Chapter 10 The Scantegrity Voting System and Its Use in the Takoma Park Elections Richard T. Carback Draper Labs David Chaum Voting Systems Institute Jeremy Clark Concordia University Aleksander Essex Western University Travis Mayberry US Naval Academy Stefan Popoveniuc The George Washington University Ronald L. Rivest MIT 237 238 Real-World Electronic Voting: Design, Analysis and Deployment Emily Shen MIT Alan T. Sherman UMBC Poorvi L. Vora The George Washington University John Wittrock AppNexus Filip Zagórski Wrocław University of Technology CONTENTS 10.1 Introduction ::::::::::::::::::::::::::::::::::::::::::::::::::::::: 239 10.1.1 Key Properties :::::::::::::::::::::::::::::::::::::::::::: 239 10.1.2 Outline ::::::::::::::::::::::::::::::::::::::::::::::::::: 241 10.1.3 Organization of This Chapter :::::::::::::::::::::::::::::: 241 10.2 The Punchscan Voting System :::::::::::::::::::::::::::::::::::::: 242 10.2.1 Voter Experience :::::::::::::::::::::::::::::::::::::::::: 242 10.2.2 Election Set-Up ::::::::::::::::::::::::::::::::::::::::::: 242 10.2.3 Ballot Printing and Voter Privacy ::::::::::::::::::::::::::: 244 10.2.4 The First Binding E2E Election :::::::::::::::::::::::::::: 244 10.2.5 Lessons Learned :::::::::::::::::::::::::::::::::::::::::: 246 10.3 The Scantegrity II Voting System ::::::::::::::::::::::::::::::::::: 246 10.3.1 Ballot Features :::::::::::::::::::::::::::::::::::::::::::: 247 10.3.2 The Scantegrity Back-End ::::::::::::::::::::::::::::::::: 248 10.3.3 Election Day :::::::::::::::::::::::::::::::::::::::::::::: 249 10.3.4 Posting and Tallying ::::::::::::::::::::::::::::::::::::::: 250 10.3.5 Auditing the Results ::::::::::::::::::::::::::::::::::::::: 250 10.3.6 Dispute Resolution :::::::::::::::::::::::::::::::::::::::: 252 10.4 The 2009 Takoma Park Election :::::::::::::::::::::::::::::::::::: 254 10.4.1 Requirements ::::::::::::::::::::::::::::::::::::::::::::: 254 10.4.2 Mock Election :::::::::::::::::::::::::::::::::::::::::::: 255 10.4.3 The Election :::::::::::::::::::::::::::::::::::::::::::::: 256 10.5 The Remotegrity Voting System :::::::::::::::::::::::::::::::::::: 259 10.5.1 Voter Experience :::::::::::::::::::::::::::::::::::::::::: 260 10.5.2 Security Properties :::::::::::::::::::::::::::::::::::::::: 260 10.6 Audiotegrity ::::::::::::::::::::::::::::::::::::::::::::::::::::::: 262 10.6.1 The Voting Process with Audiotegrity :::::::::::::::::::::: 263 The Scantegrity Voting System and Its Use in the Takoma Park Elections 239 10.6.2 Dispute Resolution Properties :::::::::::::::::::::::::::::: 263 10.7 Takoma Park Election, 2011 :::::::::::::::::::::::::::::::::::::::: 264 10.8 Usability Studies ::::::::::::::::::::::::::::::::::::::::::::::::::: 266 10.8.1 Related Studies :::::::::::::::::::::::::::::::::::::::::::: 266 10.8.2 Methodology :::::::::::::::::::::::::::::::::::::::::::::: 267 10.8.3 Known Limitations :::::::::::::::::::::::::::::::::::::::: 268 10.8.4 Voter Response :::::::::::::::::::::::::::::::::::::::::::: 269 10.8.5 Observational Results ::::::::::::::::::::::::::::::::::::: 269 10.8.6 Election Judge Response :::::::::::::::::::::::::::::::::: 271 10.8.7 Study Findings :::::::::::::::::::::::::::::::::::::::::::: 272 10.9 Current Status of the Project :::::::::::::::::::::::::::::::::::::::: 274 10.10 Conclusions ::::::::::::::::::::::::::::::::::::::::::::::::::::::: 274 Acknowledgments :::::::::::::::::::::::::::::::::::::::::::::::::::: 276 10.1 Introduction The Scantegrity project began with a simple question: is it possible to design a vot- ing system offering the strong security properties of cryptographic end-to-end (E2E) election verification with the intuitive look and feel of a paper optical-scan ballot? This chapter recounts a decade-long research effort toward answering this question, from the design of Scantegrity’s precursor Punchscan, all the way to the first govern- mental election run by an E2E voting system. The main focus of this chapter is on the Scantegrity II voting system (hereafter referred to as simply Scantegrity) and its use in the municipal elections of Takoma Park, Maryland, in 2009 and 2011. To our knowledge, the Takoma Park election of 2009 was the first use of an E2E-verifiable voting system in an in-person secret-ballot governmental election anywhere in the world, as well as being the first governmental election held in the United States to run on open-source software. We also describe the Punchscan voting system and its use in the 2007 election of the University of Ottawa Graduate Students Association/Association Étudiant(e)s Diplômé(e)s (GSAÉD), which, to our knowledge, is the first time an E2E voting sys- tem was used in a binding election.1 Additionally, this chapter describes the remote voting system Remotegrity and