As redes de sensores sem fio (RSSF) são compostas por pequenos dispositivos distribuídos em uma região geográfica com a finalidade de monitorar ou interagir com o ambiente. Esse tipo de rede tem sido alvo de grande atenção da comunidade acadêmica e empresarial, dados os avanços das produções científicas e aplicações comerciais. Além disso, há grande potencial para esse modelo de rede, pois há diversos benefícios em se ter muitos dispositivos de baixo custo trabalhando em cooperação e que ainda podem interagir com o mundo real. As RSSFs apresentam novos desafios, até então inexistentes na maioria das redes modernas. A baixa capacidade de processamento dos dispositivos, a limitação do tamanho de um pacote, a baixa taxa de transferência de pacotes, a baixa capacidade da bateria de um dispositivo e o alcance limitado do rádio dificultam ou até inviabilizam muitas implementações de segurança. Neste sentido, há uma grande variedade de protocolos de segurança, os quais tentam fornecer o máximo de propriedades desejadas, como por exemplo autenticidade e confidencialidade de mensagens. Nesta dissertação, analisamos e comparamos dois pares de protocolos de segurança que possuem grande atenção da comunidade. Os protocolos foram analisados com base em seus mecanismos criptográficos e propriedades oferecidas. Além disso, com o uso de um simulador de RSSFs, realizamos experimentos que ajudam a entender o comportamento de dois protocolos, principalmente no que se relaciona com o consumo de energia dos dispositivos sensores. / Wireless Sensor Networks (WSNs) are formed by many tiny devices deployed for monitoring or interacting with the environment in a geografic area. This kind of network has received great attention in the academic and corporate community, given the advances of comercial applications and scientific developments. The benefits of having many low cost devices cooperating and interacting with the real world explain the great potential of such networks. The WSNs come with new challenges, so far not faced in modern networks. The low-power, low-processing devices, limited bandwidth and radio range make many security implementations difficult or even unfeasible. Within this context, there is a diversity of security protocols, intended to provide as many security properties as possible, like confidentiality and authentication of messages. In this master thesis, we compare and evaluate two pairs of security protocols that enjoy significant attention in the community. These protocols are evaluated based on their given properties and cryptographic primitives. Using a WSNs simulator, we performed experiments that help to understand the behavior of these two protocols, mainly for energy consumption purposes.

Yu Haitao. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 71). / Abstracts in English and Chinese. / Chapter Chapter 1. --- Introduction --- p.1 / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- Thesis organization --- p.6 / Chapter Chapter 2. --- Conventional password authentication and backdoor password schemes --- p.7 / Chapter 2.1 --- Password and password authentication --- p.7 / Chapter 2.1.1 --- Introduction to password and its security problems --- p.7 / Chapter 2.1.2 --- Front-door passwords vs. backdoor passwords --- p.8 / Chapter 2.1.3 --- Dynamic passwords vs. static passwords --- p.9 / Chapter 2.2 --- Forgotten-password problem --- p.10 / Chapter Chapter 3. --- Introduction to Cryptography --- p.12 / Chapter 3.1 --- Introduction to information security --- p.12 / Chapter 3.2 --- Conventional cryptography --- p.16 / Chapter 3.3 --- Public-key cryptography --- p.21 / Chapter 3.4 --- RSA cryptosystem --- p.24 / Chapter 3.5 --- One-way function --- p.27 / Chapter 3.6 --- Digital signature --- p.30 / Chapter 3.7 --- Secret sharing --- p.34 / Chapter 3.8 --- Zero-knowledge proof --- p.34 / Chapter 3.9 --- Key management --- p.36 / Chapter 3.9.1 --- Key distribution in conventional cryptography --- p.36 / Chapter 3.9.2 --- Distribution of public keys --- p.39 / Chapter Chapter 4. --- A secure one-use dynamic backdoor password system based on Public Key Cryptography --- p.42 / Chapter 4.1 --- System objectives --- p.42 / Chapter 4.2 --- Simple system and analysis --- p.45 / Chapter 4.2.1 --- System diagram --- p.45 / Chapter 4.2.2 --- System protocol --- p.46 / Chapter 4.2.3 --- Applied technologies --- p.50 / Chapter 4.2.4 --- System security analysis --- p.52 / Chapter 4.3 --- Multi-user system and analysis --- p.55 / Chapter 4.3.1 --- Modification to the system diagram --- p.56 / Chapter 4.3.2 --- Modification to the system protocol --- p.57 / Chapter 4.3.3 --- System analysis for multi-user system --- p.64 / Chapter 4.4 --- Applicable modes and analysis --- p.66 / Chapter 4.5 --- Conclusion --- p.68 / Chapter Chapter 5. --- Conclusion --- p.69 / Bibliography --- p.71 / Appendix --- p.72 / Chapter A. --- Algorithm of MD5 --- p.72 / Chapter B. --- Algorithm of DSA --- p.76 / Chapter C. --- Algorithm of RSA --- p.79

Tsoi Kuen Hung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 84-92). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Objectives --- p.3 / Chapter 1.3 --- Contributions --- p.3 / Chapter 1.4 --- Thesis Organization --- p.4 / Chapter 2 --- Background and Review --- p.6 / Chapter 2.1 --- Introduction --- p.6 / Chapter 2.2 --- Cryptographic Algorithms --- p.6 / Chapter 2.3 --- Cryptographic Applications --- p.10 / Chapter 2.4 --- Modern Reconfigurable Platforms --- p.11 / Chapter 2.5 --- Review of Related Work --- p.14 / Chapter 2.5.1 --- Montgomery Multiplier --- p.14 / Chapter 2.5.2 --- IDEA Cipher --- p.16 / Chapter 2.5.3 --- RC4 Key Search --- p.17 / Chapter 2.5.4 --- Secure Random Number Generator --- p.18 / Chapter 2.6 --- Summary --- p.19 / Chapter 3 --- The IDEA Cipher --- p.20 / Chapter 3.1 --- Introduction --- p.20 / Chapter 3.2 --- The IDEA Algorithm --- p.21 / Chapter 3.2.1 --- Cipher Data Path --- p.21 / Chapter 3.2.2 --- S-Box: Multiplication Modulo 216 + 1 --- p.23 / Chapter 3.2.3 --- Key Schedule --- p.24 / Chapter 3.3 --- FPGA-based IDEA Implementation --- p.24 / Chapter 3.3.1 --- Multiplication Modulo 216 + 1 --- p.24 / Chapter 3.3.2 --- Deeply Pipelined IDEA Core --- p.26 / Chapter 3.3.3 --- Area Saving Modification --- p.28 / Chapter 3.3.4 --- Key Block in Memory --- p.28 / Chapter 3.3.5 --- Pipelined Key Block --- p.30 / Chapter 3.3.6 --- Interface --- p.31 / Chapter 3.3.7 --- Pipelined Design in CBC Mode --- p.31 / Chapter 3.4 --- Summary --- p.32 / Chapter 4 --- Variable Radix Montgomery Multiplier --- p.33 / Chapter 4.1 --- Introduction --- p.33 / Chapter 4.2 --- RSA Algorithm --- p.34 / Chapter 4.3 --- Montgomery Algorithm - Ax B mod N --- p.35 / Chapter 4.4 --- Systolic Array Structure --- p.36 / Chapter 4.5 --- Radix-2k Core --- p.37 / Chapter 4.5.1 --- The Original Kornerup Method (Bit-Serial) --- p.37 / Chapter 4.5.2 --- The Radix-2k Method --- p.38 / Chapter 4.5.3 --- Time-Space Relationship of Systolic Cells --- p.38 / Chapter 4.5.4 --- Design Correctness --- p.40 / Chapter 4.6 --- Implementation Details --- p.40 / Chapter 4.7 --- Summary --- p.41 / Chapter 5 --- Parallel RC4 Engine --- p.42 / Chapter 5.1 --- Introduction --- p.42 / Chapter 5.2 --- Algorithms --- p.44 / Chapter 5.2.1 --- RC4 --- p.44 / Chapter 5.2.2 --- Key Search --- p.46 / Chapter 5.3 --- System Architecture --- p.47 / Chapter 5.3.1 --- RC4 Cell Design --- p.47 / Chapter 5.3.2 --- Key Search --- p.49 / Chapter 5.3.3 --- Interface --- p.50 / Chapter 5.4 --- Implementation --- p.50 / Chapter 5.4.1 --- RC4 cell --- p.51 / Chapter 5.4.2 --- Floorplan --- p.53 / Chapter 5.5 --- Summary --- p.53 / Chapter 6 --- Blum Blum Shub Random Number Generator --- p.55 / Chapter 6.1 --- Introduction --- p.55 / Chapter 6.2 --- RRNG Algorithm . . --- p.56 / Chapter 6.3 --- PRNG Algorithm --- p.58 / Chapter 6.4 --- Architectural Overview --- p.59 / Chapter 6.5 --- Implementation --- p.59 / Chapter 6.5.1 --- Hardware RRNG --- p.60 / Chapter 6.5.2 --- BBS PRNG --- p.61 / Chapter 6.5.3 --- Interface --- p.66 / Chapter 6.6 --- Summary --- p.66 / Chapter 7 --- Experimental Results --- p.68 / Chapter 7.1 --- Design Platform --- p.68 / Chapter 7.2 --- IDEA Cipher --- p.69 / Chapter 7.2.1 --- Size of IDEA Cipher --- p.70 / Chapter 7.2.2 --- Performance of IDEA Cipher --- p.70 / Chapter 7.3 --- Variable Radix Systolic Array --- p.71 / Chapter 7.4 --- Parallel RC4 Engine --- p.75 / Chapter 7.5 --- BBS Random Number Generator --- p.76 / Chapter 7.5.1 --- Size --- p.76 / Chapter 7.5.2 --- Speed --- p.76 / Chapter 7.5.3 --- External Clock --- p.77 / Chapter 7.5.4 --- Random Performance --- p.78 / Chapter 7.6 --- Summary --- p.78 / Chapter 8 --- Conclusion --- p.81 / Chapter 8.1 --- Future Development --- p.83 / Bibliography --- p.84

by Shek Wong. / Thesis submitted in: December 1997. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 90-[94]). / Abstract also in Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Electronic Commerce --- p.3 / Chapter 1.2 --- Electronic Cash --- p.7 / Chapter 1.3 --- What This Report Contains --- p.9 / Chapter 2 --- Cryptographic Background --- p.11 / Chapter 2.1 --- Euler Totient Function --- p.12 / Chapter 2.2 --- Fermat's Little Theorem --- p.12 / Chapter 2.3 --- Quadratic Residues --- p.12 / Chapter 2.4 --- Legendre Symbol --- p.13 / Chapter 2.5 --- Jacobi Symbol --- p.14 / Chapter 2.6 --- Blum Integer --- p.16 / Chapter 2.7 --- Williams Integer --- p.18 / Chapter 2.8 --- The Quadratic Residuosity Problem --- p.19 / Chapter 2.9 --- The Factorization Problem --- p.20 / Chapter 2.10 --- The Discrete Logarithm Problem --- p.20 / Chapter 2.11 --- One-way Functions --- p.21 / Chapter 2.12 --- Blind Signature --- p.22 / Chapter 2.13 --- Cut-and-choose Methodology --- p.24 / Chapter 3 --- Anatomy and Panorama of Electronic Cash --- p.26 / Chapter 3.1 --- Anatomy of Electronic Cash --- p.26 / Chapter 3.1.1 --- Three Functions and Six Criteria --- p.28 / Chapter 3.1.2 --- Untraceable --- p.29 / Chapter 3.1.3 --- Online and Off-line --- p.30 / Chapter 3.1.4 --- Security --- p.32 / Chapter 3.1.5 --- Transferability --- p.33 / Chapter 3.2 --- Panorama of Electronic Cash --- p.34 / Chapter 3.2.1 --- First Model of Off-line Electronic Cash --- p.34 / Chapter 3.2.2 --- Successors --- p.35 / Chapter 3.2.3 --- Binary Tree Based Divisible Electronic Cash --- p.36 / Chapter 4 --- Spending Limit Enforced Electronic Cash --- p.37 / Chapter 4.1 --- Introduction to Spending Limit Enforced Electronic Cash --- p.37 / Chapter 4.2 --- The Scheme --- p.41 / Chapter 4.3 --- An Example --- p.44 / Chapter 4.4 --- Techniques --- p.47 / Chapter 4.5 --- Security and Efficiency --- p.51 / Chapter 5 --- Interest-bearing Electronic Cash --- p.53 / Chapter 5.1 --- Introduction to Interest-bearing