This thesis presents a novel work on hardware realization of symmetric image encryption utilizing chaos based continuous systems as pseudo random number generators. Digital implementation of chaotic systems results in serious degradations in the dynamics of the system. Such defects are illuminated through a new technique of generalized post proceeding with very low hardware cost. The thesis further discusses two encryption algorithms designed and implemented as a block cipher and a stream cipher. The security of both systems is thoroughly analyzed and the performance is compared with other reported systems showing a superior results. Both systems are realized on Xilinx Vetrix-4 FPGA with a hardware and throughput performance surpassing known encryption systems.

Algebraic attacks constitute an effective class of cryptanalytic attacks which have come up recently. In algebraic attacks, the relations between the input, output and the key are expressed as a system of equations and then solved for the key. The main idea is in obtaining a system of equations which is solvable using reasonable amount of resources. The new approaches proposed in this work and experimental studies on the existing algebraic attacks on stream ciphers will be presented. In the first attack on filter generator, the input-output relations are expressed in conjunctive normal form. The system of equations is then solved using modified Zakrevskij technique. This was one of the earliest algebraic attacks on the nonlinear filter generator. In the second attack, we relaxed the constraint on algebraic attack that the entire system description be known and the output sequence extension problem where the filter function is unknown is considered. We modeled the problem as a multivariate interpolation problem and solved it. An advantage of this approach is that it can be adapted to work for noisy output sequences where as the existing algebraic attacks expect the output sequence to be error free. Adding memory to filter/combiner function increases the degree of system of equations and finding low degree equations in this case is computeintensive. The method for computing low degree relations for combiners with memory was applied to the combiner in E0 stream cipher. We found that the relation given in literature [Armknecht and Krause] was incorrect. We obtained the correct equation and verified its correctness. A time-data size trade off attack for clock controlled filter generator was developed. The time complexity and the data requirements are in between the two approaches used in literature. A recent development of algebraic attacks - the Cube attack was studied. Cube attack on variants of Trivium were proposed by Dinur and Shamir where linear equations in key bits were obtained by combining equations for output bit for same key and a set of Initialization Vectors (IVs). We investigated the effectiveness of the cube attack on Trivium. We showed that the linear equations obtained were not general and hence the attack succeeds only for some specific values of IVs. A reason for the equations not being general is given and a modification to the linear equation finding step suggested.

Stream ciphers are private key cryptosystems used for security in communication and data transmission systems. Because they are used to encrypt streams of data, it is necessary for stream ciphers to use primitives that are easy to implement and fast to operate. LFSRs and the recently invented FCSRs are two such primitives, which give rise to certain security measures for the cryptographic strength of sequences, which we refer to as complexity measures henceforth following the convention. The linear (resp. N-adic) complexity of a sequence is the length of the shortest LFSR (resp. FCSR) that can generate the sequence. Due to the availability of shift register synthesis algorithms, sequences used for cryptographic purposes should have high values for these complexity measures. It is also essential that the complexity of these sequences does not decrease when a few symbols are changed. The k-error complexity of a sequence is the smallest value of the complexity of a sequence obtained by altering k or fewer symbols in the given sequence. For a sequence to be considered cryptographically ‘strong’ it should have both high complexity and high error complexity values. An important problem regarding sequence complexity measures is to determine good bounds on a specific complexity measure for a given sequence. In this thesis we derive new nontrivial lower bounds on the k-operation complexity of periodic sequences in both the linear and N-adic cases. Here the operations considered are combinations of insertions, deletions, and substitutions. We show that our bounds are tight and also derive several auxiliary results based on them. A second problem on sequence complexity measures useful in the design and analysis of stream ciphers is to determine the number of sequences with a given fixed (error) complexity value. In this thesis we address this problem for the k-error linear complexity of 2n-periodic binary sequences. More specifically: 1. We characterize 2n-periodic binary sequences with fixed 2- or 3-error linear complexity and obtain the counting function for the number of such sequences with fixed k-error linear complexity for k = 2 or 3. 2. We obtain partial results on the number of 2n-periodic binary sequences with fixed k-error linear complexity when k is the minimum number of changes required to lower the linear complexity.

