Scantegrity III: Automatic Trustworthy Receipts, Highlighting Over/Under Votes, and Full Voter Verifiability
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Scantegrity III: Automatic Trustworthy Receipts, Highlighting Over/Under Votes, and Full Voter Verifiability Alan T. Sherman Russell A. Fink Richard Carback David Chaum UMBC CDL UMBC CDL UMBC CDL Voting Systems Institute (VSI) Abstract 1 Introduction Building on lessons learned from the November 2009 The Scantegrity II voting system [14, 13] has signif- Scantegrity II election in Takoma Park, MD, we pro- icantly simplified the voter experience for End-to-End pose improvements to the Scantegrity II voting system (E2E) voter-verifiable elections. On November 4, 2009, that (1) automatically print trustworthy receipts for easier 1,723 voters elected the mayor of Takoma Park, MD, us- on-line verification, (2) highlight ballot features includ- ing this system [7]; Takoma Park will use Scantegrity ing over/under votes to comply with the Help America again in their 2011 municipal election. Although the Vote Act, and (3) achieve full voter verifiability by elim- 2009 election was a success free of any major prob- inating print audits. We call the improved voting system lems, this experience highlighted several limitations of Scantegrity III, which features a new ballot style and a Scantegrity: (1) some voters did not write down the ex- special casting station that highlights ballots and prints posed Scantegrity II codenumbers as required to ver- receipts. Scantegrity III addresses the major limitations ify their votes later on-line; (2) the print audit to check of Scantegrity II and delivers the feature most requested the correctness of the printed ballots added cost and by voters and election officials at the Takoma Park elec- complexity; and (3) the scanners used at Takoma Park tion: printing receipts automatically. were not compliant with the Help America Vote Act We present, analyze, and compare three designs for (HAVA) [1] because they did not notify the voter of over- a Scantegrity receipt printer: a simple image duplica- or under-votes. In post-election surveys and informal in- tor available to voters in an optional separate station be- terviews [8], both voters and election officials offered as fore casting; a mark sense translator, connected to the their main suggestion that Scantegrity should include a official ballot scanner, which reads encrypted codenum- way to print privacy-preserving receipts automatically. bers printed on the ballot; and the Scantegrity III cast- ing station, which is an embellished mark sense trans- Scantegrity provides End-to-End (E2E) voter verifia- lator. At the Scantegrity III station, voters cast ballots bility. Each voter uses an optical scan paper ballot with that include both Scantegrity II codes in invisible ink and special invisible printing in the markable positions. To Scantegrity I codes in conventional ink; this combination select a candidate, the voter marks her ballot using a spe- of codes enables print audits to be eliminated. We also cial pen with reactive ink that reveals a hidden confir- design a Trusted Platform Module (TPM) enhancement mation number—called a Scantegrity II code—in each to bolster privacy, to store keys and verification codes, marked position. The voter can record the numbers re- and to ensure that the correct software is booted. Elec- vealed by her selections to check on an election website tion integrity does not depend on the correct operation of later. In doing so, the voter validates that her vote was the TPM. Receipt printers reduce the amount of special recorded and tallied correctly, which helps verify elec- voter instruction required, improve accessibility, enable tion integrity without revealing how she voted. Each each voter to detect if any additional mark is added to her code is chosen randomly for each contest (race) and bal- ballot after casting, and make vote verification easier. lot, and therefore does not reveal the corresponding vote. The ballots are handled as traditional ballots, preserving Keywords. Applied cryptography, End-to-End (E2E) the ability to count them by hand and to perform other election systems, print audit, Punchscan and Scantegrity election activities associated with optical scan systems. voting systems, receipt printers, security engineering, Election integrity is verified when enough voters Trusted Platform Module (TPM), trustworthy computing. check their codes on the election website, and challenge any incorrect results. In 2009, some Takoma mayoral Trustworthy receipt printers help increase the confi- voters did not record exposed codenumbers. Some did dence of E2E election outcomes. They make it easier for not realize that they could make a receipt, and that doing voters to obtain receipts, and they facilitate the possibil- so was necessary for on-line verification.1 Some voters ity of printing multiple copies of receipts. For example, had trouble reading or writing codenumbers, and tran- an extra copy of every receipt could be printed and made scription errors always are possible when writing down available to independent auditors, who could check them the