Thwarting Vote Buying Through Decoy Ballots

There is increasing interest in promoting participa- tory democracy, in particular by allowing voting by mail or internet and through random-sample elec- tions. A pernicious concern, though, is that of vote buying, which occurs when a bad actor seeks to buy ballots, paying someone to vote against their own intent. This becomes possible whenever a voter is able to sell evidence of which way she voted. We show how to thwart vote buying through de- coy ballots, which are not counted but are indistin- guishable from real ballots to a buyer. We show that an Election Authority can significantly reduce the power of vote buying through a small number of optimally distributed decoys, and model societal processes by which decoys could be distributed.