One of the many items found in Killing Floor 2 are Crate Keys, which can be used to unlock various weapon skin and character cosmetic packs that are earned. However, these items are fairly rare and there are only two ways of earning them currently within the core game. One of those is by visiting the storefront on the menu screen and purchasing them with real world cash, which isn’t ideal. The other way is by getting them as a drop after you finish a match in either solo or online mode.
The best way to currently earn Crate Keys is by playing Killing Floor 2 for around a half hour or so each day. You can actually earn Keys regardless if you win or lose a match. Yet, if you do lose, make sure to stay until the final score screen pops up that reveals your stats for a match, only then will they be unlocked. It’s possible to unlock Keys even if you are early in a game, but the drop rate is believed to be much lower.
These will be random, and Killing Floor 2 has a variety of different crates that you can earn. You will have to match the specific key to the corresponding crate, however the actual series of the Crate does not matter. So if you earn a Horzine Supply Crate, it can be used on any Horzine Crate regardless of what number in the series it is. When you are playing through Killing Floor 2, always keep track of what keys you have, that way you know which crates to look out for if they do drop.