product on Steam Half Life CD key code where??? - Google Groups half life 1 cd key 25 digits
The release of Half-Life in November 1998 marked a paradigm shift in video game narrative and mechanics. However, beneath the surface of the Black Mesa Research Facility lay a sophisticated software licensing mechanism that would set the standard for PC gaming DRM for half a decade: the 25-digit alphanumeric CD key. While the theoretical key space is vast, the