Are Civilian GPS signals cryptographically signed?
GPS can be spoofed without decrypting or creating signals. Therefore, the system cannot be made secure by cryptographic signatures.

The conceptually simplest way to spoof is to erect a number of highly directional antennae and point each of those at a GPS satellite, such that it receives exclusively signals from that satellite. Then feed those signals through a bank of delay lines, mix them back together and use another directional antenna to send the result toward an enemy aircraft.

You can then sit in front of the delay lines and force arbitrary position errors upon the unsuspecting enemy. If you introduce a delay for the satellite that is south of your position, the enemy's receiver will consider itself further north than it actually is, about 30cm per nanosecond of delay.

Cryptography doesn't help you to detect or prevent those attacks, as the signals are only delayed but never changed. The only defense a receiver can mount is radio direction finding. If all satellites' signals come from the same direction, it's probably a spoofer. All modern military receiver employ this method, more sophisticated ones also crosscheck the directions of arrival against the known position of the satellites.