Specular: Towards Secure, Trust-minimized Optimistic Blockchain Execution

TL;DR

This paper introduces Specular, a secure and trust-minimized optimistic blockchain execution system that leverages Ethereum's client diversity to reduce trust assumptions and improve resilience.

Contribution

It proposes a methodology for designing resilient ORUs with minimal TCB using opportunistic 1-of-N-version programming, and implements a semantically-aware proof system for Ethereum.

Findings

Feasibility demonstrated with Specular implementation

Leverages existing Ethereum client diversity with minimal modifications

Reduces trust assumptions and enhances security in ORUs

Abstract

An optimistic rollup (ORU) scales a blockchain's throughput by delegating computation to an untrusted remote chain (L2), refereeing any state claim disagreements between mutually distrusting L2 operators via an interactive dispute resolution protocol. State-of-the-art ORUs employ a monolithic dispute resolution protocol that tightly couples an L1 referee with a specific L2 client binary--oblivious to the system's higher-level semantics. We argue that this approach (1) magnifies monoculture failure risk, by precluding trust-minimized and permissionless participation using operator-chosen client software; (2) leads to an unnecessarily large and difficult-to-audit TCB; and, (3) suffers from a frequently-triggered, yet opaque upgrade process--both further increasing auditing overhead, and broadening the governance attack surface. To address these concerns, we outline a methodology for designing a secure and resilient ORU with a minimal TCB, by facilitating opportunistic 1-of-N-version programming. Due to its unique challenges and opportunities, we ground this work concretely in the context of the Ethereum ecosystem--where ORUs have gained significant traction. Specifically, we design a semantically-aware proof system, natively targeting the EVM and its instruction set. We present an implementation in a new ORU, Specular, that opportunistically leverages Ethereum's existing client diversity with minimal source modification, demonstrating our approach's feasibility.