Дисертації з теми "CRYPTOGRAPHI"
Оформте джерело за APA, MLA, Chicago, Harvard та іншими стилями
Ознайомтеся з топ-50 дисертацій для дослідження на тему "CRYPTOGRAPHI".
Біля кожної праці в переліку літератури доступна кнопка «Додати до бібліографії». Скористайтеся нею – і ми автоматично оформимо бібліографічне посилання на обрану працю в потрібному вам стилі цитування: APA, MLA, «Гарвард», «Чикаго», «Ванкувер» тощо.
Також ви можете завантажити повний текст наукової публікації у форматі «.pdf» та прочитати онлайн анотацію до роботи, якщо відповідні параметри наявні в метаданих.
Переглядайте дисертації для різних дисциплін та оформлюйте правильно вашу бібліографію.
Poschmann, Axel York. "Lightweight cryptography cryptographic engineering for a pervasive world." Berlin Bochum Dülmen London Paris Europ. Univ.-Verl, 2009. http://d-nb.info/996578153/04.
Повний текст джерелаAlmeida, Braga Daniel de. "Cryptography in the wild : the security of cryptographic implementations." Thesis, Rennes 1, 2022. http://www.theses.fr/2022REN1S067.
Повний текст джерелаSide-channel attacks are daunting for cryptographic implementations. Despite past attacks, and the proliferation of verification tools, these attacks still affect many implementations. In this manuscript, we address two aspects of this problem, centered around attack and defense. We unveil several microarchitectural side-channel attacks on implementations of PAKE protocols. In particular, we exposed attacks on Dragonfly, used in the new Wi-Fi standard WPA3, and SRP, deployed in many software such as ProtonMail or Apple HomeKit. We also explored the lack of use by developers of tools to detect such attacks. We questioned developers from various cryptographic projects to identify the origin of this lack. From their answers, we issued recommendations. Finally, in order to stop the spiral of attack-patch on Dragonfly implementations, we provide a formally verified implementation of the cryptographic layer of the protocol, whose execution is secret-independent
Scerri, Guillaume. "Proof of security protocols revisited." Thesis, Cachan, Ecole normale supérieure, 2015. http://www.theses.fr/2015DENS0002/document.
Повний текст джерелаWith the rise of the Internet the use of cryptographic protocols became ubiquitous. Considering the criticality and complexity of these protocols, there is an important need of formal verification.In order to obtain formal proofs of cryptographic protocols, two main attacker models exist: the symbolic model and the computational model. The symbolic model defines the attacker capabilities as a fixed set of rules. On the other hand, the computational model describes only the attacker's limitations by stating that it may break some hard problems. While the former is quiteabstract and convenient for automating proofs the later offers much stronger guarantees.There is a gap between the guarantees offered by these two models due to the fact the symbolic model defines what the adversary may do while the computational model describes what it may not do. In 2012 Bana and Comon devised a new symbolic model in which the attacker's limitations are axiomatised. In addition provided that the (computational semantics) of the axioms follows from the cryptographic hypotheses, proving security in this symbolic model yields security in the computational model.The possibility of automating proofs in this model (and finding axioms general enough to prove a large class of protocols) was left open in the original paper. In this thesis we provide with an efficient decision procedure for a general class of axioms. In addition we propose a tool (SCARY) implementing this decision procedure. Experimental results of our tool shows that the axioms we designed for modelling security of encryption are general enough to prove a large class of protocols
Minaud, Brice. "Analyse de primitives cryptographiques récentes." Thesis, Rennes 1, 2016. http://www.theses.fr/2016REN1S066/document.
Повний текст джерелаIn this thesis, we study the security of some recent cryptographic primitives, both symmetric and asymmetric. Along the way we also consider white-box primitives, which may be regarded as a middle ground between symmetric and asymmetric cryptography. We begin by showing the existence of non-trivial linear maps commuting with the round function of some recent block cipher designs, which give rise to self-similarity and invariant subspace attacks. We then move on to the structural cryptanalysis of ASASA schemes, where nonlinear layers S alternate with affine layers A. Our structural cryptanalysis applies to symmetric, multivariate, as well as white-box instances. Focusing on the white-box model of incompressibility, we then build an efficient block cipher and key generator that offer provable security guarantees. Finally, on the purely asymmetric side, we describe a polynomial attack against a recent multilinear map proposal
Bultel, Xavier. "Mécanismes de délégation pour les primitives de cryptographie à clé publique." Thesis, Université Clermont Auvergne (2017-2020), 2018. http://www.theses.fr/2018CLFAC100.
Повний текст джерелаPaindavoine, Marie. "Méthodes de calculs sur les données chiffrées." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSE1009/document.
Повний текст джерелаNowadays, encryption and services issued of ``big data" are at odds. Indeed, encryption is about protecting users privacy, while big data is about analyzing users data. Being increasingly concerned about security, users tend to encrypt their sensitive data that are subject to be accessed by other parties, including service providers. This hinders the execution of services requiring some kind of computation on users data, which makes users under obligation to choose between these services or their private life. We address this challenge in this thesis by following two directions.In the first part of this thesis, we study fully homomorphic encryption that makes possible to perform arbitrary computation on encrypted data. However, this kind of encryption is still inefficient, and this is due in part to the frequent execution of a costly procedure throughout evaluation, namely the bootstrapping. Thus, efficiency is inversely proportional to the number of bootstrappings needed to evaluate functions on encrypted data. In this thesis, we prove that finding such a minimum is NP-complete. In addition, we design a new method that efficiently finds a good approximation of it. In the second part, we design schemes that allow a precise functionality. The first one is verifiable deduplication on encrypted data, which allows a server to be sure that it keeps only one copy of each file uploaded, even if the files are encrypted, resulting in an optimization of the storage resources. The second one is intrusion detection over encrypted traffic. Current encryption techniques blinds intrusion detection services, putting the final user at risks. Our results permit to reconcile users' right to privacy and their need of keeping their network clear of all intrusion
Wen, Weiqiang. "Contributions to the hardness foundations of lattice-based cryptography." Thesis, Lyon, 2018. http://www.theses.fr/2018LYSEN070/document.
