On the semantic security of functional encryption schemes

Functional encryption (FE) is a powerful cryptographic primitive that generalizes many asymmetric encryption systems proposed in recent years. Syntax and security definitions for FE were proposed by Boneh, Sahai, and Waters (BSW) (TCC 2011) and independently by O’Neill (ePrint 2010/556). In this paper we revisit these definitions, identify several shortcomings in them, and propose a new definitional approach that overcomes these limitations. Our definitions display good compositionality properties and allow us to obtain new feasibility and impossibility results for adaptive token-extraction attack scenarios that shed further light on the potential reach of general FE for practical applications.ENIAC Joint UndertakingFundação para a Ciência e a Tecnologia (FCT

Searchable Encryption for Cloud and Distributed Systems

The vast development in information and communication technologies has spawned many new computing and storage architectures in the last two decades. Famous for its powerful computation ability and massive storage capacity, cloud services, including storage and computing, replace personal computers and software systems in many industrial applications. Another famous and influential computing and storage architecture is the distributed system, which refers to an array of machines or components geographically dispersed but jointly contributes to a common task, bringing premium scalability, reliability, and efficiency. Recently, the distributed cloud concept has also been proposed to benefit both cloud and distributed computing. Despite the benefits of these new technologies, data security and privacy are among the main concerns that hinder the wide adoption of these attractive architectures since data and computation are not under the control of the end-users in such systems. The traditional security mechanisms, e.g., encryption, cannot fit these new architectures since they would disable the fast access and retrieval of remote storage servers. Thus, an urgent question turns to be how to enable refined and efficient data retrieval on encrypted data among numerous records (i.e., searchable encryption) in the cloud and distributed systems, which forms the topic of this thesis. Searchable encryption technologies can be divided into Searchable Symmetric Encryption (SSE) and Public-key Encryption with Keyword Search (PEKS). The intrinsical symmetric key hinders data sharing since it is problematic and insecure to reveal one’s key to others. However, SSE outperforms PEKS due to its premium efficiency and is thus is prefered in a number of keyword search applications. Then multi-user SSE with rigorous and fine access control undoubtedly renders a satisfactory solution of both efficiency and security, which is the first problem worthy of our much attention. Second, functions and versatility play an essential role in a cloud storage application but it is still tricky to realize keyword search and deduplication in the cloud simultaneously. Large-scale data usually renders significant data redundancy and saving cloud storage resources turns to be inevitable. Existing schemes only facilitate data retrieval due to keywords but rarely consider other demands like deduplication. To be noted, trivially and hastily affiliating a separate deduplication scheme to the searchable encryption leads to disordered system architecture and security threats. Therefore, attention should be paid to versatile solutions supporting both keyword search and deduplication in the cloud. The third problem to be addressed is implementing multi-reader access for PEKS. As we know, PEKS was born to support multi-writers but enabling multi-readers in PEKS is challenging. Repeatedly encrypting the same keyword with different readers’ keys is not an elegant solution. In addition to keyword privacy, user anonymity coming with a multi-reader setting should also be formulated and preserved. Last but not least, existing schemes targeting centralized storage have not taken full advantage of distributed computation, which is considerable efficiency and fast response. Specifically, all testing tasks between searchable ciphertexts and trapdoor/token are fully undertaken by the only centralized cloud server, resulting in a busy system and slow response. With the help of distributed techniques, we may now look forward to a new turnaround, i.e., multiple servers jointly work to perform the testing with better efficiency and scalability. Then the intractable multi-writer/multi-reader mode supporting multi-keyword queries may also come true as a by-product. This thesis investigates searchable encryption technologies in cloud storage and distributed systems and spares effort to address the problems mentioned above. Our first work can be classified into SSE. We formulate the Multi-user Verifiable Searchable Symmetric Encryption (MVSSE) and propose a concrete scheme for multi-user access. It not only offers multi-user access and verifiability but also supports extension on updates as well as a non-single keyword index. Moreover, revocable access control is obtained that the search authority is validated each time a query is launched, different from existing mechanisms that once the search authority is granted, users can search forever. We give simulation-based proof, demonstrating our proposal possesses Universally Composable (UC)-security. Second, we come up with a redundancy elimination solution on top of searchable encryption. Following the keyword comparison approach of SSE, we formulate a hybrid primitive called Message-Locked Searchable Encryption (MLSE) derived in the way of SSE’s keyword search supporting keyword search and deduplication and present a concrete construction that enables multi-keyword query and negative keyword query as well as deduplication at a considerable small cost, i.e., the tokens are used for both search and deduplication. And it can further support Proof of Storage (PoS), testifying the content integrity in cloud storage. The semantic security is proved in Random Oracle Model using the game-based methodology. Third, as the branch of PEKS, the Broadcast Authenticated Encryption with Keyword Search (BAEKS) is proposed to bridge the gap of multi-reader access for PEKS, followed by a scheme. It not only resists Keyword Guessing Attacks (KGA) but also fills in the blank of anonymity. The scheme is proved secure under Decisional Bilinear Diffie-Hellman (DBDH) assumption in the Random Oracle Model. For distributed systems, we present a Searchable Encryption based on Efficient Privacy-preserving Outsourced calculation framework with Multiple keys (SE-EPOM) enjoying desirable features, which can be classified into PEKS. Instead of merely deploying a single server, multiple servers are employed to execute the test algorithm in our scheme jointly. The refined search, i.e., multi-keyword query, data confidentiality, and search pattern hiding, are realized. Besides, the multi-writer/multi-reader mode comes true. It is shown that under the distributed circumstance, much efficiency can be substantially achieved by our construction. With simulation-based proof, the security of our scheme is elaborated. All constructions proposed in this thesis are formally proven according to their corresponding security definitions and requirements. In addition, for each cryptographic primitive designed in this thesis, concrete schemes are initiated to demonstrate the availability and practicality of our proposal