Electronic Cash --- p.53 / Chapter 5.2 --- An Example --- p.55 / Chapter 5.3 --- The Scheme --- p.55 / Chapter 5.4 --- Security --- p.57 / Chapter 5.5 --- An Integrated Scheme --- p.58 / Chapter 5.6 --- Applications --- p.59 / Chapter 6 --- Abacus Type Electronic Cash --- p.61 / Chapter 6.1 --- Introduction --- p.61 / Chapter 6.2 --- Abacus Model --- p.63 / Chapter 6.3 --- Divisible Abacus Electronic Coins --- p.66 / Chapter 6.3.1 --- Binary Tree Abacus Approach --- p.66 / Chapter 6.3.2 --- Multi-tree Approach --- p.57 / Chapter 6.3.3 --- Analysis --- p.69 / Chapter 6.4 --- Abacus Electronic Cash System --- p.71 / Chapter 6.4.1 --- Opening Protocol --- p.71 / Chapter 6.4.2 --- Withdrawal Protocol --- p.74 / Chapter 6.4.3 --- Payment and Deposit Protocol --- p.75 / Chapter 6.5 --- Anonymity and System Efficiency --- p.78 / Chapter 7 --- Conclusions --- p.80 / Chapter A --- Internet Payment Systems --- p.82 / Chapter A.1 --- Bare Web FORM --- p.82 / Chapter A.2 --- Secure Web FORM Payment System --- p.85 / Chapter A.3 --- Membership Type Payment System --- p.86 / Chapter A.4 --- Agent Based Payment System --- p.87 / Chapter A.5 --- Internet-based POS --- p.87 / B Papers derived from this thesis --- p.89 / Bibliography --- p.90

What formal guarantees should a secure messaging application provide? Do the most widely-used protocols provide them? Can we do better? In this thesis we answer these questions and with them give a formal study of modern secure messaging protocols, which encrypt the personal messages of billions of users. We give definitions and analyses of two protocols: one existing (Signal) and one new (ART). For Signal, we begin by extending and generalising classic computational models, in order to apply them to its complex ratcheting key derivations. With a threat model in mind we also define a security property, capturing strong secrecy and authentication guarantees including a new one which we call "post-compromise security". We instantiate Signal as a protocol in our model, stating its security theorem and sketching a computational reduction. Signal only supports encrypting messages between two devices, and so most implementers have built custom protocols on top of it to support group conversations. These protocols usually provide weaker security guarantees, and in particular usually do not have post-compromise security. We propose a new protocol called ART, whose goal is to bring Signal's strong security properties to conversations with multiple users and devices. We give a design rationale and a precise definition of ART, and again generalise existing computational models in order to formally specify its security properties and sketch a security reduction. ART has enjoyed widespread interest from industry, and we aim to turn it into an open standard for secure messaging. To that end, we have brought it to the IETF and formed a working group called Messaging Layer Security, with representatives from academia as well as Facebook, Google, Twitter, Wire, Cisco and more. Through MLS, we hope to bring ART's strong guarantees to practical implementations across industry. After concluding our analyses we pause for a moment, and start looking towards the future. We argue that for complex protocols like Signal and ART we are reaching the limits of computational methods, and that the future for their analysis lies with symbolic verification tools. To that end we return to the symbolic model and give a number of case studies, in each one showing how a traditional limitation of symbolic models can in fact be seen as a modelling artefact.

