From: fnerd@smds.com (FutureNerd Steve Witham)
Date: Thu, 24 Feb 94 16:13:48 PST
To: cypherpunks@toad.com
Subject: Re: Digitally Signing Physical Objects
Message-ID: <9402250005.AA16110@smds.com>
MIME-Version: 1.0
Content-Type: text/plain


i wrote-
> >[Physical signature]
> > seems to have a tricky dependence on the tolerance.  The forger can
> >get a valid plaintext and signed feature vector.  So, if the tolerance
> >for error is too low, you get false positives, but if it's too high, a
> >forger could create something starting from the feature vector.  An
> >interesting CAD/CAM problem.

Allen J. Baum replied-

> To keep black market forgery part off the market, a 30% tolerance is way
> more than enough. There should be no false negatives (making a real part
> look fake), but if 1/3 of the forgeries slip through (i.e. 2/3 don't), this
> has the affect of driving the forgery price up by a factor of 3,
> effectively pricing them out of the market. 

I don't see how you factor out the variables of the resolution, what
physical property is scanned, etc.  For some combinations, it would be easy 
to forge 100% matches.  For others, it would be hard to get a 30% match on
the original object...  Maybe the whole scheme could be improved by a
trap-door function that has built-in error-tolerance.  (By the way, if 
anybody knows about fuzzy hash functions, please write me; I'm curious for
other reasons.)

> (Unless the real goods are overpriced a factor of 3...:-)

Isn't Great Art overpriced by thousands of times, in terms of reproduction
cost?  Anyway, a 2/3 rejection rate would be tough.

-fnerd
quote me


- - - - - - - - - - - - - - -
We shall have to evolve
Problem solvers galore
As each problem they solve
Creates ten problems more. --Piet Hein
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