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Title:
\mathsf{FRAST}: TFHE-friendly Cipher Based on Random S-boxes
Authors: Mingyu Cho, Woohyuk Chung, Jincheol Ha, Jooyoung Lee, Eun-Gyeol Oh, Mincheol Son
Abstract: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 \mathsf{FRAST}, with a TFHE-friendly round function based on a random S-box to minimize the number of rounds.
The round function of \mathsf{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 \mathsf{FRAST} at the cost of a single S-box call.
In this way, \mathsf{FRAST} enjoys 2.768 (resp. 10.57) times higher throughput compared to \mathsf{Kreyvium} (resp. \mathsf{Elisabeth}) for TFHE keystream evaluation in the offline phase of the transciphering framework at the cost of slightly larger communication overload.
ePrint: https://eprint.iacr.org/2024/745
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