Data Encryption and Hashing Schemes for Multimedia Protection

There are millions of people using social networking sites like Facebook, Google+, and Youtube every single day across the entire world for sharing photos and other digital media. Unfortunately, sometimes people publish content that does not belong to them. As a result, there is an increasing demand for quality software capable of providing maximum protection for copyrighted material. In addition, confidential content such as medical images and patient records require high level of security so that they can be protected from unintended disclosure, when transferred over the Internet. On the other hand, decreasing the size of an image without significant loss in quality is always highly desirable. Hence, the need for efficient compression algorithms. This thesis introduces a robust method for image compression in the shearlet domain. Motivated by the outperformance of the Discrete Shearlet Transform (DST) compared to the Discrete Wavelet Transform (DWT) in encoding the directional information in images, we propose a DST-based compression algorithm that provides not only a better quality in terms of image approximation and compression ratio, but also increases the security of images via the Advanced Encryption Standard. Experimental results on a slew of medical images illustrate an improved performance in image quality of the proposed approximation approach in comparison to DWT, and also demonstrate its robustness against a variety of tests, including randomness, entropy, key sensitivity, and input sensitivity. We also present a 3D mesh hashing technique using spectral graph theory. The main idea is to partition a 3D model into sub-meshes, followed by the generation of the Laplace-Beltrami matrix of each sub-mesh, and the application of eigen-decomposition. This, in turn, is followed by the hashing of each sub-mesh using Tsallis entropy. The experimental results using a benchmark 3D models demonstrate the effectiveness of the proposed hashing scheme

Modal Analysis Applied to the Stability Study of Hydroelectric Systems with Modular Structures

Power plants experience distinct dynamic behaviors according to the primary source of energy. Whereas thermal power plants have a slow dynamic, modern renewables such as wind and solar PV are subject to very fast variations, due to environmental factors. Therefore, their availability is not guaranteed. Consequently, it is opportune to take advantage of the intrinsic flexibility of hydropower plants for balancing fast variations caused by modern renewable sources, in order to keep stability and reliability of the power grid. On the other hand, the use of hydropower plants as means of compensating constant variations between electricity generation and consumption leads to off-design operation. Such condition may cause instabilities or undesirable oscillations in the power plant whose origin lies in the hydraulic system. Furthermore, small hydropower plants play a major role in the development of emerging countries, where they may be frequently subjected to islanded or isolated operation. In such context, operating conditions are more critical in terms of reliability and stability. Considering these factors, one can readily understand the importance of predicting the dynamic behavior of power plants under various scenarios and different operating modes. This requires precise, comprehensive mathematical models and efficient computational tools, which are appropriate for planning new installations and better exploiting the existing ones. Thereupon, the purpose of the present work is the development of a novel tool for small-signal stability analysis of hydroelectric systems, with comprehensive modeling of both electrical and hydraulic elements of a hydropower plant. This tool is implemented in SIMSEN, a fully modular, efficient, user-friendly software developed at EPFL, for the simulation of electrical power networks and hydroelectric systems. The originality of this new tool lies not only on the exhaustive and detailed modeling of electrical and hydraulic systems (a multi-physics representation). It lies also on the fact that it is a modular tool, capable of treating systems with any given topology, with automatic generation of the full set of differential equations, based on circuits easily built in an user-friendly GUI. Another distinctive characteristic of the present work is that small-signal models of electrical elements are based on a,b,c-phase variables, different from the traditional d,q,o-axis representation. The procedure to be followed for the derivation of such models is presented in this document. Furthermore, case studies performed with this tool show that substantial interactions happen between electrical, mechanical, hydraulic and regulation elements. These interactions can be either positive or detrimental to the stability of the system. In case of adverse interactions, unstable behaviors may occur. Such instabilities cannot be predicted without a comprehensive, multi-physics model. These conflicting interactions are presented, and their consequences and possible solutions are discussed in this document