Specular: Towards Trust-minimized Blockchain Execution Scalability with EVM-native Fraud Proofs

An optimistic rollup (ORU) enables refereed delegation of computation from a blockchain (L1) to an untrusted remote system (L2), by allowing state updates posted on-chain to be disputed by any party via an interactive fraud proof (IFP) protocol. Existing systems that utilize this technique have demonstrated up to a 20x reduction in transaction fees. The most popular ORUs today, in active development, strive to extend existing Ethereum client software to support IFP construction, aiming to reuse prior L1 engineering efforts and replicate Ethereum Virtual Machine (EVM) semantics at L2. Unfortunately, to do so they tightly couple their on-chain IFP verifier with a specific client program binary--oblivious to its higher-level semantics. We argue that this approach (1) precludes the trust-minimized, permissionless participation of multiple Ethereum client programs, magnifying monoculture failure risk; (2) leads to an unnecessarily large and complex trusted computing base that is difficult to independently audit; and, (3) suffers from a frequently-triggered, yet opaque upgrade process--both further increasing auditing overhead and complicating on-chain access control. In this work, we aim to build a secure, trust-minimized ORU that addresses these problems, while preserving scalability and dispute resolution efficiency. To do so, we design an IFP system native to the EVM, that enforces Ethereum's specified semantics precisely at the level of a single EVM instruction. We present Specular, an ORU which leverages an off-the-shelf Ethereum client--modified minimally to support IFP construction--demonstrating the practicality of our approach.