Measuring Voter-Controlled Privacy

In voting, the notion of receipt-freeness has been proposed to express that a voter cannot gain any information to prove that she has voted in a certain way. Receipt-freeness aims to prevent vote buying, even when a voter chooses to renounce her privacy. In this paper, we distinguish various ways that a voter can communicate with the intruder to reduce her privacy and classify them according to their ability to reduce the privacy of a voter. We develop a formal framework combining knowledge reasoning and trace equivalences to formally model voting protocols and define vote privacy for the voters. Our framework is quantitative, in the sense that it defines a measure for the privacy of a voter. Therefore, the framework can precisely measure the level of privacy for a voter for each of the identified privacy classes. The quantification allows our framework to capture receipts that reduce, but not nullify, the privacy of the voter. This has not been identified and dealt with by other formal approaches.