Butterfly Escape uses a if you bought it outside of major storefronts (e.g., directly from the developer’s site, or via a bundle).

Finding a registration key for (the 2006 marble-shooter game) is difficult today because the original developer, Genimo , is no longer active.

These stores sell official keys. Humble Bundle often features indie packs where you can get Butterfly Escape plus four other games for $10. – these gray market keys are often purchased with stolen credit cards. When you buy a stolen key, the developer pays a "chargeback fee" and loses money.