Why ? I thought the military GPS signals were encrypted. They could make a new GPS version on a new frequency with at least crypto signed messages.
We can’t have such a flaw on the protocol forever.
I'm sure this is true in the sense that general publications use "encrypted" to mean, "There was some sort of cryptographic stuff involved and it went over my head" rather than actually encrypted. Like they do when they tell you that the passwords stolen in a breach were "encrypted with bcrypt" for example.
The key thing to pay attention to here is that unlike say, TLS, this is not a conversation. A GPS receiver is just that, a receiver passively observing the signals. You would usually "bend" GPS by relaying signals from somewhere or somewhen else, and the passive receiver concludes that it must be where the signals it is receiving seem to suggest it is.
Take an easy scenario, imagine I'm in a warehouse that has been designed as a Faraday cage. You've fit a handful of transmitters to the warehouse, and you play back a recording of GPS signals received three weeks ago in the desert outside Las Vegas.
How does my GPS receiver know (using some hypothetical "encrypted military signal") that it isn't in fact near Las Vegas three weeks ago? That's what the signals seem to suggest and there is literally no contradictory data available.
Wouldn't it be possible to embed a clock in a GPS receiver that gets used when decrypting the packets it receives? So instead of just receiving a encrypted packet and trusting it, it'll decrypt it and check when the message was from and if the slew is more than ~1 minute or whatever, then it is probably forged.
That would at least prevent the scenario that you're describing, but since I don't know enough about GPS, it probably introduces other problems.
Sky basically if your only authentication scheme is time, and the protocol is all about getting the time, it is very hard to detect proxing from other locations because the bound of accepted times basically needs to be big enough that you can be spoofed a meaningful amount.
GPS receiver can determine its own position (and get exact time in the process) even after internal clock battery dies, or when internal clock drifts too much. Such cold start takes longer, but is possible.
Ie sign the current data package plus the current time. Receivers ignore everything with an incorrect timestamp and/or wrong signature.
Now you only have to make sure that you don't lose your private keys.
I wonder if the problem lies in a different aspect of needing to authenticate messages quickly enough and thus requiring more processing power.
Also, be careful with assuming that late = imposter. The whole point of GPS is that signals take time to travel over distance, so, maybe the signal is late because you're further from that satellite instead, which is what you were trying to measure when you got distracted trying to prevent spoofing.
> got distracted trying to prevent spoofing
Indeed overwhelming the processing power with junk seems the important issue if crypto is added.
Mental model tweak: The signals aren't for a specific location, they are world-wide. So the "for Las Vegas" part doesn't work.
Replay attacks are just a kind of denial of service. In that case, you may as well just use jamming. Spoofing attacks are far more dangerous, and cryptographic signing defeats them effectively.
The whole point of GPS is that the differential in signal timing between the signals from the transmitters on different satellites tells you how far you are from those satellites, and thus if you know where the satellites are supposed to be, that tells you where you must be.
As a result, recording the signals in a desert outside Las Vegas three weeks ago is by far the easiest way to get... drum roll... the signals which when received by your GPS receiver right now will convince it that right now it's three weeks ago and it is just outside Las Vegas.
> Replay attacks are just a kind of denial of service
Nope, as explained they're actually a very common and effective spoofing tactic. The receiver doesn't say "No service" it will tell you that you're outside Las Vegas three weeks ago.
> Spoofing attacks are far more dangerous, and cryptographic signing defeats them effectively.
Spoofing attacks use the same principle (although of course they'll tell your receiver it's a few milli-seconds ago and it is somewhere nearby, not outside Las Vegas three weeks ago).
Signatures make no major difference, this data is authentic it's just second hand. Everything checks out, that's why they do it.
The thing with GPS is that it is also a time source, so you'd need a secondary source for your time to confirm that you are not off by three weeks.
So if the timestamp of the signal deviates of GPS's internal clock, GPS receiver knows it.
Now for commercial receivers, they assume their internal clock is wrong, and set their own time to new GSP time.
Not sure what military GPS receivers do, but I imagine, they kind of thought about this, and just ignore signals that have timestamps that differ too much. This probably means that they need pretty accurate internal clock.
Another way to encrypt a signal is to just have an encrypted sequence going on. If you get a sequence you have already seen something is wrong. Think of it as hash+salt or something similar.