Повний текст джерелаLattice-based cryptography is one of the most competitive candidates for protecting privacy, both in current applications and post quantum period. The central problem that serves as the hardness foundation of lattice-based cryptography is called the Learning with Errors (LWE). It asks to solve a noisy equation system, which is linear and over-determined modulo q. Normally, we call LWE problem as an average-case problem as all the coefficients in the equation system are randomly chosen modulo q. The LWE problem is conjectured to be hard even wtih a large scale quantum computer. It is at least as hard as standard problems defined in the lattices, such as Bounded Distance Decoding (BDD) and unique Shortest Vector Problem (uSVP). Finally, the best known algorithm for solving these problems is BKZ, which is very expensive. In this thesis, we study the quantum hardness of LWE, the hardness relations between the underlying problems BDD and uSVP, and the practical performance of the BKZ algorithm. First, we give a strong evidence of quantum hardness of LWE. Concretely, we consider a relaxed version of the quantum version of dihedral coset problem and show an computational equivalence between LWE and this problem. Second, we tighten the hardness relation between BDD and uSVP. More precisely, We improve the reduction from BDD to uSVP by a factor √2, compared to the one by Lyubashevsky and Micciancio. Third, we propose a more precise simulator for BKZ. In the last work, we propose the first probabilistic simulotor for BKZ, which can pridict the practical behavior of BKZ very precisely
Löken, Nils [Verfasser]. "Cryptography for the crowd : a study of cryptographic schemes with applications to crowd work / Nils Löken." Paderborn : Universitätsbibliothek, 2019. http://d-nb.info/1203205074/34.
Повний текст джерелаLambin, Baptiste. "Optimization of core components of block ciphers." Thesis, Rennes 1, 2019. http://www.theses.fr/2019REN1S036/document.
Повний текст джерелаAlong with new cryptanalysis techniques, the security of block ciphers is always evolving. When designing new block ciphers, we thus need to consider these new techniques during the security analysis. In this thesis, we show how to build some core operations for block ciphers to improve the security against some attacks. We first start by describing a method to find optimal (according to some criterion) even-odd permutations for a Generalized Feistel Network. Using a new characterization and an efficient algorithm, we are able to solve a 10-years old problem. We then give new cryptanalysis techniques to improve the division property, along with a new proven optimal criterion for designing S-boxes. We continue with new observations for the design of an alternative key-schedule for AES. We thus give a new key-schedule, which is both more efficient and more secure against some attacks compared to the original one. Finally, we describe a very efficient generic algorithm to break most proposals in white-box cryptography, as well as a dedicated attack on a previously not analyzed scheme, leading to a key-recovery attack in a few seconds
Delaplace, Claire. "Algorithmes d'algèbre linéaire pour la cryptographie." Thesis, Rennes 1, 2018. http://www.theses.fr/2018REN1S045/document.
Повний текст джерелаIn 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
Dugardin, Margaux. "Amélioration d'attaques par canaux auxiliaires sur la cryptographie asymétrique." Thesis, Paris, ENST, 2017. http://www.theses.fr/2017ENST0035/document.
Повний текст джерела: Since the 1990s, side channel attacks have challenged the security level of cryptographic algorithms on embedded devices. Indeed, each electronic component produces physical emanations, such as the electromagnetic radiation, the power consumption or the execution time. Besides, these emanations reveal some information on the internal state of the computation. A wise attacker can retrieve secret data in the embedded device using the analyzes of the involuntary “leakage”, that is side channel attacks. This thesis focuses on the security evaluation of asymmetric cryptographic algorithm such as RSA and ECC. In these algorithms, the main leakages are observed on the modular multiplication. This thesis presents two attacks targeting the modular multiplication in protected algorithms, and a formal demonstration of security level of a countermeasure named modular extension. A first attack is against scalar multiplication on elliptic curve implemented with a regular algorithm and scalar blinding. This attack uses a unique acquisition on the targeted device and few acquisitionson another similar device to retrieve the whole scalar. A horizontal leakage during the modular multiplication over large numbers allows to detect and correct easily an error bit in the scalar. A second attack exploits the final subtraction at the end of Montgomery modular multiplication. By studying the dependency of consecutive multiplications, we can exploit the information of presence or absence of final subtraction in order to defeat two protections : regular algorithm and blinding input values. Finally, we prove formally the security level of modular extension against first order fault attacks applied on elliptic curves cryptography
Koussa, Eliane. "Analysis and design of post-quantum cryptographic algorithms : PKP-based signature scheme and ultra-short multivariate signatures." Electronic Thesis or Diss., université Paris-Saclay, 2020. http://www.theses.fr/2020UPASG027.
Повний текст джерелаThe construction of large quantum computers would endanger most of the public-key cryptographic schemes in use today. Therefore, there is currently a large research effort to develop new post-quantum secure schemes. In particular, we are interested in post-quantum cryptographic schemes whose security relies on the hardness of solving some mathematical problems such as thePermuted Kernel Problem (PKP) and the Hidden Field Equations (HFE). This work investigates first the complexity of PKP. And after a thorough analysis of the State-of-theart attacks of PKP, we have been able to update some results that were not accurate, and to provide an explicit complexity formula which allows us to identify hard instances and secure sets of parameters of this problem. PKP was used in 1989 to develop the first Zero-Knowledge Identification Scheme (ZK-IDS) that has an efficient implementation on low-cost smart cards. In a second step, we optimize the PKP-based ZK-IDS and then we introduce PKP-DSS:a Digital Signature Scheme based on PKP. We construct PKP-DSS from the ZK-IDS based on PKP by using the traditional Fiat-Shamir (FS) transform that converts Identification schemes into Signature schemes. We develop a constant time implementation of PKP-DSS. It appears that our scheme is very competitive with other post-quantum FS signature schemes. Since that PKP is an NP-Complete problem and since there are no known quantum attacks for solving PKP significantly better than classical attacks, we believe that our scheme is post-quantum secure. On the other hand, we study multivariate public-key signature schemes that provide“ultra”-short signatures. We first analyze the most known attacks against multivariate signatures, and then define the minimal parameters that allow ultra-short signature. We also design some specific newmodes of operations in order to avoid particular attacks.Second, we provide various explicit examples of ultra-short signature schemes that are based on variants of HFE. We present parameters for several level of classical security: 80, 90, 100 bits in addition to 128, 192, and 256 bits; foreach level, we propose different choices of finite fields
Kosek, Amy. "An Exploration of Mathematical Applications in Cryptography." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1428944810.
Повний текст джерелаBost, Raphaël. "Algorithmes de recherche sur bases de données chiffrées." Thesis, Rennes 1, 2018. http://www.theses.fr/2018REN1S001/document.
Повний текст джерелаSearchable encryption aims at making efficient a seemingly easy task: outsourcing the storage of a database to an untrusted server, while keeping search features. With the development of Cloud storage services, for both private individuals and businesses, efficiency of searchable encryption became crucial: inefficient constructions would not be deployed on a large scale because they would not be usable. The key problem with searchable encryption is that any construction achieving ''perfect security'' induces a computational or a communicational overhead that is unacceptable for the providers or for the users --- at least with current techniques and by today's standards. This thesis proposes and studies new security notions and new constructions of searchable encryption, aiming at making it more efficient and more secure. In particular, we start by considering the forward and backward privacy of searchable encryption schemes, what it implies in terms of security and efficiency, and how we can realize them. Then, we show how to protect an encrypted database user against active attacks by the Cloud provider, and that such protections have an inherent efficiency cost. Finally, we take a look at existing attacks against searchable encryption, and explain how we might thwart them
Vial, prado Francisco. "Contributions to design and analysis of Fully Homomorphic Encryption schemes." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLV107/document.
