Two-Factor Authentication using Google Authenticator in asp.net mvc

http://www.dotnetawesome.com/2016/04/two-factor-authentication-in-aspnet-mvc.html

ASP.NET Two-Factor Auth with Google's Authenticator App

https://lachlanbarclay.net/2016/02/asp-dot-net-two-factor-auth-with-google-authenticator-app

https://lachlanbarclay.net/2016/02/asp-dot-net-two-factor-auth-with-google-authenticator-app-part-2