writes "German researchers have created a new DRM feature that changes the text and punctuation of an e-book ever so slightly. Called SiDiM, which Google translates to “secure documents by individual marking,” the changes are unique to each e-book sold. These alterations serve as a digital watermark that can be used to track books that have had any other DRM layers stripped out of them before being shared online. The researchers are hoping the new DRM feature will curb digital piracy by simply making consumers paranoid that they’ll be caught if they share an e-book illicitly.
Seems like I recall reading about this in Tom Clancy's Hunt for Red October when Jack Ryan used this technique to identify someone who was leaking secrets to the Russians. It would be so very difficult for someone to write a little program that, when stripping the DRM, randomized a couple of pieces of punctuation to break the hash that the vendor is storing along with the sales record of the individual book."Link to Original Source