Block and stream ciphers are widely used to protect the privacy of digital information. A variety of attacks against block and stream ciphers exist; the most recent being the algebraic attacks. These attacks reduce the cipher to a simple algebraic system which can be solved by known algebraic techniques. These attacks have been very successful against a variety of stream ciphers and major efforts (for example eSTREAM project) are underway to design and analyze new stream ciphers. These attacks have also raised some concerns about the security of popular block ciphers. In this thesis, apart from designing new stream ciphers, we focus on analyzing popular nonlinear transformations (Boolean functions and S-boxes) used in block and stream ciphers for various cryptographic properties, in particular their resistance against algebraic attacks. The main contribution of this work is the design of two new stream ciphers and a thorough analysis of the algebraic immunity of Boolean functions and S-boxes based on power mappings. First we present WG, a family of new stream ciphers designed to obtain a keystream with guaranteed randomness properties. We show how to obtain a mathematical description of a WG stream cipher for the desired randomness properties and security level, and then how to translate this description into a practical hardware design. Next we describe the design of a new RC4-like stream cipher suitable for high speed software applications. The design is compared with original RC4 stream cipher for both security and speed. The second part of this thesis closely examines the algebraic immunity of Boolean functions and S-boxes based on power mappings. We derive meaningful upper bounds on the algebraic immunity of cryptographically significant Boolean power functions and show that for large input sizes these functions have very low algebraic immunity. To analyze the algebraic immunity of S-boxes based on power mappings, we focus on calculating the bi-affine and quadratic equations they satisfy. We present two very efficient algorithms for this purpose and give new S-box constructions that guarantee zero bi-affine and quadratic equations. We also examine these S-boxes for their resistance against linear and differential attacks and provide a list of S-boxes based on power mappings that offer high resistance against linear, differential, and algebraic attacks. Finally we investigate the algebraic structure of S-boxes used in AES and DES by deriving their equivalent algebraic descriptions.

Block and stream ciphers are widely used to protect the privacy of digital information. A variety of attacks against block and stream ciphers exist; the most recent being the algebraic attacks. These attacks reduce the cipher to a simple algebraic system which can be solved by known algebraic techniques. These attacks have been very successful against a variety of stream ciphers and major efforts (for example eSTREAM project) are underway to design and analyze new stream ciphers. These attacks have also raised some concerns about the security of popular block ciphers. In this thesis, apart from designing new stream ciphers, we focus on analyzing popular nonlinear transformations (Boolean functions and S-boxes) used in block and stream ciphers for various cryptographic properties, in particular their resistance against algebraic attacks. The main contribution of this work is the design of two new stream ciphers and a thorough analysis of the algebraic immunity of Boolean functions and S-boxes based on power mappings. First we present WG, a family of new stream ciphers designed to obtain a keystream with guaranteed randomness properties. We show how to obtain a mathematical description of a WG stream cipher for the desired randomness properties and security level, and then how to translate this description into a practical hardware design. Next we describe the design of a new RC4-like stream cipher suitable for high speed software applications. The design is compared with original RC4 stream cipher for both security and speed. The second part of this thesis closely examines the algebraic immunity of Boolean functions and S-boxes based on power mappings. We derive meaningful upper bounds on the algebraic immunity of cryptographically significant Boolean power functions and show that for large input sizes these functions have very low algebraic immunity. To analyze the algebraic immunity of S-boxes based on power mappings, we focus on calculating the bi-affine and quadratic equations they satisfy. We present two very efficient algorithms for this purpose and give new S-box constructions that guarantee zero bi-affine and quadratic equations. We also examine these S-boxes for their resistance against linear and differential attacks and provide a list of S-boxes based on power mappings that offer high resistance against linear, differential, and algebraic attacks. Finally we investigate the algebraic structure of S-boxes used in AES and DES by deriving their equivalent algebraic descriptions.

