On the Necessity of Auditing for Election Privacy in e-Voting Systems

The importance of voter auditing in order to ensure election integrity has been extensively studied in the e-voting literature. On the other hand, the necessity of auditing to protect voter privacy in an e-voting system has been mostly overlooked. In this work, we investigate election privacy issues that appear in the state-of-the-art implementations of e-voting systems that apply threshold public key encryption (TPKE) in the client like Helios and use a bulletin board (BB). More specifically, we show that without PKI support or -more generally- authenticated BB “append” operations, such systems are vulnerable to attacks where the malicious election server can act as a man-in-the-middle between the election trustees and the voters, hence it can learn how the voters have voted. We suggest compulsory trustee auditing as countermeasure for this type of man-in-the-middle attacks. Furthermore, we propose a list of guidelines to avoid some common, subtle, yet important problems that may appear during the implementation of any TPKE-based e-voting system.