Classical results on secure multi-party computation (MPC) imply that fully secure computation, including fairness (either all parties get output or none) and robustness (output delivery is guaranteed), is impossible unless a majority of the parties is honest. Recently, cryptocurrencies like Bitcoin where utilized to leverage the fairness loss in MPC against a dishonest majority. The idea is that when the protocol aborts in an unfair manner (i.e., after the adversary receives output) then honest parties get compensated (in coins) by the adversarially controlled parties.
We describe the first robust MPC protocol with compensation, i.e., an MPC protocol where not only fairness is guaranteed (via compensation) but additionally the protocol is guaranteed to deliver output to the parties that get engaged. This means that the adversary, after an initial round of deposits, is not even able to mount a denial-of-service attack without having to suffer a monetary penalty. Our robust MPC protocol requires only a constant number of (coin-transfer and communication) rounds. To prove the security of our construction, we put forth a new formal model of secure MPC with compensation and show how the introduction of suitable ledger and synchronization functionalities makes it possible to describe such protocols using standard interactive Turing machines (ITM) circumventing the need for the use of extra features that are outside the standard model as in previous works. Our model is expressed in the universal composition setting with global setup and is equipped with a composition theorem that enables the design of protocols, which compose safely with each other and within larger environments alongside other protocols with compensation; such a composition theorem for MPC protocols with compensation was not known before.
This is joint work with Aggelos Kiayias and Hong-Sheng Zhou.