From: jim bell <jimbell@pacifier.com>
Date: Fri, 12 Apr 1996 20:41:41 +0800
To: pmacdorn@isrinc.com
Subject: Re: GPS-based authentication
Message-ID: <m0u7bv8-0008ykC@pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


At 11:33 AM 4/11/96 -6, Peter Trei wrote:
>I've read with interest your proposed GPS-based authentication 
>mechanism (it was posted to the cypherpunks mailing list). Can you
>confirm that you wrote this? Some people on the list think it may be
>a forgery.
[deleted]

>You say:
>" The signature ... is formed from bandwidth compressed raw 
>observations of all the GPS satellites in view."
>
>" 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)."

I think that Denning's paragraph is misleading.  S/A is stated; what I think 
they probably mean is closer to A/S, the anti-spoofing signal.  The S/A 
error is small and changes only slowly, the A/S signal is far faster and 
could hold the data to implement the signatures.  Even so, I think it would 
be comparatively easy to fake a signal if you are in view of most of the 
satellites in the area you wish to fake; attempting to fake something around 
the world would be harder.  

Aside from the technical difficulties associated with this  system (everyone 
has to have a GPS receiver, for instance) there is an obvious political 
problem associated with trusting the government.  After all, only its agents 
are supposed to know the dithering code which will be transmitted by the GPS 
satellite; its agents would presumably be able to fake their location since 
they can anticipate the data being transmitted.

Which raises an interesting issue, I think:  Would it be possible to remove 
the ability of the government to fake these signals, even if the rest of the 
system worked?  The goal would be to prevent anybody (including the 
operators of the GPS system) from being able to anticipate the dithering 
codes the GPS satellites would send.  One way to do that is to combine 
multiple random/pseudorandom bit streams (from hundreds, thousands, or maybe 
even millions of independent sources, perhaps you and me) into an overall 
stream, in such a way (XOR) that no data contributor could know how the 
result came out until he saw it.  Each contributor would be able to verify, 
however, that his data stream was used to form the eventual bit stream, and 
he is confident of the randomness of the system he uses to generate that 
stream.  (If he isn't, he should just change systems, or add another system 
to his equipment and XOR the results before he sends them off, crypted, to 
the central data combiner.)  Release of the decrypt keys could be delayed 
"just long enough" to prevent faking.

BTW, I'm not endorsing the underlying idea.  I think it's a leap backwards 
for freedom.


Jim Bell
jimbell@pacifier.com