Yuen Tsz Hon. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves 109-122). / Abstracts in English and Chinese. / Abstract --- p.i / Acknowledgement --- p.iii / List of Notations --- p.viii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Identity Based Cryptography --- p.3 / Chapter 1.2 --- Hierarchical Identity Based Cryptosystem --- p.4 / Chapter 1.3 --- Our contributions --- p.5 / Chapter 1.4 --- Publications --- p.5 / Chapter 1.4.1 --- Publications Produced from This Thesis --- p.5 / Chapter 1.4.2 --- Publications During Author's Study in the Degree --- p.6 / Chapter 1.5 --- Thesis Organization --- p.6 / Chapter 2 --- Background --- p.8 / Chapter 2.1 --- Complexity Theory --- p.8 / Chapter 2.1.1 --- Order Notation --- p.8 / Chapter 2.1.2 --- Algorithms and Protocols --- p.9 / Chapter 2.1.3 --- Relations and Languages --- p.11 / Chapter 2.2 --- Algebra and Number Theory --- p.12 / Chapter 2.2.1 --- Groups --- p.12 / Chapter 2.2.2 --- Elliptic Curve --- p.13 / Chapter 2.2.3 --- Pairings --- p.14 / Chapter 2.3 --- Intractability Assumptions --- p.15 / Chapter 2.4 --- Cryptographic Primitives --- p.18 / Chapter 2.4.1 --- Public Key Encryption --- p.18 / Chapter 2.4.2 --- Digital Signature --- p.19 / Chapter 2.4.3 --- Zero Knowledge --- p.21 / Chapter 2.5 --- Hash Functions --- p.23 / Chapter 2.6 --- Random Oracle Model --- p.24 / Chapter 3 --- Literature Review --- p.26 / Chapter 3.1 --- Identity Based Signatures --- p.26 / Chapter 3.2 --- Identity Based Encryption --- p.27 / Chapter 3.3 --- Identity Based Signcryption --- p.27 / Chapter 3.4 --- Identity Based Blind Signatures --- p.28 / Chapter 3.5 --- Identity Based Group Signatures --- p.28 / Chapter 3.6 --- Hierarchical Identity Based Cryptography --- p.29 / Chapter 4 --- Blind Identity Based Signcryption --- p.30 / Chapter 4.1 --- Schnorr's ROS problem --- p.31 / Chapter 4.2 --- BIBSC and Enhanced IBSC Security Model --- p.32 / Chapter 4.2.1 --- Enhanced IBSC Security Model --- p.33 / Chapter 4.2.2 --- BIBSC Security Model --- p.36 / Chapter 4.3 --- Efficient and Secure BIBSC and IBSC Schemes --- p.38 / Chapter 4.3.1 --- Efficient and Secure IBSC Scheme --- p.38 / Chapter 4.3.2 --- The First BIBSC Scheme --- p.43 / Chapter 4.4 --- Generic Group and Pairing Model --- p.47 / Chapter 4.5 --- Comparisons --- p.52 / Chapter 4.5.1 --- Comment for IND-B --- p.52 / Chapter 4.5.2 --- Comment for IND-C --- p.54 / Chapter 4.5.3 --- Comment for EU --- p.55 / Chapter 4.6 --- Additional Functionality of Our Scheme --- p.56 / Chapter 4.6.1 --- TA Compatibility --- p.56 / Chapter 4.6.2 --- Forward Secrecy --- p.57 / Chapter 4.7 --- Chapter Conclusion --- p.57 / Chapter 5 --- Identity Based Group Signatures --- p.59 / Chapter 5.1 --- New Intractability Assumption --- p.61 / Chapter 5.2 --- Security Model --- p.62 / Chapter 5.2.1 --- Syntax --- p.63 / Chapter 5.2.2 --- Security Notions --- p.64 / Chapter 5.3 --- Constructions --- p.68 / Chapter 5.3.1 --- Generic Construction --- p.68 / Chapter 5.3.2 --- An Instantiation: IBGS-SDH --- p.69 / Chapter 5.4 --- Security Theorems --- p.73 / Chapter 5.5 --- Discussions --- p.81 / Chapter 5.5.1 --- Other Instantiations --- p.81 / Chapter 5.5.2 --- Short Ring Signatures --- p.82 / Chapter 5.6 --- Chapter Conclusion --- p.82 / Chapter 6 --- Hierarchical IBS without Random Oracles --- p.83 / Chapter 6.1 --- New Intractability Assumption --- p.87 / Chapter 6.2 --- Security Model: HIBS and HIBSC --- p.89 / Chapter 6.2.1 --- HIBS Security Model --- p.89 / Chapter 6.2.2 --- Hierarchical Identity Based Signcryption (HIBSC) --- p.92 / Chapter 6.3 --- Efficient Instantiation of HIBS --- p.95 / Chapter 6.3.1 --- Security Analysis --- p.96 / Chapter 6.3.2 --- Ordinary Signature from HIBS --- p.101 / Chapter 6.4 --- Plausibility Arguments for the Intractability of the OrcYW Assumption --- p.102 / Chapter 6.5 --- Efficient HIBSC without Random Oracles --- p.103 / Chapter 6.5.1 --- Generic Composition from HIBE and HIBS --- p.104 / Chapter 6.5.2 --- Concrete Instantiation --- p.105 / Chapter 6.6 --- Chapter Conclusion --- p.107 / Chapter 7 --- Conclusion --- p.108 / Bibliography --- p.109

