On 28 August 2020 at 4.15 p.m., in Zoom room, Janno Siim will defend his thesis Non-Interactive Shuffle Arguments for obtaining the degree of Doctor of Philosophy (Computer Science).
Prof. Helger Lipmaa (Institute of Computer Science UT and Simula UiB Norway);
Prof. Eran Tromer (Tel Aviv University, Israel, and Columbia University, USA); Prof. Dario Fiore (IMDEA Software Institute, Spain).
A secure voting system is essential for any democratic government. Online voting has the potential to offer enhanced convenience and lower administrative costs, but this comes with increased security risks and many technical challenges. One such challenge is ballot shuffling that should remove the link between a voter and her vote. Paper-based voting systems achieve this by having someone shake the ballot box. Online voting systems often use a distributed system called a mix-network that lets each server shuffle (permute and rerandomize) the ciphertexts. Assuming that at least one server is not corrupted by an adversary, it will be computationally difficult to trace the initial ciphertexts to the final output of the mix-network. It is paramount that each server also proves that it shuffled correctly to avoid ciphertexts being substituted. Proof should be difficult to forge but at the same time should not reveal how the shuffling was done. Such a proof is called a zero-knowledge shuffle argument. The main contribution of this thesis is an efficient zero-knowledge shuffle argument. On modest hardware, it takes, for example, less than 3 minutes to shuffle, prove, and verify 100,000 ciphertexts. Compared to other efficient shuffle arguments, we avoid (incorrectly) treating hash functions as a random function, but instead, require that a public key of the argument is generated honestly. We propose a secure computation protocol for key generation to distribute trust among multiple parties. Finally, we propose a modification of our initial shuffle argument that is compatible with the secure computation protocol (among other improvements). Since the publication, this argument has been adopted by the Greek e-voting platform Zeus that is widely used for organizational elections.