Last month we learned how the anti-piracy outfit BREIN is able to track down
suspected e-book pirates by matching digital watermarks inside e-books with the transaction records they receive from e-book sellers (not without causing a political controversy
). If you've caught yourself wondering how such a digital watermark looks like, a computer science student at the University of Twente was able to get his hands on a watermarked EPUB file and dissected it into pieces
. The result is astonishingly simple: embedded on every page in the book is a minuscule image that turns out to be an ordinary barcode. This barcode contains the transaction code which uniquely identifies the e-book purchase.
Other pages in the book all have the same image at the bottom that is nearly impossible to notice with your naked eye. The image is embedded as a Based64 encoded string (see the image depicted above), decoding (using the openssl base64 command) it results in an image with the dimensions of 1 by a few hundred pixels. When seen with a naked eye it looks like a thin white line. [...]
When mapping the 7-bit blocks to characters you can see almost immediately that it results in a string equal to the transaction code. So, the barcode you find on each page in the book represents the transaction code encoded in 7-bit ASCII.
So is that really all? The author of the post doesn't exclude the possibility of other "invisible" watermarks, such as random variations in text or punctuation. How to know for sure? By comparing two separate purchases of the same book.