An election is a process through which citizens in liberal democracies select their governing bodies, usually through voting. For elections to be truly honest, people must be able to vote freely without being subject to coercion; that is why voting is usually done in a private manner. In this paper we analyze the security offered by a paper-ballot voting system that is used in Israel, as well as in several other countries around the world. we provide an algorithm which, based on publicly available information, breaks the privacy of the voters participating in such elections. Simulations based on real data collected in Israel show that our algorithm performs well, and can correctly recover the vote of up to 96 of the voters.
Each ballot box in the Israeli voting system can be modeled as a certain kind of a mix, namely a timed-mix. In such a mix, a buffer of messages is mixed once in each time period. The set of voters in each polling station corresponds to the set of senders, while the set of parties contesting in an election corresponds to the set of receivers. There are various known attacks on mixes @cite_9 @cite_20 @cite_21 @cite_18 @cite_3 and we refer the interested reader to a recent survey @cite_15 .
@cite_18 @cite_9 @cite_21 @cite_3 @cite_15 @cite_20
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A passive attacker can compromise a generic anonymity protocol by applying the so called disclosure attack, i.e. a special traffic analysis attack. In this work we present a more efficient way to accomplish this goal, i.e. we need less observations by looking for unique minimal hitting sets. We call this the hitting set attack or just HS-attack. In general, solving the minimal hitting set problem is NP-hard. Therefore, we use frequency analysis to enhance the applicability of our attack. It is possible to apply highly efficient backtracking search algorithms. We call this approach the statistical hitting set attack or SHS-attack. However, the statistical hitting set attack is prone to wrong solutions with a given small probability. We use here duality checking algorithms to resolve this problem. We call this final exact attack the HS*-attack. Anonymity services hide user identity at the network or address level but are vulnerable to attacks involving repeated observations of the user. Quantifying the number of observations required for an attack is a useful measure of anonymity. A user is only anonymous within a set of other users. Hence, the core functionality of an anonymity providing technique is to establish an anonymity set. In open environments, such as the Internet, the established anonymity sets in the whole are observable and change with every anonymous communication. We use this fact of changing anonymity sets and present a model where we can determine the protection limit of an anonymity technique, i.e. the number of observations required for an attacker to "break" uniquely a given anonymity technique. In this paper, we use the popular MIX method to demonstrate our attack. The MIX method forms the basis of most of the today's deployments of anonymity services (e.g. Freedom, Onion Routing, Webmix). We note that our approach is general and can be applied equally well to other anonymity providing techiques. An improvement over the previously known disclosure attack is presented that allows, using statistical methods, to effectively deanonymize users of a mix system. Furthermore the statistical disclosure attack is computationally efficient, and the conditions for it to be possible and accurate are much better understood. The new attack can be generalized easily to a variety of anonymity systems beyond mix networks.