Résumé
Threshold encryption schemes provide a common tool to secure a public-key encryption scheme against single point of failure attacks. Despite the success of lattices in building fully-homomorphic and presumably quantum-resistant encryption schemes, the task of thresholdizing those schemes remains challenging. The major bottleneck in the standard approach is the use of statistical noise flooding, leading to a significant efficiency loss and the need of stronger hardness assumptions. Recent works have replaced the heavy statistical noise flooding by a lighter one using the Rényi divergence. The new Rényi noise flooding both improves the efficiency and allows to use weaker hardness assumptions. However, arguing semantic security of lattice-based threshold schemes in the presence of Rényi noise flooding showed to be challenging. Chowdhury et al. (IACR ePrint'22) argued in the fully-homomorphic case that the Rényi divergence directly applies for semantic security by making use of an existing framework called public sampleability. In this work, we argue that their public sampleability framework was neither sufficient nor correctly used. To address both issues, we strengthen the framework and thoroughly apply it to prove semantic security of generic lattice-based threshold encryption constructions. We distinguish between the plain public-key and the fully-homomorphic settings, as different security notions are achieved. As a byproduct, this shows that the proof detour via one-way security made by Boudgoust and Scholl (Asiacrypt'23) was superfluous, now leading to tighter proofs in the standard model.