Generic Construction of Forward Secure Public Key Authenticated Encryption with Keyword Search

Forward security is a fundamental requirement in searchable encryption, where a newly generated ciphertext is not allowed to be searched by previously generated trapdoors. However, forward security is somewhat overlooked in the public key encryption with keyword search (PEKS) context and there are few proposals, whereas forward security has been stated as a default security notion in the (dynamic) symmetric searchable encryption (SSE) context. In the PEKS context, forward secure PEKS (FS-PEKS) is essentially the same as public key encryption with temporary keyword search (PETKS) proposed by Abdalla et al. (JoC 2016) which can be constructed generically from hierarchical identity-based encryption (HIBE) with level-1 anonymity. Alternatively, Zeng et al. (IEEE Transactions on Cloud Computing 2022) also proposed a generic construction of FS-PEKS from attribute-based searchable encryption supporting OR gates. In the public key authenticated encryption with keyword search (PAEKS) context, a concrete forward secure PAEKS (FS-PAEKS) construction has been proposed by Jiang et al. (The Computer Journal 2022). As an independent work, thought Xu et al. proposed a generic construction of FS-PAEKS (ePrint 2023), they employed the Liu et al. generic construction of PAEKS (AsiaCCS 2022) that requires random oracles. Thus, a generic construction of FS-PAEKS without random oracles has not been proposed so far. In this paper, we propose a generic construction of FS-PAEKS from PAEKS. In addition to PAEKS, we employ 0/1 encodings proposed by Lin et al. (ACNS 2005). We also show that the Jiang et al. FS-PAEKS scheme does not provide forward security, and thus our generic construction yields the first secure FS-PAEKS schemes. Our generic construction is quite simple, and it can also be applied to construct FS-PEKS. Our generic construction yields a comparably efficient FS-PEKS scheme compared to the previous scheme. Moreover, it eliminates the hierarchical structure or attribute-based feature of the previous generic constructions which is meaningful from a feasibility perspective

