Digital Watermarks: What Hides Within

No longer just distinctive designs in paper, watermarks now are also patterns of bits embedded in digital content

July 01, 2004 — CSO — Watermarking began in Italy in the 13th century when papermakers used these marks to identify their handiwork. Eventually, specialized watermarks were commissioned. To impede forgery, for instance, governments created distinctive watermarks for official documents. To be effective, this required controlling the distribution of the resulting watermarked paper.

The modern watermarking process is relatively simple: Papermakers spray wet pulp onto a moving belt and, before the mixture dries, press a design into the pulp using a device called a dandy roll. This dandy roll looks like an oversized printer's roller covered with a patterned wire mesh. The roll rearranges the paper fibers when it presses against the wet pulp, creating the watermark. Holding a finished sheet of paper up to the light reveals the pattern.

Paper watermarks are less common today than they used to be. Most currencies have watermarks on them, as does expensive bond paper. (For example, some universities use special bond stationery bearing their official watermarks to send out acceptance letters.) However, such bond paper doesn't work well with laser printers or ink-jets; the result is that watermarks on paper will surely become a thing of the past.

Desktop computers are quite good at mimicking the look and feel of paper watermarks. Some printer drivers allow you to print on every page a so-called background watermarka faint gray image that does a pretty good job of simulating a traditional paper watermark. And Adobe's Acrobat Professional has an "Add Watermark" option under the program's "Document" menu that lets you add the watermark image of your choice to a PDF file.

Printed watermarks offer nothing in the way of security or document authentication, of coursethey're just there to satisfy aesthetic sensibilities. As in the pre-digital era, real security comes only with a mark that's hard to forge.

"Digital watermarking" is an emerging concept, but don't let the name fool you. It's not used for authenticating documents. (That's the job of digital signatures.) Unlike a paper watermark, a digital watermark plays off that other sense of the word. It refers to the ability to unobtrusively include information in a file, and is commonly executed through a variety of cryptographic techniques, collectively known as "steganography." But instead of gently rearranging the paper fibers, digital watermarks gently rearrange bits scattered through a piece of digital content.

One common use of watermarks is for embedding copyright information inside digital images, audio and video files. Watermarks are not text that might be put into a file's "comments" field; the watermark instead is put directly into the file's data, typically by making minor variations to pixel brightness. These variations are too subtle to be noticed by the human eye. The patterns are repeated many times, allowing the information contained in the watermark to be recovered even if the image is cropped. The best watermarks can even survive a limited amount of image manipulation, such as contrast adjustments or the use of sharpening filters.