Prover segurança às informações trafegadas nos mais diversos tipos de redes é algo essencial. Entretanto, redes altamente dependentes de dispositivos com recursos limitados (como sensores, tokens e smart cards) apresentam um desafio importante: a reduzida disponibilidade de memória e energia destes dispositivos, bem como sua baixa capacidade de processamento, dificultam a utilização de diversos algoritmos criptográficos considerados seguros atualmente. Este é o caso não apenas de cifras simétricas, que proveem confidencialidade aos dados, mas também de MACs (Message Authentication Code, ou Código de Autenticação de Mensagem), que garantem sua integridade e autenticidade. De fato, algumas propostas recentes de cifras de bloco dedicadas a plataformas limitadas (e.g., o Curupira-2) proveem segurança e desempenho em um nível mais adequado a este tipo de cenário do que soluções tradicionais. Seguindo uma linha semelhante, o presente trabalho concentra-se no projeto e análise MACs leves e seguros voltados a cenários com recursos limitados, com foco especial em redes de sensores sem fio (RSSF). Marvin é o nome do algoritmo de MAC proposto neste trabalho. Marvin adota a estrutura Alred, que reutiliza porções de código de uma cifra de bloco subjacente e, assim, introduz um reduzido impacto em termos de ocupação de memória. Este algoritmo apresenta uma estrutura bastante flexível e é altamente paralelizável, permitindo diversas otimizações em função dos recursos disponíveis na plataforma alvo. Como vantagem adicional, Marvin pode ser usado tanto em cenários que necessitam apenas da autenticação de mensagens quanto em esquemas de AEAD (Authenticated- Encryption with Associated Data, ou Encriptação Autenticada com Dados Associados), que aliam encriptação e autenticação. O esquema de AEAD proposto neste trabalho, denominado LetterSoup, explora as características da estrutura do Mar vin e adota uma cifra de bloco operando no modo LFSRC (Linear Feedback Shift Register Counter, ou Contador-Registrador de Deslocamento Linear com Retroalimentação). Além da especificação de ambos os algoritmos, este documento apresenta uma análise detalhada da segurança e desempenho dos mesmos em alguns cenários representativos. / Security is an important concern in any modern network. However, networks that are highly dependent on constrained devices (such as sensors, tokens and smart cards) impose a difficult challenge: their reduced availability of memory, processing power and (specially) energy hinders the deployment of many modern cryptographic algorithms known to be secure. This inconvenience affects not only the deployment of symmetric ciphers, which provide data confidentiality, but also Message Authentication Codes (MACs), used to attest the messages integrity and authenticity. Due to the existence of dedicated block ciphers whose performance and security are adequate for use in resource-constrained scenarios (e.g., the Curupira-2), the focus of this document is on the design and analysis of message authentication algorithms. Our goal is to develop a secure and lightweight solution for deployment on resource constrained scenarios, with especial focus on Wireless Sensor Networks (WSNs). Marvin is the name of the MAC algorithm proposed in this document. Marvin adopts the Alred structure, allowing it to reuse parts of an underlying block cipher machinery; as a result, Marvins implementation builds on the ciphers efficiency and introduces little impact in terms of memory occupation. Moreover, this algorithm presents a flexible and highly parallelizable structure, allowing many implementation optimizations depending on the resources available on the target platform. Marvin can be used not only as an authentication-only function, but also in an Authenticated- Encryption with Associated Data (AEAD) scheme, combining authentication and encryption. In this document, we define a new AEAD proposal called LetterSoup, which is based on the LFSRC (Linear Feedback Shift Register Counter) mode of operation and builds on Marvin. Together with the specification of both algorithms, we provide a detailed security analysis and evaluate their performance in some representative scenarios.

