Les travaux de cette thèse portent sur les critères de sécurité des<br />algorithmes de chiffrement à clé secrète et ont été menés suivant deux<br />axes. Le premier concerne la sécurité des chiffrements symétriques<br />itératifs par blocs contre les attaques par distingueur sur le dernier<br />tour. Les résultats portent en particulier sur la généralisation d'une<br />attaque différentielle d'ordre supérieur menée sur l'algorithme<br />MISTY1. L'origine de cette attaque ainsi que de sa généralisation a pu<br />être expliquée grâce aux propriétés du spectre de Walsh des fonctions<br />de non-linéarité maximale utilisées. Ainsi il a été possible<br />d'élaborer une attaque générique sur tous les chiffrements de Feistel<br />à cinq tours utilisant des fonctions dont le spectre de Walsh est<br />divisible par une grande puissance de 2 car cette propriété permet<br />d'obtenir une borne supérieure sur le degré de la composition de<br />telles fonctions, nettement plus faible que la borne<br />triviale. Cette attaque suggère ainsi un nouveau critère de sécurité<br />qui porte sur la divisibilité du spectre de Walsh des fonctions de<br />tour utilisées dans les chiffrements itératifs par blocs. La deuxième<br />partie de la thèse porte sur l'étude des fonctions booléennes<br />symétriques, et en particulier sur l'existence éventuelle de<br />propriétés cryptographiques. À partir d'une propriété structurelle de<br />périodicité d'une représentation d'une fonction booléenne symétrique,<br />les propriétés de degré algébrique, d'équilibre, de résilience, de<br />critère de propagation et de non-linéarité ont été étudiées, ce qui a<br />permis d'améliorer les résultats existants. Par ailleurs, le calcul<br />explicite du spectre de Walsh des fonctions booléennes symétriques de<br />degré 2 et 3 a été réalisé, ainsi que la détermination de toutes les<br />fonctions symétriques équilibrées de degré inférieur ou égal à 7,<br />indépendamment du nombre de variables.

We consider the transmission of correlated sources over a multiple access channel(MAC). Multiple access channels are important building blocks in many practical communication systems, e.g., local area networks(LAN), cellular systems, wireless multi-hop networks. Thus this topic has been studied for last several decades. One recent motivation is estimating a random field via wireless sensor networks. Often the sensor nodes are densely deployed resulting in correlated observations. These sensor nodes need to transmit their correlated observations to a fusion center which uses this data to estimate the sensed random field. Sensor nodes have limited computational and storage capabilities and very limited energy. Since transmission is very energy intensive, it is important to minimize it. This motivates our problem of energy efficient transmission of correlated sources over a sensor network. Sensor networks are often arranged in a hierarchical fashion. Neighboring nodes can first transmit their data to a cluster head which can further compress information before transmission to the fusion center. The transmission of data from sensor nodes to their cluster-head is usually through a MAC. At the fusion center the underlying physical process is estimated. The main trade-off possible is between the rates at which the sensors send their observations and the distortion incurred in estimation at the fusion center. The availability of side information at the encoders and/or the decoder can reduce the rate of transmission. In this thesis, the above scenario is modeled as an information theoretic problem. Efficient joint source-channel codes are discussed under various assumptions on side information and distortion criteria. Sufficient conditions for transmission of discrete/continuous alphabet sources with a given distortion over a discrete/continuous alphabet MAC are given. We recover various previous results as special cases from our results. Furthermore, we study the practically important case of the Gaussian MAC(GMAC) in detail and propose new joint source-channel coding schemes for discrete and continuous sources. Optimal schemes are identified in different scenarios. The protocols like TDMA, FDMA and CDMA are widely used across systems and standards. When these protocols are used the MAC becomes a system of orthogonal channels. Our general conditions can be specialized to obtain sufficient conditions for lossy transmission over this system. Using this conditions, we identify an optimal scheme for transmission of Gaussian sources over orthogonal Gaussian channels and show that the Amplify and Forward(AF) scheme performs close to the optimal scheme even at high SNR. Next we investigate transmission of correlated sources over a fast fading MAC with perfect or partial channel state information available at both the encoders and the decoder. We provide sufficient conditions for transmission with given distortions. We also provide power allocation policies for efficient transmission. Next, we use MAC with side information as a building block of a hierarchical sensor network. For Gaussian sources over Gaussian MACs, we show that AF performs well in such sensor network scenarios where the battery power is at a premium. We then extend this result to the hierarchical network scenario and show that it can perform favourably to the Slepian-Wolf based source coding and independent channel coding scheme. In a hierarchical sensor network the cluster heads often need to send only a function of the sensor observations to the fusion center. In such a setup the sensor nodes can compress the data sent to the cluster head exploiting the correlation in the data and also the structure of the function to be computed at the cluster head. Depending upon the function, exploiting the structure of the function can substantially reduce the data rate for transmission. We provide efficient joint source-channel codes for transmitting a general class of functions of the sources over the MAC.

