The system has three tiers of box and key: Bronze, Silver and Golden. To open one, use the key on the box. Both are consumed and you get one rolled item from that combination's list.

The Bronze Special Box and Bronze Special Key drop from any monster in the game, at 4% each per kill. They are the base of the system: everything above comes from them, through crafting.

At the craft machine, tab Chest/Box And Keys, you move up a tier by combining 5 of the previous one. No money cost and no failure chance.

Mind the combination: the Silver Key also opens the Golden Box, but with rare item chances heavily reduced. If you have a Golden Box, it is worth holding it until you get a Golden Key — same table, very different odds.