Firebase Authentication Specialist

Comprehensive Firebase Authentication implementation covering Google ecosystem integration, social authentication providers, phone authentication, anonymous auth, custom claims, and Security Rules integration.

Quick Reference (30 seconds)

Firebase Auth Core Features:

• Google Sign-In: Native Google ecosystem integration with Cloud Identity
• Social Auth: Facebook, Twitter/X, GitHub, Apple, Microsoft, Yahoo
• Phone Auth: SMS-based verification with international support
• Anonymous Auth: Guest access with account linking
• Custom Claims: Role-based access and admin privileges
• Security Rules: Firestore, Storage, and Realtime Database integration

Context7 Library Access:

• Firebase Documentation: /firebase/firebase-docs
• Use resolve-library-id with "firebase" then get-library-docs for latest API

Platform SDK Support:

• Web: firebase/auth with modular SDK (v9+)
• iOS: FirebaseAuth with Swift and SwiftUI
• Android: firebase-auth with Kotlin
• Flutter: firebase_auth package
• React Native: @react-native-firebase/auth

Quick Decision Tree:

• Need Google ecosystem integration? Use Firebase Auth
• Building mobile-first application? Use Firebase Auth
• Need serverless Cloud Functions? Use Firebase Auth
• Need anonymous guest access? Use Firebase Auth
• Existing Firebase infrastructure? Use Firebase Auth

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Implementation Guide

Google Sign-In Integration

Google Sign-In provides seamless authentication within the Google ecosystem with access to Google APIs and services.

Web Implementation:

Step 1: Enable Google Sign-In provider in Firebase Console under Authentication then Sign-in method Step 2: Configure OAuth consent screen in Google Cloud Console Step 3: Import and configure Firebase Auth SDK

import { getAuth, signInWithPopup, GoogleAuthProvider } from 'firebase/auth';
const auth = getAuth();
const provider = new GoogleAuthProvider();
provider.addScope('https://www.googleapis.com/auth/calendar.readonly');
const result = await signInWithPopup(auth, provider);
const credential = GoogleAuthProvider.credentialFromResult(result);

Flutter Implementation:

Future<UserCredential> signInWithGoogle() async {
  final googleUser = await GoogleSignIn().signIn();
  final googleAuth = await googleUser?.authentication;
  final credential = GoogleAuthProvider.credential(
    accessToken: googleAuth?.accessToken, idToken: googleAuth?.idToken);
  return await FirebaseAuth.instance.signInWithCredential(credential);
}

Mobile Configuration Requirements:

• iOS: Configure URL schemes in Info.plist with reversed client ID
• Android: Add SHA-1 fingerprint to Firebase project settings
• Web: Configure authorized domains in Firebase Console

Social Authentication Providers

Firebase Auth supports major social identity providers with unified API.

Facebook Login:

import { FacebookAuthProvider, signInWithPopup } from 'firebase/auth';
const provider = new FacebookAuthProvider();
provider.addScope('email');
provider.addScope('public_profile');
const result = await signInWithPopup(auth, provider);

Apple Sign-In (Required for iOS apps with third-party login):

let provider = OAuthProvider(providerID: "apple.com")
provider.scopes = ["email", "fullName"]
provider.getCredentialWith(nil) { credential, error in
    if let credential = credential {
        Auth.auth().signIn(with: credential) { result, error in }
    }
}

Twitter/X and GitHub Authentication:

// Twitter
const twitterProvider = new TwitterAuthProvider();
await signInWithPopup(auth, twitterProvider);

// GitHub
const githubProvider = new GithubAuthProvider();
githubProvider.addScope('repo');
await signInWithPopup(auth, githubProvider);

Phone Number Authentication

SMS-based phone authentication with international support and reCAPTCHA verification.

Web Implementation:

import { RecaptchaVerifier, signInWithPhoneNumber } from 'firebase/auth';
const recaptchaVerifier = new RecaptchaVerifier(auth, 'recaptcha-container', {
  size: 'normal', callback: () => { /* reCAPTCHA solved */ }
});
const confirmationResult = await signInWithPhoneNumber(auth, '+1234567890', recaptchaVerifier);
const credential = await confirmationResult.confirm(verificationCode);

Flutter Implementation:

await FirebaseAuth.instance.verifyPhoneNumber(
  phoneNumber: '+1234567890',
  verificationCompleted: (credential) async {
    await FirebaseAuth.instance.signInWithCredential(credential);
  },
  verificationFailed: (e) => print('Failed: ${e.message}'),
  codeSent: (verificationId, resendToken) { /* Store verificationId */ },
  codeAutoRetrievalTimeout: (verificationId) {},
);

Phone Auth Best Practices:

• Use E.164 format for phone numbers
• Handle auto-verification on Android
• Provide manual code entry fallback
• Consider SMS costs and rate limits

Anonymous Authentication

Anonymous auth enables guest access with account upgrade path.

