Paper 2021/066

A Deep Learning Approach for Active S-box Prediction of Lightweight Generalized Feistel Block Ciphers

Mohamed Fadl Idris and Je Sen Teh and Jasy Liew Suet Yan and Wei-Zhu Yeoh

Abstract

Block cipher resistance against differential cryptanalysis is commonly assessed by counting the number of active substitution boxes (S-boxes) using search algorithms or mathematical solvers that incur high computational costs. In this paper, we propose an alternative approach using deep neural networks to predict the number of active S-boxes, trading off exactness for real-time efficiency as the bulk of computational work is brought over to pre-processing (training). Active S-box prediction is framed as a regression task whereby neural networks are trained using features such as input and output differences, number of rounds, and permutation pattern. We first investigate the feasibility of the proposed approach by applying it on a reduced (4-branch) generalized Feistel structure (GFS) cipher. Apart from optimizing a neural network architecture for the task, we also explore the impact of each feature and its representation on prediction error. We then extend the idea to 64-bit GFS ciphers by first training neural networks using data from five different ciphers before using them to predict the number of active S-boxes for TWINE, a lightweight block cipher. The best performing model achieved the lowest root mean square error of 1.62 and R$^2$ of 0.87, depicting the feasibility of the proposed approach.

Note: This paper is currently not under consideration for publication in any journal at the moment of uploading the latest version of the preprint.