Dans ce travail nous nous intéressons aux systèmes dynamiques du point de vue de l'entropie. Nous rappellons tout d'abord le formalisme des structures d'entropie introduit par T.Downarowicz. Dans ce cadre on donne en particulier une preuve élémentaire du principe variationnel pour l'entropie de queue et on généralise certaines structures d'entropie aux endomorphismes.<br>Dans un deuxième temps, nous reprenons l'approche semi-algébrique de Y. Yomdin et M. Gromov pour contrôler la dynamique locale des applications de classe $C^r$. On présente une preuve complète du lemme algébrique de Gromov, qui est un point clé de la théorie de Yomdin. Aussi nous déduisons de nouvelles applications dynamiques de cette théorie : d'une part nous bornons l'entropie de queue mesurée en fonction de l'exposant de Lyapounov ; d'autre part nous généralisons une formule due à J.Buzzi pour l'entropie k-dimensionnelle d'un produit d'applications de classe $C^{\infty}$.<br>On s'intéresse enfin à la théorie des extensions symboliques due à M.Boyle et T.Downarowicz pour les applications $C^r$ et affines par morceaux du plan. On exhibe en particulier des exemples de dynamique $C^r$ de l'intervalle ayant une grande entropie d'extension symbolique. Nous donnerons aussi une borne de l'entropie d'extensions symboliques pour les applications affines par morceaux du plan.

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Web browsers are increasingly designed to be extensible to keep up with the Web's rapid pace of change. This extensibility is typically implemented using script-based extensions. Script extensions have access to sensitive browser APIs and content from untrusted web pages. Unfortunately, this powerful combination creates the threat of privilege escalation attacks that grant web page scripts the full privileges of script extensions and control over the entire browser process. This thesis describes the pitfalls of script-based extensibility based on our study of the Firefox Web browser, and is the first to offer a classification of script-based privilege escalation vulnerabilities. We propose a taint-based system to track the spread of untrusted data in the browser and to detect the characteristic signatures of privilege escalation attacks. We show that this approach is effective by testing our system against exploits in the Firefox bug database and finding that it detects the vast majority of attacks with no false alarms.

Web browsers are increasingly designed to be extensible to keep up with the Web's rapid pace of change. This extensibility is typically implemented using script-based extensions. Script extensions have access to sensitive browser APIs and content from untrusted web pages. Unfortunately, this powerful combination creates the threat of privilege escalation attacks that grant web page scripts the full privileges of script extensions and control over the entire browser process. This thesis describes the pitfalls of script-based extensibility based on our study of the Firefox Web browser, and is the first to offer a classification of script-based privilege escalation vulnerabilities. We propose a taint-based system to track the spread of untrusted data in the browser and to detect the characteristic signatures of privilege escalation attacks. We show that this approach is effective by testing our system against exploits in the Firefox bug database and finding that it detects the vast majority of attacks with no false alarms.

The interest in extensions of the logic programming paradigm beyond the class of normal logic programs is motivated by the need of an adequate representation and processing of knowledge. One of the most difficult problems in this area is to find an adequate declarative semantics for logic programs. In the present paper a general preference criterion is proposed that selects the ‘intended’ partial models of generalized logic programs which is a conservative extension of the stationary semantics for normal logic programs of [Prz91]. The presented preference criterion defines a partial model of a generalized logic program as intended if it is generated by a stationary chain. It turns out that the stationary generated models coincide with the stationary models on the class of normal logic programs. The general wellfounded semantics of such a program is defined as the set-theoretical intersection of its stationary generated models. For normal logic programs the general wellfounded semantics equals the wellfounded semantics.

In this thesis we propose to realize the performance benefits of applicationspecific hardware optimizations in a general-purpose, multi-user system environment using a dynamically extensible microprocessor architecture. We have called our dynamically extensible microprocessor design the Extensible Microprocessor without Interlocked Pipeline Stages, or eMIPS. The eMIPS architecture uses the interaction of fixed and configurable logic available in modern Field Programmable Gate Array (FPGA). This interaction is used to address the limitations of current microprocessor architectures based solely on Application Specific Integrated Circuits (ASIC). These limitations include inflexibility, size, and application specific performance optimization. The eMIPS system allows multiple secure extensions to load dynamically and to plug into the stages of a pipelined central processing unit (CPU) data path, thereby extending the core instruction set of the microprocessor. Extensions can also be used to realize on-chip peripherals, and if area permits, even multiple cores. Extension instructions reduce dramatically the execution time of frequently executed instruction patterns. These new functionalities we have developed can be exploited by patching the binaries of existing applications, without any changes to the compilers. A FPGA based workstation prototype and a flexible simulation system implementating this design demonstrates speedups of 2x-3x on a set of applications that include video games, real-time programs and the SPEC2000 integer benchmarks. eMIPS is the first realized workstation based entirely on a dynamically extensible microprocessor that is safe for general purpose, multi-user applications. By exposing the individual stages of the data path, eMIPS allows optimizations not previously possible. This includes permitting safe and coherent accesses to memory from within an extension, optimizing multi-branched blocks, and throwing precise and restart able exceptions from within an extension. This work describes a simplified implementation of an extensible microprocessor architecture based on the Microprocessor without Interlocked Pipeline Stages (MIPS) Reduced Instruction Set Computer (RISC) architecture. The concepts and methods contained within this thesis may be applied to other similar architectures. Given this simplified prototype we look forward to propose how this architecture will be expanded as it matures.

Thesis (Ph. D.)--Oregon State University, 2003. / Typescript (photocopy). Includes bibliographical references (leaves 43-46). Also available on the World Wide Web.

