Every Sheet of Paper Has a Unique 'Fingerprint'S

What you see here is an 8.5x11 piece of paper scanned by an ordinary scanner (with the contrast bumped up). From shots like this one, researchers claim they can "fingerprint" any piece of paper.

The complete method is explained in the paper Fingerprinting Blank Paper Using Commodity Scanners. Here's the 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.

Essentially, by scanning a piece of paper from several angles, it's possible to use shadow readings to reconstruct the paper's unique 3D features—what becomes the basic fingerprint. This fingerprint data can then be stored in something as simple as a barcode for later validation.

Now if we could only authenticate our digital data with such foolproof measures. [Freedom to Tinker via boingboing]