International Association
for Cryptologic Research

The universal composability (UC) model provides strong security guarantees for protocols used in arbitrary contexts. While these guarantees are highly desirable, in practice, schemes with a standalone proof of security, such as the Groth16 proof system, are preferred. This is because UC security typically comes with undesirable overhead, sometimes making UC-secure schemes significantly less efficient than their standalone counterparts. We establish the UC security of Groth16 without any significant overhead. In the spirit of global random oracles, we design a global (restricted) observable generic group functionality that models a natural notion of observability: computations that trace back to group elements derived from generators of other sessions are observable. This notion turns out to be surprisingly subtle to formalize. We provide a general framework for proving protocols secure in the presence of global generic groups, which we then apply to Groth16.

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.

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.