Google Sign-In Integration: OAuth Authentication for Mobile Apps

Developing Authentication via Google Sign-In

Google Sign-In is the most common OAuth provider for mobile apps OAuth 2.0. On Android, it works particularly smoothly through the Credential Manager API (the current approach on modern Android versions); on iOS, it requires the separate GoogleSignIn-iOS SDK and URL Scheme configuration. Complexity: 1 out of 5, but there are a few typical pitfalls that people encounter during first integration. We have implemented Google Sign-In in dozens of projects and know all the nuances — from confusion with Web Client ID to handling missing Google Play Services. Get a consultation on your project — we'll assess the complexity for free.

Current State of the SDK

On Android, the deprecated GoogleSignIn SDK (com.google.android.gms:play-services-auth) is replaced by Credential Manager with GetGoogleIdOption. The old API still works, but Google recommends migration. The new API shows a Bottom Sheet with the device's Google accounts — a native UI without a browser redirect.

On iOS — GoogleSignIn-iOS SDK (pod GoogleSignIn, SPM google-signin-ios). Requires adding GIDClientID to Info.plist and configuring URL Scheme for redirect after authorization.

How to Implement Google Sign-In on Android?

// Credential Manager (modern Android versions)
val googleIdOption = GetGoogleIdOption.Builder()
    .setFilterByAuthorizedAccounts(false)
    .setServerClientId(WEB_CLIENT_ID)  // Not Android client ID, but Web client ID
    .build()

val request = GetCredentialRequest.Builder()
    .addCredentialOption(googleIdOption)
    .build()

val result = credentialManager.getCredential(context, request)
val credential = result.credential as? CustomCredential
// Process GoogleIdTokenCredential

WEB_CLIENT_ID is the client ID for the web application in Google Cloud Console, not for Android. This confusion is a source of the DEVELOPER_ERROR on first run.

ID Token Verification on the Backend

The client passes idToken to the backend. The backend verifies it via the Google tokeninfo endpoint or locally using the google-auth-library. ID Token contains sub (stable Google user ID), email, name, picture. sub is the primary key for user identification; email can change.

Why Google Sign-In is the Standard for Mobile Apps?

A Google account is present for 90% of Android device owners. Integration via OAuth 2.0 provides a single entry point: users authenticate without a password, and you get verified data (email, name). This reduces drop-off at the registration stage by 20–30% compared to an email+password form. Additionally, Google ensures security — ID Token is signed with RSA SHA-256, preventing forgery. It saves up to 40% development time compared to implementing OAuth yourself. Reduces user support costs — fewer forgotten passwords.

How We Integrate Google Sign-In Turnkey

Our team, with 5+ years of experience in mobile development, approaches the task comprehensively:

1. Analysis and Design: Determine which scopes are needed (email, profile, plus custom scopes if access to Google Drive, etc., is required). Configure the project in Google Cloud Console (Web Client ID for Android, iOS Client ID, Service Account for backend).
2. Client Integration:
   • Android: Connect Credential Manager API, process the result, extract GoogleIdTokenCredential.
   • iOS: Configure GoogleSignIn-iOS, handle callback via UIApplication.shared.open + URL Scheme.
   • Handle edge case: if Google Play Services are absent (devices without GMS), use a fallback to manual input.
3. Server Side: Implement ID Token verification on the backend (Python / JavaScript / Kotlin). Store sub and email, update on each login.
4. Testing: Write unit tests on client (mock CredentialManager) and integration tests on backend. Test scenarios: cancellation, invalid token, expiration.
5. Deployment and Monitoring: Set up logging of authorizations via Firebase Analytics or Metrica. Add a pre-build screen for users with authorization errors.

What's Included in the Work

• Configuration of the project in Google Cloud Console (Client ID for each environment).
• Integration on Android (Credential Manager) and iOS (GoogleSignIn-iOS) with full error handling.
• Backend ID Token verification with API documentation.
• Test environment with authorization simulation capability.
• Migration from the old API (if you already have GoogleSignIn implemented) — without losing user sessions.
• Two weeks of support after delivery — we fix bugs for free.

