Power analysis attacks pose a serious threat to the security of many trusted systems. The principle of power analysis attacks is based on the assumption that the power consumption of an electronic device is proportional to the Hamming weight (HW) of the data being processed. However, this power model is defective as it is deviated from the CMOS circuit power consumption theory where power consumption is largely dependent on the switching ability, i.e., the Hamming distance (HD) of the data being processed. This paper presents an HD power analysis model which emulates Hamming distance based on probability distribution of HW, thus conjointly achieving better feasibility and accuracy. The experiment of CPA analysis on smart card chips running DES (Data Encryption Standard) and AES (Advanced Encryption Standard) encryption demonstrates that the proposed model can achieve 10% ∼ 18% better results compared to the existing HW model, which suggests higher success rate of discerning the secret key from the trusted systems. Copyright © 2010 John Wiley & Sons, Ltd.