Building your app
Authentication (sign-up and login)
Authentication is how your app checks who someone is: sign-up creates their account once, and login proves it is them each time they come back.
Also called login, sign-up, sign in, auth
People usually prove who they are with a password, an emailed link, or an account they already have elsewhere. Authentication answers who you are. A separate step, often called authorization, decides what that person may see. Building your own login from scratch is risky, so most founders use a ready-made service.
When this shows up
Example, not a real founder: your login page works in testing. Then a user resets their password and never receives the email. The login itself was fine, but the email it depends on was never set up.
What to do next
On your live app, sign up with a fresh email address, log out, log back in, and reset the password. Fix the first step that fails.
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