Paper 2025/2024

A Note Comparing Three Incentive Designs Against Privacy-Targeted Collusion

Abstract

We compare three recent works on collusion deterrence mechanisms [SP'24, Eurocrypt'25, CCS'25] in privacy-preserving multi-party computations. They follow the same whistleblowing structure where an evidence collection module collects collusion evidence, and a mechanism assigns payments to deter incentive-driven parties from collusion. For evidence collection module, two works [SP'24, Eurocrypt'25] provide a general method for generating collusion evidence while tolerating pre-existing leakage. The other work [CCS'25] abstracts evidence generation away, except for transparent service applications where the output is treated as the evidence. For the incentive mechanisms, two works [SP'24, Eurocrypt'25] consider a mix of rational and malicious parties, and rational parties can act as an individual or as a member of a strong coalition, inside which parties trust each other and never harm other members. When parties act as individuals, given bounded malicious parties, one can design mechanisms to disincentivize collusion. When parties act as a coalition, the mechanisms can only limit the size of coalitions for exclusive secrets, i.e., more parties learning the secret reduces the value received by individuals. The most recent work [CCS'25] only models rational parties but considers colluding parties establishing retaliatory contracts to discourage betrayal among colluders. It was shown to be impossible to maintain non-collusion outcome if retaliatory contracts can impose unbounded penalties, and feasible to guarantee non-collusion otherwise. This is weaker than a strong coalition but admits mechanisms protecting secrets of a general nature.

Note: This note compares three existing works.