No matter where you go, there they are.

[Tim Dierks]

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 at consensus.com  --  www.consensus.com
Head of Thing-u-ma-jig Engineering, Consensus Development