## Cryptology ePrint Archive: Report 2020/1334

Compact Simulation-Sound NIZK Arguments of Composite Residuosity and Applications to Logarithmic-Size Ring Signatures

Benoît Libert and Khoa Nguyen and Thomas Peters and Moti Yung

Abstract: The standard model security of the Fiat-Shamir transform has been an active research area for many years. In breakthrough results, Canetti {\it et al.} (STOC'19) and Peikert-Shiehian (Crypto'19) showed that, under the Learning-With-Errors ($\LWE{}$) assumption, it provides soundness by applying correlation-intractable (CI) hash functions to so-called trapdoor $\Sigma$-protocols. In order to be compatible with CI hash functions based on standard LWE assumptions with polynomial approximation factors, all known such protocols have been obtained via parallel repetitions of a basic protocol with binary challenges. In this paper, we consider languages related to Paillier's composite residuosity assumption (DCR) for which we give the first trapdoor $\Sigma$-protocols providing soundness in one shot, via exponentially large challenge spaces. This improvement is analogous to the one enabled by Schnorr over the original Fiat-Shamir protocol in the random oracle model. Using the correlation-intractable hash function paradigm, we then obtain simulation-sound NIZK arguments showing that an element of $\mathbb{Z}_{N^2}^\ast$ is a composite residue. As a main application, we build logarithmic-size ring signatures (assuming a common reference string) which yield the shortest signature length among schemes based on standard assumptions in the standard model. We prove security under the DCR and LWE assumptions, while keeping the signature size comparable with that of random-oracle-based schemes.

Category / Keywords: public-key cryptography / NIZK arguments, composite residuosity, Fiat-Shamir, ring signatures, anonymity, standard model