In this thesis, we investigate in detail, certain important problems in cryptography and coding theory. In the first part of this thesis, we discuss the number field sieve and compare the two ways in which the sieving step is implemented -one method using the line sieve and the other using the lattice sieve. We discuss why the lattice sieve performs better than the line sieve in the presence of large primes -this has not been attempted before. In the second part of this thesis, we design low-correlation CDMA sequences over the Quadrature Amplitude Modulation (QAM) alphabet. The sequences proposed in this thesis have the lowest value of the maximum correlation parameter as compared to any other family in the literature. In the third part of this thesis, we design large families of optimal two-dimensional optical orthogonal codes for optical CDMA. The size of these codes is larger than any other code in the literature.

This degree project deals with license management for EBITool. It´s about providing protection and monitoring for a Java Application via a license server, and the construction of it. An analysis that discusses the approach and other possible courses of action is also included. Additionally, it covers a discussion of a prototype implementation of the model solution from the analysis. The prototype is a Java EE application that deploys to JBoss AS7. It´s developed using the JBoss Developer Studio 5.0.0, an Eclipse IDE with JBoss Tools preinstalled. It exposes web services to Java Applications through SOAP via JAX-WS. Using Hibernate, the web service Enterprise Java Beans get access to a PostgreSQL 9.1 database via entity classes mapped to the database through the Java Persistence API.

In this work, a multilayer security solution for digital communication systems is provided by considering the joint effects of physical-layer security channel codes with application-layer cryptography. We address two problems: first, the cryptanalysis of error-prone ciphertext; second, the design of a practical physical-layer security coding scheme. To our knowledge, the cryptographic attack model of the noisy-ciphertext attack is a novel concept. The more traditional assumption that the attacker has the ciphertext is generally assumed when performing cryptanalysis. However, with the ever-increasing amount of viable research in physical-layer security, it now becomes essential to perform the analysis when ciphertext is unreliable. We do so for the simple substitution cipher using an information-theoretic framework, and for stream ciphers by characterizing the success or failure of fast-correlation attacks when the ciphertext contains errors. We then present a practical coding scheme that can be used in conjunction with cryptography to ensure positive error rates in an eavesdropper's observed ciphertext, while guaranteeing error-free communications for legitimate receivers. Our codes are called stopping set codes, and provide a blanket of security that covers nearly all possible system configurations and channel parameters. The codes require a public authenticated feedback channel. The solutions to these two problems indicate the inherent strengthening of security that can be obtained by confusing an attacker about the ciphertext, and then give a practical method for providing the confusion. The aggregate result is a multilayer security solution for transmitting secret data that showcases security enhancements over standalone cryptography.

We address the challenge of providing low-cost, universal access of useful information to people in different parts of the globe. We achieve this by following two strategies. First, we focus on the delivery of information through computerized devices and prototype new methods for making that delivery possible in a secure, low-cost, and universal manner. Second, we focus on the use of participatory media, such as blogs, in the context of news related content, and develop methods to recommend useful information that will be of interest to users. To achieve the first goal, we have designed a low-cost wireless system for Internet access in rural areas, and a smartphone-based system for the opportunistic use of WiFi connectivity to reduce the cost of data transfer on multi-NIC mobile devices. Included is a methodology for secure communication using identity based cryptography. For the second goal of identifying useful information, we make use of sociological theories regarding social networks in mass-media to develop a model of how participatory media can offer users effective news-related information. We then use this model to design a recommender system for participatory media content that pushes useful information to people in a personalized fashion. Our algorithms provide an order of magnitude better performance in terms of recommendation accuracy than other state-of-the-art recommender systems. Our work provides some fundamental insights into the design of low-cost communication systems and the provision of useful messages to users in participatory media through a multi-disciplinary approach. The result is a framework that efficiently and effectively delivers information to people in remote corners of the world.

We address the challenge of providing low-cost, universal access of useful information to people in different parts of the globe. We achieve this by following two strategies. First, we focus on the delivery of information through computerized devices and prototype new methods for making that delivery possible in a secure, low-cost, and universal manner. Second, we focus on the use of participatory media, such as blogs, in the context of news related content, and develop methods to recommend useful information that will be of interest to users. To achieve the first goal, we have designed a low-cost wireless system for Internet access in rural areas, and a smartphone-based system for the opportunistic use of WiFi connectivity to reduce the cost of data transfer on multi-NIC mobile devices. Included is a methodology for secure communication using identity based cryptography. For the second goal of identifying useful information, we make use of sociological theories regarding social networks in mass-media to develop a model of how participatory media can offer users effective news-related information. We then use this model to design a recommender system for participatory media content that pushes useful information to people in a personalized fashion. Our algorithms provide an order of magnitude better performance in terms of recommendation accuracy than other state-of-the-art recommender systems. Our work provides some fundamental insights into the design of low-cost communication systems and the provision of useful messages to users in participatory media through a multi-disciplinary approach. The result is a framework that efficiently and effectively delivers information to people in remote corners of the world.

