about physical watermarking, redux

[Sampo Syreeni]

Many years ago I posted an idea of mine about cryptographic physical 
watermarking of things, such as paper money, or maybe missiles. Whatnot.

The idea was that you'd do some chaotic physical process in order to lay 
down a physical watermark, then image it, and finally digitally sign 
what was seen via asymmmetric cryptography. I imagined you'd then print 
the signature "on the bill" as a 2D barcode, to be verified. But I never 
worked out how you would deal with the inevitable "broken bill". I 
thought it'd take some kind of high end error correcting code.

Now it finally came to me you don't need that at all. Instead, just 
repeat what you imaged from the bill, on the bill, verbatim, using 
whatever level of ECC you want, and then the signature. The verifier can 
utilize the digital, error corrected replica for hard crypto purposes, 
while separately verifying that it matches -- in any soft statistical 
knee -- a hard to mechanically replicate, unique signature, embedded in 
the "bill".

My first and best idea about how to make this physical nonce is to mix a 
couple of dozen well cut differently fluorescent plastic fibers into the 
paper or plastic fiber pulp from which the bill/artifact is made. It 
would be rather difficult to replicate such a random arrangement of many 
fibers over, say, a 1200dpi scanned bill. Especially if the highest end 
scanner interferometrically made sure, that the fibers/chaff really are 
embedded in the paper, instead of having been printed on it.

Any problems with my idea? I'd like to hear, especially since it has 
been a couple of decades coming.
-- 
Sampo Syreeni, aka decoy - decoy at iki.fi, http://decoy.iki.fi/front
+358-40-3751464, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2