Повний текст джерелаFully Homomorphic Encryption schemes allow public processing of encrypted data. Since the groundbreaking discovery of the first FHE scheme in 2009 by Craig Gentry, an impressive amount of research has been conducted to improve efficiency, achieve new levels of security, and describe real applications and connections to other areas of cryptography. In this Dissertation, we first give a detailed account on research these past years. Our contributions include a key-recovery attack on the ideal lattices FHE scheme and a new conception of hierarchic encryption, avoiding at some extent betrayal between users while maintaining the flexibility of FHE. We also describe some implementations. This research was done in the Laboratoire de Mathématiques de Versailles, under supervision of Prof. Louis Goubin
Lashermes, Ronan. "Etude de la sécurité des implémentations de couplage." Thesis, Versailles-St Quentin en Yvelines, 2014. http://www.theses.fr/2014VERS0021/document.
Повний текст джерелаPairings are cryptographic algorithms allowing new protocols for public-key cryptography. After a decade of research which led to a dramatic improvement of the computation speed of pairings, we focused on the security of pairing implementations.For that purpose, we evaluated the resistance to fault attacks. We have sent electromagnetic pulses in the chip computing a pairing at a precise instant. It allowed us to recover the cryptographic secret which should be protected in the computation. Our study was both theoretical and practical; we did implement actual fault attacks. Finally, we proposed countermeasures in order to protect the algorithm in the future
Zuber, Martin. "Contributions to data confidentiality in machine learning by means of homomorphic encryption." Electronic Thesis or Diss., université Paris-Saclay, 2020. http://www.theses.fr/2020UPASG048.
Повний текст джерелаWe aim to provide a set tools allowing for machine learning algorithms to yield their intended results while ensuring confidentiality properties are achieved for the underlying data. This can be achieved through regulatory measures such as prohibiting the use of a sensitive database in certain cases and restricting its access to certain law enforcement agencies. The fundamental reason for the existence of our work - and every other work like it - is the following: why trust that an outside entity will not misuse personal data when you can have assurances of that fact ? This applies both in the case of a private company that may use/sell your data for profit, legally or illegally. It also applies to use by a government which may or may not have the proper safeguards against abuse, as well as the proper security for data storage and access. In our case, we provide such confidentiality properties though the use of Fully Homomorphic Encryption (FHE). Precisely, most of our work focuses on finding new algorithms for secure outsourced machine learning evaluation using FHE. While other privacy and confidentiality preserving methods are touched upon briefly, we focused our research on homomorphic encryption and strive to explain our choice and its general context. We present three main novel secure machine learning applications: a confidentiality-preserving recursive discrete neural network; a model-confidential embedding-based neural network; a confidentiality-preserving k-NN classifier. Notably, our secure k-NN classifier is the only such algorithm in the literature obtaining a result noninteractively. We evaluate the accuracy and efficiency of these three applications on real-world machine learning problems. We show that our secure schemes compare very favorably to their non-secure counterparts in terms of accuracy, while still running in realistic time. Beyond these schemes themselves, this thesis promotes a specific research direction for secure machine learning. We argue for less (though still some) focus on deep convolutional neural networks and show that looking at somewhat lesser known machine learning algorithms can yield promising results
Bert, Pauline. "Signatures reposant sur les réseaux euclidiens : de la construction à l'implémentation." Thesis, Rennes 1, 2019. http://www.theses.fr/2019REN1S126.
Повний текст джерелаLattice-based cryptography is one of the major line of research to build post-quantum public key primitives. In this thesis, we discuss about digital signatures constructions and their implementation. We first describe a Fiat-Shamir transformation from an identification scheme using rejection sampling to a digital signature secure in the random oracle model. Then we describe an identity-based encryption scheme and we prove its security in the standard model. An identity-based encryption scheme is like a classical public key where the public key is the identity of a user such as its email address or its social security number. A user contacts a third trusted party to get a secret key associated to its identity. In our construction, a secret key consists essentially in a signature of the identity of the user. We also describe this underlying digital signature scheme associated to our identity based encryption scheme. Finally, we present implementation results of these two schemes and how we choose concrete parameters
Connolly, Aisling. "Try Again. Fail Again. Fail Better : New notions of security, broken assumptions, and increased efficiency in cryptography." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEE034.
Повний текст джерелаThis thesis presents new results addressing three fundamental areas of cryptography: security notions, assumptions, and efficiency. The first part encompasses the security of symmetric primitives. We give a new security notion that provides the strongest security for symmetric primitives proven in the random oracle model (ROM). Key-correlated attacks (KCA) model the scenario where all inputs (keys, messages, and possibly nonces and headers) are correlated with the secret key. Under mild assumptions, we prove KCA security of blockciphers, and show that 3-rounds of Even-Mansour are necessary to achieve this. Then, we define a KCA-security notion for nonce-based authenticated encryption (AE), and provide a black-box transformation that turns a multiuser-secure AE into an AE scheme that is provably KCA secure in the ROM. We show relations and separations with older notions (related-key and key-dependent message security) to show that security under KCA is strictly stronger, and implies the others. The next part turns to public-key cryptography, and analyses the assumptions underlying the new public-key cryptosystem of AJPS17. Cryptanalysis of their assumption, based on arithmetic modulo a Mersenne prime, allowed us to reconstruct the secret key, given only the public key. Based on a modified version of the assumption, we propose a variant post-quantum secure public-key cryptosystem. The last part turns to efficiency, and studies the Schnorr Signature scheme. Exploiting the group structure we can generate multiple nonces (instead of just one) with which we can then generate a batch of signatures. This, together with some preprocessing tricks, allow us to increase efficiency of Schnorr signature generation. Security is maintained under a new assumption shown intractable in the Generic Group Model
Landais, Gregory. "Mise en oeuvre de cryptosystèmes basés sur les codes correcteurs d'erreurs et de leurs cryptanalyses." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066602/document.
