Nowadays, a user authentication is very important in network environments. For safe authentication, they came up with six essential conditions in earlier studies. And a variety of mechanisms is presented by research scientists. However, they could not achieve the PFS. Because, though all these schemes are assumed that the communication between a smart card and a host is safe, actually it is not. Therefore, in this paper, we will point out what the communication between a smart card and a host is not safe, and propose a new user authentication mechanism that can reach to the PFS. And also, an encryption algorithm is used about 45% less than earlier studies in our proposed scheme. Thus, we can say that enhance the efficiency.