MD5算法是一种消息摘要算法(Message Digest Algorithm)，此算法以任意长度的信息(message) 作为输入进行计算，产生一个128-bit(16-byte)的指纹或报文摘要(fingerprint or message digest)。两个不同的message产生相同message digest的几率相当小，从一个给定的message digest逆向产生原始message更是困难(不过也有可能从message digest构造（message)，因此MD5算法适合用在数字签名应用中。MD5实现简单，在32位的机器上运行速度也相当快，实际应用也不仅仅局限于数字签名。-MD5 algorithm is a message digest algorithm (Message Digest Algorithm), this algorithm is an arbitrary length message (message) is calculated as input to produce a 128-bit (16-byte) of the fingerprint or message digest (fingerprint or message digest .) Two different message likely to produce the same message digest is quite small, from a given message digest produced reverse the original message is even more difficult (but may also be constructed from the message digest (message), so use the MD5 algorithm is suitable for applications in digital signature . MD5 simple, in the 32-bit machine running fairly fast, practical applications are not limited to digital signatures.