Format and Order Revealing Encryption

As more and more cloud services emerge so does the need for new methods for securing the data these services consume, especially since data leaks have become the norm rather than the exception. Since most cloud services require some kind of access to our private data in order to perform searches and provide services, new ways of securing our data in the cloud is needed. This dissertation examines the current state of the cryptographic world in order to try to and understand and resume what solutions currently exist for this particular type of problem. This work is motivated by a particular problem of data delegation to a cloud infrastructure. This problem involves the protection of sensitive data whilst it’s analysed by a third party. While there is no simple approach to solve this particular problem, this dissertation discusses three main approaches to tackle this problem. One approach attempts to define a new cryptographic scheme with a leakage profile that would allow a third party to only have access to some information of the plaintext but, at the same time, keep the plaintext safe from attackers. Another approach attempts to use already existing cryptographic schemes, such as, Format Preserving Encryption and Order Revealing Encryption to solve this particular problem. A final approach tries to solve this problem by utilising cryptographic tools, such as hash-functions and hash-based message authentication codes. An extended study was also conducted in many cryptographic schemes, both current and old cryptographic schemes. This study allowed for a better view of the cryptographic world and how these schemes could help us achieve a solution. For this dissertation, a prototype was also implemented of some recent cryptographic schemes. These prototype implementations allowed for a deeper understanding of how these schemes work and also allowed us to conduct some experiments while trying to combine two cryptographic schemes. The results of this dissertation show that that trying to solve a problem via creating a new cryptographic scheme is not an easy feat especially when one wants to define correctly the strict security requirements and also the work needed to understand the mathematical workings of similar schemes. Lastly we conclude that solving the problem with the help of already existing tools may be the easiest solution, but, it may also only work for a specific scenario and hence is of no use in other similar situations. A solution to the particular problem studied in this thesis is also presented at the end of this dissertation, although, it only applies to this specific problem and does not solve the more general problem of privacy of data delegation to the cloud.Com a explosão de serviços baseados na nuvem que ocorre nos dias de hoje, torna-se imperativo que os dados que são consumidos por este tipo de serviços sejam de alguma forma protegidos contra ataques ou roubos[Cen18]. O principal problema com este tipo de serviços é que, normalmente, estes serviços precisam de acesso aos dados para conseguirem fazer pesquisas e correlacionar dados de forma a que seja possível fornecer diversos serviços. Esta dissertação tem como objetivo estudar o mundo da criptografia de forma a perceber que tipo de garantias são oferecidas pelos esquemas criptográficos existentes nos dias de hoje para serviços baseados na nuvem. Este trabalho é motivado por um problema real de delegação de dados para a nuvem. Este problema envolve a proteção de dados sensíveis que precisam de ser analisados por entidades externas. Embora não haja uma abordagem simples para resolver este tipo de problemas, nesta dissertação iremos discutir três abordagens que, potencialmente, poderão resolver este problema. Uma abordagem tenta definir o que poderia ser a estrutura geral de um novo esquema criptográfico que pudesse lidar com o problema específico em análise. Numa outra abordagem iremos utilizar ferramentas existentes para tentar resolver o problema em questão. Iremos também tentar unir dois esquemas criptográficos existentes, de forma a tentar combater este problema em específico. Foi também realizado um estudo a vários esquemas criptográficos de forma a perceber quais as soluções que existem hoje em dia para problemas relacionados com a delegação de dados para entidades externas, como também, tentar perceber que esquemas criptográficos que ainda são resultados meramente teóricos mas que possam vir, no futuro, a ser úteis para combater esta problemática. Os resultados desta dissertação mostram que resolver um problema relacionado com criptografia nem sempre é fácil, uma vez que, a má utilização destes esquemas poderá levar a uma falha grave de segurança. Por fim, concluímos que, resolver um problema desta natureza através de ferramentas existentes é bastante mais fácil do que tentar desenvolver esquemas criptográficos novos, mas que irá perder o poder de poder ser aplicado a outros problemas semelhantes

Secure data storage and retrieval in cloud computing

Nowadays cloud computing has been widely recognised as one of the most inuential information technologies because of its unprecedented advantages. In spite of its widely recognised social and economic benefits, in cloud computing customers lose the direct control of their data and completely rely on the cloud to manage their data and computation, which raises significant security and privacy concerns and is one of the major barriers to the adoption of public cloud by many organisations and individuals. Therefore, it is desirable to apply practical security approaches to address the security risks for the wide adoption of cloud computing