Blockchain technology has attracted significant attention in academia and industry for establishing trust among participants without a centralized third party, even in the presence of malicious nodes. However, current blockchain systems still face scalability and privacy issues which bring limitations in decentralized applications (DApps), hindering wider adoption. In this thesis, we aim to address the blockchain scalability and privacy issues, along with enhancing DApp performance through the following detailed aspects:

Aspect 1: The original blockchain design requires all participants to keep the same copy of all historical data in order to verify newly-generated transactions and prevent record tampering. To address the ever-growing storage requirement restricting the system scalabili...[ Read more ]

Blockchain technology has attracted significant attention in academia and industry for establishing trust among participants without a centralized third party, even in the presence of malicious nodes. However, current blockchain systems still face scalability and privacy issues which bring limitations in decentralized applications (DApps), hindering wider adoption. In this thesis, we aim to address the blockchain scalability and privacy issues, along with enhancing DApp performance through the following detailed aspects:

Aspect 1: The original blockchain design requires all participants to keep the same copy of all historical data in order to verify newly-generated transactions and prevent record tampering. To address the ever-growing storage requirement restricting the system scalability, we introduce Consensus Unit (CU) to organize nodes to store at least one copy of blockchain data together. We optimize the block storage scheme to fully utilize the storage space with minimized query cost for each node. Meanwhile, we present solutions to address dynamic scenarios when new blocks arrive and nodes join or depart from the CU. Extensive experiments confirm the superiority of CU in saving storage and maintaining the system throughput.

Aspect 2: Transparent records are crucial for public verifiability of blockchain transactions, but compromise user privacy. Zero-knowledge (set membership) proof (ZKP) enables private transaction attestation, but existing mechanisms lack scalability. Frequent addition/removal of set elements incurs significant costs and impacts efficiency, such as proof generation and verification time. We use sharding techniques to enhance on-chain state management, employing element-set assignment algorithms for both element addition and removal cases, minimizing information leakage of frequently used elements. Our solution is implemented on both Merkle tree and RSA-based state sets to demonstrate its efficiency and effectiveness.

Aspect 3: Finally, we introduce an innovative blockchain framework that focuses on optimizing DApp performance with transaction privacy guarantees. We propose L2chain, a novel blockchain framework that further scales the system by utilizing a layer-2 network for DApps. The proposed split-execute-merge (SEM) transaction processing workflow, facilitated by the RSA accumulator, allows DApps to lock and update a part of the state digest in parallel, enhancing system throughput. We design a witness cache mechanism for DApp executors to reduce transaction processing latency. To ensure confidentiality, we leverage the trusted execution environment (TEE) for DApps to execute encrypted transactions off-chain. To guarantee transaction execution and order correctness, we propose a two-step execution process for DApps to prevent attacks (i.e., rollback attacks) from subverting the state transition. Extensive experiments demonstrate that L2chain can achieve 1.5X to 42.2X and 7.1X to 8.9X throughput improvements in permissioned and permissionless settings respectively.

This thesis provides a detailed roadmap of the problems, solutions, methodologies, and experimental studies based on relevant datasets and real systems, aiming to design a new system architecture to achieve a scalable blockchain system with user privacy guarantees and improved DApp performance.