One of the readers of the ID book said, "Why do we need an access token? Can't we just save the password?" That's right, now that I think about it, I didn't explain such a basic thing. So, I'll add it if I have a chance to revise it, but for now I'll put it on my blog.
Further display I heard a powerful phrase: "Why do we need an access token? Can't we just store the password?" So I'll explain it here.