  Voting mechanisms are widely accepted and used methods for decentralized
decision-making. Ensuring the acceptance of the voting mechanism's outcome is a
crucial characteristic of robust voting systems. Consider this scenario: A
group of individuals wants to choose an option from a set of alternatives
without requiring an identification or proof-of-personhood system. Moreover,
they want to implement utilitarianism as their selection criteria. In such a
case, players could submit votes multiple times using dummy accounts, commonly
known as a Sybil attack (SA), which presents a challenge for decentralized
organizations. Is there a voting mechanism that always prevents players from
benefiting by casting votes multiple times (SA-proof) while also selecting the
alternative that maximizes the added valuations of all players (efficient)?
One-person-one-vote is neither SA-proof nor efficient. Coin voting is SA-proof
but not efficient. Quadratic voting is efficient but not SA-proof. This study
uses Bayesian mechanism design to propose a solution. The mechanism's structure
is as follows: Players make wealth deposits to indicate the strength of their
preference for each alternative. Each player then receives an amount based on
their deposit and the voting outcome. The proposed mechanism relies on two main
concepts: 1) Transfers are influenced by the outcome in a way that each
player's optimal action depends only on individual preferences and the number
of alternatives; 2) A player who votes through multiple accounts slightly
reduces the expected utility of all players more than the individual benefit
gained. This study demonstrates that if players are risk-neutral and each
player has private information about their preferences and beliefs, then the
mechanism is SA-proof and efficient. This research provides new insights into
the design of more robust decentralized decision-making mechanisms.
