You don’t need any key for GSM, since the network/base station only started authenticating itself to the phone/SIM with 3G.
That’s why it would be good to shut down GSM at some point: It would raise the difficulty of such attacks significantly.
What I don’t understand is how they managed to actually intercept any SMS with an IMSI catcher. They’d need to get the network to send these through their infrastructure, so I wonder how that worked?
Update: Ah, they were just sending out texts themselves, not intercepting anything.