The work described in this dissertation was largely motivated by the aim of producing a survey on the theory of group extensions. From the broad scope of the theory of group extensions we single out two aspects to discuss, namely the study of the split and the non-split cases and give examples of both. A great part of this dissertation is dedicated to the study of split extensions. After setting the background theory for the study of the split extensions we proceed in exploring the ramifications of this concept within the development of the group structure and consequently investigate well known products which are its derived namely the holomorph, and the wreath product. The theory of group presentations provides in principle the necessary tools that permit the description of a group by means of its generators and relators. Through this knowledge we give presentations for the groups of order pq,p2q and p3. Subsequently using a classical result of Gaschutz we investigate the split extensions of non-abelian groups in which the normal subgroup is either a non-abelian normal nilpotent group or a non-abelian normal solvable group. We also study other cases of split extensions such as the affine subgroups of the general linear and the symplectic groups. It is expected that some of the results obtained will provide a theoretical algorithm to describe these affine subgroups. A particular case of the non-split extensions is discussed as the Frattini extensions. In fact a simplest example of a Frattini extension is a non-split extension in which the kernel of an epimorphism e is an irreducible G-module. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.

The character table of a finite group provides considerable amount of information about the group, and hence is of great importance in Mathematics as well as in Physical Sciences. Most of the maximal subgroups of the finite simple groups and their automorphisms are of extensions of elementary abelian groups, so methods have been developed for calculating the character tables of extensions of elementary abelian groups. Character tables of finite groups can be constructed using various techniques. However Bernd Fischer presented a powerful and interesting technique for calculating the character tables of group extensions. This technique, which is known as the technique of the Fischer-Clifford matrices, derives its fundamentals from the Clifford theory. If G=N.G is an appropriate extension of N by G, the method involves the construction of a nonsingular matrix for each conjugacy class of G/N~G. The character table of G can then be determined from these Fischer-Clifford matrices and the character table of certain subgroups of G, called inertia factor groups. In this dissertation, we described the Fischer-Clifford theory and apply it to both split and non-split group extensions. First we apply the technique to the split extensions 2,7:Sp6(2) and 2,8:SP6(2) which are maximal subgroups of Sp8(2) and 2,8:08+(2) respectively. This technique has also been discussed and used by many other researchers, but applied only to split extensions or to the case when every irreducible character of N can be extended to an irreducible character of its inertia group in G. However the same method can not be used to construct character tables of certain non-split group extensions. In particular, it can not be applied to the non-split extensions of the forms 3,7.07(3) and 3,7.(0,7(3):2) which are maximal subgroups of Fischer's largest sporadic simple group Fi~24 and its automorphism group Fi24 respectively. In an attempt to generalize these methods to such type of non-split group extensions, we need to consider the projective representations and characters. We have shown that how the technique of Fischer-Clifford matrices can be applied to any such type of non-split extensions. However in order to apply this technique, the projective characters of the inertia factors must be known and these can be difficult to determine for some groups. We successfully applied the technique of Fischer-Clifford matrices and determined the Fischer-Clifford matrices and hence the character tables of the non-split extensions 3,7.0,7(3) and 3,7.(0,7(3):2). The character tables computed in this thesis have been accepted for incorporation into GAP and will be available in the latest versions. / Thesis (Ph.D)-University of Natal, Pietermaritzburg, 2001.

The smallest Fischer sporadic simple group Fi22 is generated by a conjugacy class D of 3510 involutions called 3-transpositions such that the product of any noncommuting pair is an element of order 3. In Fi22 there are exactly three conjugacy classes of involutions denoted by D, T and N and represented in the ATLAS [26] by 2A, 2B and 2C, containing 3510, 1216215 and 36486450 elements with corresponding centralizers 2·U(6,2), (2 x 2~+8:U(4,2)):2 and 25+8:(83 X 32:4) respectively. In Fi22 , we have Npi22(26) = 26:8P(6,2), where 26 is a 2B-pure group, and thus the maximal subgroup 26:8P(6, 2) of Fi22 is a 2-local subgroup. The full automorphism group of Fi22 is denoted by Fi22 . In Fi22 , there are three involutory outer automorphisms of Fi22 which are denoted bye, f and 0 and represented in the ATLAS [26] by 2D, 2F and 2E respectively. We obtain that Fi22 = Fi22 :(e) and it can be easily shown that Fi22 = Fi22 :(e) = Fi22 :(f) = Fi22 :(0). As e, f and 0 act on Fi22 , then we obtain the subgroups CPi22 (e) rv 0+(8,2):83, CPi22 (f) rv 8P(6,2) x 2 and CPi22 (()) rv 26:0-(6,2) of Fi22 which are generated by CD(e), Cn(f) and CD(0) respectively. In this thesis we are concerned with the construction of the character tables of certain groups which are associated with Fi22 and its automorphism group Fi22 . We use the technique of the Fischer-Clifford matrices to construct the character tables of these groups, which are split extensions. These groups are 26:8P(6, 2), 26:0-(6,2) and 27:8P(6, 2). The study of the group 26:8P(6, 2) is essential, as the other groups studied in this thesis are related to it. The groups 8P(6,2) and 0- (6,2) of 6 x 6 matrices over GF(2), played crucial roles in our construction of the group 8P(6, 2) as a group of 7 x 7 matrices over GF(2) which would act on 27 . Also the character table of 25:86 , the affine subgroup of 8P(6, 2) fixing a nonzero vector in 26 , is constructed by using the technique of the Fischer-Clifford matrices. This character table is used in the construction of the character table 26:SP(6, 2). The character tables computed in this thesis have been accepted for incorporation into GAP and will be available in the latest version of GAP. / Thesis (Ph.D.) - University of Natal, Pietermaritzburg, 1998.