Protection against unapproved copying

It is deeply recognized that digital data such as computer programs, photographs or music can be easily copied without any loss of quality. Then these copies can be redistributed without permission from the original rights holders. This problem is well-known by producers of programs and there are some methods (or attempts) of protection against unapproved copying. One way of countering unauthorized redistribution is to uniquely mark each individual instance of the originally distributed computer program, so that the recipient (user) can be identified if that content appears on “black market”. The authorized distributor embeds a unique mark, also called a “forensic watermark” or a “fingerprint,” into each instance of the program before transmitting it to the user. The embedding algorithm ensures that the mark is imperceptible, i.e., the program is not spoilt by the mark. Moreover, the location and nature of the mark is kept secret from the user to prevent him from locating and altering the mark. Another way stopping unauthorized redistribution is to include marks in such a way that the program authorization is impossible for “illegal” copy. There are two main implementation problems disturbing to realization of the described program protection scheme in practice. First, it is difficult to include marks in such a way that they cannot be found out. (The point is that it is difficult to find “natural” way to mark the computer programs.) Second, there exists a so-called collusion attack. A group of recipients (called “colluders” or a “coalition”) can collaborate to escape identification. Comparing their program copies, they can find the locations where their content, and thus their marks, differ. By cleverly manipulating the content at those locations, the colluders can attempt to create a version of the content that cannot be traced back to any of them.