How to Write Your Own Authorization and Authentication System

I was surprised at how Easy ish it was to write my own authorization and authentication system. It's pretty simple maybe a couple hundred lines of code to do that. But then you think about like some of the more complicated instances where you want to have a single sign-on for All of your services across 18 different applications. You need to send somebody at sms token and then what happens if they lose that? I regret ever using sign in with twitter or sign in with facebook for anything. Yeah, the only thing I like is signing with github.