Side channel attack (SCA) has the problems of high sample quantity and long analysis time. Noise of side channel leakage and its differential reduction method are researched combined with the architecture of micro control unit (MCU). Signal difference variability and hamming weight difference variability are defined, whose inverse proportion is checked. Method of key analysis based on difference variability is proposed, which uses its change property in encrypt process. Data encryption standard (DES) is cracked in experiment, which only needs 150 power tracks and 1.03 s analysis time. The attack method can be extended to crack other block ciphers which implemented in general purpose MCU.%针对旁路攻击方法存在的样本量大、分析时间长等问题，结合微控制器的系统结构，分析了旁路泄漏信号的噪声来源及其差分抑制方法；定义了信号差异度和汉明重量差异度，分析了二者间的反比映射关系；利用加密过程中差异度的变化特征，提出了基于差异度的密钥分析方法；以 DES 密码算法为验证目标，仅用150组功耗轨迹，分析用时1.03 s破解了密钥，可推广应用于以通用微控制器作为实现载体的其他分组密码系统。
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