Guess my vote : a study of opacity and information flow in voting systems

With an overall theme of information flow, this thesis has two main strands. In the first part of the thesis, I review existing information flow properties, highlighting a recent definition known as opacity [25]. Intuitively, a predicate cP is opaque if for every run in which cP is true, there exists an indistinguishable run in which it is false, where a run can be regarded as a sequence of events. Hence, the observer is never able to establish the truth of cPo The predicate cP can be defined according to requirements of the system, giving opacity a great deal of flexibility and versatility. Opacity is then studied in relation to several well-known definitions for information flow. As will be shown, several of these properties can be cast as variations of opacity, while others have a relationship by implication with the opacity property [139]. This demonstrates the flexibility of opacity, at the same time establishing its distinct character. In the second part of the thesis, I investigate information flow in voting systems. Pret a Voter [36] is the main exemplar, and is compared to other schemes in the case study. I first analyse information flow in Pret a Voter and the FOO scheme [59], concentrating on the core protocols. The aim is to investigate the security requirements of each scheme, and the extent to which they can be captured using opacity. I then discuss a systems-based analysis of Pret a Voter [163], which adapts and extends an earlier analysis of the Chaum [35] and Neff [131]' [132]' [133] schemes in [92]. Although this analysis has identified several potential vulnerabilities, it cannot be regarded as systematic, and a more rigorous approach may be necessary. It is possible that a combination of the information flow and systems- based analyses might be the answer. The analysis of coercion-resistance, which is performed on Pret a Voter and the FOO scheme, may exemplify this more systematic approach. Receipt-freeness usually means that the voter is unable to construct a proof of her vote. Coercion-resistance is a stronger property in that it accounts for the possibility of interaction between the coercer and the voter during protocol execution. It appears that the opacity property is ideally suited to expressing the requirements for coercion-resistance in each scheme. A formal definition of receipt-freeness cast as a variation of opacity is proposed [138], together with suggestions on how it might be reinforced to capture coercion-resistance. In total, the thesis demonstrates the remarkable flexibility of opacity, both in expressing differing security requirements and as a tool for security analysis. This work lays the groundwork for future enhancement of the opacity framework.EThOS - Electronic Theses Online ServiceDSTL : EPSRCGBUnited Kingdo

Reusable anonymous return channels

Mix networks are used to deliver messages anonymously to recipients, but do not straightforwardly allow the recipient of an anonymous message to reply to its sender. Yet the ability to reply one or more times, and to further reply to replies, is essential to a complete anonymous conversation. We propose a protocol that allows a sender of anonymous messages to establish a reusable anonymous return channel. This channel enables any recipient of one of these anonymous messages to send back one or more anonymous replies. Recipients who reply to different messages can not test whether two return channels are the same, and therefore can not learn whether they are replying to the same person. Yet the fact that multiple recipients may send multiple replies through the same return channel helps defend against the counting attacks that defeated earlier proposals for return channels. In these attacks, an adversary traces the origin of a message by sending a specific number of replies and observing who collects the same number of messages. Our scheme resists these attacks because the replies sent by an attacker are mixed with other replies submitted by other recipients through the same return channel. Moreover, our protocol straightforwardly allows for replies to replies, etc. Our protocol is based upon a re-encryption mix network, and requires four times the amount of computation and communication of a basic mixnet