Повний текст джерелаThis thesis is about algorithmic problems arising when someone wants to implement a cryptosystem based on error correcting codes or a cryptanalysis of such a system. The benefits of these systems come from their excellent algorithmic complexity, better of several orders than the classical public key schemes. They also bring a credible alternative to the current systems that for most of them rely on number theory and on the problems of factorisation and discrete logarithm. P.Shor showed that these two problems could be solved in polynomial time in the quantum computer model. This computer is far from being operational but we will need alternatives we can trust and that have efficient implementations
Roux-Langlois, Adeline. "Lattice - Based Cryptography - Security Foundations and Constructions." Thesis, Lyon, École normale supérieure, 2014. http://www.theses.fr/2014ENSL0940/document.
Повний текст джерелаLattice-based cryptography is a branch of cryptography exploiting the presumed hardness of some well-known problems on lattices. Its main advantages are its simplicity, efficiency, and apparent security against quantum computers. The principle of the security proofs in lattice-based cryptography is to show that attacking a given scheme is at least as hard as solving a particular problem, as the Learning with Errors problem (LWE) or the Small Integer Solution problem (SIS). Then, by showing that those two problems are at least as hard to solve than a hard problem on lattices, presumed polynomial time intractable, we conclude that the constructed scheme is secure.In this thesis, we improve the foundation of the security proofs and build new cryptographic schemes. We study the hardness of the SIS and LWE problems, and of some of their variants on integer rings of cyclotomic fields and on modules on those rings. We show that there is a classical hardness proof for the LWE problem (Regev's prior reduction was quantum), and that the module variants of SIS and LWE are also hard to solve. We also give two new lattice-based group signature schemes, with security based on SIS and LWE. One is the first lattice-based group signature with logarithmic signature size in the number of users. And the other construction allows another functionality, verifier-local revocation. Finally, we improve the size of some parameters in the work on cryptographic multilinear maps of Garg, Gentry and Halevi in 2013
Dold, Florian. "The GNU Taler system : practical and provably secure electronic payments." Thesis, Rennes 1, 2019. http://www.theses.fr/2019REN1S008/document.
Повний текст джерелаWe describe the design and implementation of GNU Taler, an electronic payment system based on an extension of Chaumian online e-cash with efficient change. In addition to anonymity for customers, it provides the novel notion of income transparency, which guarantees that merchants can reliably receive a payment from an untrusted payer only when their income from the payment is visible to tax authorities. Income transparency is achieved by the introduction of a refresh protocol, which gives anonymous change for a partially spent coin without introducing a tax evasion loophole. In addition to income transparency, the refresh protocol can be used to implement Camenisch-style atomic swaps, and to preserve anonymity in the presence of protocol aborts and crash faults with data loss by participants. Furthermore, we show the provable security of our income-transparent anonymous e-cash, which, in addition to the usual anonymity and unforgeability proper- ties of e-cash, also formally models conservation of funds and income transparency. Our implementation of GNU Taler is usable by non-expert users and integrates with the modern Web architecture. Our payment platform addresses a range of practical issues, such as tipping customers, providing refunds, integrating with banks and know-your-customer (KYC) checks, as well as Web platform security and reliability requirements. On a single machine, we achieve transaction rates that rival those of global, commercial credit card processors. We increase the robustness of the exchange—the component that keeps bank money in escrow in exchange for e-cash—by adding an auditor component, which verifies the correct operation of the system and allows to detect a compromise or misbehavior of the exchange early. Just like bank accounts have reason to exist besides bank notes, e-cash only serves as part of a whole payment system stack. Distributed ledgers have recently gained immense popularity as potential replacement for parts of the traditional financial industry. While cryptocurrencies based on proof-of-work such as Bitcoin have yet to scale to be useful as a replacement for established payment systems, other more efficient systems based on Blockchains with more classical consensus algorithms might still have promising applications in the financial industry. We design, implement and analyze the performance of Byzantine Set Union Consensus (BSC), a Byzantine consensus protocol that agrees on a (super-)set of elements at once, instead of sequentially agreeing on the individual elements of a set. While BSC is interesting in itself, it can also be used as a building block for permissioned Blockchains, where—just like in Nakamoto-style consensus—whole blocks of transactions are agreed upon at once, increasing the transaction rate
Cauchois, Victor. "Couches de diffusion linéaires à partir de matrices MDS." Thesis, Rennes 1, 2018. http://www.theses.fr/2018REN1S077/document.
Повний текст джерелаThis thesis focuses on two aspects of symmetric cryptology related to the use of MDS matrices as building blocks of linear layers for symmetric primitives. A first part handles designs of linear layers for symmetric ciphers based upon MDS matrices. Associations between recursive, respectively circulant, matrices and polynomials are reproduced between other matrix structures and elements in non-commutative polynomial rings of Ore. As for recursive and circulant matrices, those structures come along with lightweight hardware implementations. From Gabidulin codes are derived direct constructions of MDS matrices with properties close to involution from hardware perspectives. The second part is about distinguishing attacks on an exemple of AES-like permutations. The use of some MDS matrix to build the linear layer induces a macroscopic description of differential trails through the different steps of the algorithm computing the permutation. Truncated differential path appears, from which rebound attack are built. Original work here generalizes rebound attack applied on permutations of GROSTL-512 from structured differential path not raised from free propagations of differences. This structure allows not to consume all degrees of freedom in a simple algorithmic step but to divide this comsumption into three algorithmic steps. An attack of a reduced-round version with 11 rounds of one permutation of GROSTL-512 can then be mounted
Daubignard, Marion. "Formalisation de preuves de sécurité concrète." Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00721776.
Повний текст джерелаThomas, Gael. "Design et Analyse de sécurité pour les constructions en cryptographie symétrique." Thesis, Limoges, 2015. http://www.theses.fr/2015LIMO0042/document.
Повний текст джерелаThe work done during this Ph.D. lies at the crossroads of symmetric cryptography and constraints environments. The goal of such cryptography, called lightweight cryptography, is to propose and evaluate symmetric algorithms that can be implemented on very ressource limited devices. The contributions of this thesis are first on the security evaluations of feedback with carry shift registers (FCSR) to some new attacks and second on a unified vision of generalized Feistel networks (GFNs) that allows to better understand their cryptographic properties. These studies gave rise to two new lightweight algorithms: first GLUON a hash function based upon FCSRs and second the cipher LILLIPUT based on a family further extanding the notion of generalized Feistel network. Finally, a generic method for carrying out a differential fault attack on GFNs is outlined
Brunet, Solenn. "Conception de mécanismes d'accréditations anonymes et d'anonymisation de données." Thesis, Rennes 1, 2017. http://www.theses.fr/2017REN1S130/document.
