Vote-Independence: A Powerful Privacy Notion for Voting Protocols

Recently an attack on ballot privacy in Helios has been discovered [20], which is essentially based on copying other voter’s votes. To capture this and similar attacks, we extend the classical threat model and introduce a new security notion for voting protocols: Vote-Independence. We give a formal definition and analyze its relationship to established privacy properties such as Vote-Privacy, Receipt-Freeness and CoercionResistance. In particular we show that even Coercion-Resistant protocols do not necessarily ensure Vote-Independence.