I was going to say "GPS predates signing", but it turns out that the first GPS launch and the invention of RSA signing were in the same year: 1978. Of course the signing technology was also considered to be subject to US export control for decades ...

Besides, signing doesn't necessarily help - the easiest way to GPS spoof is simply to re-broadcast the signals received at a nearby point at higher power, so the victim receiver thinks it's at that point.

There is recent work on doing this: https://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Lo_I...

1978 RSA invention is when it happened in public domain, 5 years before that a guy at GCHQ invented a similar asymmetric scheme, which i'm sure was well in use by 1978.

replay attack does make sense, but isn't it identified uniquely by time? i mean the content of the payload is basically very accurate time. if a device notices the time doesn't change - it can detect spoofing.

You continually receive signals at point A, send them to point B and transmit them there at higher power.

yeah, you're right, it's sort of a brain in the jar problem, isn't it. but it does seem like it's not that hard to detect because of latency between A and B, though it would require a pretty accurate clock on GPS device.

No! Differential latency from the satellites is entirely how GPS works, and there is no way of having an accurate enough clock to be able to spot that without (a) it being an atomic clock and (b) synced to the satellite time at a known location.

But accuracy of GPS signal is in microseconds, so if you suddenly detect a change in level of milliseconds (latency between A and B) - that would totally imply spoofing shenanigans?

Yes, which would manifest as a sudden movement of place detected. What I'm not sure about is what this looks like as you approach the jammer and its signal gradually overwhelms some or all of the signals; does it just look like multipath error.

Obviously I don’t know what I’m talking about, but it seems like such attack is only undetectable by cold gps device that has been offline for longer than some period of time, long enough to explain time shift by clock drift.

It would be fairly easy to rebroadcast continuously, then it's a valid signal, just in the wrong place.

Receiving the same signal over and over would be suspicious, but unless you have some other method to determine time and position, what are you going to do?

You could say this about any legacy network. Like the mobile phone network. The designers just didn't think about it/it was expensive/it's expensive to fix/there's legacy hardware.

well if the hardware is advanced enough to do encryption, it is certainly is capable of producing signatures. are GPS satellites not upgradeable? maybe they didn't think about it 40 years ago, but in the last decade - it's about time..

For what purpose? Risk someone crafting fake signatures?

From what i can understand, this separation of concerns is a safer option

so that no device under any circumstances can be spoofed. what kind of risk is there in crafted signatures if right now you're 100% vulnerable to spoofing without any effort required to produce said signatures?