unforgeable optical tokens?

[Ian Clelland]

> Not really. Illuminating the device at different locations and
> angles is certainly not as good as a cryptographical challenge.
> Since the location and angle is done by some mechanical device,
> the numers of locations and angles is certainly "small"

I think you're right here; in order for the challenges to be 
reproducable, the locations / angles that the reader uses would 
have to be discrete, probably by some sort of stepper motor. 
However, if the readers are autonomous (and each one needs to 
see the physical token once in order to identify it later,) 
then every reader could be calibrated differently, and would 
therefore use one relatively small subset of locations / angles 
out of a large number of subsets.

> and once you are in posession of the token (e.g. as a clerk ini
> the shop), it might be possible to generate a complete table of
> all location/angle/response triples.

I wonder if an analysis of the diffraction patterns produced by 
passing light though a token like this would provide enough 
information to reconstruct the internal 3-D shape... it strikes 
me as being a problem similar to X-ray crystallography.


Ian Clelland
<ian at veryfresh.com>

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