International Association
for Cryptologic Research

We study ballot secrecy and ballot independence for election schemes. First, we propose a definition of ballot secrecy as an indistinguishability game in the computational model of cryptography. Our definition builds upon and strengthens earlier definitions to ensure that ballot secrecy is preserved in the presence of an adversary that controls the bulletin board and communication channel. Secondly, we propose a definition of ballot independence as a straightforward adaptation of a non-malleability definition for asymmetric encryption. We also provide a simpler, equivalent definition as an indistinguishability game. Thirdly, we prove that ballot independence is necessary in election schemes satisfying ballot secrecy. Finally, we demonstrate the applicability of our results by analysing Helios. Our analysis identifies a new attack against Helios, which enables an adversary to determine if a voter did not vote for the adversary\'s chosen candidate. The attack requires the adversary to control the bulletin board or communication channel, thus, it could not have been detected by earlier definitions of ballot secrecy.