MD5就是采用单向加密的加密算法，所谓单向加密就是只能对数据进行加密，也就是说，没有办法对加密以后的数据进行解密。在实际中的一个应用就是数据库中的用户信息加密，当用户创建一个新的账号或者密码，他的信息不是直接保存到数据库，而是经过一次加密以后再保存，这样，即使这些信息被泄露，也不能立即理解这些信息的真正含义。

-MD5 encryption is one-way encryption algorithm, the so-called one-way encryption is only for data encryption, In other words, there is no way to encrypt the data after decryption. In the actual application is a database of user information encryption, when users create a new account or password, his message is not stored directly to the database, but after a subsequent preservation of encryption, so that even if such information was compromised, and can not immediately understand the true meaning of the information.