Vote Validatability in Mix-Net-Based eVoting

One way to build secure electronic voting systems is to use Mix-Nets, which break any correlation between voters and their votes. One of the characteristics of Mix-Net-based eVoting is that ballots are usually decrypted individually and, as a consequence, invalid votes can be detected during the tallying of the election. In particular, this means that the ballot does not need to contain a proof of the vote being valid. However, allowing for invalid votes to be detected only during the tallying of the election can have bad consequences on the reputation of the election. First, casting a ballot for an invalid vote might be considered as an attack against the eVoting system by non-technical people, who might expect that the system does not accept such ballots. Besides, it would be impossible to track the attacker due to the anonymity provided by the Mix-Net. Second, if a ballot for an invalid vote is produced by a software bug, it might be only detected after the election period has finished. In particular, voters would not be able to cast a valid vote again. In this work we formalize the concept of having a system that detects invalid votes during the election period. In addition, we give a general construction of an eVoting system satisfying such property and an efficient concrete instantiation based on well-studied assumptions.