密码S盒的一种新自动搜索方法

密码S盒是许多对称密码算法的核心部件，通常决定算法的安全强度.如何设计并确保密码S盒具有一定能力抵御侧信道攻击(如功耗攻击)一直是业界研究的难点.在密码S盒的设计中，除了传统的代数构造外，采用自动搜索工具(如元胞自动机(cellular automata, CA))进行搜索设计也是当前研究热点之一.基于CA规则，采用变元分量部分固定和分别搜索的策略，提出了一种S盒新搜索方法.研究结果表明：更多的4×4最优S盒被发现，实现S盒的扩展；特别地，该方法还可以将CA规则下3类4×4次优S盒转化为4×4最优S盒.与已有结果相比较，新发现的4×4最优S盒具有较低透明阶等优点，能更好地抵御侧信道攻击.

A New Automatic Search Method for Cryptographic S-Box

The cryptographic S-boxes are core component in too many symmetric encryption algorithms, which usually determine the security strength of these algorithms. The secure evaluation indicators for these cryptographic S-boxes contain balancedness, algebraic degree, nonlinearity, and differential uniformity etc. How to design the cryptographic S-boxes that have some robust abilities (indicators) against both the traditional attacks and the side channel attacks such as power attacks appears to be a rather difficult task. Currently, the automatic search tools, such as CA(cellular automata), neural network, etc, have became the research hotspots regarding to the design of the cryptographic S-box, except to the classical algebraic construction. Based on the CA rules, a new search method for S-box is proposed, which uses the strategy of partial fixed and separate searching for the variable components. More specifically, in the first place, the features of CA rules of this method is described. Moreover, the strategy of partial fixed and separate searching for the variable components according to the properties of cryptographic S-boxes is constructed. Finally, some new S-boxes are achieved and their features of these S-boxes are also evaluated. It is shown that too many 4×4 optimal S-boxes are attained. In particular, three classes of 4×4 sub-optimal S-boxes can also be transformed to some 4×4 optimal S-boxes under the CA rules of this method. Compared with the previous well-known results, these new 4×4 optimal S-boxes have lower transparency order so that they have a robuster ability against side channel attacks.