Paper 2025/951 Original: eprint.iacr.org/2025/951 ↗

Enhancing Provable Security and Efficiency of Permutation-based DRBGs

Abstract

We revisit the security analysis of the permutation-based deterministic random bit generator~(DRBG) discussed by Coretti et al. at CRYPTO 2019. Specifically, we prove that their construction, based on the sponge construction, and hence called Sponge-DRBG in this paper, is secure up to $O\left(\min \left\{2^{\frac{c}{2}}, 2^{\frac{\lambda}{2}}\right\}\right)$ queries in the seedless robustness model, where $\lambda$ is the required min-entropy and $c$ is the sponge capacity. This significantly improves the provable security bound from the existing $O\left(\min \left\{2^{\frac{c}{3}}, 2^{\frac{\lambda}{2}}\right\}\right)$ to the birthday bound. We also show that our bound is tight by giving matching attacks. As the Multi-Extraction game-based reduction proposed by Chung et al. at Asiacrypt 2024 is not applicable to Sponge-DRBG in a straightforward manner, we further refine and generalize the proof technique so that it can be applied to a broader class of DRBGs to improve their provable security. We also propose a new permutation-based DRBG, dubbed POSDRBG, with almost the optimal output rate $1$, outperforming the output rate $\frac{r}{n}$ of Sponge-DRBG, where $n$ is the output size of the underlying permutation and $r=n-c$. We prove that POSDRBG is tightly secure up to $O\left(\min \left\{2^{\frac{c}{2}}, 2^{\frac{\lambda}{2}}\right\}\right)$ queries. Thus, to the best of our knowledge, POSDRBG is the first permutation-based DRBG that achieves the optimal output rate of 1, while maintaining the same level of provable security as Sponge-DRBG in the seedless robustness model.