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A Game-Based Definition of Coercion-Resistance and its Applications

Ralf Küsters, Tomasz Truderung, and Andreas Vogt

Coercion-resistance is one of the most important and intricate security requirements for voting protocols. Several definitions of coercion-resistance have been proposed in the literature, both in cryptographic settings and more abstract, symbolic models. However, unlike symbolic approaches, only very few voting protocols have been rigorously analyzed within the cryptographic setting. A major obstacle is that existing cryptographic definitions of coercion-resistance tend to be complex and limited in scope: They are often tailored to specific classes of protocols or are too demanding. In this paper, we therefore present a simple and intuitive, yet widely applicable cryptographic definition of coercion-resistance, in the style of game-based definitions. This definition allows to precisely measure the level of coercion-resistance a protocol provides. As a proof of concept, we apply our definition to two voting systems, namely, the Bingo voting system and ThreeBallot. The results we obtain are out of the scope of existing approaches. We show that the Bingo voting system provides the same level of coercion-resistance as an ideal voting system. We also precisely measure the degradation of coercion-resistance of ThreeBallot in case the so-called short ballot assumption does not hold and show that the level of coercion-resistance ThreeBallot provides is significantly lower than that of an ideal system, even in case of short ballots.