Logical Analysis and Verification of Cryptographic Protocols - Loria

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178 CHAPTER 7. VOTER VERIFIABILITY FOR E-VOTING PROTOCOLS which are not: we show that the protocols due to Fujioka, Okamoto & Ohta [115] and Lee et al. [141] are voter verifiable. Intuitively, voter verifiability may appear to contradict another important property of voting systems, namely coercion-resistance. If a voter is capable of verifying that her vote has been included in the tally, she may be able to use that capability to convince a coercer that she voted as he ordered. However, there are systems that satisfy both, such as the system by Lee et al. [141] which we consider in Section 7.5.4. This is achieved by ensuring that what constitutes a proof for the voter will not be a proof for the coercer. Therefore, it is important to ensure that voter verifiability is not defined so strongly that it is incompatible with coercion-resistance. This work has been done in collaboration with Mark Ryan, Ben Smyth, and Steve Kremer during my internship at the School of Computer Science (University of Birmingham) from October, 2008 until January, 2009. A more recent version will be presented in the 4th Benelux Workshop on Information and System Security (WISSec 2009) [191]. Outline. We introduce in Section 7.1 the electronic voting protocols. In Section 7.2, we introduce the applied pi calculus. The formalisation of electronic voting protocols and voter verifiability properties are given in Section 7.3 and Section 7.4. We give in Section some examples, and the related works are given in Section 7.6. 7.1 Electronic voting protocols Electronic voting, also known as e-voting, is a term encompassing several different types of voting, embracing both electronic means of casting a vote and electronic means of counting votes. Electronic voting systems can include punch cards, optical scan voting systems and specialised voting kiosks, including selfcontained Direct-recording electronic (DRE) voting systems. It can also involve transmission of ballots and votes via telephones, private computer networks, or the Internet. Electronic voting may offer advantages compared to other voting techniques. It promises the possibility of a convenient, efficient and secure facility for recording and tallying votes. It can be used for a variety of types of elections, from small committees or on-line communities through to full-scale national elections. Electronic voting for electorates have been in use since the 1960. However, the electronic voting machines used in recent US elections have been fraught with problems. In [132], the authors have analysed the source code of the machines sold by the second largest and fastest-growing vendor, which
7.1. ELECTRONIC VOTING PROTOCOLS 179 are in use in 37 US states. This analysis has produced a catalogue of vulnerabilities and possible attacks. A potentially much more secure system could be implemented, based on cryptographic protocols that specify the messages sent between the voters and administrators. Such systems are called electronic voting protocols. Abstractly, electronic voting protocols are cryptographic protocols that specify the messages sent between the voters and administrators, they can be written as a sequence of messages sent between voters and administrators. Such protocols have been studied for several decades [97, 136, 30, 39, 62, 63, 65], and a various types of electronic voting protocols have been proposed in the literature [61, 64, 39, 92, 62, 115, 141, 181]. These protocols aim to provide security properties which go beyond those that can be achieved by paper-based voting systems. Some of these properties are given next. 7.1.1 Properties of electronic voting protocols Some properties commonly sought for electronic voting protocols are the following: • Fairness: no early results can be obtained which could influence the remaining voters. • Vote-privacy: the fact that a particular voter voted in a particular way is not revealed to anyone. • Receipt-freeness: a voter does not gain any information which can be used to prove to a coercer that she voted in a certain way. • Coercion-resistance: a voter can not cooperate with a coercer to prove to him that she voted in a certain way. • Inalterability: no one can change a voter’s vote. • Declared tally: the published outcome is a correct sum of the votes cast. • Eligibility: only legitimate voters can vote, and only once. • Individual verifiability: a voter can check that her ballot was included in the tally. • Universal verifiability: anybody can check the correctness of the published outcome. • Eligibility verifiability: anybody can check that each vote cast was created by a unique legitimate voter.
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Logical Analysis and Verification of Cryptographic Protocols - Loria

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