摘要： 分析了由Schneier提出的FCSR和线性反馈移位寄存器（LFSR）相结合的密钥流生成器的结构特性，给出了其可生成密钥流的周期和线性复杂度的理论上界，讨论如何选择LFSR和FCSR的参数以使产生的密钥流具有较好的伪随机特性，并使其周期和线性复杂度尽可能接近理论上界。利用美国技术与标准局（NIST）提供的STS软件包进行生成器选定参数下输出的密钥流的8项随机性测试，结果表明，在该文论述的参数选择方法下，生成器产生的序列具有良好的伪随机特性。利用FPGA实现了该密钥流生成器，并通过与5种现有流密码方案实现结果的性能比较发现，该方案具有较高的密钥流吞吐量和性价比，可在移动终端实施。

关键词: 带进位反馈移位寄存器, 线性移位寄存器, 2-adic复杂度, 线性复杂度, 随机性检测

Abstract: A novel stream cipher based on the combination of FCSR and LFSR is proposed by Schneier, which is paid close attention by researchers. In this paper, the properties of this stream cipher are analyzed and the theoretical upper bound of period and that of linear complexity are presented. Then, how to select the parameters of FCSR and LFSR is discussed so that the output sequences can access the theory up bound as much as possible. Meanwhile, the pseudorandom properties of generated sequence are checked by eight tests in NIST STS package. According to the testing results, the generated sequences have good pseudorandom properties if the parameters are selected as the proposed rule. The stream cipher is realized in FPGA and compared with the implementation result of other stream ciphers, which proves this stream cipher is very efficient and can be employed in mobile equipment.

Key words: FCSR, LFSR, 2-adic span, Linear span, Test of pseudo-randomness