Paper 2026/1152

To Catch a Cheater: Some notes on the malicious (in)security of ORQ

Abstract

Malicious-secure multiparty computation protocols protect against an adversary's arbitrary misbehavior. In the honest-majority four-party setting, one common technique relies on all communication between parties being duplicated. Under this approach, all correct messages are sent twice, while corrupted messages are only sent by the adversary and will not match concurrent correct messages. When a receiver observes that inconsistency, it announces cheating has occurred (and possibly aborts). Existing implementations often optimize this procedure by batching many such checks into a single hash and running a final consistency check just before revealing the result of a computation. Brüggemann and Schneider (Eurocrypt 2026) recently showed that these delayed consistency checks in honest-majority, malicious-secure protocols can violate privacy. Adversaries can introduce errors such that subsequent incorrect hashes reveal their missing secret share and allow plaintext secrets to be recovered just before the honest parties abort. Their suggested fix evaluates the hash comparison under multiparty computation, rather than in plaintext. In this report, we detail our fix for the Fantastic Four protocol in ORQ, a recent system for secure analytics that is vulnerable to the attack. The new implementation has a modest overhead that amortizes with larger inputs. The complexity of the modified protocol highlights the difficulty of implementing malicious-secure systems in practice; even seemingly harmless optimizations can break privacy.