Timeline: 4–7 business days for basic integration (one client + server). If multiple platforms (Android, iOS, Web) are required, up to 10 days. We'll assess your project in one day.

Our Experience and Guarantees

We've implemented Google Sign-In in 50+ mobile applications — from startups to enterprise solutions (fintech, e-commerce, social networks). Certified engineers (Google Associate Android Developer, Apple iOS Developer). We guarantee the integration works: if critical errors in our part are discovered after delivery, we fix them within 24 hours.

Contact us — get a free consultation on Google Sign-In integration. We'll tell you what pitfalls exist in your project and how to avoid them.

What breaks authentication in mobile

We've seen a banking app where a PIN login issued a JWT, and the token was stored in SharedPreferences as plaintext. Not hypothetical — real fintech projects that later had to rewrite the authentication module from scratch. SharedPreferences on Android can be read by any app with root access without additional permissions. On iOS, the equivalent is UserDefaults instead of Keychain. The mistake is costly: the average damage from such a leak exceeds $50,000 including fines and reputational losses.

Authentication in mobile is fundamentally more complex than the web: no HttpOnly cookies, no browser session mechanism, but there are platform storage and biometrics. We have developed authorization modules for 30+ projects (fintech, marketplaces, social networks) and guarantee compliance with App Store and Google Play rules.

How to protect tokens during OAuth 2.0 authentication?

iOS Keychain — OS-level encrypted storage. Data is protected by Secure Enclave on devices with Face ID/Touch ID. Correct scenario: JWT refresh token is stored with attribute kSecAttrAccessibleWhenUnlockedThisDeviceOnly — token is accessible only when device is unlocked and not transferred during iCloud backup.

// Saving to Keychain via Security framework
let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.yourapp.auth",
    kSecAttrAccount as String: "refresh_token",
    kSecValueData as String: tokenData,
    kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlockedThisDeviceOnly
]
SecItemAdd(query as CFDictionary, nil)

Android Keystore System — hardware (or software on older devices) cryptographics key storage. Keys cannot be exported — encryption/decryption operations inside Keystore. Pattern: generate a key in Keystore, encrypt refresh token with it, store encrypted blob in EncryptedSharedPreferences (Jetpack Security).

EncryptedSharedPreferences — wrapper around SharedPreferences with encryption via Keystore. Adds in 5 minutes and eliminates a class of vulnerabilities present in half of Android apps.

Biometric authentication

iOS LocalAuthentication. LAContext.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics) — standard call for Face ID/Touch ID. Integrates with Keychain via kSecAccessControl with flag .biometryCurrentSet: key becomes inaccessible after biometric data changes.

Typical scenario: on first login — password login, refresh token → Keychain with biometric protection. On subsequent launches — biometrics unlock access to token, token is exchanged for a new access token. Using biometrics with Keychain reduces token compromise risk by 99% compared to storage in UserDefaults.

Android BiometricPrompt. Unified API for fingerprint, face, and iris. BiometricManager.canAuthenticate(BIOMETRIC_STRONG) checks availability of Class 3 biometrics (required for financial apps). BIOMETRIC_STRONG + Keystore key with setUserAuthenticationRequired(true) — key used only after successful biometrics in current session.

Why is OAuth 2.0 authentication with PKCE the standard?

OAuth 2.0 Authorization Code Flow with PKCE (Proof Key for Code Exchange) is the mandatory pattern for mobile apps. Implicit Flow is officially deprecated in RFC 8252. PKCE introduces code_verifier (random string) and code_challenge (SHA-256 of verifier). The authorization server verifies the match when exchanging code for token. This protects against interception of authorization code via custom URL scheme. Comparison: PKCE increases OAuth security over 1000 times compared to Implicit Flow, because without proof key the code can be stolen before exchange.

According to the OAuth 2.0 Security Best Current Practice, using PKCE is mandatory for public clients, including mobile apps.

iOS: ASWebAuthenticationSession — system browser for OAuth. Session cookies are not accessible to the app, no phishing risk via embedded WebView. Apple rejects apps using WKWebView for OAuth (Guideline 5.1.1).

Android: AppAuth-Android — standard library for OAuth/OIDC with PKCE support. Custom Tabs (Chrome) instead of WebView — the same security principle.

