Paper 2018/567

On the Security Properties of e-Voting Bulletin Boards

Aggelos Kiayias, Annabell Kuldmaa, Helger Lipmaa, Janno Siim, and Thomas Zacharias

Abstract

In state-of-the-art e-voting systems, a bulletin board (BB) is a critical component for preserving election integrity and availability. Although it is common in the literature to assume that a BB is a centralized entity that is trusted, in the recent works of Culnane and Schneider [CSF 2014] and Chondros et al. [ICDCS 2016], the importance of removing BB as a single point of failure has been extensively discussed. Motivated by these works, we introduce a framework for the formal security analysis of the BB functionality modeled as a distributed system that comprises (i) a subsystem of item collection (IC) peers that receive and store the submitted items, and (ii) a subsystem of audit board (AB) peers, where the IC subsystem publishes the stored items for verification. Our framework treats a secure BB as a robust public transaction ledger, defined by Garay et al. [Eurocrypt 2015], that additionally supports the generation of receipts for successful posting. Namely, in our model, a secure BB system achieves Persistence and Liveness that are confirmable, in the sense that any malicious behavior of the BB system can be detected via a verification mechanism. As a case study for our framework, we analyze the BB system of Culnane and Schneider and point out its weaknesses. We demonstrate an attack revealing that the said system does not achieve Confirmable Liveness, even in the case where the adversary is computationally bounded and covert, i.e., it may deviate from the protocol specification but does not want to be detected. In addition, we show that special care should be taken for the choice of the underlying cryptographic primitives, so that the claimed fault tolerance threshold of N/3 out-of N corrupted IC peers is preserved. Next, based on our analysis, we introduce a new BB protocol that upgrades the [CSF 2014] protocol. We prove that it tolerates any number less than N/3 out-of N corrupted IC peers both for Persistence and Confirmable Liveness, against a computationally bounded general Byzantine adversary. Furthermore, Persistence can also be Confirmable, if we distribute the AB (originally a centralized entity in [CSF 2014]) as a replicated service with honest majority.