How can I implement Google OAuth to replace email/password authentication in a self-hosted app?

Below is an overview of how to integrate Google OAuth into your self-hosted app. The steps are similar regardless of the backend language, but I’ll use a Node.js/Express example with Passport as a concrete illustration.

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1. Create and Configure Your Google Cloud Project

• Create a Project: You’ve already done this.
• Enable OAuth Consent Screen: Configure the consent screen details.
• Create Credentials: Generate a Client ID and Client Secret.
• Set Authorized Redirect URI: For example, http://yourdomain.com/auth/google/callback

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2. Install Necessary Packages

If you’re using Node.js, install passport and the Google OAuth strategy:

BASHCopied
npm install passport passport-google-oauth20 express express-session

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3. Setup Passport with Google OAuth Strategy

Create your server code. Here’s an example of how you can integrate Google authentication:

JAVASCRIPTCopied
const express = require('express');
const session = require('express-session');
const passport = require('passport');
const GoogleStrategy = require('passport-google-oauth20').Strategy;

const app = express();

// Session middleware setup
app.use(session({
  secret: 'your-session-secret', // Change this secret!
  resave: false,
  saveUninitialized: true
}));

// Passport middleware initialization
app.use(passport.initialize());
app.use(passport.session());

// Configure the Google strategy for use by Passport.
passport.use(new GoogleStrategy({
    clientID: 'YOUR_GOOGLE_CLIENT_ID',
    clientSecret: 'YOUR_GOOGLE_CLIENT_SECRET',
    callbackURL: '/auth/google/callback'
  },
  (accessToken, refreshToken, profile, done) => {
    // This callback is called after successful authentication.
    // You should search for an existing user or create a new one using profile info.
    // For demonstration, just pass the profile object.
    return done(null, profile);
  }
));

// Serialize user information into session storage.
passport.serializeUser((user, done) => {
  done(null, user);
});

// Deserialize user from session storage.
passport.deserializeUser((user, done) => {
  done(null, user);
});

// Route to initiate authentication with Google.
app.get('/auth/google',
  passport.authenticate('google', { scope: ['profile', 'email'] })
);

// Google OAuth callback route.
app.get('/auth/google/callback', 
  passport.authenticate('google', { failureRedirect: '/login' }),
  (req, res) => {
    // Successful authentication
    res.redirect('/');
  }
);

// Basic protected route example.
app.get('/', (req, res) => {
  if (!req.isAuthenticated()) {
    return res.redirect('/auth/google');
  }
  res.send(`Hello, ${req.user.displayName}`);
});

// Start the server.
const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

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4. Additional Considerations

• User Management:
  In the Google strategy callback, replace the placeholder logic with calls to your database. Look up the user via profile.id and store/retrieve any additional user information you need.

• Security:
  Make sure your app supports HTTPS in production. Also, keep your Client Secret safe – use environment variables or a secure vault.

• Error Handling:
  Improve error handling in the callback and authentication flows as needed.

• Frontend Integration:
  If needed, adjust routes or authentication flows based on your frontend architecture.

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By following these steps and adapting the provided example to your chosen stack, you can replace your email/password login with Google OAuth for authentication. If you use another language or framework, look for an equivalent OAuth2 client library and adapt the above logic accordingly.