I'd also like to know lol. I try to figure it out but also get shot before I can figure it out!
EDIT: Apparently I found this answer around the web:
The solution is simple but not very intuitive.
You have to pick the two "Cipher Keys" that make up the one in the middle, in the example above, these are keys C & F. What isn't very clear is how you pick them. You need to tap one of the keys, tap the middle symbol, then tap the other key and again the middle symbol.