How to get keys:
- Capture missions (any)
- Raid missions (any)
- Defense missions (any) - depends on wave level
- Shop
The way defense missions work is that rewards are based on mobs' level in 5/10/15/... waves. Depending on level it assigns it a tier. I don't know what the thresholds are at the moment.
Below is a list of all the rewards you can get depending on mission type. Note that capture, raid and intel (spy?) missions aren't based on level.
Duplicate entries aren't a mistake on my part.
Drop tables:


CAPTURE & RAID
Ammo Drum: 7.37%
Banshee Chassis Blueprint: 7.37%
Fast Deflection: 8.43%
Fast Deflection: 8.43%
Fast Hands: 8.43%
Large Health Restore: 8.43%
Magazine Warp: 8.43%
Molten Impact: 0.17%
North Wind: 0.17%
Omni Ammo Restore: 0.17%
Orokin Tier 1 Extermination Key: 0.17%
Orokin Tier 1 Raid Key: 0.17%
Orokin Tier 2 Extermination Key: 0.17%
Orokin Tier 2 Raid Key: 0.17%
Orokin Tier 3 Extermination Key: 0.17%
Pressure Point: 0.17%
Quick Rest: 8.43%
Quickdraw: 8.43%
Rush: 7.37%
Shocking Touch: 0.17%
Stretch: 0.17%
Trick Mag: 8.43%
Vitality: 0.17%
Vitality: 8.43%


INTEL
Ammo Drum: 7.37%
Banshee Systems Blueprint: 7.37%
Fast Deflection: 8.43%
Fast Deflection: 8.43%
Fast Hands: 8.43%
Large Health Restore: 8.43%
Magazine Warp: 8.43%
Molten Impact: 0.17%
North Wind: 0.17%
Omni Ammo Restore: 0.17%
Orokin Tier 1 Extermination Key: 0.17%
Orokin Tier 1 Raid Key: 0.17%
Orokin Tier 2 Extermination Key: 0.17%
Orokin Tier 2 Raid Key: 0.17%
Orokin Tier 3 Extermination Key: 0.17%
Pressure Point: 0.17%
Quick Rest: 8.43%
Quickdraw: 8.43%
Rush: 7.37%
Shocking Touch: 0.17%
Stretch: 0.17%
Trick Mag: 8.43%
Vitality: 0.17%
Vitality: 8.43%

OROKIN

Tier 1:
Forma Blueprint: 0.67%
Latron Prime Barrel: 11%
Latron Prime Blueprint: 25.3%
Latron Prime Receiver: 0.67%
Latron Prime Stock: 25.3%
Scythe Blade: 0.67%
Scythe Blueprint: 11%
Sycthe Handle: 25.3%

Tier 2:
Forma Blueprint: 12.65%
Forma: 0.5%
Frost Prime Blueprint: 0.5%
Frost Prime Chassis Blueprint: 12.65%
Frost Prime Helmet Blueprint: 12.65%
Frost Prime Systems Blueprint: 7.37%
Latron Prime Barrel: 7.37%
Latron Prime Blueprint: 12.65%
Latron Prime Receiver: 0.5%
Latron Prime Stock: 12.65%
Scythe Blade: 0.5%
Scythe Blueprint: 7.37%
Sycthe Handle: 12.65%

Tier 3:
Forma Blueprint: 12.65%
Forma: 2.01%
Frost Prime Blueprint: 7.37%
Frost Prime Chassis Blueprint: 12.65%
Frost Prime Helmet Blueprint: 12.65%
Frost Prime Systems Blueprint: 7.37%
Latron Prime Blueprint: 12.65%
Latron Prime Receiver: 12.65%
Scythe Blade: 12.65%
Scythe Blueprint: 7.37%


VOID KEY PACK:
First key:
Orokin Tier 1 Extermination Key: 47.39%
Orokin Tier 1 Raid Key: 47.39%
Orokin Tier 2 Extermination Key: 2.37%
Orokin Tier 2 Raid Key: 2.37%
Orokin Tier 3 Extermination Key: 0.47%
Next 3 keys:
Orokin Tier 1 Extermination Key: 39.22%
Orokin Tier 1 Raid Key: 39.22%
Orokin Tier 2 Extermination Key: 9.8%
Orokin Tier 2 Raid Key: 9.8%
Orokin Tier 3 Extermination Key: 1.96%
Last key:
Orokin Tier 3 Extermination Key: 100%

New Defense Loot table: (hotfix8.1.0)

Taken from reddit[img]