Recently I received a check and across the top was the legend, "This document contains a watermark. Hold at an angle to view." Watermarking-impressing a design into the fibers of paper during its manufacture-has been well-known since the earliest days of paper making. It is used for security, to indicate paper quality, or in business environments, as a subtle promotional technique. Generally, there is no attempt to hide a watermark, and especially in security applications, one's attention is often deliberately drawn to it to validate the genuineness of a document.
Currently, much research and development is focused on digital watermarking technology, which shows promise as a way to protect the rights of digital information owners. Digital watermarks differ from printed ones because they are usually hidden from the user and can be applied to several different types of media-text, images, and even sound. Such versatility makes digital watermarks very attractive to many people who would like to add security to their products, such as publishers, music producers, and photographers.
TECHNOLOGY OF DIGITAL WATERMARKING
Digital watermarking can be considered part of the science of steganography which literally means "covered writing." (The Greek root stega- means "to cover.") According to Johnson's excellent review of the history and theory of steganography , Steganography is the art and science of communicating in a way which hides the existence of communication. In contrast to cryptography, where the enemy is allowed to detect, intercept, and modify messages...the goal of steganography is to hide messages inside other harmless messages in a way that does not allow the enemy to even detect that there is a second secret message present.
Other terms relevant to digital watermarking are data hiding and information hiding. Two international conferences on information hiding were held, one in 1996 and another in April 1998. Many papers describing research on digital watermarking and steganography can be found in the proceedings of those conferences.
In researching this article, I found several excellent Web sites dealing with digital watermarking and steganography (see the sidebar). Some of these sites are additionally useful because they offer extensive directories of links to additional relevant sites.
Digital watermarking uses naturally occurring variations of text and images, and thus cannot be seen by the user unless special techniques are employed. A watermark can be embedded into an electronic image, for example, by slightly changing the brightness of some of its pixels in a regular or even random pattern. Because the watermarking changes are so slight, the human eye cannot readily detect them. The watermark can be dispersed throughout the image so that it is very difficult to remove even if detected. In the case of text, the same effect can be achieved by slightly varying the spacing between the lines and characters that make up the document. With sounds, watermarking software can disperse messages throughout the sound file that can be read by a computer, but are imperceptible to human ears.
Proponents of digital watermarks claim that they are not only difficult to detect, but they will survive copying, printing, and electronic manipulations or transformations into other file types because they cause only slight changes to the original medium. Thus, digital watermarking may offer the potential of tamper-proof security for intellectual property distributed over the Internet.
Digital watermarking has attracted the attention of some major corporations, which are actively conducting research and developing applications. For example:
Researchers at AT&T Research Laboratories are working with textual material and have published several articles in technical journals discussing their results.
IBM is using watermarking in a large project to digitize the Vatican library . …