Since only StarCraft (not its expansion, StarCraft: Brood War) comes with a CD key, when Blizzard released the game to be downloaded from their website, they could combine the two games. An "Anthology" CD key, like a 13-digit StarCraft key, can be used as both an original StarCraft client or a Brood War client. The new Anthology CD key uses the same algorithm as the 26-character WarCraft III keys.
StarCraft Anthology can be downloaded from Blizzard's web-based Battle.net account system.
StealthBot has a known issue with some of these keys (specifically those purchased recently), and using them may get you an IP ban for two weeks. Please use StealtBot 2.7 Build 469 or later to resolve this issue.