How should my api handle login via auth0? - rest

Question

I'm trying to learn how to utilize auth0 to handle user authentication for an api I am currently creating.
My api has two endpoints:
Login endpoint: /api/login
Request access token endpoint: /api/auth?code={code}
Here the authentication flow is:
User goes to the login endpoint of my api.
User is redirected to auth0 ui.
User inputs their login credentials.
Auth0 redirects back to /api/auth where a request for an access_token is made using the login code.
Firstly, is my understanding the Oauth authentication flow correct? If so, how best should my api handle the initial login redirect to auth0?
Because at the moment when I hit up /api/login from the front-end ui it just returns the html of the login page at auth0. Should I instead return a 302 with the redirect url or is it possible to create an endpoint where the user inputs the username & password via my api and avoids the redirect?
---update---
After a user has authenticated via auth0 they receive a access_token and id_token which should my api use to verify the user is who they say they are?
Not sure if my understanding is correct but I belive that my frontend ui is the OAuth client application and my API service is an OAuth resource server. As such does my api need to call out to auth0 /userinfo to verify the user?

Answer 1

Assuming you are trying to protect an end-user application (your question wasn't clear on that), my understanding is if you are using Auth0, you likely won't need an /api/login and api/auth API. If you are using Auth0 you can get those things during your authentication via Auth0.
I would say your APPLICATION (not API) would redirect the user to the Auth0 login endpoint. You would do that by incorporating the Auth0 SDK of choice, depending on what you're building. For example, if you're building a web app, you may choose to incorporate auth0.js and call webAuth.authorize() to trigger the login. During that login, if you have configured an API within Auth0, and you provide the proper Scope and Audience during your login, your response will return an API token.
Then your user is in a state on the client side where you are logged in, and you have a token. You can then provide that token to your API, and your API can validate that token as needed. Auth0 also has various libraries for token validation (like this spring security one, for example).
Lastly, the question on which oAuth flow to use, that also depends on what type of app you're protecting. There are again Auth0 docs to help. The flow depends on if you're building a server-side web app, a SPA, a native app, etc. Your question was a little confusing, and it sounded a bit like you are building an API and want to protect that. If there is no client-side app (only machine-to-machine API calls), then you wouldn't be dealing with HTML and login pages. You'd likely be getting into the Client Credentials flow, which last I checked was only included for Enterprise Auth0 users.