Google One Tap Login: Web vs Flutter Deep Dive

In the age of seamless digital experiences, user authentication should be quick, secure, and painless. Google One Tap Login delivers just that on the web—but what about mobile apps built with Flutter? This blog dives deep into the technology behind Google One Tap, comparing its implementation on the web and how to replicate its behavior in Flutter.

The Use Case: Frictionless Authentication

Google One Tap Login enables users to authenticate without needing to fill out forms or remember passwords. The main benefits:

• One-click login/signup

• Reduced drop-off rates during registration

• Improved user retention

• Higher security using OAuth 2.0 + OpenID Connect (OIDC)

Web: Native Google One Tap

How It Works

Google One Tap on the web uses:

• The Google Identity Services (GIS) JavaScript library

• Browser sessions, cookies, and Chrome account info

• OIDC-based ID tokens sent back via JavaScript callback

Flow

1. Load the GIS script:

<script src="https://accounts.google.com/gsi/client" async defer></script>

2. Initialize and show the prompt:

google.accounts.id.initialize({

  client_id: 'YOUR_CLIENT_ID',

  callback: handleCredentialResponse

});

google.accounts.id.prompt();

3. On user consent, Google returns a JWT ID token.

4. Your backend verifies it using Google's public keys.

Behind the Scenes

Google One Tap relies on browser cookies, active Chrome sessions, third-party cookies, and the Federated Credential Management API.

Flutter: Native Equivalent of One Tap

Flutter doesn’t have direct support for the web-based One Tap prompt. Instead, we use the google_sign_in package, optionally paired with Firebase Authentication.

Tools Used

• google_sign_in package

• (Optional) firebase_auth for Firebase integration

Flow

1. Configure OAuth Client IDs for Android/iOS on Google Cloud Console.

2. Add google_sign_in to your Flutter project.

3. Use signInSilently() to mimic auto-login:

final GoogleSignIn _googleSignIn = GoogleSignIn();

final account = await _googleSignIn.signInSilently();

4. If needed, fall back to manual login:

final account = await _googleSignIn.signIn();

5. Send the ID token to your backend or authenticate with Firebase.

Where It Checks for Sessions?

On the web, One Tap checks the login status using browser cookies and the user's Chrome session. On Android, it checks system Google accounts through Play Services. On iOS, it uses app-specific tokens stored in the Keychain.

If the user previously signed in and hasn't signed out, signInSilently() can restore the session without prompting.

Auto-Login Best Practices

• Always call signInSilently() at startup to attempt seamless login

• Securely cache tokens for session persistence

• Use Firebase or a custom backend to manage authenticated sessions

Web vs Flutter: A Conceptual Comparison

In web apps, One Tap provides a floating UI prompt powered by JavaScript and browser sessions. In Flutter, you create your own UI (e.g., a "Sign in with Google" button) and use signInSilently() to replicate the auto-login behavior.

While the web version depends on the user's browser session and cookies, the Flutter version checks the device's saved login tokens—via Play Services on Android and Keychain on iOS.

Summary

Google One Tap Login is a powerful tool for the web that dramatically improves user experience. On mobile, Flutter apps can replicate this behavior using the google_sign_in package, leveraging silent sign-in and token caching.

By understanding the platform differences and session sources, you can create a consistent, low-friction login experience across your web and mobile apps.

Need help setting up a full cross-platform login system or backend token verification? Drop a comment or get in touch!