实现数字签名，首先用MD5对要发送的文件进行摘要，产生MD5码，然后将文件和摘要一同发送给对方进行验证，若两次的摘要相同，则说明文件在发送过程中没有被篡改，反之，篡改了。-Realize digital signature, first for sending in MD5 documents are the, produce the MD5 yards, then will file and the send to each other together for verification, i