As far as I understand, receiving enough GPS signals at the same time enables to deduce the position and the time.
So I guess it is possible to use an offline receiver as a very precise clock?
If so, is it possible to flood this offline receiver with fake signals to make it believe it's 12:00:01 when it's actually 12:00:00?
Or more specifically, is it possible to design a receiver who can't be attacked this way?
If GPS signals (or Galileo's, …) are cryptographically signed, it's easy to reject non-signed signals by saving the public key of the satellites beforehand. But are the GPS signals cryptographically signed?
Edit: My question is not about the civilian signal being encrypted or not (meaning unreadable for people not having a secret key), but signed or not (meaning the authenticity of the signal being verifiable thanks to a public information: the public key of this satellite).
Best Answer
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.