Countering Ballot Stuffing and Incorporating Eligibility Verifiability in Helios

Helios is a web-based end-to-end verifiable electronic voting system which has been said to be suitable for low-coercion environments. Although many Internet voting schemes have been proposed in the literature, Helios stands out for its real world relevance. It has been used in a number of elections in university campuses around the world and it has also been used recently by the IACR to elect its board members. It was noted that a dishonest server in Helios can stuff ballots and this seems to limit the claims of end-to-end verifiability of the system. In this work, we investigate how the issue of ballot stuffing can be addressed with minimum change to the current vote casting experience in Helios and we argue formally about the security of our techniques. Our ideas are intuitive and general enough to be applied in the context of other Internet voting scheme and they also address recent attacks exploiting the malleability of ballots in Helios

Extending Helios Towards Private Eligibility Verifiability

We show how to extend the Helios voting system to provide eligibility verifiability without revealing who voted which we call private eligibility verifiability. The main idea is that real votes are hidden in a crowd of null votes that are cast by others but are indistinguishable from those of the eligible voter. This extended Helios scheme also improves Helios towards receipt-freeness

Extending the Helios Internet Voting Scheme Towards New Election Settings

Internet voting has long been a topic both of public discussion and also of scientific research. While the introduction of Internet voting may bring many advantages, it is further important to ensure an adequate level of security of the systems and underlying schemes that are used for casting and tallying the votes in order to encourage faith and acceptance for this relatively new way of voting. A number of cryptographic schemes have been proposed, that enable secure Internet voting. One of the most established and well-researched solutions is the Helios scheme, which is also implemented as an open-source system. Both its implementation and the scheme behind it has been extensively studied in the literature, and the Helios system has been used for numerous elections in practice, such as the IACR elections. However, there are election settings for which Helios is currently not appropriate, either due to infrastructure demands, required functionality for the voters or assurance of the security requirements. These kinds of election settings could benefit from the advantages that secure Internet voting provides. In this thesis we identify the election settings not currently supported by Helios, propose our extensions for each one of these settings and evaluate their security. Hence, this work describes four Internet voting schemes that are build upon Helios, with each scheme developed towards a specific setting. The first scheme presented here enables elections within the so-called boardroom voting setting. This setting is characterized by its decentralization, whereby all the tasks within the election are distributively performed by the voters themselves, without the support of a centralized infrastructure. The election in the boardroom voting setting are further conducted in an ad-hoc manner, so that limited time is available for preparation beforehand. We propose an extension of Helios that distributes the tasks of the voting system components in Helios among the voters. For this, we use cryptographic primitives such as decentralized key exchange with short authentication strings, distributed secret sharing and distributed decryption and Byzantine agreement. The second scheme extends Helios with proxy voting functionality. Proxy voting, as a newly emerged form of voting, enables the voter to delegate her voting right in the election to a trusted third-party, the so-called proxy, who is authorized to vote on the voter's behalf. This extension facilitates such delegation while assuring the security for delegating voters and for the proxies and preserves the security guarantees provided by Helios for the voters who vote directly (instead of delegating). For ensuring the security of our extension, we introduce the so-called delegation credentials that are assigned to the voters and are used to compute anonymized delegation tokens sent to the proxies to enable delegation. We further use cryptographic primitives such as proofs of knowledge and signatures of knowledge. The third scheme combines the first two settings to extend Helios towards the proxy boardroom voting setting, namely, a setting in which the elections are performed in a decentralized way as in boardroom voting, yet the voters who cannot participate in the election themselves are allowed to delegate their voting right to a trusted proxy before the election. The security of our extension is assured with threshold secret sharing and Pedersen commitments. The fourth scheme extends Helios by improving its security. As such, it introduces participation privacy, meaning that the voting system does not reveal which voters have participated in the election, while supporting verification that only the eligible voters have cast their ballots in the election. The extension furthermore introduces receipt-freeness, ensuring that the voter cannot create a receipt that proves to a third party how she voted, thus preventing vote selling. To ensure the security of the extension, a new kind of entity is introduced, the posting trustee, and a new kind of ballot, the so-called dummy ballot that is indistinguishable from a normal ballot cast by the voter, but does not modify the election result. We furthermore use disjunctive zero-knowledge proofs and proofs of signature knowledge to prove, that a sender of a particular ballot knows the private signature key of an eligible voter, or that the ballot is a dummy ballot. For each one of the extensions, the security model is provided, which describes the security requirements and the assumptions that are necessary for ensuring the security requirements (i.e. vote privacy or vote integrity), is provided. For the first three extensions, the security model is used as a base for the informal security evaluation, in which an informal argument is used to show, that the security requirements hold under the described assumptions. Conducting a formal security evaluation for these extensions is considered an important part of the future work, in which new formal definitions have to be developed. For the fourth extension, we provide a formal security analysis that relies on the formal definitions for the security requirements of vote privacy, vote integrity and eligibility, available in the literature. We furthermore introduce new formal definitions for participation privacy, receipt-freeness and fairness, which we also use for the formal proofs of our extension

Secure multi party computations for electronic voting

Στην παρούσα εργασία, μελετούμε το πρόβλημα της ηλεκτρονικής ψηφοφορίας. Θεωρούμε ότι είναι έκφανση μιας γενικής διαδικασίας αποφάσεων που μπορεί να υλοποιηθεί μέσω υπολογισμών πολλαπλών οντοτήτων, οι οποίοι πρέπει να ικανοποιούν πολλές και αντικρουόμενες απαιτήσεις ασφαλείας. Έτσι μελετούμε σχετικές προσεγγίσεις οι οποίες βασιζονται σε κρυπτογραφικές τεχνικές, όπως τα ομομορφικά κρυπτοσυστήματα, τα δίκτυα μίξης και οι τυφλές υπογραφές. Αναλύουμε πώς προσφέρουν ακεραιότητα και ιδιωτικότητα (μυστικότητα) στην διαδικασία και την σχέση τους με την αποδοτικότητα. Εξετάζουμε τα είδη λειτουργιών κοινωνικής επιλογής που μπορούν να υποστηρίξουν και παρέχουμε δύο υλοποιήσεις. Επιπλέον ασχολούμαστε με την αντιμετώπιση ισχυρότερων αντιπάλων μη παρέχοντας αποδείξεις ψήφου ή προσφέροντας δυνατότητες αντίστασης στον εξαναγκασμό. Με βάση την τελευταία έννοια προτείνουμε μια τροποποίηση σε ένα ευρέως χρησιμοποιούμενο πρωτόκολλο. Τέλος μελετούμε δύο γνωστές υλοποιήσεις συστημάτων ηλεκτρονικής ψηφοφοριας το Helios και το Pret a Voter .In this thesis, we study the problem of electronic voting as a general decision making process that can be implemented using multi party computations, fulfilling strict and often conflicting security requirements. To this end, we review relevant cryptographic techniques and their combinations to form voting protocols. More specifically, we analyze schemes based on homomorphic cryptosystems, mixnets with proofs of shuffles and blind signatures. We analyze how they achieve integrity and privacy in the voting process, while keeping efficiency. We examine the types of social choice functions that can be supported by each protocol. We provide two proof of concept implementations. Moreover, we review ways to thwart stronger adversaries by adding receipt freeness and coercion resistance to voting systems. We build on the latter concept to propose a modification to a well known protocol. Finally, we study two actual e-Voting implementations namely Helios and Pret a Voter