Exploit. Infinity void keys.

Simple exploit. Exists since void keys were introduced. Got already 10m credits and 40 forma from there.
Need 2 players.
A-key owner. B-exploit user(You).
1.Owner creates lobby and invites B.
2.B accepts invite, and right after lobby has downloaded he must press the button where you can choose game mode(solo, private, etc.). You have less then 1 sec to do so. Not as hard as it sounds.
3. Player B will see that menu with solo/private/etc. modes. Sometimes it disappears, sometimes not. Main idea is to choose solo mode. If you can see menu, it is not a problem. If you don't see you should press under that big blue button(where "waiting for vote" is written).
4. Player B will be moved to another void lobby. But before you close pop-uped message that tells you about disconnect you should change mode from solo to private and invite your friends and player A.
If player B doesn't have the key same type of player A used, B wont lose anything. Player A wont lose any keys. Player B will be host.

Enjoy.

P.S. Probably player B will have negative count of keys. I mean, if player B has 0 keys, after mission is done he will have -1 keys and so on. So he wont be able to obtain these keys in future. But sometimes it is possible to reset counter via support tickets(huge risk) and when new patches are released.