On Scalable Blockchain Systems with User Privacy Guarantees

The Hong Kong University of Science and Technology
Department of Computer Science and Engineering


PhD Thesis Defence


Title: "On Scalable Blockchain Systems with User Privacy Guarantees"

By

Mr. Zihuan XU


Abstract:

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.


Date: 			Friday, 16 June 2023

Time: 			4:00pm - 6:00pm

Venue: 			Room 3494
			lifts 25/26

Chairperson: 		Prof. Ralf VAN DER LANS (MARK)

Committee Members: 	Prof. Lei CHEN (Supervisor)
			Prof. Yangqiu SONG
			Prof. Qian ZHANG
			Prof. Can YANG (MATH)
			Prof. Jianliang XU (HKBU)


**** ALL are Welcome ****