GoogleAuthenticator

Simple, easy to use server-side two-factor authentication library for .NET that works with Google Authenticator

Install-Package GoogleAuthenticator

See blog post for usage instructions (1.x only - does not apply to 2.x, see Wiki for 2.x):

https://csharprookie.wordpress.com/2015/03/17/implementing-free-two-factor-authentication-in-net-using-google-authenticator/

Notes

On linux, you need to ensure libgdiplus is installed if you want to generate QR Codes. See codebude/QRCoder#227.