Synchronous stream ciphers constitute an important class of symmetric ciphers. After the call of the eSTREAM project in 2004, 34 stream ciphers with different design approaches were proposed. In this thesis, we aim to provide a general framework to analyze stream ciphers statistically. Firstly, we consider stream ciphers as pseudo random number generators and study the quality of their output. We propose three randomness tests based on one dimensional random walks. Moreover, we theoretically and experimentally analyze the relations of various randomness tests. We focus on the ideas of algebraic, time memory tradeoff (TMTO) and correlation attacks and propose a number of chosen IV distinguishers. We experimentally observe statistical weaknesses in some of the stream ciphers that are believed to be secure.

La sécurité des transmissions et du stockage des données est devenue un enjeu majeur de ces dernières années et la cryptologie, qui traite de la protection algorithmique de l'information, est un sujet de recherche extrêmement actif. Elle englobe la conception d'algorithmes cryptographiques, appelée cryptographie, et l'analyse de leur sécurité, appelée cryptanalyse.Dans cette thèse, nous nous concentrons uniquement sur la cryptanalyse, et en particulier celle des algorithmes de chiffrement symétrique, qui reposent sur le partage d'un même secret entre l'entité qui chiffre l'information et celle qui la déchiffre. Dans ce manuscrit, trois attaques contre des algorithmes de chiffrement symétriques sont présentées. Les deux premières portent sur deux candidats de l'actuelle compétition cryptographique CAESAR, les algorithmes AEZ et NORX, tandis que la dernière porte sur l'algorithme Kravatte, une instance de la construction Farfalle qui utilise la permutation de la fonction de hachage décrite dans le standard SHA-3. Les trois algorithmes étudiés présentent une stratégie de conception similaire, qui consiste à intégrer dans une construction nouvelle une primitive, i.e. une fonction cryptographique élémentaire, déjà existante ou directement inspirée de travaux précédents.La compétition CAESAR, qui a débuté en 2015, a pour but de définir un portefeuille d'algorithmes recommandés pour le chiffrement authentifié. Les deux candidats étudiés, AEZ et NORX, sont deux algorithmes qui ont atteint le troisième tour de cette compétition. Les deux attaques présentées ici ont contribué à l'effort de cryptanalyse nécessaire dans une telle compétition. Cet effort n'a, en l'occurrence, pas permis d'établir une confiance suffisante pour justifier la présence des algorithmes AEZ et NORX parmi les finalistes.AEZ est une construction reposant sur la primitive AES, dont l'un des principaux objectifs est d'offrir une résistance optimale à des scénarios d'attaque plus permissifs que ceux généralement considérés pour les algorithmes de chiffrement authentifié. Nous montrons ici que dans de tels scénarios il est possible, avec une probabilité anormalement élevée, de retrouver l'ensemble des secrets utilisés dans l'algorithme.NORX est un algorithme de chiffrement authentifié qui repose sur une variante de la construction dite en éponge employée par exemple dans la fonction de hachage Keccak. Sa permutation interne est inspirée de celles utilisées dans BLAKE et ChaCha. Nous montrons qu'il est possible d'exploiter une propriété structurelle de cette permutation afin de récupérer la clé secrète utilisée. Pour cela, nous tirons parti du choix des concepteurs de réduire les marges de sécurité dans le dimensionnement de la construction en éponge.Enfin, la dernière cryptanalyse remet en cause la robustesse de l'algorithme Kravatte, une fonction pseudo-aléatoire qui autorise des entrées et sorties de taille variable. Dérivée de la permutation Keccak-p de SHA-3 au moyen de la construction Farfalle, Kravatte est efficace et parallélisable. Ici, nous exploitons le faible degré algébrique de la permutation interne pour mettre au jour trois attaques par recouvrement de clé : une attaque différentielle d'ordre supérieur, une attaque algébrique "par le milieu" et une attaque inspirée de la cryptanalyse de certains algorithmes de chiffrement à flot. / Nowadays, cryptology is heavily used to protect stored and transmitted data against malicious attacks, by means of security algorithms. Cryptology comprises cryptography, the design of these algorithms, and cryptanalysis, the analysis of their security.In this thesis, we focus on the cryptanalysis of symmetric encryption algorithms, that is cryptographic algorithms that rely on a secret value shared beforehand between two parties to ensure both encryption and decryption. We present three attacks against symmetric encryption algorithms. The first two cryptanalyses target two high profile candidates of the CAESAR cryptographic competition, the AEZ and NORX algorithms, while the last one targets the Kravatte algorithm, an instance of the Farfalle construction based on the Keccak permutation. Farfalle is multipurpose a pseudo-random function (PRF) developed by the same designers' team as the permutation Keccak used in the SHA-3 hash function.The CAESAR competition, that began in 2015, aims at selecting a portfolio of algorithms recommended for authenticated encryption. The two candidates analysed, AEZ and NORX, reached the third round of the CAESAR competition but were not selected to be part of the finalists. These two results contributed to the cryptanalysis effort required in such a competition. This effort did not establish enough confidence to justify that AEZ and NORX accede to the final round of the competition.AEZ is a construction based on the AES primitive, that aims at offering an optimal resistance against more permissive attack scenarios than those usually considered for authenticated encryption algorithms. We show here that one can recover all the secret material used in AEZ with an abnormal success probability.NORX is an authenticated encryption algorithm based on a variant of the so-called sponge construction used for instance in the SHA-3 hash function. The internal permutation is inspired from the one of BLAKE and ChaCha. We show that one can leverage a strong structural property of this permutation to recover the secret key, thanks to the designers' non-conservative choice of reducing the security margin in the sponge construction.Finally, the last cryptanalysis reconsiders the robustness of the Kravatte algorithm. Kravatte is an efficient and parallelizable PRF with input and output of variable length. In this analysis, we exploit the low algebraic degree of the permutation Keccak used in Kravatte to mount three key-recovery attacks targeting different parts of the construction: a higher order differential attack, an algebraic meet-in-the-middle attack and an attack based on a linear recurrence distinguisher.