// Anonymous sign-in
const result = await signInAnonymously(auth);
console.log('Anonymous UID:', result.user.uid);

// Account linking (upgrade to permanent)
import { linkWithCredential, EmailAuthProvider } from 'firebase/auth';
const credential = EmailAuthProvider.credential(email, password);
const linked = await linkWithCredential(auth.currentUser, credential);

// Link with social provider
const googleProvider = new GoogleAuthProvider();
await linkWithPopup(auth.currentUser, googleProvider);

Anonymous Auth Use Cases:

• E-commerce cart before checkout
• Content preview before signup
• Game progress before account creation

Custom Claims and Tokens

Custom claims enable role-based access control and admin privileges.

Server Side (Admin SDK):

import { getAuth } from 'firebase-admin/auth';
await getAuth().setCustomUserClaims(uid, { admin: true });
await getAuth().setCustomUserClaims(uid, {
  role: 'editor', organizationId: 'org_123', permissions: ['read', 'write']
});

Client Side:

const idTokenResult = await auth.currentUser.getIdTokenResult();
if (idTokenResult.claims.admin === true) { /* Show admin UI */ }

// Force token refresh after claim update
await auth.currentUser.getIdToken(true);

Custom Claims Best Practices:

• Keep claims small (under 1000 bytes total)
• Use claims for access control, not user data
• Token refresh required after claim changes

Security Rules Integration

Firebase Security Rules use authentication state for access control.

Firestore Security Rules:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /admin/{document=**} {
      allow read, write: if request.auth.token.admin == true;
    }
    match /organizations/{orgId}/documents/{docId} {
      allow read, write: if request.auth.token.organizationId == orgId;
    }
  }
}

Cloud Storage Security Rules:

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/{allPaths=**} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /public/{allPaths=**} {
      allow read: if true;
      allow write: if request.auth != null;
    }
  }
}

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Advanced Patterns

Cloud Functions Auth Triggers

Firebase Cloud Functions respond to authentication lifecycle events.

import { auth } from 'firebase-functions';
import { getFirestore } from 'firebase-admin/firestore';

export const onUserCreate = auth.user().onCreate(async (user) => {
  await getFirestore().collection('users').doc(user.uid).set({
    email: user.email, displayName: user.displayName,
    createdAt: FieldValue.serverTimestamp(), role: 'member'
  });
});

Blocking Functions:

import { beforeUserCreated } from 'firebase-functions/v2/identity';
export const validateUserCreate = beforeUserCreated((event) => {
  if (!event.data.email?.endsWith('@company.com')) {
    throw new HttpsError('invalid-argument', 'Unauthorized email domain');
  }
  return { customClaims: { role: 'employee' } };
});

Multi-Factor Authentication

Firebase Auth supports SMS-based second factor.

import { multiFactor, PhoneMultiFactorGenerator, PhoneAuthProvider } from 'firebase/auth';

// Enroll MFA
const mfUser = multiFactor(auth.currentUser);
const session = await mfUser.getSession();
const verificationId = await new PhoneAuthProvider(auth)
  .verifyPhoneNumber({ phoneNumber: '+1234567890', session }, recaptchaVerifier);
const credential = PhoneAuthProvider.credential(verificationId, code);
await mfUser.enroll(PhoneMultiFactorGenerator.assertion(credential), 'Phone');

// Sign-in with MFA challenge
try { await signInWithEmailAndPassword(auth, email, password); }
catch (error) {
  if (error.code === 'auth/multi-factor-auth-required') {
    const resolver = getMultiFactorResolver(auth, error);
    // Complete MFA verification with resolver.hints
  }
}

Session Management

import { setPersistence, browserLocalPersistence, browserSessionPersistence,
         inMemoryPersistence, onAuthStateChanged } from 'firebase/auth';

// Persistence options
await setPersistence(auth, browserLocalPersistence);  // Default - persist across sessions
await setPersistence(auth, browserSessionPersistence); // Clear on tab close
await setPersistence(auth, inMemoryPersistence);       // Memory only

// Auth state listener
const unsubscribe = onAuthStateChanged(auth, (user) => {
  if (user) { console.log('Signed in:', user.uid); }
  else { console.log('Signed out'); }
});

Firebase Auth Emulator

import { connectAuthEmulator } from 'firebase/auth';
if (process.env.NODE_ENV === 'development') {
  connectAuthEmulator(auth, 'http://localhost:9099');
}

Emulator Features:

• Test phone auth without real SMS
• Create test users programmatically
• Reset auth state between tests
• Debug token generation

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Resources

Context7 Documentation Access:

Use resolve-library-id with "firebase" then get-library-docs with topic "authentication" for comprehensive API documentation.

Firebase Official Resources:

• Firebase Console: console.firebase.google.com
• Authentication Documentation: firebase.google.com/docs/auth
• Security Rules Reference: firebase.google.com/docs/rules

Works Well With:

• moai-platform-firestore: Firestore database integration with auth-based security
• moai-lang-flutter: Flutter SDK for mobile Firebase Auth
• moai-lang-typescript: TypeScript patterns for Firebase SDK
• moai-domain-backend: Backend architecture with Firebase Admin SDK
• moai-quality-security: Security best practices for authentication

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Status: Production Ready Generated with: MoAI-ADK Skill Factory v1.0 Last Updated: 2025-12-07 Provider Coverage: Firebase Authentication Only