L'Internet des Objets (IdO) permet à des milliards de dispositifs informatiques embarqués de se connecter les uns aux autres. Les objets concernés couvrent la plupart de nos appareils de la vie quotidienne, tels que les thermostats, les réfrigérateurs, les fours, les machines à laver et les téléviseurs. Il est facile d'imaginer l'ampleur du danger, si ces dispositifs venaient à nous espionner et révélaient nos données personnelles. La situation serait encore pire si les applications critiques IdO, par exemple, le système de contrôle des réacteurs nucléaires, le système de sécurité du véhicule ou les dispositifs médicaux, étaient compromis. Afin de garantir la sécurité et lutter contre des menaces de sécurité dans l'IdO, des solutions de sécurité robustes doivent être considérées. Cependant, les appareils pour l’IdO sont limités en mémoire, capacités de calcul et énergie, et disposent de moyens de communication peu fiables, ce qui les rend vulnérables à des attaques variées. Dans ce contexte, nous nous concentrons sur deux défis majeurs, à savoir des protocoles de sécurité légers en termes de calculs et d’infrastructure, et des mécanismes d'établissement de clés légers, les solutions existantes actuellement étant beaucoup trop coûteuses pour les dispositifs IdO. En réponse au premier défi, nous avons, d'une part, proposé ECKSS - un nouveau schéma de signcryption léger qui évite l'utilisation de PKI. Cette proposition permet de chiffrer et signer simultanément des messages en garantissant la confidentialité et la non-falsification du canal de communication. De plus, les échanges de message sont authentifiés sans recourir à des certificats. Par ailleurs, nous avons aussi proposé OEABE qui est un mécanisme de délégation pour le chiffrement à base d’attributs CP-ABE (Ciphertext-Policy Attribute-Based Encryption). CP-ABE est un schéma de chiffrement par attributs qui permet aux utilisateurs de préciser au moment du chiffrement qui pourra déchiffrer leurs données. Notre solution, OEABE, permet à un dispositif contraint en ressources de générer rapidement un chiffré CP-ABE tout en précisant les droits d’accès à ses données. Cette solution est d’autant plus utile que le volume de données générées par les dispositifs IdO est en augmentation exponentielle chaque année. Quant au deuxième défi, nous avons proposé tout d'abord deux modes de distribution de clés pour le protocole standard de gestion de clés MIKEY. Ils s’appuient sur notre schéma de signcryption ECKSS et héritent ainsi de la légèreté d'ECKSS à la fois en termes de calculs et de dispensent d'utilisation de PKI. Les résultats expérimentaux, obtenus à partir d’une plateforme de capteurs Openmote, ont prouvé l'efficacité de nos solutions comparativement aux autres méthodes de MIKEY. Nous avons aussi proposé un schéma d'échange de clés, appelé AKAPR qui est très adapté dans le cas où les deux parties qui participent à la négociation de clés sont très contraintes en ressources / The Internet of Things (IoT) enables billions of embedded computing devices to connect to each other. The smart things cover our everyday friendly devices, such as, thermostats, fridges, ovens, washing machines, and TV sets. It is easy to imagine how bad it would be, if these devices were spying on us and revealing our personal information. It would be even worse if critical IoT applications, for instance, the control system in nuclear reactors, the vehicle safety system or the connected medical devices in health-care, were compromised. To counteract these security threats in the IoT, robust security solutions must be considered. However, IoT devices are limited in terms of memory, computation and energy capacities, in addition to the lack of communication reliability. All these inconvenients make them vulnerable to various attacks, as they become the weakest links of our information system. In this context, we seek for effective security mechanisms in order to establish secure communications between unknown IoT devices, while taking into account the security requirements and the resource constraints of these devices. To do so, we focus on two major challenges, namely, lightweight security protocols in terms of processing and infrastructure and lightweight key establishment mechanisms, as existing solutions are too much resource consuming. To address this first challenge, we first propose ECKSS - a new lightweight signcryption scheme which does not rely on a PKI. This proposal enables to encrypt and sign messages simultaneously while ensuring the confidentiality and unforgeability of the communication channels. In addition, the message exchanges are authenticated without relying on certificates. Moreover, we also propose OEABE which is a delegation-based mechanism for the encryption of the Ciphertext-Policy Attribute-Based Encryption (CP-ABE). CP-ABE is anattribute-based public key encryption scheme that gives users the flexibility to determine who can decrypt their data at runtime. Our solution enables a resource-constrained device to generate rapidly a CP-ABE ciphertext with authorization access rights to its data. This solution is particularly useful as the volume of data issued from IoT devices grows exponentially every year. To solve the second challenge, we first propose two new key distribution modes for the standard key management protocol MIKEY, based on our signcryption scheme ECKSS. These modes inherit the lightness of ECKSS and avoid the use of PKI. The experimental results, conducted in the Openmote sensor platform, have proven the efficiency of our solutions compared with other existing methods of MIKEY. Then, we propose a new key agreement scheme, named AKAPR. In case the two communicating parties are involved in the key negotiation procedure, AKAPR is very suitable in the context of IoT. As such, it can operate even if the two communicating parties are highly resource-constrained

