Think two blank sheets of paper are the same? they aren't.
Researchers at Princeton University and University College London say they can identify unique information, essentially like a fingerprint, from any sheet of paper using a 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 California in May.
"We've found a way to identify documents even when there was nothing additional printed on them," says Alex Halderman, 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 when held to a light, it can be seen that 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 software they've written.
By turning the page by 90 degrees and scanning it again and again, it's possible to 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 to figure out what the surface texture of the document looks like," says William Clarkson, a Princeton graduate student. "Then it can extract essentially a fingerprint of the document."
With a well-preserved sheet of paper, the researchers say that their fingerprints are close to 100% 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 says. "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.
On the downside, it could be used to track anonymous surveys or to monitor voting done on paper ballots. Ballot tracking wouldn't be easy, however; 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", says 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 [a] need for precautions that we might not have been aware of before," he says.