The fingerprint of paper

At a time of rising concern about terrorism, British scientists last week announced that they had discovered a relatively inexpensive and surprisingly powerful new way to defeat forgers attempting to fake birth certificates, passports, and other documents.

The scientists built a laser scanner that sweeps across the surface of paper, cardboard, or plastic, recording all of the unique microscopic imperfections that are a natural part of manufacturing such materials.

This scan serves as a fingerprint which, the scientists said, has two surprising properties: The fingerprints are robust, surviving scorching, dousing in water, crumpling, and scribbling over with pens. And these fingerprints depend on structures that are so complex and so small -- on the scale of between one tenth and one ten-thousandth the diameter of a human hair -- that nobody on the planet will be able to copy one for the foreseeable future. Unlike other methods such as using holograms or special inks, the fingerprint is already there.

To authenticate a passport, for example, someone would scan one of the pages at a predetermined spot, and compare the results to a scan made when the passport was first issued.

''This is a whole new approach to countering fraud and counterfeiting," said Russell Cowburn, who led the research and is a professor of nanotechnology at Imperial College London.

If the technique, which is described in the current issue of the journal Nature, proves commercially practical, then it could find a wide range of applications, from securing official documents like birth certificates, passports, and identification cards, to preventing fraud by checking the packaging of pharmaceuticals, cigarettes, and other items.

Already, there are security technologies, such as human fingerprinting and hand and iris scans, which identify people by the natural differences among them. This seeks to expand that idea -- using a laser scanner that costs less than $1,000 -- to the world of products.

''If it all pans out, the implications could be huge," said Peter Schroeder, a professor of computer science and applied and computational mathematics at the California Institute of Technology. ''It is a very neat idea and like a lot of ingenuity, it seems obvious in retrospect."

A British company commercializing the technology, Ingenia Technology, has been talking with officials at the U.S. Government Printing Office, which prints passports, and the National Institute of Standards and Technology, a federal agency that advises the government on security and a wide range of technology. The Government Printing Office is planning to conduct an independent test of the scanner in the next few months, according to Michael L. Walsh, the chief technical officer there.

The scientists made the discovery, said Cowburn, after a bad glue job. The laboratory was looking for a new method of securing documents using chips with very small features that could be read by a laser. The chips are difficult to manufacture, Cowburn said, and thus were seen as a way to discourage forgers. One day, though, a scientist got a signal that was strong, but completely different from what was expected. The reason: The chip had fallen off the paper.

Seen at extreme magnification, a seemingly smooth sheet of paper looks like a tortured landscape of sharp cliffs and deep valleys, made by the way the individual paper fibers fell together when the paper was manufactured. This landscape is what gives each piece of paper its unique fingerprint, with no need to manufacture a special chip.

The scanner works by shining a line of laser light on the paper, according to the Nature report. As the light sweeps across the paper, the intensity of light varies at the scanner's four light detectors. A record of these intensities is the fingerprint.

The team showed that they can verify a document -- matching a scan to a previous scan of the same document -- with very high accuracy, as long as the area scanned is the same. The method is flexible enough, though, that the center of the area scanned only has to be close to the center of the one scanned before -- within a few centimeters horizontally and a few millimeters vertically, according to Cowburn.

The research was funded by investors, and Cowburn has a financial stake in Ingenia Technology, where he serves as one of the directors.

There are two main ways that the technology might be used to secure a document, such a birth certificate, Cowburn said. When the birth certificate is issued, the scanner would be used on one spot of the certificate, generating a code that could be kept in a central computer file somewhere. When someone attempts to use a birth certificate, for example to get a passport, then it would be scanned again and compared to the database to authenticate it.

Alternately, the birth certificate could be scanned and the fingerprint, in numerical form, could be encrypted and then printed on the birth certificate as a bar code. When the birth certificate was presented, it would be scanned and compared with the bar code. There are bar codes in commercial use now that could hold enough information to store the fingerprint, Cowburn said.

In both cases, it would be virtually impossible for a forger to get past the scanner except by somehow breaking into the computer system that stores the information and modifying it. Cowburn said that he envisioned the technique would be used in combination with other security methods, the same way currency or passports have multiple security features.

Gareth Cook can be reached at cook@globe.com.