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Compared with modern touch-screen voting systems, it may seem low tech. But according to its creators, the scratch-and-vote (S&V) system is a good way to let voters check that their ballot papers have been counted as they intended.

Using a current touch-screen system, “there is no way for an individual voter to know that his or her vote has been properly counted,” says Josh Benaloh, a cryptographer who pioneered the development of cryptography in elections, and who now works for Microsoft Research in Redmond, WA. “Even election officials cannot be certain that the systems are free of errors.”

Some of these machines are now designed to print paper receipts for each vote that’s cast. This procedure is a little better, says Benaloh; but voters are still dependent on other people and procedures. “In practice, voters have no way to ensure that their votes are being counted properly or that they are being counted at all,” he says.

With encryption-based voting systems, end-to-end verifiability is possible, because any voter should be able to “audit” the entire voting process. At the same time, such auditing processes must be balanced against the need for anonymity, says Ben Adida at MIT’s Computer Science and Artificial Intelligence Laboratory. Adida created the S&V system with Ronald Rivest, professor of electrical engineering and computer science at MIT, who co-created RSA, one of the most widely used encryption algorithms.

Traditional paper-based systems do not provide sufficient anonymity because the unique number printed on the ballot to ensure that it is legitimate can be traced back to the voter’s name. Therefore, a number of researchers have tried using cryptographic techniques to keep a voter’s identity a secret while ensuring that all votes cast are legitimate.

The S&V approach builds on this idea and can be used in conjunction with a number of existing voting schemes. One recently proposed scheme, called Prêt-à-Voter, involves the listing the candidates’ names in random order on one half of the ballot, with the tick boxes on the opposite side. After votes have been cast, a voter tears along a perforated line separating the list of names from the tick boxes. Developed by Peter Ryan at the University of Newcastle-upon-Tyne in England and David Chaum, a cryptographer who founded DigiCash, the system depends on a cryptographic code on the tick-box side of the ballot to encode the list of candidates’ names in the order they appeared on the original ballot.

The concern with this kind of system is how to ensure that the information encrypted matches the order of the candidates’ names. This can be achieved by giving each voter two ballot papers. The voters choose which ballot is audited and which they’ll use to cast their vote. This audit process tells them nothing about the validity of the ballot paper itself – but it does provide a 50:50 chance of spotting a rigged ballot paper. And, given such a high probability, illegitimate ballot papers would quickly show up in an entire electorate.

The S&V approach makes this auditing process secure because it allows a ballot paper to be checked without having to involve an election official (who in theory could be corrupt and tamper with a ballot). When applied to the Prêt-à-Voter scheme, S&V adds a scratch surface on the side bearing the candidates’ names, while the order of the candidates’ names is encoded cryptographically beneath the tick boxes. “This scratch surface is exactly like a lottery card,” says Adida.

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