Today Will Clarkson, Tim Weyrich, Adam Finkelstein, Nadia Heninger, Alex Halderman and I released a paper, Fingerprinting Blank Paper Using Commodity Scanners. The paper will appear in the Proceedings of the IEEE Symposium on Security and Privacy, in May 2009.
Here's the paper's abstract:
This paper presents a novel technique for authenticating physical documents based on random, naturally occurring imperfections in paper texture. We introduce a new method for measuring the three-dimensional surface of a page using only a commodity scanner and without modifying the document in any way. From this physical feature, we generate a concise fingerprint that uniquely identifies the document. Our technique is secure against counterfeiting and robust to harsh handling; it can be used even before any content is printed on a page. It has a wide range of applications, including detecting forged currency and tickets, authenticating passports, and halting counterfeit goods. Document identification could also be applied maliciously to de-anonymize printed surveys and to compromise the secrecy of paper ballots.
Viewed under a microscope, an ordinary piece of paper looks like this:
The microscope clearly shows individual wood fibers, laid down in a pattern that is unique to this piece of paper.
If you scan a piece of paper on an ordinary desktop scanner, it just looks white. But pick a small area of the paper, digitally enhance the contrast and expand the image, and you see something like this:
The light and dark areas you see are due to two factors: inherent color variation in the surface, and partial shadows cast by fibers in the paper surface. If you rotate the paper and scan again, the inherent color at each point will be the same, but the shadows will be different because the scanner's light source will strike the paper from a different angle. These differences allow us to map out the tiny hills and valleys on the surface of the paper.
Here is a visualization of surface shape from one of our experiments:
This part of the paper had the word "sum" printed on it. You can clearly see the raised areas where toner was applied to the paper to make the letters. Around the letters you can see the background texture of the paper.
Computing the surface texture is only one part of the job. From the texture, you want to compute a concise, secure "fingerprint" which can survive ordinary wear and tear on the paper, such as crumpling, scribbling or printing, and moisture. You also want to understand how secure the technology will be in various applications. Our full paper addresses these issues too. The bottom-line result is a sort of unique fingerprint for each piece of paper, which can be determined using an ordinary desktop scanner.
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