本文讨论由多个线性反馈移位寄存器,通过非线性组合逻辑产生的二元序列作为密钥流时的破译难度问题。文中给出了衡量这种二元序列“复杂度”的一个特征,进而指出使这种非线性生成器序列增大破译难度所应满足的条件。针对组合函数,文中给出了线性统计独立性的概念。运用Walsh函数和频谱分析方法研究这一问题,是本文的初次尝试。 This article discusses the difficulty of deciphering when a binary sequence generated by nonlinear combinational logic is used as a key stream by multiple linear feedback shift registers. In this paper, we give a measure of the “complexity” of this binary sequence, and then point out the conditions that must be met to make this kind of nonlinear generator sequence more difficult to decipher. For combinatorial functions, the concept of linear statistical independence is given. The use of Walsh function and spectral analysis of this issue, is the first attempt of this article.