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With identity theft on the rise, there is more reason than ever to ensure the authenticity of important documents such as passports and birth certificates. Now physicists in England have discovered that many items, including paper documents, plastic cards and product packaging, have intrinsic patterns that can be used for identification purposes. And because the configurations are virtually impossible to modify in a controllable manner, they could form the basis of a new tool in the fight against fraud.

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."