codes. Further, ballots with many races (or with many and post them on-line. candidates when instant runoff is used) would burden the Contributions of our work include: voter with recording multiple codes. • Three design concepts for a Scantegrity receipt Securely adding a receipt printer to Scantegrity is a printer, including a simple image duplicator, a challenging task: for the system to preserve E2E voter mark-sense translator, and a Scantegrity III casting verifiability, the voters must be able to verify that the station. printed receipts are valid. If a malicious receipt printer could generate improper receipts unnoticed by voters, • Solutions to three additional limitations of Scant- changes in election outcomes might go undetected. Fur- egrity: eliminating the need for separate print au- thermore, a printer presents a potential vulnerability dits, mitigating the threat of adding marks to ballots against voter privacy and adds system complexity. after casting (this vulnerability also exists for tradi- Fink [18] and Carback [9] proposed a simple receipt tional optical scan systems), and notifying the voter printer with scanner, which in an optional separate step of over- and under-votes prior to casting. before ballot casting, duplicates the scanned images of the marked ovals, letting the voter compare her ballot and • A new user interface for any optical scan voting printed receipt without doing any character recognition. system featuring backlighting of ballots to highlight This design does not guarantee that the ballot cast is the certain characteristics, including but not limited to same ballot scanned by the receipt printer. Fink [18] and over- and under-votes. Carback [9] also proposed a more complex mark-sense • Design enhancements for bolstering the receipt translator design in which the official precinct-count op- printers with a Trusted Platform Module (TPM), to tical scan (PCOS) scanner is connected to a secure re- safeguard privacy, detect problems sooner, and en- ceipt printer. From the marked oval positions detected force election policy. by the scanner, the receipt printer reconstructs the asso- ciated codenumbers independently, providing a power- The rest of the paper is organized as follows. Section 2 ful check on the PCOS scanner. Disadvantages include reviews previous and related work. Section 3 presents the need for the printer to know the codenumbers and our three receipt printer designs: image duplicator, mark the need for a physical mechanism securing the ballot sense translator, and the Scantegrity III casting station. against modifications before casting while the voter com- Section 4 lists requirements for a basic receipt printer. pares the printed receipt and ballot. Section 5 suggests how each design can be improved Embellishing the mark-sense translator, Chaum pro- with a TPM to protect privacy and to detect problems posed Scantegrity III, addressing all of the main limita- sooner. Section 6 presents security arguments and dis- tions of Scantegrity noted above and enabling safe au- cusses a variety of issues raised by our designs, and Sec- tomated receipt generation. In the Scantegrity III cast- tion 7 concludes our work. Appendix A provides addi- ing station, the voter places the marked ballot under a tional design and use details. glass panel, where it is scanned and where ballot charac- teristics (including possible under- and over-voting) can 2 Previous and Related Work be highlighted with backlighting. The device prints a receipt in one of two different styles verifiably chosen Scantegrity II [14, 13] is an End-to-End (E2E) voting at random and permits the voter to compare the receipt system, meaning that it provides high assurance that the and ballot before casting while they remain locked under tally is computed properly while maintaining ballot se- the glass. Using a combination of ideas from Punchscan crecy [33]. It has been deployed in a mock election [39, and Scantegrity I, this receipt construction eliminates the 40] and a real election at Takoma Park, MD [7, 8]. Print- need for separate print audits. Significantly, the back- ing the codenumbers in invisible ink simplifies dispute lighting feature of this design is of separate interest and procedures of the earlier Scantegrity I system [12]. Re- can perform more than simply identifying possible over- sponding to suggestions from Takoma Park voters and and under-votes. election officials to automate receipt printing, in a joint 1Some voters later explained that they chose not to read any instruc- chapter of their dissertations, Fink [18] and Carback [9] tions because they knew how to vote [8]. describe two receipt printer designs for Scantegrity: an 2 image duplicator and mark sense translator. Circa Febru- TCG specifications [42] and the overview by Pearson ary 2011, Chaum drew preliminary sketches for a Scant- et al. [31]. To developers using the TPM, we recom- egrity III casting station. This paper integrates and re- mend the practical guide by Challener et al. [10] and the fines these ideas. TrouSerS software stack and test suite for understanding Aspects of our designs were inspired by Scantegrity’s programming details [23].