accessible Scantegrity variant Audiotegrity, and their use in the 2011 Takoma Park election. We also briefly recount a number of smaller, non-binding elections run during the course of this project.2 1A binding election is one whose outcome is binding on a constituency, though it need not be a political election. It could be, e.g., a primary, party, union, stockholder or student government election. Whereas a non-binding election is effectively an opinion poll. 2These elections are documented on punchscan.org and scantegrity.org. 240 Real-World Electronic Voting: Design, Analysis and Deployment 10.1.1 Key Properties End-to-End Verifiable Loosely speaking, a cryptographically end-to-end verifiable (E2E) voting system is designed to provide the following security properties: (a) A voter has the means to check that her ballot was included unmodified in the set of cast ballots, and receives evidence that convinces her if it is not. (b) Anyone has the means to check that the set of cast ballots were counted cor- rectly, and receives convincing evidence if they were not. (c) The preceding checks are performed in a way that upholds the secrecy of every voter’s ballot. Additionally, Scantegrity provides an additional security property: (d) If a voter’s ballot was recorded incorrectly, she can prove this fact to others without revealing her vote. Similarly, if her vote was recorded correctly, but the voter makes a spurious claim, the election officials can prove this fact to others. We refer to this last property as dispute resolution. This particular property is impor- tant in an E2E setting because it allows the electorate as a whole to attribute fault to the actual at-fault party: the elections officials or the voter. It is also special in the sense that not all E2E systems provide it. Most of the assumptions made by the Scantegrity voting system and its variants are standard to E2E voting systems: a threshold of election trustees is required to be honest for privacy, cryptographic primitives used are assumed secure and an append- only, public bulletin board with authenticated write-access is assumed to exist. In addition, Scantegrity’s privacy and dispute resolution properties require that symbols printed in invisible ink or beneath scratch-off are not visible. Practical Aspects From a practical perspective, a major design goal of Scantegrity is to provide vot- ers with an intuitive interface. We focused on paper optical-scan ballots for a variety of reasons: it is a predominant mode of ballot casting, and is legally mandated in a number of jurisdictions, such as the United States; it provides a degree of recover- ability in the event of an electronic failure; used in a precinct-scan configuration, it does not require electricity at the polling place. Scantegrity itself offers a number of advantages over other optical-scan based E2E systems: (a) The verifiability elements are constructed as an overlay on the optical scan ballot, making them compatible with pre-existing layouts. The Scantegrity Voting System and Its Use in the Takoma Park Elections 241 (b) Participation in the verification process is opt-in; voters can (in principle) ig- nore the Scantegrity-related components and vote as they would otherwise on a plain paper optical-scan ballot. 10.1.2 Outline In this chapter, we trace through a couple of iterations of deploying, redesigning, and redeploying an E2E system. We begin by describing the Punchscan system and its use in an election in 2007. We then describe the lessons learned from that election, which led to the development of Scantegrity and an early variant, Scantegrity I. The former improved on the dispute resolution properties of the latter, and was used by Takoma Park in 2009 and 2011. We recount some of the modifications we made to the original Scantegrity proposal for the election, and how feedback from a preliminary mock election further refined the system, as well as the results of the election itself. Scantegrity was deployed again in the municipal election of Takoma Park in 2011. For this election, we developed and deployed the remote voting system Re- motegrity and the accessibility-enhanced Scantegrity variant Audiotegrity. Both of these systems were tested in an open test before the 2011 election, and we document the feedback and how it contributed to changes to the systems finally fielded in 2011. We end this chapter with a description of the results of usability tests that demon- strate high levels of confidence in Scantegrity, and high levels of support for the ver- ification receipt, even when voters indicated they did not understand the underlying cryptographic mechanisms. While E2E system usability is an understudied subject, the data we collected after the Takoma Park mock and municipal elections