Abstract
Cast-as-intended verification seeks to prove to a voter that their vote was cast according to their intent. In case ballot casting is made remotely through a voting client, one of the most important dangers a designer faces are malicious voting clients (e.g. infected by a malware), which may change the voter’s selections. A previous approach for achieving cast-as-intended verification in this setting uses the so-called Return Codes. These allow a voter to check whether their voting options were correctly received by the ballot server, while keeping these choices private. An essential ingredient of this approach is a mechanism that allows a voter to discard a vote that does not represent their intent. This is usually solved using multiple voting, namely, if the return codes received by the voter do not match their choices, they cast a new vote. However, what happens if voters are not allowed to cast more than one ballot (aka single vote casting)? In this paper we propose a simple ballot casting protocol, using return codes, for allowing a voter to verify votes in a single vote casting election. We do so without significantly impacting the number of operations in the client side. This voting protocol has been implemented in a binding election in the Swiss canton of Neuchâtel in March 2015, and will be the canton’s new voting platform.
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Notes
- 1.
For example, the risk of voter coercion or vote selling in Switzerland is assumed to be affordable given the fact that many voters already use the postal channel.
- 2.
As usual, the terms “scheme” and “protocol” can be read interchangeably without much loss of precision.
- 3.
How this key is provided to the voting device in the Neuchâtel protocol is explained in Sect. 6.1.
- 4.
Note that \(sk_{c}\) is not considered to be divided in shares in this protocol. This is due to the fact that the secrets for computing the return codes (\(sk_{c}\) and \(sk_{\mathtt {id}}\)) belong to two different entities that are assumed not to collude for providing vote secrecy. However, distributing \(sk_{c}\) might be considered to weaken the trust assumptions.
- 5.
Notice that this is formally an encryption key pair, but it is being used here differently.
- 6.
As explained in Sect. 2, return codes have to be computed between two entities which are assumed not to collude, in order to ensure vote secrecy. In this implementation, the voting device computes a partial computation in the \(\mathsf {Vote}\) algorithm, while the voting server computes the final values in the \(\mathsf {ProcessBallot}\) algorithm.
- 7.
Since the voting device has only one chance to show the values to the voter, a brute force attack succeeds with probability at most \(10^{-4t}\) in changing the value of t voting choices without detection.
- 8.
The SHA-256 hash and the AES-128 symmetric encryption algorithms are used in the implementation made in Neuchâtel.
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We are thankful to the comments and suggestions made by the anonymous reviewers.
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Galindo, D., Guasch, S., Puiggalí, J. (2015). 2015 Neuchâtel’s Cast-as-Intended Verification Mechanism. In: Haenni, R., Koenig, R., Wikström, D. (eds) E-Voting and Identity. Vote-ID 2015. Lecture Notes in Computer Science(), vol 9269. Springer, Cham. https://doi.org/10.1007/978-3-319-22270-7_1
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