A while ago I thought about how one might apply public key signatures to physical currency. It came to me that if there was a way to generate measurable properties in the notes which couldn't easily be reproduced (essentially, physical randomness in the notes' composure), it'd be possible to make counterfeit notes detectable by signing a hash of a measurement of such randomness by a well known public key and barcoding it on the bill: if one possessed the key, one could produce arbitrary looking bills and just sign the hash. If one didn't have the key, and couldn't cheaply enough reproduce whatever gave rise to the measurement, one couldn't reproduce a valid-looking note. The randomness could come from things like turbulent deposition of magnetic/fluorescent materials on the bill, something one might assume to be fairly expensive to reproduce exactly.
Is there some "prior art" to be found, anywhere? Does the idea sound reasonable? (I tend to be of the opinion that paper money is here to stay for quite a while, and improving on the state of the art of anti-counterfeit measures seems like a fairly good idea. Over that background, Chaumian cash and the like do not make a lot of a difference.) Sampo Syreeni, aka decoy - mailto:[EMAIL PROTECTED], tel:+358-50-5756111 student/math+cs/helsinki university, http://www.iki.fi/~decoy/front openpgp: 050985C2/025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
