International Association
for Cryptologic Research

A transciphering framework, also known as hybrid homomorphic encryption, is a practical method of combining a homomorphic encryption (HE) scheme with a symmetric cipher in the client-server model to reduce computational and communication overload on the client side. As a server homomorphically evaluates a symmetric cipher in this framework, new design rationales are required for “HE-friendly” ciphers that take into account the specific properties of the HE schemes. In this paper, we propose a new TFHE-friendly cipher, dubbed FRAST, with a TFHE-friendly round function based on a random S-box to minimize the number of rounds. The round function of FRAST can be efficiently evaluated in TFHE by a new optimization technique, dubbed double blind rotation. Combined with our new WoP-PBS method, the double blind rotation allows computing multiple S-box calls in the round function of FRAST at the cost of a single S-box call. In this way, FRAST enjoys 2.768 (resp. 10.57) times higher throughput compared to Kreyvium (resp. Elisabeth) for TFHE keystream evaluation in the offline phase of the transciphering framework at the cost of slightly larger communication overload.

This paper studies the provable security of the deterministic random bit generator~(DRBG) utilized in Linux 6.4.8, marking the first analysis of Linux-DRBG from a provable security perspective since its substantial structural changes in Linux 4 and Linux 5.17. Specifically, we prove its security up to $O(\min\{2^{\frac{n}{2}},2^{\frac{\lambda}{2}}\})$ queries in the seedless robustness model, where $n$ is the output size of the internal primitives and $\lambda$ is the min-entropy of the entropy source. Our result implies $128$-bit security given $n=256$ and $\lambda=256$ for Linux-DRBG. We also present two distinguishing attacks using $O(2^{\frac{n}{2}})$ and $O (2^{\frac{\lambda}{2}})$ queries, respectively, proving the tightness of our security bound.