Firebase Auth Basics

---
name: firebase-auth-basics
description: Guide for setting up and using Firebase Authentication. Use this
  skill when the user's app requires user sign-in, user management, or secure
  data access using auth rules.
compatibility: This skill is best used with the Firebase CLI, but does not
  require it. Firebase CLI can be accessed through `npx -y
  firebase-tools@latest`.
metadata:
  category: Identity
---
## Prerequisites

- **Firebase Project**: Created via
  `npx -y firebase-tools@latest projects:create` (see `firebase-basics`).
- **Firebase CLI**: Installed and logged in (see `firebase-basics`).

## Core Concepts

Firebase Authentication provides backend services, easy-to-use SDKs, and
ready-made UI libraries to authenticate users to your app.

### Users

A user is an entity that can sign in to your app. Each user is identified by a
unique ID (`uid`) which is guaranteed to be unique across all providers. User
properties include:

- `uid`: Unique identifier.
- `email`: User's email address (if available).
- `displayName`: User's display name (if available).
- `photoURL`: URL to user's photo (if available).
- `emailVerified`: Boolean indicating if the email is verified.

### Identity Providers

Firebase Auth supports multiple ways to sign in:

- **Email/Password**: Basic email and password authentication.
- **Federated Identity Providers**: Google, Facebook, Twitter, GitHub,
  Microsoft, Apple, etc.
- **Phone Number**: SMS-based authentication.
- **Anonymous**: Temporary guest accounts that can be linked to permanent
  accounts later.
- **Custom Auth**: Integrate with your existing auth system.

Google Sign In is recommended as a good and secure default provider.

### Tokens

When a user signs in, they receive an ID Token (JWT). This token is used to
identify the user when making requests to Firebase services (Realtime Database,
Cloud Storage, Firestore) or your own backend.

- **ID Token**: Short-lived (1 hour), verifies identity.
- **Refresh Token**: Long-lived, used to get new ID tokens.

## Workflow

### 1. Provisioning

#### Option 1. Enabling Authentication via CLI

Only Google Sign In, anonymous auth, and email/password auth can be enabled via
CLI. For other providers, use the Firebase Console.

Configure Firebase Authentication in `firebase.json` by adding an 'auth' block:

```
{
  "auth": {
    "providers": {
      "anonymous": true,
      "emailPassword": true,
      "googleSignIn": {
        "oAuthBrandDisplayName": "Your Brand Name",
        "supportEmail": "support@example.com",
        "authorizedRedirectUris": [
          "https://my-project-id.firebaseapp.com/__/auth/handler",
          "http://localhost:4000"
        ]
      }
    }
  }
}
```

> [!NOTE] If the Google Sign-In popup opens and immediately closes with the
> error `[firebase_auth/unauthorized-domain]`, it means the domain is not
> authorized. For local development, ensure `localhost` is included in the
> **Authorized Domains** list in the Firebase Console (Authentication >
> Settings > Authorized domains). **CRITICAL**: Do NOT include the protocol or
> port number in the Authorized Domains list (e.g., use `localhost`, NOT
> `http://localhost:9090`).

**CRITICAL**: After configuring `firebase.json`, you MUST deploy the auth
configuration to the Firebase backend for the changes to take effect. This is
essential for auth providers like Google Sign-In, email/password, etc. to
auto-generate the necessary OAuth clients for your app platforms. Run:

```bash
npx -y firebase-tools@latest deploy --only auth
```

#### Option 2. Enabling Authentication in Console

Enable other providers in the Firebase Console.

1. Go to the
   https://console.firebase.google.com/project/_/authentication/providers
1. Select your project.
1. Enable the desired Sign-in providers (e.g., Email/Password, Google).

### 2. Client Setup & Usage

**Web** See [references/client_sdk_web.md](references/client_sdk_web.md).

**Flutter** See [references/flutter_setup.md](references/flutter_setup.md).
**Android (Kotlin)** See
[references/client_sdk_android.md](references/client_sdk_android.md).

**iOS (Swift)** See [references/ios_setup.md](references/ios_setup.md).

### 3. Security Rules

Secure your data using `request.auth` in Firestore/Storage rules.

See [references/security_rules.md](references/security_rules.md).