Nevertheless, it's still possible to jam (see LightSquared). I actually think this is how Iran did it.

According to this wired article it is possible, but still unlikely.

http://www.wired.com/dangerroom/2011/12/iran-drone-hack-gps/

Resistent to accidental jamming, but jamming a broad range of frequencies would easily jam it, wouldn't it?

That might require more hardware, but it do not think it require superior technology.

Without going into details, it really isn't.

Just wondering what is harder - interfering with multiple frequency signals or cracking a big crypto key.

Can't you get into details?

No, I really can't (or at least I should not) go into details.