Distributed ElGamal à la Pedersen
Véronique Cortier, David Galindo, Stéphane Glondu, Malika Izabachène · 2013
Real-world elections often require threshold cryptosystems so that any t out of l trustees can proceed to tallying. This is needed to protect the confidentiality of the voters' votes against curious authorities (at least t+1 trustees must collude to learn individual votes) as well as to increase the robustness of the election (in case some trustees become unavailable, t+1 trustees suffice to compute the election result). We describe a fully distributed (with no dealer) threshold cryptosystem suitable for the Helios voting system (in particular, suitable to partial decryption), and prove it secure under the Decisional Diffie-Hellman assumption. Secondly, we propose a fully distributed variant of Helios, that allows for arbitrary threshold parameters l,t, together with a proof of ballot privacy when used for referendums.