Повний текст джерелаThe emergence of personal mobile devices, with communication and positioning features, is leading to new use cases and personalized services. However, they imply a significant collection of personal data and therefore require appropriate security solutions. Indeed, users are not always aware of the personal and sensitive information that can be inferred from their use. The main objective of this thesis is to show how cryptographic mechanisms and data anonymization techniques can reconcile privacy, security requirements and utility of the service provided. In the first part, we study keyed-verification anonymous credentials which guarantee the anonymity of users with respect to a given service provider: a user proves that she is granted access to its services without revealing any additional information. We introduce new such primitives that offer different properties and are of independent interest. We use these constructions to design three privacy-preserving systems: a keyed-verification anonymous credentials system, a coercion-resistant electronic voting scheme and an electronic payment system. Each of these solutions is practical and proven secure. Indeed, for two of these contributions, implementations on SIM cards have been carried out. Nevertheless, some kinds of services still require using or storing personal data for compliance with a legal obligation or for the provision of the service. In the second part, we study how to preserve users' privacy in such services. To this end, we propose an anonymization process for mobility traces based on differential privacy. It allows us to provide anonymous databases by limiting the added noise. Such databases can then be exploited for scientific, economic or societal purposes, for instance
Milio, Enea. "Calcul de polynômes modulaires en dimension 2." Thesis, Bordeaux, 2015. http://www.theses.fr/2015BORD0285/document.
Повний текст джерелаModular polynomials on elliptic curves are a fundamental tool used for the computation of graph of isogenies, class polynomials or for point counting. Thus, they are fundamental for the elliptic curve cryptography. A generalization of these polynomials for principally polarized abelian surfaces has been introduced by Régis Dupont in 2006, who has also described an algorithm to compute them, while theoretical results can been found in an article of Bröker– Lauter of 2009. But these polynomials being really big, they have been computed only in the minimal case p = 2. In this thesis, we continue the work of Dupont and Bröker–Lauter by defining and giving theoretical results on modular polynomials with new invariants, based on theta constants. Using these invariants, we have been able to compute the polynomials until p = 7 but bigger examples look intractable. Thus we define a new kind of modular polynomials where we restrict on the surfaces having real multiplication by the maximal order of a real quadratic field. We present many examples and theoretical results
Sanders, Olivier. "Conception et optimisation de mécanismes cryptographique anonymes." Thesis, Paris, Ecole normale supérieure, 2015. http://www.theses.fr/2015ENSU0027/document.
Повний текст джерелаNew technologies offer greater convenience for end-users but usually at the cost of a loss in terms of privacy, which is often underestimated by the latter. For example, knowledge by a third party of the information related to a transaction is far from insignificant since it may reveal intimate details such as whereabouts, religious beliefs or health status.In this thesis, we are interested in cryptographic technics allowing to reconcile both security requirements and user’s privacy. In a first part, we will focus on two specific cases: anonymous payment and anonymous authentication. We propose new constructions, improving the efficiency of state-of-the-art solutions, which make all the features of these primitives more accessible for practical applications. We provide a detailed security analysis for each scheme, proving that they achieve the expected properties under reasonable assumptions.However, to fulfill the strong technical constraints of these use cases, it may be necessary to optimize these constructions which are usually rather complex. To this end, we propose in a second part, new solutions to improve the efficiency of most common operations and algorithms. Each of these contributions is not restricted to anonymous systems and thus may be of independent interest
Truneček, Petr. "Kryptografické protokoly v praxi." Master's thesis, Vysoké učení technické v Brně. Fakulta elektrotechniky a komunikačních technologií, 2009. http://www.nusl.cz/ntk/nusl-218171.
Повний текст джерелаPrest, Thomas. "Gaussian sampling in lattice-based cryptography." Thesis, Paris, Ecole normale supérieure, 2015. http://www.theses.fr/2015ENSU0045/document.
Повний текст джерелаAlthough rather recent, lattice-based cryptography has stood out on numerous points, be it by the variety of constructions that it allows, by its expected resistance to quantum computers, of by its efficiency when instantiated on some classes of lattices. One of the most powerful tools of lattice-based cryptography is Gaussian sampling. At a high level, it allows to prove the knowledge of a particular lattice basis without disclosing any information about this basis. It allows to realize a wide array of cryptosystems. Somewhat surprisingly, few practical instantiations of such schemes are realized, and the algorithms which perform Gaussian sampling are seldom studied. The goal of this thesis is to fill the gap between the theory and practice of Gaussian sampling. First, we study and improve the existing algorithms, byboth a statistical analysis and a geometrical approach. We then exploit the structures underlying many classes of lattices and apply the ideas of the fast Fourier transform to a Gaussian sampler, allowing us to reach a quasilinearcomplexity instead of quadratic. Finally, we use Gaussian sampling in practice to instantiate a signature scheme and an identity-based encryption scheme. The first one yields signatures that are the most compact currently obtained in lattice-based cryptography, and the second one allows encryption and decryption that are about one thousand times faster than those obtained with a pairing-based counterpart on elliptic curves
Hauteville, Adrien. "Nouveaux protocoles et nouvelles attaques pour la cryptologie basée sur les codes en métrique rang." Thesis, Limoges, 2017. http://www.theses.fr/2017LIMO0088/document.
Повний текст джерелаSecurity of public keys cryptography is based on difficult mathematic problems, especially in number field theory, such as the factorization for RSA or the discrete logarithm for ElGamal. However, algorithms are more and more efficient to solve these problems. Furthermore, quantum computers would be able to easily break these cryptosystems. Code-based cryptography in rank metric is a solid candidate to design new postquatum cryptosystems since it is fast and has low weight keysize. The goals of this thesis are to study hard problems in rank metric and algorithms which solve them, also to search for new attacks and new primitives based on these problems
Spini, Gabriele. "Unconditionally Secure Cryptographic Protocols from Coding-Theoretic Primitives." Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0817/document.
