Proposed in this paper is a side-channel attack method based on chosen plaintext-timing difference.This method,which is called chosen plaintext-timing attack for short and is adopted to evaluate the security of cryptography algorithms,extracts the secret key by selecting a few appropriate plaintexts which are sent to the cryptosystem and by measuring the execution time of the corresponding code,and uses the secret key to attack block cryptosystem and public cryptosystem.Moreover,DES block cipher algorithm is taken as an example to explain the implementation of the chosen plaintext-timing attack.Experimental results on a microcontroller unit indicate that the proposed attack method is effective and practical.In addition,the reason for the execution time diffe-rence existing in cryptosystem implementation is analyzed,and the corresponding countermeasures are presented.