Abstract
Although blockchain-based smart contracts aim at proposing an open, transparent and decentralized system, they suffer from multiple security issues, adoption limitations, and unaccountable participants. Firstly, they are usually deployed over permissionless and immutable networks, therefore, the buggy smart contracts cannot be patched easily. An effective and fine-grained access control system is seen as an intuitive way to mitigate it. Secondly, scalability is regarded as a main limitation in terms of transaction per seconds (TPS) due to underlying consensus protocols. The recent deployment of off-chain payment channel solutions aim to address this issue. However, a long timeout of termination is required and the off-chain system participants do not operate efficiently nor fail-safe, leaving multiple attack spaces. Lastly, with the recent emergence of Decentralized Finance (DeFi) that combines blockchain technologies with interoperable financial spaces, off-chain oracle is proposed to feed any information into DeFi platforms. However, the legacy schemes are criticized for intransparent services, operational failures and inadequate incentive implementations. Our works make following contributions in the aspects of security, scalability and accountability with off-chain systems: (1) we propose a fine-grained, updatable offchain access control framework over smart contracts that shifts the burden of on-chain access policy validation to lightweight token-based access verification, in addition to integrating multiple vulnerability detection tools to enhance runtime security of smart contracts (see. Chapter 2). (2) we present a novel, fail-safe off-chain watchtower for payment channels. In contrast to prior schemes, our work can observe numerous payment channels simultaneously yet tolerate multiple attacks. In addition, our work achieves quick termination and supports about 120 tx/s (see. Chapter 3). (3) we empirically study the behavior of DeFi oracles and report massive operational failures and unaccounatbilities. Furthermore, we devise a fair incentive protocol that aims at transparency, accountability and compatibility for DeFi oracles with reward, penalty, dispute and resolution mechanisms (see. Chapter 4 and Chapter 5). (4) we move forward our academic contributions above by proposing a decentralized content compliance validation design for real-world applications to demonstrate the generalizability of our academic contributions (see. Chapter 6).