Optimistic Mixing for Exit-Polls

Abstract:
We propose a new mix network that is optimized to produce a correct output very fast when all mix servers execute the mixing protocol correctly (the usual case). Our mix network only produces an output if no server cheats. However, in the rare case when one or several mix servers cheat, we convert the inputs to a format that allows "back-up" mixing. This back-up mixing can be implemented using any one of a wide array of already proposed (but slower) mix networks. When all goes well, our mix net is the fastest, both in real terms and asymptotically, of all those that offer standard guarantees of privacy and correctness. In practice, this benefit far outweighs the drawback of a comparatively complex procedure to recover from cheating. Our new mix is ideally suited to compute almost instantly the output of electronic elections, whence the name "exit-poll" mixing.