Researchers discover way to 'fingerprint' paper.

Think two blank sheets of paper are the same? Look closer.

Researchers at Princeton University and University College London say they can identify unique information, essentially like a fingerprint, from any sheet of paper using any reasonably good scanner. The technique could be used to crack down on counterfeiting or even keep track of confidential documents.

The researchers' paper on the finding is set to be presented at an IEEE (Institute of Electrical and Electronics Engineers) security conference in Oakland, California, next May.

"We've found a way to identify documents even when there was nothing additional printed on them," said Alex Halderman, now an assistant professor at the University of Michigan, who was part of the Princeton team. "This is like an invisible serial number printed on every piece of paper ever made."

Two blank pieces of paper may look identical, but if you hold them to a light, you can see that in fact they're unique mashups of fibres. The researchers say that they can measure this unique texture using a standard 1200 DPI (dots per inch) scanner and some custom software they've written.

By turning the page by 90 degrees and scanning it again and again, the researchers can pluck out subtle distinctions in the paper's texture and create a unique digital map of its surface.

"You scan it four times and then the software is able - from these four scans - to figure out what the surface texture of the document looks like," said William Clarkson, a Princeton graduate student. "Then it can extract essentially a fingerprint of the document."

This isn't the first time these researchers have found interesting data in unlikely places. Four of the paper's six co-authors, including Clarkson and Halderman, helped develop what's known as the cold-boot attack, which showed how to get information out of a computer's memory, even after it has been turned off. This technique could be used to skirt some hard-drive encryption systems.

With a well-preserved sheet of paper, the researchers say that their fingerprints are pretty close to 100 percent accurate. If the paper is soaked or marked up, things become trickier, but with error-correction software it's still easy to make a definitive ID, Clarkson said. "You have to significantly modify the document to make it become unidentifiable."

The researchers believe their technique could be used to identify counterfeit money, tickets and even packaging containers. A drug company like Pfizer, for example, could take fingerprints of their labels when they are shipped, and this data could be verified later by a government or company representative in order to spot fakes. Using public key encryption, companies could even create "self-authenticating" packages that could be checked by special scanners.

Art dealers could use the technique to fingerprint original works of art, they say.

More troubling, however, the technique could be used to track anonymous surveys or to monitor voting done on paper ballots. Ballot tracking wouldn't be easy. Someone would have to scan the ballots before Election Day and then have a way of tracking the order in which these ballots were given out.

Vote tracking is "the possibility that is most troubling to us," said Halderman, who has done extensive research into the security of computerised voting systems. This work shows some new problems with paper ballots too. "There are limitations and need for precautions that we might not have been aware of before," he said.