Many web sites offer users the option to use a streamlined single-click registration and login built on third party authentication services, typically run by the big social networks. In this article I want to give you an introduction to the OAuth protocol, which is one of the most used third-party authentication mechanism. I will also show you a complete Flask application that implements "Sign In
![OAuth Authentication with Flask](https://cdn-ak-scissors.b.st-hatena.com/image/square/2926cb824bfb8bda5581b77a123230374f1dce3d/height=288;version=1;width=512/https%3A%2F%2Fblog.miguelgrinberg.com%2Fstatic%2Fcards%2Foauth-authentication-with-flask.jpg)