IMHO its impossible ! RSA is asymmetric. So u can obtain the decryption key from the game ,but reencrypt...?! Only the devs knows the encryption key (Maybe its buried accidentally somewhere in a gamefile )...
Or am i wrong ?
btw Masquerade's answer seems good....
Edit:
Maybe start looking here :
https://www.unknowncheats.me/forum/arma-3-a/