Security Proofs for Participation Privacy, Receipt-Freeness and Ballot Privacy for the Helios Voting Scheme

Abstract

The Helios voting scheme is well studied including formal proofs for verifiability and ballot privacy. However, depending on its version, the scheme provides either participation privacy (hiding who participated in the election) or verifiability against malicious bulletin board (preventing election manipulation by ballot stuffing), but not both at the same time. It also does not provide receipt-freeness, thus enabling vote buying by letting the voters contstruct receipts proving how they voted. Recently, an extension to Helios, further referred to as KTV-Helios, has been proposed that claims to provide these additional security properties. However, the authors of KTV-Helios did not prove their claims. Our first contribution is to provide formal definition for participation privacy and receipt-freeness, that can be applied to KTV-Helios. These definitions were used to also prove the corresponding claims of KTV-Helios. Our second contribution is to use the existing definitions of ballot privacy and verifiability against malicious bulletin board as applied to Helios in order to prove that both security properties also hold for KTV-Helios.

A Proxy Voting Scheme Ensuring Participation Privacy and Receipt-Freeness

“…The idea of the extension in [8,7] is to use the so-called dummy ballots cast by the posting trustee in order to obfuscate the presence of ballots cast by voters, ensuring participation privacy or receipt-freeness (by allowing voters to update their vote via casting an additional ballot without the adversary noticing). These dummy ballots are designed to be indistinguishable from non-dummy ballots while having no effect on the tally result.…”

“…These dummy ballots are designed to be indistinguishable from non-dummy ballots while having no effect on the tally result. The scheme in [8,7] runs as follows:…”

“…The assumption (A-NoUnknownPlaintext), in particular, is crucial for the design of the scheme in [8,7] in order to ensure that the voter can still vote for her preferred voting option in case of coercion or vote buying. If the assumption is not fulfilled, the voter can change the ballot cast by an adversary to an invalid vote without being noticed, but not to a vote for a specific voting option of voter's choice.…”

“…Posting Trustee and Dummy Ballots As in the scheme proposed in [8,7], our extension relies on the so-called dummy ballots cast during the voting by a special kind of entity, the posting trustee. The dummy ballots in [8,7] are designed to be indistinguishable from non-dummy ballots and have no effect on the tally result. They are meant to obfuscate the presence of ballots cast by voters, ensuring participation privacy or receipt-freeness (by allowing voters to update their vote via casting an additional ballot without the adversary noticing).…”

“…Our contribution is addressing these weaknesses and proposing an Internet voting scheme for proxy voting that ensures participation privacy and receipt-freeness. In particular, we use the idea of dummy ballots introduced in the Helios extension that has been proposed in [7] and its security proven in [8,9]. Our extension ensures participation privacy by hiding the information, whether a given eligible voter participated in the election, cast her ballot directly or delegated to some proxy.…”