Stephanie Delaune , Steve Kremer and Mark D . Ryan Coercion - Resistance and Receipt - Freeness in Electronic Voting Research Report LSV - 06 - 08 April 2006

In this paper we formally study important properties of electronic voting protocols. In particular we are interested in coercion-resistance and receipt-freeness. Intuitively, an election protocol is coercion-resistant if a voter A cannot prove to a potential coercer C that she voted in a particular way. We assume that A cooperates with C in an interactive way. Receipt-freeness is a weaker property, for which we assume that A and C cannot interact during the protocol: to break receipt-freeness A later provides evidence (the receipt) of how she voted. While receipt-freeness can be expressed using observational equivalence from the applied pi calculus, we need to introduce a new relation to capture coercion-resistance. Our formalization of coercionresistance and receipt-freeness are quite different. Nevertheless, we show in accordance with intuition that coercionresistance implies receipt-freeness, which implies privacy, the basic anonymity property of voting protocols, as defined in previous work. Finally we illustrate the definitions on a simplified version of the Lee et al. voting protocol.