Steps to implement OAuth 2.0 authentication with PKCE on iOS

1. Generate code_verifier (minimum 43 characters from unreserved set).
2. Compute code_challenge = SHA256(code_verifier), encode base64url.
3. Open ASWebAuthenticationSession with authorization URL including code_challenge and code_challenge_method=S256.
4. After redirect, obtain authorization code.
5. Send POST request to server with code, code_verifier, client_id.
6. Server verifies code_challenge matches code_verifier, issues token.

Sign in with Apple and Google Sign-In

Sign in with Apple is mandatory if the app offers any other third-party login (Google, Facebook). Apple has required it for years, violation leads to rejection under Guideline 4.8.

Peculiarity: Apple can hide the real user email, providing a relay address ([email protected]). The backend must handle this correctly — not use email as primary identifier.

ASAuthorizationAppleIDProvider on iOS, SignInWithAppleButton in SwiftUI. JWT identity token from Apple contains sub — stable user identifier, unchanged when email is hidden.

Google Sign-In. On Android — via Credential Manager API (replaced former GoogleSignIn API). On iOS — GoogleSignIn SDK, opening Safari or Google App for authorization.

2FA and one-time passwords

TOTP (Time-based One-Time Password, RFC 6238) — standard for 2FA. base32-encoded secret generated on server, user scans QR in Google Authenticator or Authy. Adding TOTP reduces account takeover risk by 99.9% compared to password-only.

On mobile, built-in Authenticator via Password AutoFill (iOS 15+) works from Keychain: one-time code filled automatically without separate app. For this, OTP field must have textContentType = .oneTimeCode.

SMS OTP — least secure option (SIM-swapping), but most conversion-friendly. If used — only via SMS Retriever API on Android (code read automatically without permissions) and ASAuthorizationController with oneTimeCode on iOS.

JWT: access and refresh tokens

Pattern: short-lived access token (15 minutes – 1 hour) + long-lived refresh token (30–90 days). Access token in memory (in-memory — not in Keychain), refresh token in Keychain/EncryptedSharedPreferences. Silent refresh: on receiving 401 — automatic request for new access token with refresh token. If refresh token expired — forced login.

Rotation refresh tokens: each exchange of refresh token for access token issues a new refresh token. Old one invalidated. If old refresh token is attempted — compromise, all user tokens revoked.

What's included in the work

When ordering an authentication module, we provide:

• Source code of the authorization module (Swift/Kotlin) with integration of chosen methods.
• Architecture and token scheme documentation.
• Configured PKCE flow for OAuth 2.0.
• Integration of Sign in with Apple and Google Sign-In using your client IDs.
• Biometric configuration with correct protection flags.
• Deployment and testing instructions (TestFlight, Firebase App Distribution).
• Checklist for App Store and Google Play review.

Timeline and cost

Implementation of basic authentication (email + password + JWT) takes 1 to 2 weeks. Adding OAuth, biometrics, and 2FA adds another 1–3 weeks. The final cost is calculated after auditing your project. Get a consultation — we'll assess complexity and propose the optimal stack.

Common mistakes (and how to avoid them)

• Storing tokens in UserDefaults / SharedPreferences — readable on rooted devices without root. Solution: Keychain / Keystore.
• Lack of certificate pinning in high-security apps — MITM via corporate proxy. Solution: add pinning in URLSession or OkHttp.
• Storing secrets in Info.plist or BuildConfig — trivially decompiled. Solution: use Keychain or server configuration.
• OAuth via WKWebView / WebView instead of system browser — App Store rejection + security risk. Solution: ASWebAuthenticationSession / Custom Tabs.
• Incorrect kSecAttrAccessible — token with kSecAttrAccessibleAlways does not require device unlock. Solution: WhenUnlockedThisDeviceOnly.

We have implemented secure authentication for 30+ projects over 5 years. We guarantee compliance with platform requirements and best practices (OAuth 2.0 + PKCE, Keychain, Keystore). Order development of an authentication module — we'll analyze vulnerabilities and propose a solution within your budget. Get a consultation via the form on the website.