unforgeable optical tokens?

[Ed Gerck]

bear wrote:

> Anyway; it's nothing particularly great for remote authentication;
> but it's *extremely* cool for local authentication.

Local authentication still has several optical issues that need to be answered,
and which may limit the field usefullness of a device based on laser speckle.

For example, optical noise by both diffraction and interference effects is a
large problem -- a small scratch, dent, fiber, or other mark (even invisible,
but producing an optical phase change) could change all or most all of
the speckle field. The authors report that a 0.5mm hole produces a large
overall change -- which can be easily understood since the smaller the defect,
the larger the spatial effect (Fourier transform).

But temperature/humidity/cycle differences might be worse -- any dilation or
contraction created by a temperature/humidity/cycle difference between recording
time (in lab conditions) and the actual validation time (in field conditions) would
change the entire speckle field in a way which is not "geometric" -- you can't just
scale it up and down to search for a fit.

Also, one needs to recall that this is not a random field -- this IS a speckle field.
There is a definite higher probability for bunching at dark and white areas
(because of the scatter's form, sine function properties, laser coherence length,
etc). This intrinsic regularity can be used to reduce the search space to a much
lower space than what I saw suggested.  Taking into account loss of resolution
by vibration and positioning would also reduce the search space.

Finally, the speckle field will show autocorrelation properties related to the sphere's
size and size distribution, which will further reduce randomness. In fact, this is a
standard application of speckle: to measure the diameter statistics of small spheres.

Cheers,
Ed Gerck


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