The expansion of worldwide digital networks gives consumers extensive access to (and lets them reuse) audio-visual material regardless of the context they're taken from - such as advertising, entertainment, education, or research. In this kind of environment, the question of who owns the content is extremely relevant. Generally, we accept that universal access to material is indispensable for the progress of our society. However, we recognize that creators of information aren't only responsible for the content - and thus must have some control related to accessibility and use issues - but should also be able to draw commercial value out of their work. Unauthorized taping, reading, manipulating or removing of data might lead to financial loss or legal problems for producers and creators. Thus, designers, producers and publishers of digital data, such as images, videos, audio sources or multimedia material (e.g. games or virtual environments) need technical solutions to deal with the problems associated with copyright protection of their data. They require systems in which digital data can be easily signed by authors or producers to ensure and prove ownership rights on the produced audio-visual material after it has been publicly released.