https://medium.freecodecamp.org/how-to-set-up-two-factor-authentication-on-asp-net-core-using-google-authenticator-4b15d0698ec9

In this article, we are going to learn how to perform two-factor authentication in an ASP.NET Core application using the Google Authenticator app.

To use it, you need to configure the Google Authenticator app on your smartphone using the QR code generated in the web app. When you login to the web application, you have to enter a six-digit pin that will be generated in the app to finish the two-factor authentication. The key generated in the app will be unique to your userID, and is a time-based one-time password (TOTP) — that is, it will expire after a certain time.