Two factor authentication using Authenticator apps is getting more popular. One advantage of Authenticator 2FA is that you don't need to use a service nor do users have to provide additional bits of personal information. It's easy to implement, doesn't cost anything and also very secure as it uses one-time codes that can't easily be corrupted short of physical take over of a device. In this article I describe how Authenticator based 2FA works in the context of an application without using ASP.NET Identity.