Design of a Masked S-Box for SM4 Based on Composite Field

This paper propose a new masking scheme for SM4 s-box based on composite field. Through isomorphism bit matrices, we simplify the calculation by changing finite field inversion from GF(28) toGF(((22)2)2) to reduce the computational difficulty. We carefully modify the inversion to ensure every intermediate value is masked during the process. The theoretical analysis and simulated CPA proves the effectiveness of this method. Thus our method can eliminate the need to pre-compute the s-box every time when the mask is updated, as a result, saves a lot of time and storage room. This method is suitable for implementations with limited resources such as smart cards.