A criptografia de chave pública sem certificado (certificateless) é uma alternativa ao modelo convencional de criptografia assimétrica, pois a autenticação da chave pública ocorre implicitamente durante a execução dos protocolos, sem a necessidade de gerenciamento e distribuição de certificados digitais. Potencialmente reduz custos computacionais e o nível de segurança alcançado é maior quando comparado ao modelo baseado em identidade. Nesta tese de doutorado, modelos formais de segurança para acordo de chave com autenticação sem certificado são aprimorados visando dois objetivos paralelos: (1) aumentar o nível de confiança que usuários podem depositar na autoridade geradora de chaves secretas parciais e (2) viabilizar protocolos que sejam eficientes computacionalmente e com propriedades de segurança relevantes, dentre as quais se inclui resistência a ataques de adversários que têm total controle do canal de comunicação e que podem substituir chaves públicas de usuários por valores arbitrários. Para atestar que as melhorias efetuadas são praticáveis e possibilitam que os objetivos sejam alcançados, novos protocolos são propostos para o caso que envolve dois participantes na comunicação. Os protocolos são provados seguros, usando-se técnica de redução de problemas computacionais. / Certificateless public key cryptography is an alternative model to traditional asymmetric key cryptography, because the public key authentication occurs implicitly during a protocol run, with no need of digital certificates management and distribution. It has the potential to reduce computing costs, and it allows a higher security level than the one in the identity-based model. In this PhD thesis, formal security models for certificateless authenticated key agreement are improved with two independent objectives: (1) to increase the trust level for the partial secret key generating authority on which users rely, and (2) to enable computationally efficient protocols, with significant security properties, such as resistance against attacks from adversaries with full control of the communication channel, and from adversaries who are able to replace users\' public keys by any chosen value. In order to demonstrate that these improvements made are feasible and achieve the objectives, new protocols are proposed in the two-party case. These protocols are proved secure by using reduction techniques for provable security.

A criptografia de chave pública sem certificado (certificateless) é uma alternativa ao modelo convencional de criptografia assimétrica, pois a autenticação da chave pública ocorre implicitamente durante a execução dos protocolos, sem a necessidade de gerenciamento e distribuição de certificados digitais. Potencialmente reduz custos computacionais e o nível de segurança alcançado é maior quando comparado ao modelo baseado em identidade. Nesta tese de doutorado, modelos formais de segurança para acordo de chave com autenticação sem certificado são aprimorados visando dois objetivos paralelos: (1) aumentar o nível de confiança que usuários podem depositar na autoridade geradora de chaves secretas parciais e (2) viabilizar protocolos que sejam eficientes computacionalmente e com propriedades de segurança relevantes, dentre as quais se inclui resistência a ataques de adversários que têm total controle do canal de comunicação e que podem substituir chaves públicas de usuários por valores arbitrários. Para atestar que as melhorias efetuadas são praticáveis e possibilitam que os objetivos sejam alcançados, novos protocolos são propostos para o caso que envolve dois participantes na comunicação. Os protocolos são provados seguros, usando-se técnica de redução de problemas computacionais. / Certificateless public key cryptography is an alternative model to traditional asymmetric key cryptography, because the public key authentication occurs implicitly during a protocol run, with no need of digital certificates management and distribution. It has the potential to reduce computing costs, and it allows a higher security level than the one in the identity-based model. In this PhD thesis, formal security models for certificateless authenticated key agreement are improved with two independent objectives: (1) to increase the trust level for the partial secret key generating authority on which users rely, and (2) to enable computationally efficient protocols, with significant security properties, such as resistance against attacks from adversaries with full control of the communication channel, and from adversaries who are able to replace users\' public keys by any chosen value. In order to demonstrate that these improvements made are feasible and achieve the objectives, new protocols are proposed in the two-party case. These protocols are proved secure by using reduction techniques for provable security.

