Proving Coercion-Resistance of Scantegrity II

Abstract. By now, many voting protocols have been proposed that, among others, are designed to achieve coercion-resistance, i.e., resis-tance to vote buying and voter coercion. Scantegrity II is among the most prominent and successful such protocols in that it has been used in several elections. However, almost none of the modern voting proto-cols used in practice, including Scantegrity II, has undergone a rigorous cryptographic analysis. In this paper, we prove that Scantegrity II enjoys an optimal level of coercion-resistance, i.e., the same level of coercion-resistance as an ideal voting protocol (which merely reveals the outcome of the election), ex-cept for so-called forced abstention attacks. This result is obtained under the (necessary) assumption that the workstation used in the protocol is honest. Our analysis is based on a rigorous cryptographic definition of coercion-resistance we recently proposed. We argue that this definition is in fact the only existing cryptographic definition of coercion-resistance suitable for analyzing Scantegrity II. Our case study should encourage and facili-tate rigorous cryptographic analysis of coercion-resistance also for other voting protocols used in practice.