Vote-Independence: A Powerful Privacy Notion for Voting Protocols

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