It's not so much that the military signal (ranging code) is encrypted, per se, but it uses a DRNG for which the parameters are kept secret. To be able to spoof

It's not so much that the military signal (ranging code) is encrypted, per se, but it uses a DRNG for which the parameters are kept secret. To be able to spoof GPS, you need to be able to duplicate the ranging code for a given satellite. This is easy for civilian GPS, where the code repeats ~1000 times/sec and the details are public; much harder for military code where it repeats 1 time/week and the parameters are secret. You can still jam either one, though, just by overwhelming appropriate frequencies with noise. This is what the spot beam in newer satellites is for -- getting more power to areas of conflict to make the signal harder to jam.