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Made available in DSpace on 2015-03-05T14:01:20Z (GMT). No. of bitstreams: 0 Previous issue date: 25 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O avanço tecnológico e o valor que as informações representam no mundo moderno conduzem a uma constante busca por cifradores mais velozes, seguros e adaptáveis à diversos ambientes. Enquanto a segurança se baseia no comportamento complicado dos algoritmos, a eficiência se baseia na simplicidade das operações matemáticas envolvidas. Os sistemas caóticos têm a propriedade de exibir comportamento complicado mesmo sendo definido de modo determinístico por funções algebricamente simples. Portanto, a proposta de algoritmos de cifração baseados em sistemas caóticos é um tema de ativa pesquisa. Este trabalho estuda a eficiência e a segurança de um dos mais completos cifradores caóticos. Com base nesse estudo, melhorias são propostas tanto quanto à sua eficiência quanto à sua segurança. Com isto, chega-se a uma versão otimizada, a qual é referida ao longo deste trabalho como cifrador caótico (CC). O cifrador resultante não possui as fraquezas dos algoritmos caóticos propostos até então. Além disso, comparações exper / The technological advance and value of information in the preset days lead to a constant search for more efficient, secure and adaptable ciphers. While de security is based on the complicated behavior of the algorithms, its efficiency is based on the simplicity of the mathematical operations involved. The chaotic systems are characterized by the property of showing complex behavior even being defined by simple algebraic mathematical expressions. Therefore, the proposal of ciphers based on chaotic dynamics has became an active research field. This work deals with the efficiency and security of one of the most complete chaotic cipher proposed so far. Based on this study, improvements are proposed for both efficiency and security. The final result is an optimized version, which is denominated chaotic cipher (CC) towards this work. This cipher does not suffer from the weaknesses of the chaotic ciphers proposed so far. Moreover, experimental comparisons shows that the CC performance is comparable with those of s

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1978. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaf 51. / B.S.

Dans cette thèse, nous discutons d’aspects algorithmiques de trois différents problèmes, en lien avec la cryptographie. La première partie est consacrée à l’algèbre linéaire creuse. Nous y présentons un nouvel algorithme de pivot de Gauss pour matrices creuses à coefficients exacts, ainsi qu’une nouvelle heuristique de sélection de pivots, qui rend l’entière procédure particulièrement efficace dans certains cas. La deuxième partie porte sur une variante du problème des anniversaires, avec trois listes. Ce problème, que nous appelons problème 3XOR, consiste intuitivement à trouver trois chaînes de caractères uniformément aléatoires de longueur fixée, telles que leur XOR soit la chaîne nulle. Nous discutons des considérations pratiques qui émanent de ce problème et proposons un nouvel algorithme plus rapide à la fois en théorie et en pratique que les précédents. La troisième partie est en lien avec le problème learning with errors (LWE). Ce problème est connu pour être l’un des principaux problèmes difficiles sur lesquels repose la cryptographie à base de réseaux euclidiens. Nous introduisons d’abord un générateur pseudo-aléatoire, basé sur la variante dé-randomisée learning with rounding de LWE, dont le temps d’évaluation est comparable avec celui d’AES. Dans un second temps, nous présentons une variante de LWE sur l’anneau des entiers. Nous montrerons que dans ce cas le problème est facile à résoudre et nous proposons une application intéressante en re-visitant une attaque par canaux auxiliaires contre le schéma de signature BLISS. / In this thesis, we discuss algorithmic aspects of three different problems, related to cryptography. The first part is devoted to sparse linear algebra. We present a new Gaussian elimination algorithm for sparse matrices whose coefficients are exact, along with a new pivots selection heuristic, which make the whole procedure particularly efficient in some cases. The second part treats with a variant of the Birthday Problem with three lists. This problem, which we call 3XOR problem, intuitively consists in finding three uniformly random bit-strings of fixed length, such that their XOR is the zero string. We discuss practical considerations arising from this problem, and propose a new algorithm which is faster in theory as well as in practice than previous ones. The third part is related to the learning with errors (LWE) problem. This problem is known for being one of the main hard problems on which lattice-based cryptography relies. We first introduce a pseudorandom generator, based on the de-randomised learning with rounding variant of LWE, whose running time is competitive with AES. Second, we present a variant of LWE over the ring of integers. We show that in this case the problem is easier to solve, and we propose an interesting application, revisiting a side-channel attack against the BLISS signature scheme.