Digital watermarks
With the advent of digital works (pictures, music, film and so on) digital watermarks have also been developed, however they are not always used in quite the same way as watermarks in documents. This is because, at least for the moment, it seems impractical to try and prevent people from making copies of computerised files, as computers will readily make perfect copies of the files they hold – something essential for backup and recovery.
The first type of digital watermarks you will see are those which are visible or obvious, and are intended to be so. These are usually images that are superimposed upon a still picture or a moving picture. The intention is either that in the event the images are copied then the ownership is not in dispute, or to prevent any realistic commercial use of the images if they are copied because their quality would not be acceptable.
The second type of watermarks are invisible. These digital watermarks are created by embedding extra information, commonly in the form of digital patterns, into the computer files containing the images or sounds to be protected. For this to be successful, the addition of the image to the information in the file must have no noticeable effect as far as the person seeing the subsequent picture or hearing the sound. There are two reasons for this. The first is not to reduce user satisfaction. The second is to make it much more difficult for someone to remove the pattern because they do not know what they are looking for.
In this case, the owner is using the watermark to help identify the source of the copy, since they cannot prevent the copying. The principle is to ‘scan’ the images or sounds and recognize the pattern, thus ‘prove’ the ownership of the original so that the owner can prosecute those found making or storing illegal copies. This type of watermark is often used in the film and music industries to identify pirated copies.
However, it is not a very useful method if you are trying to protect information such as text or similar. This is because to hide these kinds of watermarks in a file you need to be able to alter lots of bits without anyone being able to notice. But in text files there is nowhere to ‘hide’ the watermark. Also, whilst these watermarks act to identify the owner of the original they don’ t tell you anything about who was actually authorized to use them. See invisible vs visible watermarks.
Several patents appear to have been granted over the use of digital watermarks as a method of demonstrating that a document that is encoded as a picture is genuine (but not necessarily original). These rely upon mathematical calculations performed on the picture, which has embedded in it, a unique numeric identification. This has a similar characteristic to the watermark manufactured into paper documents.