Prêt à Voter

Prêt à Voter is an E2E voting system devised by Peter Ryan of Newcastle University. It aims to provide guarantees of accuracy of the count and ballot privacy that are independent of software, hardware etc. Assurance of accuracy flows from maximal transparency of the process, consistent with maintaining ballot privacy. In particular, Prêt à Voter enables voters to confirm that their vote is accurately included in the count whilst avoiding dangers of coercion or vote buying.

The key idea behind the Prêt à Voter approach is to encode the vote using a randomized candidate list. The randomisation of the candidate list on each ballot form ensures the secrecy of each vote. Incidentally, it also removes any bias towards the top candidate that can occur with a fixed ordering.

The value printed on the bottom of the receipt is the key to extraction of the vote. Buried cryptographically in this value is the information needed to reconstruct the candidate order and so extract the vote encoded on the receipt. This information is encrypted with secret keys shared across a number of tellers. Thus, only the set of tellers acting in together are able to interpret the vote encoded on the receipt.

After the election, voters (or perhaps proxies acting on their behalf) can visit the Web Bulletin Board (WBB) and confirm their receipts appear correctly. Once this is over, the tellers take over and perform anonymising mixes and decryption of the receipts. All the intermediate stages this is process are posted to the WBB and are audited later.

There are various auditing mechanisms to ensure that all the steps, the creation of the ballot forms, the mixing and decryption etc all performed correctly, but these are carefully designed so as not to impinge on ballot privacy.

Example

Suppose that our voter is called Anne. At the polling station, Anne chooses at random a ballot form sealed in an envelope, and example of such a form is shown in Figure 1.

Anne now exits the booth clutching her receipt, registers with an official and casts her receipt. Her receipt is placed over an optical reader or similar device that records the random value at the bottom of the strip and records in which the cell her X is marked. Her original, paper receipt is digitally signed and franked and returned to her to keep.

Origin

Prêt à Voter was inspired by the earlier, voter-verifiable scheme by David Chaum. It replaces the visual cryptographic encoding the voter's choice in Chaum's scheme by the conceptually and technologically simpler candidate randomization. The Prêt à Voter idea of encoding the vote through permutations has subsequently been incorporated in Chaum's Punchscan scheme. However Punchscan uses a permutation of indirection symbols instead of candidate names allowing it to comply with voting laws that require a specific ordering of candidates. Prêt à Voter won second place at the 2007 University Voting Systems Competition, after a the winning team, Punchscan, uncovered a security flaw in the random number generator portion of the Prêt à Voter source codecite web |url= http://blog.wired.com/27bstroke6/2007/07/us-team-wins-vo.html|title= US/Canada Team Wins Voting Machine Competition|accessdate=2008-08-19 |last= Zetter|first= Kim |date= July 19, 2007|publisher= Wired] .

References

* An accessible account of Prêt à Voter can be found in "The Computer Ate my Vote", chapter to appear in "Formal Methods: State of the Art and New Directions", Ed. Paul Boca, Springer 2007, also available as Newcastle University Technical Report 988 [http://www.cs.ncl.ac.uk/research/pubs/trs/papers/988.pdf] .
* Further technical details can be found in Newcastle University Technical Reports 864, 880, 929, 956 and 965.