Повний текст джерелаThe topic of this dissertation is Cryptography, and its connections with Coding Theory. Concretely, we make use of techniques from Coding Theory to construct and analyze cryptographic protocols with new and/or enhanced properties. We first focus on Secret Sharing, an important topic with many applications to modern Cryptography, which also forms the common ground for most of the concepts discussed in this thesis. In the flavor we are interested in, a secret-sharing scheme takes as input a secret value, and produces as output n shares in such a way that small enough sets of shares yield no information at all on the secret (privacy), while large enough sets of shares allow to recover the secret (reconstruction). A secret-sharing scheme can thus be seen as a solution to a secure communication problem where a sender Alice is connected to a receiver Bob via $n$ distinct channels, some of which are controlled by an adversary Eve. Alice can use a secret-sharing scheme to communicate a secret message to Bob in such a way that Eve learns no information on the message by eavesdropping on the channels she controls, while Bob can receive the message even if Eve blocks the channels under her control. Our contributions to Secret Sharing concern its connection with Coding Theory; since the two fields share the goal of recovering data from incomplete information, it is not surprising that Secret Sharing and Coding Theory have known a long and fruitful interplay. In particular, Massey initiated a very successful analysis on how to construct and study secret-sharing schemes from error-correcting codes. The downside of this analysis is that the privacy of secret-sharing schemes is estimated in terms of the dual of the underlying code; this can be problematic as it might not be possible to obtain codes with desirable properties that have good duals as well. We circumvent this problem by establishing a new connection between the two fields, where the privacy of secret-sharing schemes is no longer controlled by the dual of the underlying code. This allows us to fully harness the potential of recent code constructions to obtain improved schemes; we exemplify this by means of two applications. First, by making use of linear-time encodable and decodable codes we obtain a family of secret-sharing schemes where both the sharing (computation of the shares from the secret) and the reconstruction can be performed in linear time; for growing privacy and reconstruction thresholds, this was an hitherto open problem. Second, we make use of list-decodable codes to construct robust secret-sharing schemes, i.e., schemes that can recover the secret even if some of the shares are incorrect, except with a small error probability. The family we present optimizes the trade-off between the extra data that needs to be appended to the share to achieve robustness and the error probability in the reconstruction, reaching the best possible value. etc
Hugounenq, Cyril. "Volcans et calcul d'isogénies." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLV050/document.
Повний текст джерелаIsogeny computation problem appeared in the SEA algorithm to count the number of points on an elliptic curve defined over a finite field. Algorithms using ideas of Elkies (1998) solved this problem with satisfying results in this context. The appearance of new applications of the isogeny computation problem (trapdoor crypto system, hash function, scalar multiplication acceleration, post quantic crypto system) motivated the search for a faster algorithm outside the SEA context. Couveignes's algorithm (1996) offers the best complexity in the degree of the isogeny but, despite improvements by DeFeo (2011), it proves being unpractical with great characteristic.The aim of this work is to present a modified version of Couveignes's algorithm (1996) that maintains the same complexity in the degree of the isogeny but is practical with any characteristic.Two approaches contribute to the improvement of Couveignes's algorithm (1996) : firstly, the construction of towers of degree $ell$ extensions which are efficient for faster arithmetic operations, as used in the work of De Feo (2011), and secondly, the specification of sets of points of order $ell^k$ that are stable under the action of isogenies.The main contribution of this document is done following the second approach. Our work uses the graph of isogeny where the vertices are elliptic curves and the edges are isogenies. We based our work on the previous results of David Kohel (1996), Fouquet and Morain (2001), Miret emph{& al.} (2005,2006,2008), Ionica and Joux (2001). We therefore present in this document, through the study of the action of the Frobenius endomorphism on points of order $ell^k$, a new way to specify directions in the isogeny graph (volcano)
Battistello, Alberto. "On the security of embedded systems against physical attacks." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLV047/document.
Повний текст джерелаThe subject of this thesis is the security analysis of cryptographic implementations. The need for secure communications has always been a primary need for diplomatic and strategic communications. Cryptography has always been used to answer this need and cryptanalysis have often been solicited to reveal the content of adversaries secret communications. The advent of the computer era caused a shift in the communication paradigms and nowadays the need for secure communications extends to most of commercial and economical exchanges. Modern cryptography provides solutions to achieve such new security goals but also open the way to a number of new threats. It is the case of fault and side-channel-attacks, which today represents the most dangerous threats for embedded cryptographic implementations. This thesis resumes the work of research done during the last years as a security engineer at Oberthur Technologies. Most of the results obtained have been published as research papers [9,13-17] or patents [1-6]. The security research goals of companies around the world working in the embedded domain are twofold. The security engineer has to demonstrate the ability to correctly evaluate the security of algorithms and to highlight possible threats that the product may incur during its lifetime. Furthermore it is desirable to discover new techniques that may provide advantages against competitors. It is in this context that we present our work.This manuscript is divided into four main chapters.The first chapter presents an introduction to various mathematical and computational aspects of cryptography and information theory. We also provide an introduction to the main aspects of the architecture of secure micro-controllers.Afterwards the second chapter introduces the notion of fault attacks and presents some known attack and countermeasure [15-17]. We then detail our work on asymmetric and symmetric infective fault countermeasures as long as on elliptic curves fault attacks [13].The third chapter discusses about side-channels, providing a brief introduction to the subject and to well-known side-channel attacks and countermeasures. We then present two new attacks on implementations that have been considered secure against side channels [9,14]. Afterwards we discuss our combined attack which breaks a state-of-the-art secure implementation [10].Finally, the fourth chapter concludes this works and presents some perspectives for further research.During our investigations we have also found many countermeasures that can be used to thwart attacks. These countermeasures have been mainly published in the form of patents [1-6]. Where possible some of them are presented along with the attack they are conceived to thwart
Traore, Moussa. "Privacy-preserving and secure location authentication." Phd thesis, Toulouse, INPT, 2015. http://oatao.univ-toulouse.fr/14595/1/traore.pdf.
Повний текст джерелаAtighehchi, Kevin. "Contributions à l'efficacité des mécanismes cryptographiques." Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4037.
Повний текст джерелаThe need for continuing innovation in terms of performances and resource savings impel us to optimize the design and the use of cryptographic mechanisms. This leads us to consider several aspects in this dissertation: parallel cryptographic algorithms, incremental cryptographic algorithms and authenticated dictionaries.In the context of parallel cryptography we are interested in hash functions. In particular, we show which tree structures to use to reach an optimal running time. For this running time, we show how to decrease the amount of involved processors. We also explore alternative (sub-optimal) tree structures which decrease the number of synchronizations in multithreaded implementations while balancing at best the load of the work among the threads.Incremental cryptographic schemes allow the efficient updating of cryptographic forms when we change some blocks of the corresponding documents. We show that the existing incremental schemes restrict too much the possible modification operations. We then introduce new algorithms which use these ones as black boxes to allow a broad range of modification operations, while preserving a privacy property about these operations.We then turn our attention to authenticated dictionaries which are used to authenticate answers to queries on a dictionary, by providing to users an authentication proof for each answer. We focus on authenticated dictionaries based on hash trees and we propose a solution to remedy their main shortcoming, the size of proofs provided to users
Chaulet, Julia. "Etude de cryptosystèmes à clé publique basés sur les codes MDPC quasi-cycliques." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066064/document.
