Rails Authentication From Scratch. Going Beyond Railscasts

If you have to implement authentication in your rails app, Devise is your safest option. Rolling your own is shooting yourself in the foot.But using Devise didn't feel like coding to me. It's like setting up your new mac by reading instructional blog posts around the net. Devise has great documentation and has all of your questions covered. You just follow them and you get industry level security.But it would be good coding practice if we can understand how Devise, and authentication in general works.