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 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 fibers. 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 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.