Russell Cowburn and co-workers at Imperial College London, Durham University and the University of Sheffield used a phenomenon called "laser speckle" to examine the structure of different surfaces. This technique is already routinely used to measure surface roughness in metal and paper and for visualizing blood in vivo. They scanned a focused laser beam over a sheet of white paper and used photodetectors to record the intensity of the light reflected from four different angles.

The physicists then quantified how much random fluctuations on the paper differed from the mean value (called the zero positional shift) and converted these values into 1s and 0s to obtain the fingerprint code. They obtained different codes for different sheets from the same pack (see figure), and achieved similar results for plastic credit and identity cards and cardboard packaging. Moreover, a sheet of paper could be identified even after it had been screwed up into a ball, submerged in water, baked at 180°C, scribbled on with ballpoint and black marker pens, and scrubbed with abrasives.

"Our findings open the way to a new and much simpler approach to authentication and tracking," says Cowburn. "This is a system so secure that not even the inventors would be able to crack it since there is no known manufacturing process for copying surface imperfections at the necessary level of precision."

The probability of two pieces of paper sharing the same fingerprint is less than 1 in 1027 and for smoother surfaces, such as plastic cards and cardboard, it is 1 in 1020 says the team. Furthermore, each fingerprint only takes up between 200 and 500 bytes of storage space in a database.

"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 -- it's as if documents and packaging have their own unique DNA," adds Cowburn. "This makes protection covert, low-cost, simple to integrate into the manufacturing process and immune to attacks against the security feature itself."