At 4:22 PM 4/10/96, Peter Trei wrote:
Peter - didn't they say that the checking station is also listening to the satellites? That way they can tell that you are playing back signals that you taped earlier because they won't match what the satellites are broadcasting right now.
I think your idea would work if you wanted to pretend to be at a point which was _farther_ from each of the satellites than where you actually are. Then you could delay all of the signals. But the only way to be farther would be to be deep underground. You might be able to pretend to be at the center of the earth, but that is not very useful.
Actually I suppose this only applies to those satellites which are shared between you and the checkin station. If you are far away then maybe you only share one or two. If you know which ones those are, you can lie to your heart's content about other ones, and for the shared ones you can again delay the signal and claim to be farther than you are.
If their authenticated repeaters are used then you have to assume the checking station has all the satellite signals and again the best you can do is pretend to be a Mole Man.
Hal
Denning hasn't thought this through. Do the math.
While we may disagree with Ms. Denning on a number of political matters, she's quite intelligent; I suspect the paper is well-founded.
The diameter of the earth is 12,576 km The speed of light is about 3e5 km/sec
GPS receivers are line-of-sight only; only a small portion of the earth can see the same satellites.
[...] Therefore any site that can see the same set of satellites as the site it is trying to simulate can do so, buffering less than 50 ms of waveforms and pretending to be on the end of a slow link.
GPS works by measuring the differing distances to a number of satellites. Thus, a crucial factor of GPS reception is not just the signals from satellites, but the different times at which these signals were received. It might be possible to seperately record the signals from several different satellites, delay them each just the right amount of time, and then recombine them to simulate being at another nearby location (within several hundred miles). However, this might not be possible. Examine the following quote from Denning's paper: :The location signature is virtually impossible to forge at the :required accuracy. This is because the GPS observations at any given time :are essentially unpredictable to high precision due to subtle satellite :orbit perturbations, which are unknowable in real-time, and intentional :signal instabilities (dithering) imposed by the U.S. Department of Defense :selective availability (SA) security policy. It's possible that the orbit perturbations may be enough to screw up an attempt to forge a signal; the variations in signal timings won't provide enough information to an attacker to be able to accurately replicate what the signal would look like at another location. It remains to be seen whether it is reliably possible for the secure host, at its location, to distinguish between an accurate signature and an inaccurate but plausible forged signature. Selective Availability doesn't really seem to matter that much, especially since it's going to be phased out. (There was an announcement on this last week, but I can't find a reference right now). - Tim Tim Dierks -- timd@consensus.com -- www.consensus.com Head of Thing-u-ma-jig Engineering, Consensus Development