如何在ASP中实现自定义认证。净MVC 5

I'm developing an ASP.NET MVC 5 application. I have an existing DB, from which I created my ADO.NET Entity Data Model. I have a table in that DB which contains "username" and "password" column, and I want to use them to implement authentication and authorization in my Webapp; I cannot create any other database or table or column and I cannot use the standard Identity authentication, because of customer's requirements. I don't need to manage signup, password changing or other stuffs: just login with password and username. How can I do that? I'm developing an ASP.NET MVC 5 application. I