I'm sorry, but im not english so if i have a mistake i hope you'll understand.
You can farm keys in zombies by reaching high rounds. So if you use cheat engine to increase rounds you can farm 30 keys per game, each game in 10 minutes.
Steps:
1. Start a new game in zombies.
2. Open Cheat Engine.
3. Reach round 2. Then scan ''2'' value.
4. Reach round 3 and then do the next scan ''3''.
5. Continue doing the same until round 6 and let just one zombie alive.
6. Then select all adresses form the left and click the red arrow bellow.
7. Select all adresses and press right click. Press change value to 100 or something like that.
8. Kill the zombie and next round will be 101 (or whatever u have changed).
9. Die and enjoy.