Abstract: Two key indicators to descript randomness of cipher chip power leakage were obtained through a large number of engineering experiments:one is the distribution law of gate-level flip number,another one is the transition matrix of the number of gate-level flip．Based on the two key indicators,we introduced the concept of entropy rate in information theory．Through the entropy rate,we can dynamicly measure the speed of the entropy increasing about the power consumption waveform in the encryption process,and effectively measure the defensive performance of the cipher chip under SPA attack.

Key words: SPA attack,Entropy rate,Quantify of the defence capability,Information security