Past few years have witnessed a dramatic shift for IT infrastructures from a self-sustained model to a centralized and multi-tenant elastic computing paradigm -- Cloud Computing, which significantly reshapes the landscape of existing data utilization services. In truth, public cloud service providers (CSPs), e.g. Google, Amazon, offer us unprecedented benefits, such as ubiquitous and flexible access, considerable capital expenditure savings and on-demand resource allocation. Cloud has become the virtual ``brain" as well to support and propel many important applications and system designs, for example, artificial intelligence, Internet of Things, and so forth; on the flip side, security and privacy are among the primary concerns with the adoption of cloud-based data services in that the user loses control of her/his outsourced data. Encrypting the sensitive user information certainly ensures the confidentiality. However, encryption places an extra layer of ambiguity and its direct use may be at odds with the practical requirements and defeat the purpose of cloud computing technology. We believe that security in nature should not be in contravention of the cloud outsourcing model. Rather, it is expected to complement the current achievements to further fuel the wide adoption of the public cloud service. This, in turn, requires us not to decouple them from the very beginning of the system design. Drawing the successes and failures from both academia and industry, we attempt to answer the challenges of realizing efficient and useful secure data services in the public cloud. In particular, we pay attention to security and privacy in two essential functions of the cloud ``brain", i.e. data storage and processing. Our first work centers on the secure chunk-based deduplication of encrypted data for cloud backup and achieves the performance comparable to the plaintext cloud storage deduplication while effectively mitigating the information leakage from the low-entropy chunks. On the other hand, we comprehensively study the promising yet challenging issue of search over encrypted data in the cloud environment, which allows a user to delegate her/his search task to a CSP server that hosts a collection of encrypted files while still guaranteeing some measure of query privacy. In order to accomplish this grand vision, we explore both software-based secure computation research that often relies on cryptography and concentrates on algorithmic design and theoretical proof, and trusted execution solutions that depend on hardware-based isolation and trusted computing. Hopefully, through the lens of our efforts, insights could be furnished into future research in the related areas. / Ph. D. / Past few years have witnessed a dramatic shift for IT infrastructures from a self-sustained model to a centralized and multi-tenant elastic computing paradigm – Cloud Computing, which significantly reshapes the landscape of existing data utilization services. In truth, public cloud service providers (CSPs), e.g. Google, Amazon, offer us unprecedented benefits, such as ubiquitous and flexible access, considerable capital expenditure savings and on-demand resource allocation. Cloud has become the virtual “brain” as well to support and propel many important applications and system designs, for example, artificial intelligence, Internet of Things, and so forth; on the flip side, security and privacy are among the primary concerns with the adoption of cloud-based data services in that the user loses control of her/his outsourced data. Encryption definitely provides strong protection to user sensitive data, but it also disables the direct use of cloud data services and may defeat the purpose of cloud computing technology. We believe that security in nature should not be in contravention of the cloud outsourcing model. Rather, it is expected to complement the current achievements to further fuel the wide adoption of the public cloud service. This, in turn, requires us not to decouple them from the very beginning of the system design. Drawing the successes and failures from both academia and industry, we attempt to answer the challenges of realizing efficient and useful secure data services in the public cloud. In particular, we pay attention to security and privacy in two essential functions of the cloud “brain”, i.e. data storage and processing. The first part of this research aims to provide a privacy-preserving data deduplication scheme with the performance comparable to the existing cloud backup storage deduplication. In the second part, we attempt to secure the fundamental information retrieval functions and offer effective solutions in various contexts of cloud data services.

In recent years, the desire for financial privacy and anonymity spurred the growth of electronic cash and cryptocurrencies. The introduction of decentralized cryptocurrencies, such as Bitcoin, accelerated their adoption in society. Since digital information is easier to reproduce, digital currencies are vulnerable to be spent more than once – this is called a double-spending attack. In order to prevent double-spending, Bitcoin records transactions in a tamper-resilient shared ledger called the blockchain. However, the time required to generate new blocks in the blockchain causes a delay in the transaction confirmation. This delay, typically around one hour in Bitcoin, is impractical for real world trade and limits the wide-spread use of blockchain-based cryptocurrencies. In this thesis, we propose a solution to prevent double-spending attacks and thus enable fast transaction confirmations using the security guarantees of Trusted Execution Environments (TEEs). We achieve this by enforcing sign-once semantics that prevent the payer from reusing designated signing keys to sign more than one transaction. We also provide a way for the payee to verify whether a specific signing key is subject to sign-once semantics. The payee, however still receives the funds later, once the transaction is verified similarly to existing credit card payments. In this way, our solution reduces transaction confirmation times of blockchain-based cryptocurrencies and is also compatible with existing deployments since it does not require any modifications to