Let’s say you want to verify the authenticity of a signal. The signal can take any form like an image, audio, video, or any other kind of bit stream. By now, everybody would have heard the term “watermark” being used in the general sense. The most common example would be currency notes. Watermarks are embedded to verify the authenticity of the notes. But how do we achieve that with more complicated signals? As things move into the virtual world, where the threats are elevated to a much higher and abstract level, we need a way to verify the authenticity of different forms of digital signals. How do we do it?
What is watermarking used for?
When we talk about watermarking, the first thing that comes to mind is copyright protection of digital media. In the past, duplicating artwork was quite complicated and required a high level of expertise for the counterfeit to look like the original. However, this is not true in the digital world. It is now possible for almost anyone to duplicate or manipulate digital data and not lose data quality. Similar to the process when artists creatively signed their paintings with a brush to claim copyrights, artists of today can watermark their work by hiding their name within the image. Hence, the embedded watermark permits identification of the owner of the work. It is clear that this concept is also applicable to other media such as digital video and audio. Currently, the unauthorized distribution of digital audio over the internet in the MP3 format is a big problem (*wink* Napster *wink*!). In this scenario, digital watermarking may be useful to set up controlled audio distribution and to provide efficient means for copyright protection, usually in collaboration with international registration bodies.
What is a digital watermark?
A digital watermark is a digital signal that is embedded into the digital file. A digital watermark is basically an image laid over another image and leaves the carrier image in tact. Most watermarks are used redundantly throughout an image. An algorithm is used that inserts the watermark inside the digital image. The “author” can decide whether or not they would like the watermark to be seen. Many companies would choose to have the watermark plainly visible as a way to advertise their products. The intensity of the watermark will determine if the watermark is going to be visible or not. Other things to consider would be the placement of the watermark as well as the size. One type of watermarking uses color separation that allows the watermark to only be seen in one of the color bands. In this way, the watermark is not seen. But if the image is printed where the colors need to be separated, the embedded image will appear, thus making the actual image useless for commercial use.
If an image is manipulated or altered, it will usually destroy the watermark. The algorithm that is used to insert the watermark is the same one that will be needed to remove it. If the watermark is removed from the image and is completely intact, then it could be assumed that the watermark is authentic. If the watermark has been destroyed then a company or “author” could use this evidence in court to show copyright infringement. There have been some problems with using watermarks on the internet for copy protection. When a tracking service is used to try and protect against illegal copies being used on the internet, the tracking service can only access sites that are not password protected.
How do we achieve it?
Digital Watermarking describes methods and technologies that hide information, for example a number or text, in digital media, such as images, video or audio. The embedding takes place by manipulating the content of the digital data, which means the information is not embedded in the frame around the data. The hiding process has to be such that the modifications of the media are imperceptible. For images, this means that the modifications of the pixel values have to be invisible. Furthermore, the watermark must be either robust or fragile, depending on the application. By “robust”, I mean the capability of the watermark to resist manipulations of the media, such as lossy compression (where compressing data and then decompressing it retrieves data that may well be different from the original, but is close enough to be useful in some way), scaling, and cropping. In some cases the watermark may need to be fragile. “Fragile” means that the watermark should not resist tampering, or would resist only up to a certain, predetermined extent.