This could make an enormous difference in security against forgeries:
The scientists built a laser scanner that sweeps across the surface of paper, cardboard, or plastic, recording all of the unique microscopic imperfections that are a natural part of manufacturing such materials.
This scan serves as a fingerprint which, the scientists said, has two surprising properties: The fingerprints are robust, surviving scorching, dousing in water, crumpling, and scribbling over with pens. And these fingerprints depend on structures that are so complex and so small—on the scale of between one tenth and one ten-thousandth the diameter of a human hair—that nobody on the planet will be able to copy one for the foreseeable future. Unlike other methods such as using holograms or special inks, the fingerprint is already there.
Scientific American has more details:
All nonreflective surfaces are rough on a microscopic level. James D. R. Buchanan and his colleagues at Imperial College London report today in the journal Nature on the potential for this characteristic to “provide strong, in-built, hidden security for a wide range of paper, plastic or cardboard objects.” Using a focused laser to scan a variety of objects, the team measured how the light scattered at four different angles. By calculating how far the light moved from a mean value, and transforming the fluctuations into ones and zeros, the researchers developed a unique fingerprint code for each object. The scanning of two pieces of paper from the same pack yielded two different identifiers, whereas the fingerprint for one sheet stayed the same even after three days of regular use. Furthermore, when the team put the paper through its paces—screwing it into a tight ball, submerging it in cold water, baking it at 180 degrees Celsius, among other abuses—its fingerprint remained easily recognizable.
The team calculates that the odds of two pieces of paper having indistinguishable fingerprints are less than 10-72. For smoother surfaces such as matte-finished plastic cards, the probability increases, but only to 10-20. “Our findings open the way to a new and much simpler approach to authentication and tracking,” co-author Russell Cowburn remarks. “This is a system so secure that not even the inventors would be able to crack it since there is no known manufacturing process for copying surface imperfections at the necessary level of precision.”
To ensure the security of currency, you could fingerprint every bill and store the fingerprints in a large database. Or you can digitally sign the fingerprint and print it on the bill itself. The fingerprint is large enough to use as an encryption key, which opens up a bunch of other security possibilities.
This idea isn’t new. I remember currency anti-counterfeiting research in which fiber-optic bits were added to the paper pulp, and a “fingerprint” was taken using a laser. It didn’t work then, but it was clever.