the base protocol, peers, or miners. We designed and implemented a proof-of-concept of our solution using Intel SGX technology and integrated it with Copay, a popular Bitcoin wallet from BitPay. This thesis also presents the security evaluation of our system along with other possible extensions and enhancements. / De senaste åren har begäran efter sekretess och anonymitet för ekonomisk transaktioner sporrat tillväxten av elektroniska kontanter och kryptovalutor. Introducerandet av decentraliserade kryptovalutor, som t.ex. Bitcoin, har accelereratibruktagningen av dylika valutasystem. Digitala valutor är dock sårbara för dubbelspenderande (eng.double spending) eftersom digital information är lättare attreproducera. För att förhindra dubbelspenderande bokför Bitcoin valutatrans-aktioner i en distribuerad databas, den så kallade blockkedjan (eng.blockchain), som kan motstå förvanskling av bokförda transaktioner. Tiden som krävs för attgenerera nya block i Bitcoins blockkedja leder dock till en fördröjningen företransaktioner som skapas i databasen kan bekräftas. Denna fördröjning, som oftas varar kring en timme, är opraktisk för handel i verkliga världen och begränsardärför den allmänna spridningen av blockkedgebaserade kryptovalutor. I denna avhandlingen föreslår vi en lösningen som hindrar dubbelspenderandegenom att utnyttja säkerhetsgarantier hos anförtrodda exekveringsmiljöer (eng.Trusted Execution Environments). Vi åstadkommer detta genom att hindra beta-laren från att återanvända specifika kryptografiska nycklar för att digitalt signera flera transaktioner. Vi möjligjör också ett sätt för mottagaren att bekräfta ifall en kryptografisk underteckningsnyckel är skyddad på ovannämnda sätt. Mottagaren erhåller dock betalningen först senare, likt existerande kreditkortsbetalningar. Vår lösningen förminskar transaktionstiden för Bitcoin-betalningar på ett sätt som är kompatibelt med existerande användningssätt eftersom lösningen inte kräver modifikationer i grundläggande Bitcoin-protokollet. Vi utvecklade en prototyp av vår lösning genom att utnyttja Intel SGX teknologi och integrerade prototypen med CoPay, en popular plånboksapplikation för Bitcoin utveckald av företaget BitPay. Vi presenterar även en utvärdering av säkerheten i vårt system och beskriver möjliga utökningar och förbättringar. / <p>This thesis is part of Erasmus Mundus 2-Year Masters NordSecMob program. </p>

Consumers of digital contents are empowered with numerous technologies allowing them to produce perfect copies of these contents and distribute them around the world with little or no cost. To prevent illegal copying and distribution, a technology called Digital Rights Management (DRM) is developed. With this technology, consumers are allowed to access digital contents only if they have purchased the corresponding licenses from license issuers. The problem, however, is that those consumers are not allowed to resell their own licenses- a restriction that goes against the first-sale doctrine. Enabling a consumer to buy a digital license directly from another consumer and allowing the two consumers to fairly exchange the license for a payment are still an open issue in DRM research area. This thesis investigates existing security solutions for achieving digital license reselling and analyses their strengths and weaknesses. The thesis then proposes a novel Reselling Deal Signing (RDS) protocol to achieve fairness in a license reselling. The idea of the protocol is to integrate the features of the concurrent signature scheme with functionalities of a License Issuer (LI). The security properties of this protocol is informally analysed and then formally verified using ATL logic and the model checker MOCHA. To assess its performance, a prototype of the RDS protocol has been developed and a comparison with related protocols has been conducted. The thesis also introduces two novel digital tokens a Reselling Permission (RP) token and a Multiple Reselling Permission (MRP) token. The RP and MRP tokens are used to show whether a given license is single and multiple resalable, respectively. Moreover, the thesis proposes two novel methods supporting fair and secure digital license reselling. The first method is the Reselling Deal (RD) method which allows a license to be resold once. This method makes use of the existing distribution infrastructure, RP, License Revocation List (LRL), and three protocols: RDS protocol RD Activation (RDA) protocol, and RD Completion (RDC) protocol. The second method is a Multiple License Reselling (MLR) method enabling one license to be resold N times by N consumers. The thesis presents two variants of the MLR method: RRP-MR (Repeated RP-based Multi-Reselling) and HC-MR (Hash Chain-based Multi-Reselling). The RRP-MR method is designed such that a buyer can choose to either continue or stop a multi-reselling of a license. Like the RD method, the RRP-MR method makes use of RP, LI, LRL, and the RDS, RDA, and RDC protocols to achieve fair and secure reselling. The HC-MR method allows multiple resellings while keeping the overhead on LI at a minimum level and enable a buyer to check how many times a license can be further resold. To do so, the HC-MR utilises MRP and the hash chain cryptographic primitive along with LRL, LI and the RDS, RDA and RDC protocols. The analysis and the evaluation of these three methods have been conducted. While supporting the license reselling, the two methods are designed to prevent a reseller from (1) continuing using a resold license, (2) reselling a non-resalable license, and (3) reselling one license a unauthorised number of times. In addition, they enable content owners of resold contents to trace a buyer who has violated any of the usage rights of a license bought from a reseller. Moreover, the methods enable a buyer to verify whether a license he is about to buy is legitimate for re-sale. Furthermore, the two methods support market power where a reseller can maximise his profit and a buyer can minimise his cost in a reselling process. In comparison with related works, our solution does not make use of any trusted hardware device, thus it is more cost-effective, while satisfying the interests of both resellers and buyers, and protecting the content owner's rights.