International Association for Cryptologic Research

International Association
for Cryptologic Research


Jan Bobolz


A Generic Construction of an Anonymous Reputation System and Instantiations from Lattices
With an anonymous reputation system one can realize the process of rating sellers anonymously in an online shop. While raters can stay anonymous, sellers still have the guarantee that they can only be reviewed by raters who bought their product. We present the first generic construction of a reputation system from basic building blocks, namely digital signatures, encryption schemes, non-interactive zero-knowledge proofs, and linking indistinguishable tags. We then show the security of the reputation system in a strong security model. Among others, we instantiate the generic construction with building blocks based on lattice problems, leading to the first module lattice-based reputation system.
On the Impossibility of Surviving (Iterated) Deletion of Weakly Dominated Strategies in Rational MPC
Rational multiparty computation (rational MPC) provides a framework for analyzing MPC protocols through the lens of game theory. One way to judge whether an MPC protocol is rational is through weak domination: Rational players would not adhere to an MPC protocol if deviating never decreases their utility, but sometimes increases it. Secret reconstruction protocols are of particular importance in this setting because they represent the last phase of most (rational) MPC protocols. We show that most secret reconstruction protocols from the literature are not, in fact, stable with respect to weak domination. Furthermore, we formally prove that (under certain assumptions) it is impossible to design a secret reconstruction protocol which is a Nash equlibrium but not weakly dominated if (1) shares are authenticated or (2) half of all players may form a coalition.