MD5是让大容量信息在用数字签名软件签署私人密匙前被"压缩"成一种保密的格式(就是把一个任意长度的字节串变换成一定长的大整数)。不管是MD2、MD4还是MD5，它们都需要获得一个随机长度的信息并产生一个128位的信息摘要。虽然这些算法的结构或多或少有些相似，但MD2的设计与MD4和MD5完全不同，那是因为MD2是为8位机器做过设计优化的，而MD4和MD5却是面向32位的电脑。(MD5 allows large capacity information to be compressed into a secret format before signing a private key with a digital signature software (that is, converting a string of any length into a large integer of a certain length). Whether it's MD2, MD4, or MD5, they all need to get a random length of information and produce a 128 bit summary of the information. Although these algorithms are more or less similar in structure, the design of MD2 is quite different from that of MD4 and MD5, because MD2 has been designed for 8 bit machines, while MD4 and MD5 are 32 bit computers.)