Goldfish: No More Attacks on Proof-of-Stake Ethereum

The latest message driven (LMD) greedy heaviest observed sub-tree (GHOST) consensus protocol is a critical component of proof-of-stake (PoS) Ethereum. In its current form, the protocol is brittle, as evidenced by recent attacks and patching attempts. We report on Goldfish, a considerably simplified candidate under consideration for a future Ethereum protocol upgrade. We prove that Goldfish satisfies the properties required of a drop-in replacement for LMD GHOST: Goldfish is secure in synchronous networks under dynamic participation, assuming a majority of the nodes (called validators) follows the protocol. Goldfish is reorg resilient (i.e., honestly produced blocks are guaranteed inclusion in the ledger) and supports fast confirmation (i.e., the expected confirmation latency is independent of the desired security level). We show that subsampling validators can improve the communication efficiency of Goldfish, and that Goldfish is composable with finality gadgets and accountability gadgets, which improves state-of-the-art ebb-and-flow protocols. Attacks on LMD GHOST exploit lack of coordination among honest validators, typically provided by a locking mechanism in classical BFT protocols. However, locking requires votes from a quorum of all participants and is not compatible with dynamic availability. Goldfish is powered by a novel coordination mechanism to synchronize the honest validators' actions under dynamic participation. Experiments with our implementation of Goldfish demonstrate the practicality of this mechanism for Ethereum.