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On Scalable Blockchain Systems with User Privacy Guarantees
PhD Thesis Proposal Defence Title: "On Scalable Blockchain Systems with User Privacy Guarantees" by Mr. Zihuan XU Abstract: Blockchain technology has attracted much attention in both academia and industry due to its success in building trust among participants without a centralized third party, even with the existence of malicious nodes. However, compared to traditional databases, current blockchain systems still cannot handle a massive number of transactions to accommodate more users (a.k.a., scalability issues). In this thesis, we aim to address the blockchain scalability issue from two 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. However, the ever-growing storage requirement can easily become a barrier to accommodate more users, especially for low-end devices such as mobile. To address this challenge, we introduce Consensus Unit (CU) to organize nodes to store at least one copy of blockchain data together. Moreover, we further optimize the block storage scheme to fully utilize the storage space with minimized query cost for each node. Meanwhile, we present solutions to address the dynamic scenarios when new blocks arrive and nodes join or depart from the CU. To verify the effectiveness of CU, we have conducted extensive experiments which confirm the superiority of CU in saving storage and maintaining the system throughput. Aspect 2: Transparent records ever are the key to making blockchain transactions publicly verifiable. However, it also breaks user privacy. Recently, Zero-knowledge (set membership) proof (ZKP) is widely used to enable private transaction attestation. However, existing mechanisms do not fully consider scalability. Particularly, frequent addition/removal of set elements, not only brings the significant cost to keep public parameters up to date to provers and verifiers but also affects mechanism efficiency (e.g., generation time of the proof and verification, etc.). To address this challenge, we leverage the sharding technique to assist the on-chain state management with specially designed element-set assignment algorithms under both element addition and removal cases to minimize the potential information leakage of the frequently used elements. We implement our solution on both Merkle tree and RSA-based state sets to evaluate its efficiency and effectiveness and use a real ZKP-based application named zkSync to demonstrate its applicability. Results show that our solution can notably reduce the system latency with a growing number of users. This proposal will provide a detailed research roadmap of the aforementioned topics including the formal problem definitions, corresponding solutions as well as the methodology of experimental studies. Especially, we will discuss related works in scaling blockchain systems to motivate our future direction, aiming to design a new system architecture to achieve a scalable blockchain system with user privacy guarantees. Date: Monday, 13 March 2023 Time: 12:00noon - 2:00pm Venue: Room 4472 lifts 25/26 Committee Members: Prof. Lei Chen (Supervisor) Prof. Bo Li (Chairperson) Dr. Xiaojuan Ma Dr. Yangqiu Song **** ALL are Welcome ****