Повний текст джерелаConsidering the McEliece cryptosystem using quasi-cylcic MDPC (Moderate Density Parity Check matrix) codes allows us to build a post-quantum encryption scheme with nice features. Namely, it has reasonable key sizes and both encryption and decryption are performed using binary operations. Thus, this scheme seems to be a good candidate for embedded and lightweight implementations. In this case, any information obtained through side channels can lead to an attack. In the McEliece cryptosystem, the decryption process essentially consists in decoding. As we consider the use of an iterative and probabilistic algorithm, the number of iterations needed to decode depends on the instance considered and some of it may fail to be decoded. These behaviors are not suitable because they may be used to extract information about the secrets. One countermeasure could be to bound the number of encryptions using the same key. Another solution could be to employ a constant time decoder with a negligible decoding failure probability, that is to say which is about the expected security level of the cryptosystem. The main goal of this thesis is to present new methods to analyse decoder behavior in a cryptographic context.Second, we explain why a McEliece encryption scheme based on polar code does not ensure the expected level of security. To do so, we apply new techniques to resolve the code equivalence problem. This allows us to highlight several common properties shared by Reed-Muller codes and polar codes. We introduce a new family of codes, named decreasing monomial codes, containing both Reed-Muller and polar codes. These results are also of independent interest for coding theory
Yerushalmi, Yoav. "Incremental cryptography." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/42789.
Повний текст джерелаIncludes bibliographical references (leaves 147-148).
by Yoav Yerushalmi.
M.Eng.
Shamonin, K. E. "Quantum cryptography." Thesis, Sumy State University, 2018. http://essuir.sumdu.edu.ua/handle/123456789/66837.
Повний текст джерелаLopez, Samuel. "MODERN CRYPTOGRAPHY." CSUSB ScholarWorks, 2018. https://scholarworks.lib.csusb.edu/etd/729.
Повний текст джерелаLegendre, Florian. "Exploitation de la logique propositionnelle pour la résolution parallèle des problèmes cryptographiques." Thesis, Reims, 2014. http://www.theses.fr/2014REIMS006/document.
Повний текст джерелаDemocratization of increasingly high-Performance digital technologies and especially the Internet has considerably changed the world of communication. Consequently, needs in cryptography are more and more numerous and the necessity of verifying the security of cipher algorithms is essential.This thesis deals with a new cryptanalysis, called logical cryptanalysis, which is based on the use of logical formalism to express and solve cryptographic problems. More precisely, works presented here focuses on a particular category of ciphers, called cryptographic hash functions, used in authentication and data integrity protocols.Logical cryptanalysis is a specific algebraic cryptanalysis where the expression of the cryptographic problem is done through the satisfiabilty problem, fluently called sat problem. It consists in a combinatorial problem of decision which is central in complexity theory. In the past years, works led by the scientific community have allowed to develop efficient solvers for industrial and academical problems.Works presented in this thesis are the fruit of an exploration between satisfiability and cryptanalysis, and have enabled to display new results and innovative methods to weaken cryptographic functions.The first contribution is the modeling of a cryptographic problem as a sat problem. For this, we present some rules that lead to describe easily basic operations involved in cipher algorithms. Then, a section is dedicated to logical reasoning in order to simplify the produced sat formulas and show how satisfiability can help to enrich a knowledge on a studied problem. Furthermore, we also present many points of view to use our smooth modeling to apply a probabilistic reasoning on all the data associated with the generated sat formulas. This has then allowed to improve both the modeling and the solving of the problem and underlined a weakness about the use of round constants.Second, a section is devoted to practical attacks. Within this framework, we tackled preimages of the most popular cryptographic hash functions. Moreover, the collision problem is also approached in different ways, and particularly, the one-Bloc collision attack of Stevens on MD5 was translated within a logical context. It's interesting to remark that in both cases, logical cryptanalysis takes a new look on the considered problems
Ghammam, Loubna. "Utilisation des couplages en cryptographie asymétrique pour la micro-électronique." Thesis, Rennes 1, 2016. http://www.theses.fr/2016REN1S081/document.
Повний текст джерелаLes couplages sont des outils mathématiques introduits par André Weil en 1948. Ils sont un sujet très en vogue depuis une dizaine d'années en cryptographie asymétrique. Ils permettent en effet de réaliser des opérations cryptographiques impossible à réaliser simplement autrement tel que la signature courte et la cryptographie basée sur l'identité. Ces dernières années, le calcul des couplages est devenu plus facile grâce à l'introduction de nouvelles méthodes de calculs mathématiques particulièrement efficaces sur les courbes elliptiques dites les courbes bien adaptées aux couplages. Aujourd'hui, nous sommes au stade de transfert de cette technologie, de la théorie vers la mise en œuvre pratique, sur des composants électroniques. Ce transfert soulève de nombreuses problématiques qui s'avèrent difficile à surmonter à cause de la différence de culture scientifique entre mathématiciens et micro-électroniciens. Dans le présent document, en premier lieu, nous avons étudié le problème de l'implémentation du couplage dans des environnements restreints. En effet, le calcul du couplage de Tate, ou aussi de l'une de ses variantes, nécessite plusieurs variables pour être implémenté, par conséquent, il nécessite une bonne partie de la mémoire du composant électronique sur lequel nous souhaitons implémenter un tel couplage.Dans ce contexte, en faisant des optimisations mathématiques, nous avons pu implémenté ces couplages dans des environnements retreints. Le deuxième problème que nous avons traité dans cette thèse est celui de la sécurité des protocoles cryptographiques basés sur les couplages. Dans ce contexte, puisque les couplages sur les courbes elliptiques sont censés d'être matériellement attaqués, nous devons le protéger contre ces attaques. Nous avons étudié les attaques sur les couplages et nous avons proposé une contre-mesure
Roşie, Răzvan. "On the achievability of white-box cryptography." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEE010/document.
Повний текст джерелаThis thesis investigates the realizability of white-box implementations for secure pseudorandom permutations. Concretely, we show that multi-input functional encryption achieving a natural definition of one-wayness is instrumental in building implementations that are secure against key-extraction attacks. As a contribution of independent interest, we extend the notion of robustness to a larger set of primitives. Roughly speaking, robust encryption guarantees that a ciphertext cannot be decrypted under different keys. Initially formalized in a public-key context, we introduce compelling definitions for authentication and functional encryption schemes
Zhang, Wei. "Improvements and generalisations of signcryption schemes." Thesis, Royal Holloway, University of London, 2014. http://repository.royalholloway.ac.uk/items/b9117fa7-54ef-45c0-bbc4-6e6c114ceba1/1/.
Повний текст джерелаLac, Benjamin. "Cryptographie légère intrinsèquement résistante aux attaques physiques pour l'Internet des objets." Thesis, Lyon, 2018. http://www.theses.fr/2018LYSEM021.
