An Epistemic Approach to Coercion-Resistance for Electronic Voting Protocols

Coercion resistance is an important and one of themost intricate security requirements of electronicvoting protocols. Several definitions of coercionresistance have been proposed in the literature,including definitions based on symbolic models.However, existing definitions in such models arerather restricted in their scope and quite complex.In this paper, we therefore propose a new definitionof coercion resistance in a symbolic setting, basedon an epistemic approach. Our definition isrelatively simple and intuitive. It allows for afine-grained formulation of coercion resistance andcan be stated independently of a specific, symbolicprotocol and adversary model. As a proof of concept,we apply our definition to three votingprotocols. In particular, we carry out the firstrigorous analysis of the recently proposed Civitassystem. We precisely identify those conditions underwhich this system guarantees coercion resistance orfails to be coercion resistant. We also analyzeprotocols proposed by Lee et al. and Okamoto.