In the 21st century, many valuable materials such as music and movies are stored in various form of digital media and delivered over the Internet. To prevent these copyrighted materials from illegal duplication, copyright protection technologies have to be employed. An effective copyright protection technology will reduce the risk of large revenue loss in the industries concerned.; DVD, or Digital Video Disk, represents a quality digital medium with great market value. The copyright protection system used in DVD applications is essentially a cryptosystem. A cryptosystem is comprised of two parts: a set of cryptographic protocols and underlying ciphers. In this thesis; research is carried out on both parts of the cryptosystem.; We use a formal method called Coloured Petri Nets to model and analyze the cryptographic protocol in DVD. The Petri Nets modeling of the protocol offers graphical representations and achieves a certain degree of formalization of it. A weakness is found and remedial suggestions are made. The stream cipher in DVD applications consists of two linear feedback shift Registers (LFSRs) with a non-linear combining function. In general, LFSR based stream ciphers are vulnerable to various versions of the correlation attack. A substitution box (s-box) based stream cipher can offer a more secure solution. Inspired by the design concept of the RC4 stream cipher, we propose a new family of stream ciphers that makes use of a cascade of small s-boxes. Cycle structures and output statistical properties of the cascaded s-box ciphers are studied in this thesis. Our experimental results give the indication that the cascaded s-box stream cipher develops more resistance to attacks as we increase the number of cells in the cascade.
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