Повний текст джерелаThe Internet of Things has many application areas and offers huge potentials for businesses, industries and users. Our study deals with the cryptographic requirements and the security issues of connected objects, which specificities are the large number of data they handle every day, and the fact they are often fielded in hostile environment, physically accessible to any type of potential attacker.Side-channel attacks and fault attacks are the two main categories of physical attacks. In our research works, we analyze these different techniques of physical attacks and the existing countermeasures to thwart them, and we introduce two new attack paths that we have experimentally validated in laboratory on a recent family of symmetric encryption schemes: the interleaving structures.In order to meet the security needs and the high performance constraints of the connected objects, we propose a new generic software countermeasure based on redundancy to thwart most of the physical attacks that we called the IRC. We then study the deployment of the IRC on the existing encryption schemes, and its resistance to physical attacks.Finally, we introduce GARFIELD: a new lightweight block cipher adapted to the IRC in order to ensure a good compromise between security and performance. We check its resistance to conventional mathematical attacks and we propose several implementations with different security levels, for the applications of the Internet of Things, for which we analyze the resulting performances and the validity in practice
PRIYADHARSHINI, THIRUTHUVADOSS ANGELINE. "Comparison and Performance Evaluation of Modern Cryptography and DNA Cryptography." Thesis, KTH, Skolan för informations- och kommunikationsteknik (ICT), 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-120103.
Повний текст джерелаNyman, Ellinor. "Cryptography : A study of modern cryptography and its mathematical methods." Thesis, Uppsala universitet, Analys och sannolikhetsteori, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-447460.
Повний текст джерелаLajoie-Mazenc, Paul. "Réputation et respect de la vie privée dans les réseaux dynamiques auto-organisés." Thesis, Rennes 1, 2015. http://www.theses.fr/2015REN1S039/document.
Повний текст джерелаReputation mechanisms are very powerful mechanisms to foster trust between unknown users, by rewarding good behaviors and punishing bad ones. Reputation mechanisms must guarantee that the computed reputation scores are precise and robust against attacks; to guarantee such properties, existing mechanisms require information that jeopardize users' privacy: for instance, clients' interactions might be tracked. Privacy-preserving reputation mechanisms have thus been proposed, protecting both clients' privacy and the providers' one. However, to guarantee strong privacy properties, these mechanisms provide imprecise reputation scores, particularly by preventing clients to testify about their negative interactions. In this thesis, we propose a new distributed privacy-preserving reputation mechanism allowing clients to issue positive as well as negative feedback. Such a construction is made possible thanks to tools from the distributed systems community -- distributed third parties that allow for a distribution of trust and that tolerate malicious behaviors -- as well as from the cryptographic one -- for instance zero-knowledge proofs of knowledge or anonymous proxy signatures. Furthermore, we prove that our mechanism guarantees the required privacy and security properties, and we show with theoretical and practical analysis that this mechanism is usable
Vialla, Bastien. "Contributions à l'algèbre linéaire exacte sur corps finis et au chiffrement homomorphe." Thesis, Montpellier, 2015. http://www.theses.fr/2015MONTS112.
Повний текст джерелаThis thesis is composed of two independent parts.The first one is related to homomorphic encryption and the second part deal with sparse linear algebra on finite fields.Homomorphic encryption extends traditional encryption in the sense that it becomes feasible to perform operations on ciphertexts, without the knowledge of the secret decryption key. As such, it enables someone to delegate heavy computations on his sensitive data to an untrusted third party, in a secure way. More precisely, with such a system, one user can encrypt his sensitive data such that the third party can evaluate a function on the encrypted data, without learning any information on the underlying plain data. Getting back the encrypted result, the user can use his secret key to decrypt it and obtain, in clear, the result of the evaluation of the function on his sensitive plain data. For a cloud user, the applications are numerous, and reconcile both a rich user experience and a strong privacy protection.The first fully homomorphic encryption (FHE) scheme, able to handle an arbitrary number of additions and multiplications on ciphertexts, has been proposed by Gentry in 2009.In homomorphic encryption schemes, the executed function is typically represented as an arithmetic circuit. In practice, any circuit can be described as a set of successive operation gates, each one being either a sum or a product performed over some ring.In Gentry’s construction, based on lattices, each ciphertext is associated with some noise, which grows at each operation (addition or multiplication) done throughout the evaluation of the function. When this noise reaches a certain limit, decryption is not possible anymore.To overcome this limitation, closely related to the number of operations that the HE.Eval procedure can handle, Gentry proposed in a technique of noise refreshment called“bootstrapping”.The main idea behind this bootstrapping procedure is to homomorphically run the decryptionprocedure of the scheme on the ciphertext, using an encrypted version of the secret key. In this context, our contribution is twofold. We first prove that the lmax-minimizing bootstrapping problem is APX-complete and NP-complete for lmax ≥ 3. We then propose a new method to determine the minimal number of bootstrappings needed for a given FHE scheme and a given circuit.We use linear programming to find the best outcome for our problem. The main advantage of our method over the previous one is that it is highly flexible and can be adapted for numerous types of homomorphic encryption schemes and circuits.Computing a kernel element of a matrix is a fundamental kernel in many computer algebra and cryptography algorithms. Especially, many applications produces matrices with many matrix elements equals to 0.Those matrices are named sparse matrices. Sparse linear algebra is fundamentally relying on iterative approaches such as Wiedemann or Lanczos. The main idea is to replace the direct manipulation of a sparse matrix with its Krylov subspace. In such approach, the cost is therefore dominated by the computation of the Krylov subspace, which is done by successive product of a matrix by a vector or a dense matrix.Modern processor unit characteristics (SIMD, multicores, caches hierarchy, ...) greatly influence algorithm design.In this context our work deal with the best approach to design efficient implementation of sparse matrix vector product for modern processors.We propose a new sparse matrix format dealing with the many +-1 matrix elements to improve performance.We propose a parallel implementation based on the work stealing paradigm that provide a good scaling on multicores architectures.We study the impact of SIMD instructions on sparse matrix operations.Finally, we provide a modular arithmetic implementation based on residue number system to deal with sparse matrix vector product over multiprecision finite fields
Karpman, Pierre. "Analyse de primitives symétriques." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX095/document.
Повний текст джерелаThis thesis is about block ciphers and cryptographic hash functions, which are two essential primitives of symmetric-key cryptography. In the first part of this manuscript, we study useful building blocks for block cipher design. We first consider large diffusion matrices builtfrom algebraic-geometry codes, and then construct a small S-box with good diffusion. In the second case, we show how the S-box can be used to define a compact and efficient block cipher targetting small processors. In the second part, we focus on the SHA-1 hash function, for which we develop a free start collision attack. We show how classical collision attacks can be made more efficient by exploiting the additional freedom provided by the model. This allows us in particular to compute explicit collisions for the full compression function of SHA-1