A Fast Confirmation Rule for the Ethereum Consensus Protocol (2405.00549v2)

Abstract: A Confirmation Rule, within blockchain networks, refers to an algorithm implemented by network nodes that determines (either probabilistically or deterministically) the permanence of certain blocks on the blockchain. An example of Confirmation Ruble is the Bitcoin's longest chain Confirmation Rule where a block $b$ is confirmed (with high probability) when it has a sufficiently long chain of successors, its siblings have notably shorter successor chains, the majority of the network's total computation power (hashing) is controlled by honest nodes, and network synchrony holds. The only Confirmation Rule currently available in the Ethereum protocol, Gasper, is the FFG Finalization Rule. While this Confirmation Rule works under asynchronous network conditions, it is quite slow for many use cases. Specifically, best-case scenario, it takes around 13 to 19 min to confirm a transaction, where the actual figure depends on when the transaction is submitted to the network. In this work, we devise a Fast Confirmation Rule for Ethereum's consensus protocol. Our Confirmation Rule relies on synchrony conditions, but provides a best-case confirmation time of 12 seconds only, greatly improving on the latency of the FFG Finalization Rule. Users can then rely on the Confirmation Rule that best suits their needs depending on their belief about the network conditions and the need for a quick response.

• The paper proposes novel, fast confirmation rules for Ethereum's Gasper consensus protocol, initially focusing on LMD-GHOST and extending to LMD-GHOST-HFC to enhance block permanence.
• The proposed rules are designed to ensure critical properties like safety (confirmed blocks remain in the canonical chain) and monotonicity (once confirmed, a block stays confirmed), even under adversarial conditions.
• This research has practical implications for balancing block confirmation speed and network security, potentially leading to faster and more efficient confirmations in Ethereum's Proof-of-Stake system.

Confirmation Rules for Ethereum's Gasper Protocol

This paper presents a series of confirmation rules within Ethereum's consensus protocol, Gasper, designed to enhance the protocol's efficiency in block confirmations. By refining the mechanisms underlying block permanency assurance, the authors propose rules which balance the trade-offs between confirmation speed and safety guarantees.

Overview

The paper examines the necessity of confirmation rules in validating the stability and permanency of blocks within a blockchain network. Specifically, a confirmation rule is an algorithm executed by network nodes to determine the permanence of a block. Prior to Ethereum's transition to Proof-of-Stake via The Merge, block confirmation was guided by the longest chain rule, similar to Bitcoin's approach. The transition introduced Gasper, Ethereum's PoS consensus protocol, composed of FFG-Casper and LMD-GHOST, demanding novel confirmation stipulations tailored to its architectural nuances.

Confirmation Rule Development

The analysis first addresses LMD-GHOST, the component responsible for ensuring chain liveness. The authors propose a novel confirmation rule focusing initially solely on LMD-GHOST, accounting for its dynamic availability independent of FFG-Casper's finalization process. This foundational rule uses safety indicators $Q_b^n$ and $P_b^n$ to measure block support within the validator network, ensuring high confidence in block permanence. The aim is to establish a standardized approach for fast block confirmation, thereby minimizing the latency between block proposal and confirmation.

Safety and Monotonicity

To ensure both safety and monotonicity—a confirmed block remains confirmed in subsequent epochs—the rule must accommodate two core properties:

• Safety: For a block confirmed at time $t$, it must be present in the canonical chain of any honest validator at times $t' > t$.
• Monotonicity: Once a block is confirmed, it should remain confirmed for all future times.

The authors discuss the safety guarantees provided by the LMD-GHOST confirmation framework, illustrating that suitable implementations can maintain these properties under adversarial conditions, specifically when the adversary controls a non-majority fraction of the validator weight (up to a safety decay threshold).

Extending to LMD-GHOST-HFC

The paper builds on the initial framework, integrating FFG-Casper's impact to extend the rule for LMD-GHOST-HFC. This extension accounts for the effect of validator set changes, involving validator entry, exit, rewards, and penalties. By introducing additional confirmation rule parameters, the authors propose metrics that further protect against unsafe block finalizations due to changes in validator composition or network dynamics. This version of the rule requires tracking FFG votes across multiple epochs, addressing challenges in observing past network states—a practical limitation in current implementations.

Practical Implications and Future Scope

This research signifies a potential shift in how Ethereum's consensus protocol can handle block confirmations, balancing speed and security more effectively. The authors suggest potential future works, particularly in exploring the statistical confidence levels underlying assumptions like the proportion of Byzantine validators $\beta$, which affects the robustness of their proposed mechanisms. Furthermore, simplifications or additional extensions in handling unslashed Byzantine validators could also warrant exploration, potentially reducing the complexity of assumptions needed for safe and monotonic confirmations.

In conclusion, this paper advances the discourse on efficient confirmation strategies within Proof-of-Stake systems, offering insights into achieving rapid confirmations while preserving the integrity and security of Ethereum's blockchain network. As Ethereum continues to evolve, these insights are invaluable for the ongoing refinement of its consensus mechanisms.