Feasibility Analysis for Sybil Attacks in Shard-Based Permissionless Blockchains

Committee-based permissionless blockchain approaches overcome single leader consensus protocols’ scalability issues by partitioning the outstanding transaction set into shards and selecting multiple committees to process these transactions in parallel. However, by design, shard-based blockchain solutions are vulnerable to Sybil attacks. An adversary with enough computational/hash power can easily manipulate the consensus protocol by generating multiple valid node identifiers/IDs (i.e., multiple Sybil committee members). Despite the straightforward nature of these attacks, they have not been systematically investigated. This paper fills this research gap by analyzing Sybil attacks in shard-based consensus of PoW blockchain systems. Specifically, we provide a detailed analysis for Elastco, one of the prominent shard-based blockchain models. We show that the PoW technique used for ID generation in the initial phase of such protocols is vulnerable to Sybil attacks when an adversary (could be a group of colluding nodes) possesses enough hash power. We analytically derive conditions for two different Sybil attacks and perform numerical simulations to validate our theoretical results under various parameters. Further, we utilize the BlockSim simulator to validate our mathematical computation and results confirm the correctness of the analysis.