说明：  MD5算法的实现。MD5以512位分组来处理输入的信息，且每一分组又被划分为16个32位子分组，经过了一系列的处理后，算法的输出由四个32位分组组成，将这四个32位分组级联后将生成一个128位散列值。
(MD5 algorithm. MD5 to 512 groups to deal with the importation of information and each group was also divided into 16 groups of 32 seats, After a series of processing, the output from the algorithm 4 32 subgroups, 4 32 this group will generate a cascade 128 hash value.)