Date Published: 2008-11-30
Author(s):Ronald L. Rivest, Massachusetts Institute of Technology
We present a new paper-based voting method with interesting security properties. The attempt here is to see if one can achieve the same security properties of recently proposed cryptographic voting protocols, but without using any cryptography, using only paper ballots. We partially succeed. (Initially, I thought the proposal accomplished this goal, but several readers discovered a vote-buying attack (see Section 4.4) that appears to be rather difficult to fix without making the resulting system much less usable in practice. Currently, this paper should thus be viewed more as an academic proposal than a practical proposal. Perhaps some variation on these ideas in this paper might still turn out to be of practical use. The “OneBallot with Exchanged Receipts” system sketched at the end of Section 5.3.1, looks particularly promising at the moment. . . )
The principles of ThreeBallot are simple and easy to understand.
In this proposal, not only can each voter verify that her vote is recorded as she intended, but she gets a “receipt” that she can take home that can be used later to verify that her vote is actually included in the final tally. Her receipt, however, does not allow her to prove to anyone else how she voted.
In this “ThreeBallot” voting system, each voter casts three paper ballots, with certain restrictions on how they may be filled out, so the tallying works. These paper ballots are of course “voter-verifiable.” All ballots cast are scanned and published on a web site, so anyone may correctly compute the election result.
A voter receives a copy of one of her ballots as her “receipt”, which she may take home. Only the voter knows which ballot she copied for her receipt. The voter is unable to use her receipt to prove how she voted or to sell her vote, as the receipt doesn’t reveal how she voted. A voter can check that the web site contains a ballot matching her receipt. Deletion or modification of ballots is thus detectable; so the integrity of the election is verifiable.