Risk-free Selfish Mining in Hybrid Predictability Model. A Case Study on Polkadot's NPoS

Selfish mining attack is a well-known attack in blockchains where Byzantine validators collude to gain more revenue than their fair share. Prior studies show that selfish mining attack can be risk-free in the longest-chain variants of Proof-of-Stake (PoS) protocols, i.e., Byzantine validators will never be caught and do not take any risk of losing their revenue. However, it is still unclear whether risk-free selfish mining is possible for PoS in the non-global predictability model [EC 2019], where in the global unpredictability model, validators can predict the roles of all validators in advance. In this work, we study the nominated PoS protocol by Polkadot, a cryptocurrency with a market cap of over 7 billion USD (top 20 blockchains). Polkadot's PoS protocol falls into a hybrid predictability model (i.e., non-global predictability model) that has never been well studied before. We present the first two risk-free (and meanwhile profitable) selfish mining attacks against Polkadot's PoS protocol. In our attack, a mining pool with 33% Byzantine validators can launch risk-free selfish mining and earn 7.09% extra block rewards in expectation. Our experimental results using Polkadot's implementation show that our risk-free attacks achieve similar profit as conventional selfish mining attack, such as the well-known selfish mining against Bitcoin.