A Secure and Efficient Blockchain Sharding Scheme via Hybrid Consensus and Dynamic Management

Sharding significantly enhances blockchain scalability by dividing the entire network into smaller shards that reach consensus and process transactions in parallel. Nevertheless, two new issues emerge with the adoption of sharding. One issue involves the shrinking size of consensus groups, which leads to vulnerability in consensus. Most existing works introduce periodic shuffle mechanisms to mitigate this problem. Nevertheless, these measures necessitate stronger security assumptions and can only offer a probabilistic assurance of consensus security. Another issue is the challenge in processing cross-shard transactions posed by the isolation of shards. Existing approaches utilize two-phase commit (2PC) or relay transaction mechanisms to handle cross-shard transactions. However, these approaches are vulnerable to double cross-shard attacks from malicious shards and are unable to achieve immediate atomicity. In this paper, to address the vulnerable consensus issue and achieve instant atomicity in cross-shard transactions, we design a hybrid consensus mechanism that embeds a lightweight global consensus into parallel intra-shard consensus processes. The global consensus allows all consensus nodes to jointly process cross-shard transactions, achieving cross-shard transaction instant atomicity. It also records shard snapshots to facilitate shard auditing to defend against malicious shards. Furthermore, we consider the performance of the proposed mechanism, and design a dynamic shard management mechanism. The dynamic shard management mechanism reduces transaction congestion and maintains an appropriate number of shards based on the system's state. We conduct analyses of potential attacks and prove that our approach ensures safety and liveness even in the presence of malicious shards. We also evaluate the performance of our system and compare it with both non-sharded and classic blockchain-sharding systems. The evaluation results demonstrate the efficacy of our approach in dealing with transaction congestion while astutely controlling the number of shards.