Spacecrafts are extensively used by public and private sectors to support a variety of services. Considering the cost and the strategic importance of these spacecrafts, there has been an increasing demand to utilize strong cryptographic primitives to assure their security. Moreover, it is of utmost importance to consider fault tolerance in their designs due to the harsh environment found in space, while keeping low area and power consumption. The problem of recovering spacecrafts from failures or attacks, and bringing them back to an operational and safe state is crucial for reliability. Despite the recent interest in incorporating on-board security, there is limited research in this area. This research proposes a trusted hardware module approach for recovering the spacecrafts subsystems and their cryptographic capabilities after an attack or a major failure has happened. The proposed fault tolerant trusted modules are capable of performing platform restoration as well as recovering the cryptographic capabilities of the spacecraft. This research also proposes efficient fault tolerant architectures for the secure hash (SHA-2) and message authentication code (HMAC) algorithms. The proposed architectures are the first in the literature to detect and correct errors by using Hamming codes to protect the main registers. Furthermore, a quantitative analysis of the probability of failure of the proposed fault tolerance mechanisms is introduced. Based upon an extensive set of experimental results along with probability of failure analysis, it was possible to show that the proposed fault tolerant scheme based on information redundancy leads to a better implementation and provides better SEU resistance than the traditional Triple Modular Redundancy (TMR). The fault tolerant cryptographic primitives introduced in this research are of crucial importance for the implementation of on-board security in spacecrafts.

This thesis presents a computationally efficient cryptosystem based on chaotic continuous-interval cellular automata (CCA). This cryptosystem increases data protection as demonstrated by its flexibility to encrypt/decrypt information from distinct sources (e.g., text, sound, and images). This cryptosystem has the following enhancements over the previous chaos-based cryptosystems: (i) a mathematical model based on a new chaotic CCA strange attractor, (ii) integration of modules containing dynamical systems to generate complex sequences, (iii) generation of an unlimited number of keys due to the features of chaotic phenomena obtained through CCA, which is an improvement over previous symmetric cryptosystems, and (iv) a high-quality concealment of the cryptosystem strange attractor. Instead of using differential equations, a process of mixing chaotic sequences obtained from CCA is also introduced. As compared to other recent approaches, this mixing process provides a basis to achieve higher security by using a higher degree of complexity for the encryption/decryption processes. This cryptosystem is tested through the following three methods: (i) a stationarity test based on the invariance of the first ten statistical moments, (ii) a polyscale test based on the variance fractal dimension trajectory (VFDT) and the spectral fractal dimension (SFD), and (iii) a surrogate data test. This cryptosystem secures data from distinct sources, while leaving no patterns in the ciphertexts. This cryptosystem is robust in terms of resisting attacks that: (i) identify a chaotic system in the time domain, (ii) reconstruct the chaotic attractor by monitoring the system state variables, (iii) search the system synchronization parameters, (iv) statistical cryptanalysis, and (v) polyscale cryptanalysis.

Dans cette thèse nous abordons le problème de la conception de passerelle IPsec très haut débit pour la sécurisation des communications entre réseaux locaux. Pour cela, nous proposons deux architectures : une passerelle purement logicielle sur un unique serveur, dite intégrée, et une passerelle utilisant plusieurs serveurs et un module matériel cryptographique, dite en rupture. La première partie de nos travaux étudie l'aspect performance des deux architectures proposées. Nous commençons par montrer qu'un serveur générique est limité par sa puissance de calcul pour atteindre l'objectif de chiffrement et communication à 10 Gb/s. De plus, les nouvelles cartes graphiques, bien que prometteuses en terme de puissance, ne sont pas adaptées au problème du chiffrement de paquets réseau (à cause de leur faible taille). Nous mettons alors en place une pile réseau répartie sur plusieurs machines et procédons à sa parallélisation dans le cadre de l'architecture en rupture. Dans un second temps, nous analysons l'intégration d'une passerelle dans un réseau, notamment l'interaction du protocole de contrôle ICMP avec IPsec. ICMP est particulièrement important pour atteindre le haut débit par son implication dans le mécanisme d'optimisation de la taille des paquets. Pour cela, nous avons développé IBTrack, un logiciel d'étude du comportement des routeurs, par rapport à ICMP, le long d'un chemin. Nous montrons ensuite qu'ICMP est un vecteur d'attaque contre IPsec en exploitant un défaut fondamental des normes IP et IPsec : le surcoût des paquets IP créé par le mode tunnel entre en conflit avec le minimum de la taille maximale prévue par IP.