Using Multi-dimensional Quorums for Optimal Resilience in Multi-resource Blockchains.

Permissionless blockchains commonly use resource challenges to defend against sybil attacks. For example, popular resource challenge designs include Proof-of-Work and Proof-of-Stake. It is well-known that simultaneously exploiting multiple resources can help make a permissionless blockchain more robust. For example, combining PoW and PoS can help to keep a blockchain secure, even when the attacker controls more than 50% of the computational power in the system.While there have been existing efforts for combining multiple resources in blockchains, they only provide partial solutions. Specifically, it is currently still unclear how to combine PoW and PoS, or multiple resources in general, to achieve optimal resilience. Here by optimal resilience, we mean that the blockchain can tolerate every security region, unless that security region is proven to be impossible to tolerate. Existing designs are not able to achieve such optimal resilience.As our central contribution, this work proposes the novel design and formal security analysis of a blockchain protocol that combines PoS and PoW, which can be further generalized to multiple resources. Our blockchain is the very first blockchain that can achieve optimal resilience. Our design also overcomes a common tricky issue of PoW difficulty adjustment in previous designs. We have further implemented a research prototype of our blockchain design, and experimentally demonstrate its good end-to-end performance.