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This paper analyzes the Loki e-voting protocol, which aims to improve coercion resistance and usability compared to prior work. It identifies two vulnerabilities: a brute-force attack compromising the evasion strategy and a forced abstention attack. The authors generalize the integrity attack to demonstrate that pre-agreed secret credentials are necessary to prevent last-minute coercion in e-voting systems, and propose a fix involving reverting to such credentials.
Loki, the supposedly coercion-resistant e-voting protocol, crumbles under brute-force and forced abstention attacks, revealing a fundamental need for pre-agreed secrets to prevent last-minute coercion.
Coercion-resistance (CR) is a crucial security property in e-voting systems. It ensures that an attacker cannot compel a voter to vote in a specific way by using threats or rewards. The Loki e-voting protocol, proposed by Giustolisi \emph{et al.} at IEEE S\&P (2024), introduces a novel design that mitigates last-minute coercion through a re-voting mechanism. It also aims to address the usability issues of the seminal JCJ e-voting protocol, specifically: i) the requirement that voters can store and hide pre-agreed credentials, and ii) the ability of voters to convincingly lie while being coerced. In this work, we identify two vulnerabilities in Loki. The first is a brute-force attack that compromises the integrity of the evasion strategy. Specifically, this attack allows an adversary to cast a ballot on behalf of their victim in a way that the evasion strategy cannot defend against, rendering it ineffective. The second vulnerability is a forced abstention attack, which allows an adversary to detect when their victim has complied with their instruction not to vote. We generalise the integrity attack to reveal a fundamental dilemma: without pre-agreed secret credentials, it is not possible to prevent last-minute coercion. Finally, we show how reverting to pre-agreed secret credentials fixes the aforementioned vulnerabilities and discuss the trade-off between tallying efficiency and stronger trust assumptions.