The purpose of this thesis is to design a secure and optimized cryptographic stream cipher for passive type Radio Frequency Identification (RFID) tags. RFID technology is a wireless automatic tracking and identification device. It has become an integral part of our daily life and it is used in many applications such as electronic passports, contactless payment systems, supply chain management and so on. But the information carried on RFID tags are vulnerable to unauthorized access (or various threats) which raises the security and privacy concern over RFID devices. One of the possible solutions to protect the confidentiality, integrity and to provide authentication is, to use a cryptographic stream cipher which encrypts the original information with a pseudo-random bit sequence. Besides that RFID tags require a resource constrained environment such as efficient area, power and high performance cryptographic systems with large security margins. Therefore, the architecture of stream cipher provides the best trade-off between the cryptographic security and the hardware efficiency. In this thesis, we first described the RFID technology and explain the design requirements for passive type RFID tags. The hardware design for passive tags is more challenging due to its stringent requirements like power consumption and the silicon area. We presented different design measures and some of the optimization techniques required to achieve low-resource cryptographic hardware implementation for passive tags. Secondly, we propose and implement a lightweight WG-5 stream cipher, which has good proven cryptographic mathematical properties. Based on these properties we measured the security analysis of WG-5 and showed that the WG-5 is immune to different types of attacks such as algebraic attack, correlation attack, cube attack, differential attack, Discrete Fourier Transform attack (DFT), Time-Memory-Data trade-off attack. The implementation of WG-5 was carried out using 65 nm and 130 nm CMOS technologies. We achieved promising results of WG-5 implementation in terms of area, power, speed and optimality. Our results outperforms most of the other stream ciphers which are selected in eSTREAM project. Finally, we proposed RFID mutual authentication protocol based on WG-5. The security and privacy analysis of the proposed protocol showed that it is resistant to various RFID attacks such as replay attacks, Denial-of-service (DoS) attack, ensures forward privacy and impersonation attack.

