Security Proofs for Participation Privacy and Stronger Veri ability for Helios

The Helios voting scheme is well studied including formal proofs for verifiability and ballot privacy, but it does not provide participation privacy (i.e. it reveals who participated in the election). Kulyk, Teague and Volkamer proposed an extension to Helios that is claimed to provide ballot privacy as well as participation privacy while providing stronger verifiability than Helios. However, the authors did not prove their claims. Our contribution is to provide a formal definition for participation privacy and to prove that their claims hold.