Integrating OAuth2 PKCE Flows in Flutter Apps

Introduction

OAuth2 with PKCE (Proof Key for Code Exchange) is the recommended authorization pattern for mobile development because it mitigates interception risks when using public clients. In Flutter, correctly implementing PKCE means combining secure code challenge generation, an external user-agent for authentication, and a safe token exchange and storage strategy. This tutorial walks through the core concepts, recommended packages, a minimal code-first PKCE example, redirect handling, and practical security points for production apps.

PKCE Basics And Why It Matters

PKCE extends the Authorization Code flow by adding a one-time code verifier and a derived code challenge. The client sends the code_challenge with the authorization request; the authorization server returns an authorization code. When the client redeems the code for tokens, it must present the original code_verifier. If an attacker intercepts the authorization code, they cannot exchange it without the verifier.

For mobile apps, where embedding a client secret is unsafe, PKCE provides strong protection without a static secret. Important terms:

• code_verifier: high-entropy random string generated by the client.

• code_challenge: base64url(SHA256(code_verifier)) or plain(code_verifier) if server supports plain.

• redirect_uri: app-specific URI or universal link that returns control to the app.

Choosing Packages And Platform Setup

Recommended packages for a robust PKCE implementation in Flutter:

• flutter_appauth: handles the authorization code + PKCE flows and token exchange with native integrations.

• url_launcher: if you build a custom browser flow or need to open an external browser.

• uni_links: listen for inbound redirects from custom schemes or universal links.

• crypto: for manual PKCE generation when you prefer explicit control.

Platform setup notes:

• iOS: register your custom URL scheme and (preferably) add Universal Links for secure redirects.

• Android: add an intent filter with a custom scheme or App Links and configure the redirect host.

• Use https universal links where possible to avoid collisions with other apps.

Implementing The PKCE Flow In Flutter

You can let flutter_appauth handle PKCE automatically, but understanding how to generate the verifier and challenge is valuable. Below is a concise example that generates a verifier and the SHA-256 challenge.

import 'dart:convert';
import 'dart:math';
import 'package:crypto/crypto.dart';

String createCodeVerifier([int length = 64]) {
  final rnd = Random.secure();
  final bytes = List<int>.generate(length, (_) => rnd.nextInt(256));
  return base64UrlEncode(bytes).replaceAll('=', '');
}

String createCodeChallenge(String verifier) =>
    base64UrlEncode(sha256.convert(utf8.encode(verifier)).bytes)
        .replaceAll('=', '');

To initiate authentication, build the authorization URL with response_type=code, include your client_id, redirect_uri, scope, state, code_challenge (from above) and code_challenge_method=S256. Open the URL in the system browser to ensure the OS-managed user-agent handles cookies and MFA consistently.

Handling Redirects And Token Exchange

When the user finishes authentication the authorization server redirects to your redirect_uri with ?code=... and ?state=.... Use uni_links to capture the URI and extract the authorization code. Then exchange the code for tokens by POSTing to the token endpoint including: grant_type=authorization_code, code, redirect_uri, client_id (if required), and code_verifier.

If you use flutter_appauth, the plugin performs redirect listening and token exchange for you and injects the code_verifier automatically when configured. Example flow using url_launcher + a simple HTTP POST to exchange tokens (pseudo-code takeaway): open auth URL, listen for redirect URI, extract code, POST code + code_verifier to token endpoint, then parse tokens.

Token storage: store access tokens in volatile memory and persist refresh tokens securely — on Android use EncryptedSharedPreferences or Keystore-backed storage; on iOS use Keychain. Do not store tokens in plain SharedPreferences or files.

Security Best Practices

• Prefer universal links/app links (https) to custom schemes when possible to reduce interception risk.

• Use code_challenge_method=S256 always; the plain method is weak or unsupported by many providers.

• Short-lived access tokens + refresh tokens minimize exposure. Revoke sessions on logout if the provider supports it.

• Validate the state parameter on redirect to mitigate CSRF attacks.

• Implement strict redirect URI validation on the server side and register exact redirect URIs with the authorization server.

• Use platform secure storage for refresh tokens and re-authenticate on suspicious events (device change, revoked token).

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Conclusion

Integrating OAuth2 PKCE in Flutter combines correct PKCE generation, a robust user-agent experience (system browser or native flows), careful redirect handling, and secure token storage. Whether you rely on flutter_appauth to abstract many details or implement PKCE manually with crypto, the essential steps are the same: generate a high-entropy code_verifier, use S256 code_challenge, open the authorization URL in an external browser, capture the redirect, exchange the code using the verifier, and store tokens securely. Applying these patterns will make your Flutter mobile development authentication flows both secure and user-friendly.