Ethereum Proof-of-Stake and the Probabilistic Bouncing Attack

Abstract

Ethereum has undergone a recent change called the Merge , which made Ethereum a Proof-of-Stake blockchain shifting closer to BFT consensus. Ethereum, which wishes to keep the best of the two protocol designs (BFT and Nakomoto-style), now has a convoluted consensus protocol as its core. The result is a blockchain possibly being produced in a tree-like form while participants try to finalize blocks. We categorize different attacks jeopardizing the liveness of the protocol. The Ethereum community has responded by creating patches against some of them. We discovered a new attack on the patched protocol. To support our analysis, we propose a new high-level formalization of the properties of liveness and availability of the Ethereum blockchain, and we provide pseudo-code. We believe this formalization to be useful for other analyses as well. Our results yield that the Ethereum Proof-of-Stake has safety but only probabilistic liveness. The probability of the liveness is influenced by the parameter describing the time frame allowed for validators to change their mind about the current main chain.