本文深入探討許多基於公開金鑰密碼和通行碼的密文運算方案。首先第一個主題是「公開金鑰密碼」，從其基本架構和安全定義開始，透過文獻探討逐步地討論公開金鑰密碼學的各項特性、以及討論公開金鑰密碼中兩個常見的密文運算：同態加密系統和可交換性加密系統。同態運算是針對同一把公鑰加密的不同密文間的運算：兩個以同一把公鑰加密的密文可以在不解密的前提下進行運算，進而成為另一個合法密文。這個密文運算的結果等同於兩個明文做運算後再以該公鑰加密。可交換性加密系統是一個容許重複的加密系統：已用甲方公鑰加密的密文可以再度用乙方公鑰再加密，進而之成一個多收件者的密文。第一個主題圍繞著這兩個密文運算的技巧討論相關的加密方案。接下來第二個研究的的主題是「基於公開金鑰密碼之密文相等性驗證」，「密文相等性驗證」是密文運算中一個基礎但重要的功能，經授權的測試者可以在不解密密文的前提下，驗證兩個加密後的訊息是否相等。此外，除了相等或不相等之外，測試者無法得知密文中的其他訊息。「基於公開金鑰密碼之密文相等性驗證」相當於在「公開金鑰密碼」的基礎上，再加上「授權」和「密文相等性驗證」的功能。其中「授權」的範圍和「授權」的設計，直接影響到該方案的實用性及安全性，本文提出三個關於「授權」的主題：「單一密文授權」、「相容性授權」和「語意安全授權」。第三個研究主題是「 可搜尋式加密系統」， 常被應用於以下情境：使用者一個檔案及數個「關鍵字」進行加密，然後儲存在雲端伺服器上。當使用者想要對加密檔案進行關鍵字搜尋時，他可以自訂幾個想搜尋的「關鍵字」並對雲端伺服器發出搜尋要求。在收到搜尋要求後，雖然關鍵字都是加密儲存，仍可利用「可搜尋式加密」技巧將符合關鍵字搜尋的檔案傳回給收件者。整個過程中檔案和關鍵字都被加密保護，伺服器無法得知其儲存及搜尋內容。本文提出兩個「 可搜尋式加密系統」，分別是「子集合式多關鍵字可搜尋式加密系統」和「基於通行碼的可搜尋式加密系統」 。 / This dissertation addresses the research about ciphertext computation skills over public key encryption and password-authenticated cryptosystems. The first topic is related to the public key encryption, the framework and security notions for public key encryption are revised; and two common ciphertext-computable public key encryptions including homomorphic encryption and commutative encryption are following discussed. The homomorphic encryption denotes computations over ciphertexts encrypted using the same public key. The homomorphic operation over ciphertexts may be equal to the encryption of a new message computed between two original messages. In terms of commutative encryption, it stands for a repeated encryption system that Alice’s ciphertext can be duplicated encrypted using Bob’s public key. A dual-receiver ciphertext will appear after the commutative encryption. Following, based on the public key encryption, the second topic focuses on the public key encryption with equality test schemes, the basic and fundamental ciphertext computation. Briefly, the user-authorized testers are able to verify the equivalence between messages hidden in ciphertexts after they acquire trapdoors from ciphertext receivers; and the ciphertexts were never decrypted in the whole equality testing process. The scope and architecture of the authorization directly influence the application and security for equality test schemes. Three authorizations including “cipher-bound authorization”, “compatible authorization” and “semantic secure authorization” will be proposed. The third topic is keyword search. It works in the following scenario: a user outsources encrypted files and encrypted keywords on a cloud file storage system; then, when needed, the user is able to request a search query to the file server, which is corresponding to some encrypted keywords. Although files and keywords are encrypted, the server is still able to verify the match-up and return related files to the user. Two researches about keyword search are proposed: the subset multi-keyword search based on public key encryption, and the password-authenticated keyword search.