Paper 2026/234

When Trying to Catch Cheaters Breaks the MPC: Breaking and Fixing Delayed Consistency Checks in Trident, Fantastic Four, SWIFT, and Quad (Full Version)

Thomas Schneider, Technical University of Darmstadt

Abstract

Actively secure multi-party computation in the honest-majority setting often relies on multiple parties computing the same message to be sent. This additional redundancy allows to detect when a party deviates from the protocol. Many works utilize this for efficient protocol design, with some protocols delaying and batching consistency checks to further boost efficiency. In this paper, we show multiple cases where such batched consistency checks render the protocols insecure. Our concrete attacks derive additional knowledge from the batched consistency checks, reconstructing values on intermediate wires. Specifically, we show concrete attacks on Trident (NDSS'20), Fantastic Four (USENIX Security'21) including its implementation in the popular MP-SPDZ framework (CCS'20), and Quad (PoPETS'25). Furthermore, we find how an imprecise specification of SWIFT (USENIX Security'21) can enable a similar attack and reveal a gap in their security proof. Finally, we propose a fix for all protocols with a small performance overhead. Our provably secure fix uses a generic, joint consistency check that replaces the former, insecure consistency checks.

Note: March 15, 2026: Disclosure on ePrint after responsible disclosure process. May 7, 2026: Minor updates and clarifications (random coefficients need to be secret, dot product optimization, emphasized that consistency checks for inputs in Trident are not delayed, updated responsible disclosure information)

@misc{cryptoeprint:2026/234,
      author = {Andreas Brüggemann and Thomas Schneider},
      title = {When Trying to Catch Cheaters Breaks the {MPC}: Breaking and Fixing Delayed Consistency Checks in Trident, Fantastic Four, {SWIFT}, and Quad (Full Version)},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/234},
      year = {2026},
      doi = {10.1007/978-3-032-25324-8_16},
      url = {https://eprint.iacr.org/2026/234}
}