# Firebase Auth Basics

> Guía para configurar y usar Firebase Authentication. Úsala cuando la app necesite inicio de sesión de usuarios, gestión de usuarios o acceso seguro a datos mediante reglas de auth.

Fuente: https://skillsagentes.com/skills/firebase/agent-skills/firebase-auth-basics
Markdown: https://skillsagentes.com/skills/firebase/agent-skills/firebase-auth-basics.md

Repositorio: https://github.com/firebase/agent-skills
Autor: firebase
Licencia: Apache-2.0
Actualizado: el mes pasado
Coste de contexto: 43 tok instalada, 1k tok al activarse, 6.5k tok con todos los archivos del bundle
Bundle: 6 archivos, 25 KB
Permisos que pide: ninguno declarado

## Instalación

Un skill son archivos markdown: los mismos archivos valen para cualquier agente y lo único que cambia es el directorio de destino, es decir la bandera `--agent`. Añade `-g` para instalarlo en todos los proyectos de la máquina.

```bash
# Claude Code
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent claude-code
# Cursor
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent cursor
# Codex
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent codex
# Gemini CLI
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent gemini
# Windsurf
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent windsurf
# Cline
npx -y skills add firebase/agent-skills --skill firebase-auth-basics --agent cline
```

## Qué hace

- Configura Firebase Authentication en firebase.json (anonymous, emailPassword, googleSignIn) y despliega esa configuración con firebase-tools
- Explica conceptos base: usuarios (uid, email, displayName), proveedores de identidad y tokens (ID Token, Refresh Token)
- Guía la configuración del SDK cliente en Web, Flutter y Android (Kotlin) mediante archivos de referencia
- Indica cómo asegurar datos con request.auth en reglas de seguridad de Firestore/Storage

## Cuándo usarla

- La app necesita inicio de sesión de usuarios, gestión de usuarios o acceso seguro a datos mediante reglas de auth

## Qué la activa

- "Configura Firebase Authentication con Google Sign In para mi app"
- "Añade login por email y contraseña usando Firebase Auth"
- "Cómo protejo mis datos de Firestore con reglas basadas en el usuario autenticado"

## Antes de instalar

- Requiere un proyecto Firebase ya creado y Firebase CLI instalado y logueado (ver firebase-basics).
- Necesita en el PATH: npx

## Archivos

- SKILL.md — 4 KB
- references/client_sdk_android.md — 5 KB
- references/client_sdk_web.md — 7 KB
- references/flutter_setup.md — 5 KB
- references/ios_setup.md — 2 KB
- references/security_rules.md — 1 KB

## SKILL.md

Reproducido tal cual desde firebase/agent-skills bajo Apache-2.0. Esta sección es el documento original y está en inglés.

## 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).

## Dónde encaja

- Categoría: [Seguridad](https://skillsagentes.com/categorias/seguridad.md) — Auditorías, revisión de dependencias, manejo de secretos y modelado de amenazas.
- Creador: [firebase](https://skillsagentes.com/creators/firebase.md) — 12 skills en el directorio
- [Todas las skills](https://skillsagentes.com/skills.md)
- [Ranking de instalaciones](https://skillsagentes.com/ranking.md)

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