Many security protocols rely on the assumption that the trusted third party (TTP) will behave “as it should”. However, this assumption is difficult to justify in the real world. A TTP may become malicious due to its hidden interests or having been compromised. It is publicly acknowledged that a failed TTP can easily destroy the entire security protocol. This thesis aims to provide results on how to use blockchain technologies to mitigate TTP challenges and thereby secure existing cryptographic protocols. Firstly, we formally define a smart contract-based TTP (denoted as TTP-I) and give two security protocols based on such a type of TTP as concrete instances. In this approach, a smart contract can either complement a TTP’s actions or take over the entire functions of the existing TTP. This helps to obtain many security properties such as transparency and accountability. Smart contracts, however, are not adequate to replace TTP that is capable of maintaining secret information since all the states changed by TTP-I are in plaintext and publicly accessible. To fill the gap, we propose another type of TTP (denoted as TTP-II) that enables confidential executions by combining smart contracts and Trusted Execution Environments (TEEs). To achieve this goal, we first investigate the state-of-the-art TEE-aided confidential smart contracts and then explore their core mechanisms. We further apply TTP-II to a traceable credential system and an accountable decryption system. These systems are proved secure and feasible. However, since blockchain systems suffer from scalability and performance issues, the development of blockchain-based cryptographic protocols is inevitably retarded. At last, to make better blockchain systems, we provide two core mechanisms: a weak consensus algorithm and a delegatable payment protocol. The weak consensus algorithm allows parallel block generation, improving the performance and scalability of upper-layer blockchain systems. The delegatable payment protocol creates an offline payment channel, improving the payment speed. Both proposed algorithms have been practically implemented and systematically evaluated. Notably, the weak consensus algorithm has already been taken up by industries.

Video abstract: https://youtu.be/rkAatxBRaul