Abstract: The personal identification number (PIN) and encryption technologies are commonly used to form dual protection mechanism for smart cards.In this paper, a novel power analysis attack with a cut off power technique against smart cards is proposed.This attack can bypass the restriction on the times of PIN inputs to obtain it through the exhaustive search.And the differential power analysis experiment on DES implemented in an exemplary smart card showed that the secret key of DES was recovered successfully with 40000 plaintext samples.