  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.

Cryptography and Security