Fingerprints aren’t just for fingers anymore. Now, they could be an important new tool for fighting document forgery.

All paper, as well as plastic credit and debit cards, bears a unique “fingerprint” of microscopic surface imperfections. According to Russell Cowburn, professor of nanotechnology at Imperial College London, detecting these unique patterns is easy to do with a portable laser scanner.

And it’s cheap, too: “Our field scanners could be manufactured for $1,000 or less (when made) in volume,” said Cowburn.

The detection process makes use of the optical phenomenon known as laser speckle. Light coming from a focused laser is coherent, or in phase, but when it strikes a microscopically rough surface like a piece of paper, the light is scattered, producing a pattern of light and dark “speckles.” The scanner’s photodetectors digitize and record this pattern.

According to Cowburn’s research, as published July 28 in the journal Nature, the unique speckle pattern of a sheet of paper remains recognizable even after crunching the paper into a ball, soaking it in water, baking it at 180 degrees Celsius (350 degrees Fahrenheit) for 30 minutes, scrubbing it with an abrasive cleaning pad or scribbling over it with a big black marker.

A cross-correlation algorithm that assesses the degree of similarity between the base-line scan and the new scan allows the paper’s identity to be verified. The odds of two pieces of paper having similar patterns are greater than 1,000 to one.

These fingerprints raise the possibility of securing documents without resorting to controversial solutions like RFID tags. In the future, every passport, driver’s license and birth certificate could be scanned for its unique speckle pattern by the issuing agency. Portable scanners at border crossings or police stations would read the pattern on the document in question and match it to the baseline database. A standard desktop PC could check 10 million entries per second.

This could put document forgers around the world out of business. “There is no known manufacturing process for copying surface imperfections at the necessary level of precision,” said Cowburn.

“The beauty of this system is that there is no need to modify the item being protected in any way with tags, chips or inks,” he said.

But it’s still not foolproof. This sort of security would not have prevented the 9/11 terrorists from obtaining their legal Virginia driver’s licenses with false information, said Nick Fadziewicz, an expert on security at Comter Systems. Eleven of the terrorists successfully obtained those licenses using false information. Many states, including Virginia, now have much tougher requirements.

“There is no one solution for security,” said Fadziewicz. “The goal is to put in enough strong security measures to minimize the (potential) to create fake documents.”

By Stephen Leahy

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