International Association
for Cryptologic Research

Repeated modular squaring plays a crucial role in various time-based cryptographic primitives, such as Time-Lock Puzzles and Verifiable Delay Functions. At ACM CCS 2021, Thyagarajan et al. introduced “OpenSquare”, a decentralised protocol that lets a client delegate the computation of repeated modular squaring to third-party servers while ensuring that these servers are compensated only if they deliver valid results. In this work, we unveil a significant vulnerability in OpenSquare, which enables servers to receive payments without fulfilling the delegated task. To tackle this issue, we present a series of mitigation measures.