TRIP: Coercion-resistant Registration for E-Voting with Verifiability and Usability in Votegral

TL;DR

This work tackles coercion resistance in online voting by combining a novel registration component (TRIP) with a coercion-resistant, end-to-end verifiable voting architecture (Votegral). TRIP enables voters to obtain verifiable real and indistinguishable fake credentials printed on paper in a privacy booth, using interactive zero-knowledge proofs embedded in the credentials to provide verifiability without enabling coercers to prove real credentials. The paper provides formal security arguments for coercion resistance and individual verifiability, documents an implementation and hardware-agnostic performance evaluation, and presents usability studies showing competitive ease of use. The results indicate that Votegral can operate with practical latency, scale to real-world sizes, and offer measurable usability, making coercion-resistant online voting more feasible for large elections while acknowledging limitations and policy considerations for cross-channel deployments.

Abstract

Online voting is convenient and flexible, but amplifies the risks of voter coercion and vote buying. One promising mitigation strategy enables voters to give a coercer fake voting credentials, which silently cast votes that do not count. Current systems along these lines make problematic assumptions about credential issuance, however, such as strong trust in a registrar and/or in voter-controlled hardware, or expecting voters to interact with multiple registrars. Votegral is the first coercion-resistant voting architecture that leverages the physical security of in-person registration to address these credential-issuance challenges, amortizing the convenience costs of in-person registration by reusing credentials across successive elections. Votegral's registration component, TRIP, gives voters a kiosk in a privacy booth with which to print real and fake credentials on paper, eliminating dependence on trusted hardware in credential issuance. The voter learns and can verify in the privacy booth which credential is real, but real and fake credentials thereafter appear indistinguishable to others. Only voters actually under coercion, a hopefully-rare case, need to trust the kiosk. To achieve verifiability, each paper credential encodes an interactive zero-knowledge proof, which is sound in real credentials but unsound in fake credentials. Voters observe the difference in the order of printing steps, but need not understand the technical details. Experimental results with our prototype suggest that Votegral is practical and sufficiently scalable for real-world elections. User-visible latency of credential issuance in TRIP is at most 19.7 seconds even on resource-constrained kiosk hardware. A companion usability study indicates that TRIP's usability is competitive with other e-voting systems, and formal proofs support TRIP's combination of coercion-resistance and verifiability.

Figures (19)

• Figure 1: Voter-registration workflow in the TRIP architecture In this workflow, the voter (1) authenticates to an official at a check-in desk to obtain a check-in ticket, (2) enters a supervised private environment to create a real and any potential fake credentials, (3) presents one of their credentials to an official at the check-out desk, and (4) activates their real credential on a device they trust.
• Figure 2: Registration receipt and envelope design A paper credential consists of an envelope (a) and printed receipt (b). Voters mark envelopes to discern their real credential from fake ones. They transport each credential by placing the receipt in the envelope (c), and activate it on their devices by lifting the receipt one-third of its length (d).
• Figure 3: Votegral protocol operation summary
• Figure 4: Execution latency: (L) represents resource-constrained devices while (H) represents resource-abundant devices.
• Figure 5: Comparing Phase Execution across Voting Systems. Swiss Post is end-to-end verifiable but not coercion resistant. VoteAgain is coercion resistant via deniable re-voting. Civitas implements the JCJ system with coercion resistance via fake credentials. Due to its quadratic time complexity, we extrapolate Civitas' latency beyond $10^4$ voters. Each system uses four shufflers. As this parameter was not configurable in VoteAgain, we ran its mixing and shuffling processes four times.

Theorems & Definitions (10)

• Definition 1: Coercion-resistance
• Theorem 2: Coercion-resistance
• proof
• Theorem 3
• Lemma 4
• Lemma 5
• Lemma 6
• Theorem 7
• Lemma 8
• Lemma 9