The purpose of this thesis is to design a secure and optimized cryptographic stream cipher for passive type Radio Frequency Identification (RFID) tags. RFID technology is a wireless automatic tracking and identification device. It has become an integral part of our daily life and it is used in many applications such as electronic passports, contactless payment systems, supply chain management and so on. But the information carried on RFID tags are vulnerable to unauthorized access (or various threats) which raises the security and privacy concern over RFID devices. One of the possible solutions to protect the confidentiality, integrity and to provide authentication is, to use a cryptographic stream cipher which encrypts the original information with a pseudo-random bit sequence. Besides that RFID tags require a resource constrained environment such as efficient area, power and high performance cryptographic systems with large security margins. Therefore, the architecture of stream cipher provides the best trade-off between the cryptographic security and the hardware efficiency. In this thesis, we first described the RFID technology and explain the design requirements for passive type RFID tags. The hardware design for passive tags is more challenging due to its stringent requirements like power consumption and the silicon area. We presented different design measures and some of the optimization techniques required to achieve low-resource cryptographic hardware implementation for passive tags. Secondly, we propose and implement a lightweight WG-5 stream cipher, which has good proven cryptographic mathematical properties. Based on these properties we measured the security analysis of WG-5 and showed that the WG-5 is immune to different types of attacks such as algebraic attack, correlation attack, cube attack, differential attack, Discrete Fourier Transform attack (DFT), Time-Memory-Data trade-off attack. The implementation of WG-5 was carried out using 65 nm and 130 nm CMOS technologies. We achieved promising results of WG-5 implementation in terms of area, power, speed and optimality. Our results outperforms most of the other stream ciphers which are selected in eSTREAM project. Finally, we proposed RFID mutual authentication protocol based on WG-5. The security and privacy analysis of the proposed protocol showed that it is resistant to various RFID attacks such as replay attacks, Denial-of-service (DoS) attack, ensures forward privacy and impersonation attack.

[pt] No futuro próximo, viveremos em cidades inteligentes. Nossas casas, nossos carros e a maioria dos nossos equipamentos estarão interconectados. Se a infraestrutura das cidades inteligentes não fornecerem privacidade e segurança, os cidadãos ficarão relutantes em participar e as principais vantagens de uma cidade inteligente irão se dissolver. Vários algoritmos de criptografia recentemente foram quebrados ou enfraquecidos e os comprimentos das chaves estão aumentando, conforme cresce o poder computacional. Um estudo recente descobriu que 93 porcento de 20.000 aplicações Android tinham violado uma ou mais regras de criptografia. Essas violações enfraquecem a criptografia ou as inutiliza. Outro problema é a autenticação. Uma chave privada comprometida de única autoridade de certificação intermediária pode comprometer toda cidade inteligente que utilizar certificados digitais para autenticação. Neste trabalho, investigamos por que tais violações ocorrem. Propomos o HX: um algoritmo de criptografia modular baseado em funções de hash resistentes à colisão que reduz automaticamente as violações de regras de criptografia e o HXAuth: um protocolo de autenticação de chave simétrica para trabalhar em conjunto com o SRAP ou independentemente, com um segredo previamente partilhado. Nossos experimentos apontam na direção de que a maioria dos desenvolvedores não tem o conhecimento básico necessário em criptografia para utilizar corretamente um algoritmo de criptografia. Nossos experimentos também provam que o HX é seguro, modular e é mais forte, mais eficaz e mais eficiente do que o AES, o Salsa20 e o HC-256. / [en] In the near future, we will live in smart cities. Our house, our car and most of our appliances will be interconnected. If the infrastructure of the smart cities fails to provide privacy and security, citizens will be reluctant to participate and the main advantages of a smart city will dissolve. Several encryption algorithms have been broken recently or significantly weakened and key lengths are increasing as computing power availability grows. In addition to the ever growing computing power a recent study discovered that 93 percent from 20,000 Android applications had violated one or more cryptographic rules. Those violations either weaken the encryption or render them useless. Another problem is authentication. A single compromised private key from any intermediate certificate authority can compromise every smart city which will use digital certificates for authentication. In this work, we investigate why such violations occur and we propose: HX, a modular encryption algorithm based on Collision Resistant Hash Functions that automatically mitigates cryptographic rules violations and HXAuth, a symmetric key authentication protocol to work in tandem with Secure RDF Authentication Protocol (SRAP) or independently with a pre-shared secret. Our experiments points in the direction that most developers do not have the necessary background in cryptography to correctly use encryption algorithms, even those who believed they had. Our experiments also prove HX is safe, modular and is stronger, more effective and more efficient than AES, Salsa20 and HC-256.