Paper 2021/450

RandChain: Practical Scalable Decentralized Randomness Attested by Blockchain

Gang Wang and Mark Nixon

Abstract

Reliable and verifiable public randomness is not only an essential building block in various cryptographic primitives, but also is a critical component in many distributed and decentralized protocols, e.g., blockchain sharding. A 'good' randomness generator should preserve several distinctive properties, such as public-verifiability, bias-resistance, unpredictability, and availability. However, it is a challenging task to generate such good randomness. For instance, a dishonest party may behave deceptively to bias the final randomness, which is toward his preferences. And this challenge is more serious in a distributed and decentralized system. Blockchain technology provides several promising features, such as decentralization, immutability, and trustworthiness. Due to extremely high overheads on both communication and computation, most existing solutions face an additional scalability issue. We propose a sharding-based scheme, RandChain, to obtain a practical scalable distributed and decentralized randomness attested by blockchain in large-scale applications. In RandChain, we eliminate the use of computation-heavy cryptographic operations, e.g., Publicly Verifiable Secret Sharing (PVSS), in prevalent approaches. We build a sub-routine, RandGene, which utilizes a commit-then-reveal strategy to establish local randomness, enforced by efficient Verifiable Random Function (VRF). RandGene generates the randomness based on statistical approaches, instead of cryptographic operations, to eliminate computational operations. RandChain maintains a two-layer hierarchical chain structure via a sharding scheme. The first level chain is maintained by RandGene within each shard to provide a verifiable randomness source by blockchain. The second level chain uses the randomnesses from each shard to build a randomness chain.