Based on the idea of linear approximation

this paper designs a genetic algorithm fitness function model of Hamming distance

in which we have the quantitative experiments of the genetic algorithm analysis on part of the DES keys in the 1st to the 6th round under different fixed bits lengths.The search of experimental result can acquire 21 bits unknown key of 6-round DES.Furthermore

the experimental results show that the designing of fitness function and the security of cryptanalysis’ object are important factor affecting the effect of the evolutionary cryptanalysis.Because of the intrinsic randomicity or probability for the evolutionary technology

there exist different for the running time and different success rates of acquiring results in plenty of experiments.Further test indicate that the better results can be acquired by increasing population size

the running generation or the amount of the plaintext-ciphertext pairs to some extent.