Are civilian GPS signals cryptographically signed?

Question

As far as I understand, receiving enough GPS signals at the same time enables to deduce the position and the time.

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?

More specifically, is it possible to design a receiver that 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.

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.)

Answer 1

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.

Answer 2

The last generation of Galileo systems use cryptographic authentication of geolocation transmissions, called navigation message authentication or NMA. GPS is also in progress of implementing their own NMA system. But there are limitations to these systems. Here's a more detailed look into NMA.

Galileo's implementation, called OSNMA, uses symmetric MAC -- with the MAC key also being broadcast -- making it possible to forge alternate contents for a message after it has been received from the real satellite. The GPS implementation, called Chimera, is under development with the L1C signal specification -- which is planned to be fully available in late 2020s. In this scheme, digital signatures are transmitted at most once every 3 minutes.

Crucially, it seems there is nothing to prevent an attacker from intercepting real navigation messages and re-transmitting them with a carefully chosen delay to give a particular receiver a different understanding of its location. If an attacker was able to cut off a receiver from real navigation messages entirely and make it listen to just retransmitted ones, it's still possible to credibly mess with someone's location (e.g. a man-in-the-middle attack).

How effective attacks are will also depend on how GNSS receivers are implemented and how they will deal with conflicting data. For better accuracy, consumer receivers listen to signals from all systems, including GLONASS and BeiDou. If they receive authentic messages from just a few Galileo satellites and lots of spoofed (unverifiable) messages from other systems -- are they going to reject the unverifiable messages? I suspect not.

Answer 3

So I guess it is possible to use an offline receiver as a very precise clock?

Yes, you can use the GPS PPS (pulse per second) signal available on most every receiver to sync up your clock an maintain better time accuracy.

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?

No, if someone spoofs a GPS satellite signal (which is really sophisticated kind of attack) then you are at the mercy of that signal. You could possibly work to build a system that could detect spoofing.

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?

With GPS, there are two codes, and depending on which receiver you have access to (unless you're military, you're probably not going to get access to p-codes). These determine the accuracy, but so does the receiver, so I would worry about what receiver your using and not how the signals are getting to it because that is what determines the accuracy. There is no 'cryptographic signing' of the signal and it can be spoofed.

There are two codes that are used:

C/A code: The C/A code on the GPS signal is the one used for general or Civilian Access. This code is transmitted at 10.23 million chips per second, Mcps.

P code: The P code is the precision code that can only be accessed by the US military. The P code transmits at a rate of 10.23 Mcps. Source: https://www.radio-electronics.com/info/satellite/gps/signals.php

The accuracy of the PPS signal is also determined by the receiver, and how it generates this signal from the GPS data.