Optimally-Fair Multi-party Exchange Without Trusted Parties

We present a multi-party exchange protocol that achieves optimal partial fairness even in the presence of a dishonest majority. We demonstrate how this protocol can be applied to any type of multi-party exchange scenario where the network topology is complete. When combined with standard secure multi-party computation techniques, our protocol enables SMPC with partial fairness when a dishonest majority is involved. Fairness optimality is proven in an abstract model which applies to all protocols based on the concept of concealing the point when the secrets are exchanged. Our protocol improves known results via the use of timed-release encryption and commutative blinding.