Joel Alwen

Collusion-Free Multiparty Computation in the Mediated Model

Joel Alwen, Jonathan Katz, Yehuda Lindell, Giuseppe Persiano, abhi shelat and Ivan Visconti

Collusion-free protocols prevent subliminal communication (i.e., covert
channels) between parties running the protocol. In the standard
communication model, if one-way functions exist, then protocols
satisfying any reasonable degree of privacy cannot be collusion-free. To
circumvent this impossibility, Alwen, shelat and Visconti (CRYPTO 2008)
recently suggested the mediated model where all communication passes
through a mediator. The goal is to design protocols where
collusion-freeness is guaranteed as long as the mediator is honest,
while standard security guarantees hold if the mediator is dishonest. In
this model, they gave constructions of collusion-free protocols for
commitments and zero-knowledge proofs in the two-party setting.
We strengthen the definition of Alwen et al., and resolve the main open
questions in this area by showing a collusion-free protocol (in the
mediated model) for computing any multi-party functionality.