In the picture

Thanks to computer technology, bootleggers have never had it so good. Are “digital watermarks” the answer?

Science & technologyJan 10th 1998 edition

IN A break with The Economist's general practice, this article is signed by the author. His dread of the editor's fury, however, enjoined him to keep his byline concealed. His first name is woven into this paragraph like an invisible thread, and to make it visible you need his age, since that is the number of characters (not including spaces) between each letter of his name, as well as the number of characters before the first letter.

Hiding information inside other information is called steganography (from Greek, meaning “covered writing”). It was originally devised as a way to send secret messages, by burying them in a prearranged way in innocuous material. As such it has been a favourite encryption technique since the time of the ancient Chinese, who embedded code ideograms in messages, much as in this article. But now that images and sounds can be made instantly available around the world, steganography is taking off with the opposite use in mind: using secret messages to protect the material they are buried in.

Bootlegging has always been a problem for artists—musicians, for example—whose work is meant to be mass-reproduced. But the bootleggers' customers have traditionally had to put up with second-rate goods. Once music and images are stored digitally as strings of numbers, however, they can be copied perfectly; and if they are put on the Internet, copying and distribution cost almost nothing. To prosecute mass-scale piracy, creators first have to be able to prove their creatorship, something that is much more difficult in a digital age. Thus a number of companies have been developing “digital watermarks” to help them.

Simply adding a name in digital form—printing it at the top of a photograph, say—is useless. It spoils the work, and since digital material is easy to fiddle with, pirates can simply remove it. A digital watermark has to be encoded so that it does not show, cannot be deliberately removed, and is still there when a picture or recording is altered, or when only an extract of it is used. This is quite a challenge.

Fortunately, whereas the opening of this article had to be written so that each letter of the author's name was in exactly the right place—which took ages—a picture or sound already created can have extra information added without making a perceptible difference. In a picture, for example, each pixel (ie, dot) is described by two numbers: colour and luminance, or brightness. The simplest watermarking method is to encode the information as slight changes to the luminance of the pixels in a small patch, and repeat the changes throughout the picture. The code can be read by overlaying the watermarked patches so that the small changes are multiplied and become detectable.

Digimarc, a company in Portland, Oregon, offers commercial watermarking services for images put on the Internet. It uses a similar but more sophisticated approach. It makes the watermark weaker in bland, uniform areas of a picture, and stronger in patches of sharp contrast (eg, the edges of shapes) where it is harder to see. Signum Technologies in Cheltenham, in Britain, goes yet further, arranging the watermark data in a different order in each patch. This makes the watermark pattern non-repeating, and thus less noticeable to the eye. It also makes the mark harder to remove deliberately, since doing so depends on knowing how it was scrambled.

But these methods are not impregnable. Digimarc had to issue an improved version of its software in September after its customers discovered that watermarks rarely stood up to JPEG compression, a standard Internet method of storing pictures more efficiently by cutting out less significant data. And image-editing software can be used to do things that will obliterate a watermark—for example, redrawing a photograph so that it looks like a relief cut in stone—while leaving a picture that clearly derives from the original.

A more robust technique is to convert the picture into the “frequency domain”, essentially a graph of how quickly features vary across the image. The watermark is encoded as changes to parts of this graph, which is then turned back into a picture. Fairly large changes in the frequency domain can be made without degrading the image. Signafy, a spin-off of the Japanese electronics giant NEC based in Princeton, New Jersey, is one firm developing this type of marking, and claims that its watermark will even survive in images that have been sent by fax—should anyone want to look.

In the noise

Whatever the method used, putting a watermark in is only part of the story. To spot watermarked pictures being used illicitly, a program known as a webcrawler can trawl the Internet looking for them. However, prosecuting miscreants will rarely be worth the hassle. Datamark, a start-up in London, has begun offering a more big-brotherish approach with software designed for use by picture libraries. Datamark's technique injects a unique watermark into an image whenever it is downloaded. As a result, the watermark identifies not only who owns the copyright on a picture, but also who downloaded each copy. The gallery can then hold its customers liable if copies show up somewhere they should not be.

For now, most picture libraries will probably stick to more physical means of delivery, since downloading a sharp enough image over the Internet takes a long time. In general, the pictures on websites are too small, or their quality too poor, for their copyright to be worth bothering with, and most are pilfered only for use on other sites. This does matter if, for instance, the site that has stolen them is itself an online gallery that charges for admission—almost all of these being porn sites—but webcrawlers cannot get into such sites anyway.

Watermarking music may turn out to matter more. As with pictures, this involves making unnoticed changes to the material—for instance, to sounds at frequencies that people do not hear so well. Again, compression can trip things up, because it often cuts out precisely those frequencies. But while people may want to meddle with pictures, they generally prefer their music intact (which means the marks need not be quite so robust), and they tend to buy music more than they buy pictures.

At the moment, downloading a song at hi-fi quality over the Internet can take several times as long as playing it, but a number of small companies have set up as online distributors, in the hope that Internet connections will get faster in the next few years. They watermark their merchandise—but not as the first line of defence against piracy. For example, Liquid Audio, based in Redwood City, California, relies mainly on its own brand of encryption, which ensures that each piece of music sent out is encoded so that only the intended recipient can play it. The watermark is a fallback, in case the encryption is broken.

Watermarking has its limitations. As with all crimes, the more sophisticated the deterrents, the more sophisticated the criminals become. And, given the Internet's disregard for national borders, lawyers will often be powerless. But digital watermarks may find a legal role that has nothing to do with copyright. In these days of easy fakes, pictures from police cameras or interview recordings could have watermarks put in that are fragile rather than robust. Like the name of the (26-year-old) author at the start of this article, such watermarks would disappear with the slightest tampering; their presence would guarantee that the material was authentic. Whether you are faced with corrupt policemen or marauding editors, such protection could be invaluable.

This article appeared in the Science & technology section of the print edition under the headline "In the picture"

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