Due to the fast progress of the internet technologies, electronic commerce becomes more and more popular. Many people and businesses deal with their transactions via the internet. The technologies of credit cards, electronic tickets, e-cash, and other advanced services have realized the vision of electronic commerce. In this thesis, we propose an off-line e-cash scheme with anonymity, untraceability, double-spending checking, and traceability. Anonymity and untraceability must be possessed in any e-cash scheme. In an off-line e-cash scheme, the bank or the third party (TTP) must be able to revoke the anonymity of a user who doubly spent her/his e-cash(s). In our proposed e-cash scheme, the bank can fast derive the identity of the user who doubly spent her/his e-cash(s) without the participation of TTP. If some illegal transactions are reported, TTP can also directly revoke the anonymity of the user who spent her/his e-cash(s) in the illegal transactions. In addition, the police needs to trace a specific user in some situation, and we propose a process to satisfy this requirement,called traceability.

Decentralised cryptocurrencies such as Bitcoin offer a new paradigm of electronic payment systems that do not rely on a trusted third-party. Instead, the peers forming the network handle the task traditionally left to the third-party, preventing attackers from spending twice the same resource, and do so in a publicly verifiable way through Bitcoin's main innovation, the blockchain. However, due to a lack of synchrony in the network, Bitcoin peers may transiently have conflicting views of the system: the blockchain is forked. This can happen purely by accident but attackers can also voluntarily create forks to mount other attacks on the system. In this work, we describe Bitcoin and its underlying blockchain protocol; we introduce a formal model to capture the normal operations of the system as well as forks and double-spending attacks. We use it to define Bitcoin's fundamental properties in terms of safety, liveness and validity. We present the current state of the system: first, we analyse some of the most prominent works that academia has produced between 2008 and 2016, as well as some promising leads to improve the system; then, we use the results of a measurement campaign to show that the size of the network is relatively stable because join and leave operations compensate each other, and that blocks propagate to most of the network in a matter of seconds. We further compare our results to those usually accepted by the community. We introduce a Bitcoin network simulator that we have implemented and present the experiment we have performed to validate it. Finally, we propose a modification to Bitcoin's operations that can prevent double-spending attacks and forks without giving up on its main ideological principles, decentralisation and the absence of source of trust.

Smart grid is a power supply system that uses digital communication technology to detect and react to local changes for power demand. Modern and future power supply system requires a distributed system for effective communication and management. Blockchain, a distributed technology, has been applied in many fields, e.g., cryptocurrency exchange, secure sharing of medical data, and personal identity security. Much research has been done on the application of blockchain to smart grid. While blockchain has many advantages, such as security and no interference from third parties, it also has inherent disadvantages, such as untrusted network environment, lacking data source privacy, and low network throughput.In this research, three systems are designed to tackle some of these problems in blockchain technology. In the first study, Information-Centric Blockchain Model, we focus on data privacy. In this model, the transactions created by nodes in the network are categorized into separate groups, such as billing transactions, power generation transactions, etc. In this model, all transactions are first encrypted by the corresponding pairs of asymmetric keys, which guarantees that only the intended receivers can see the data so that data confidentiality is preserved. Secondly, all transactions are sent on behalf of their groups, which hides the data sources to preserve the privacy. Our preliminary implementation verified the feasibility of the model, and our analysis demonstrates its effectiveness in securing data source privacy, increasing network throughput, and reducing storage usage. In the second study, we focus on increasing the network’s trustworthiness in an untrusted network environment. A reputation system is designed to evaluate all node’s behaviors. The reputation of a node is evaluated on its computing power, online time, defense ability, function, and service quality. The performance of a node will affect its reputation scores, and a node’s reputation scores will be used to assess its qualification, privileges, and job assignments. Our design is a relatively thorough, self-operated, and closed-loop system. Continuing evaluation of all node’s abilities and behaviors guarantees that only nodes with good scores are qualified to handle certain tasks. Thus, the reputation system helps enhance network security by preventing both internal and external attacks. Preliminary implementation and security analysis showed that the reputation model is feasible and enhances blockchain system’s security. In the third research, a countermeasure was designed for double spending. Double spending is one of the two most concerned security attacks in blockchain. In this study, one of the most reputable nodes was selected as detection node, which keeps checking for conflict transactions in two consecutive blocks. Upon a problematic transaction was discovered, two punishment transactions were created to punish the current attack behavior and to prevent it to happen in future. The experiment shows our design can detect the double spending effectively while using much less detection time and resources.

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>

近年來手機的普及率日漸增加，手機逐漸成為生活中不可或缺的工具，因此許多生活方式逐漸的偏向由手機端完成，例如:找路不需要再透過地圖,上網查資料不需要再透過電腦，人們逐漸地把實體錢包轉向利用手機支付的電子錢包，像是中國支付寶等支付系統。利用手機當作錢包已經是現今手機發展的主要方向，然而對於手機的安全支付議題也日漸重視，近年來有安全晶片的保護下使用者的手機安全也有一定程度的提升，但是在離線交易的情況下惡意使用者的操作依然是可以欺騙安全晶片並製造出雙重支付的問題。 2016年陳等人提出了一個基於NFC系統的匿名行動付款協定，然而該協定中必須要有銀行端的介入才能執行交易。在本論文中，我們基於陳等人的線上交易協定為基礎下發展了本篇論文的新交易協定，此交易協定可以適用於離線以及線上的環境。 離線環境下的雙重支付行為一直交易的過程中難以預防的攻擊，在本篇論文中我們透過安全晶片、符號化和本論文研究的雜湊鍊來預防雙重支付行為，且能保障使用者在交易過程中的匿名性。 / As the coverage of mobile phone has been constantly increased in recent years, the mobile phones have become an indispensable tool in life. Many ways of lives are gradually done through the mobile terminals, for example: No longer need to find the way through the map or search information through the computer, people have also gradually turned to electronic payment via e-wallets instead of paying via physical wallets, such as AliPay in China. Adopting the mobile phone as a wallet is nowadays the main development direction of mobile phones. Meanwhile, people are paying more and more attention to the topics on the security of mobile payment than before. In recent years, under the protection of secure element, the security of users’ mobile phone has been enhanced to a certain extent. In the case of off-line transactions, malicious users are capable of fooling secure element and making double spending. In 2016, Chen et al. proposed a NFC-Based anonymous mobile payment protocol. In that protocol the transaction can only be executed with the involvement of issuer. In this research, we introduce a new protocol which can support both on-line and off-line transactions. Our protocol is modified from that of Chen et al.’s idea. In our protocol, to prevent a malicious user, we use a secure element which stores sensitive information that cannot be altered by the user. In this way, the cheating behavior of a malicious user can be prevented. On the other hand, by using the token techniques, the anonymity of a user can be achieved from the view of a merchant. In this study, we focus on double spending which can make merchant a lot of cost at off-line transaction. We used hash chain to verify the correctness of transactions and prevent the double spending.