Collusion-Free Protocols in the Mediated Model

Prior approaches [15, 14] to building collusion-free protocols require exotic channels. By taking a conceptually new approach, we are able to use a more digitally-friendly communication channel to construct protocols that achieve a stronger collusion-free property. We consider a communication channel which can filter and rerandomize message traffic. We then provide a new security definition that captures collusion-freeness in this new setting; our new setting even allows for the mediator to be corrupted in which case the security gracefully fails to providing standard privacy and correctness. This stronger notion makes the property useful in more settings. To illustrate feasibility, we construct a commitment scheme and a zeroknowledge proof of knowledge that meet our definition in its two varia-