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).