Implement user login/out without database using Play framework

I am using play framework to develop a web app. Now I need to implement authentication/authorisation without database (really strange requirement). There will only be log in and log out function, no registration. The username/password pair will be authenticated using external service.

Due to limited experience, my current idea is to use token to authenticate and a local file to store username/password.

Is my idea feasible? Is there any recommended libs? If I use token, do I need to pass that token in Http request/response every time and authenticate the token in every controller?

Thanks!