Paper 2025/1321

Threshold Receipt-Free Voting with Server-Side Vote Validation

Abstract

Proving the validity of ballots is a central element of verifiable elections. Such proofs can however create challenges when one desires to make a protocol receipt-free. We explore the challenges raised by validity proofs in the context of protocols where threshold receipt-freeness is obtained by secret sharing an encryption of a vote between multiple authorities. In such contexts, previous solutions verified the validity of votes by decrypting them after passing them through a mix-net. This approach however creates subtle privacy risks, especially when invalid votes leak structural patterns that threaten receipt-freeness. We propose a different approach of threshold receipt-free voting in which authorities re-randomize ballot shares then jointly compute a ZK proof of ballot validity before letting the ballots enter a (possibly homomorphic) tallying phase. Our approach keeps the voter computational costs limited while offering verifiability and improving the ballot privacy of previous solutions. We present two protocols that enable a group of servers to verify and publicly prove that encrypted votes satisfy some validity properties: Minimix, which preserves prior voter-side behavior with minimal overhead, and Homorand, which requires voters to submit auxiliary data to facilitate validation over large vote domains. We show how to use our two protocols within a threshold receipt-free voting framework. We provide formal security proofs and efficiency analyses to illustrate trade-offs in our designs.