feat: Flutter Windows Desktop support

[NandanPrabhu]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR adds native Windows desktop support to the auth0_flutter SDK, enabling Auth0 Universal Login on Flutter Windows apps using the OAuth 2.0 Authorization Code Flow with PKCE. The implementation is a C++ Flutter plugin that integrates with the existing platform-interface layer without modifying the mobile (iOS/Android) code paths.

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New: WindowsWebAuthentication class

A dedicated Windows authentication class exposed via Auth0.windowsWebAuthentication(). Unlike the mobile WebAuthentication class, this:

• Requires appCallbackUrl explicitly
• Does not auto-store credentials in CredentialsManager

final auth0 = Auth0('DOMAIN', 'CLIENT_ID');

// Simple: Auth0 redirects directly to the app via custom scheme
final credentials = await auth0.windowsWebAuthentication().login(
  appCallbackUrl: 'auth0flutter://callback',
);

// Intermediary server: HTTPS endpoint forwards to auth0flutter://callback
final credentials = await auth0.windowsWebAuthentication().login(
  appCallbackUrl: 'myapp://callback',
 redirectUrl: 'https://myhosteddomain.com/callback
);

Regardless of what redirectUrl is registered with Auth0, the Windows plugin always wakes the app by listening on the myapp://callback custom scheme. When using an intermediary server, the server must forward the callback to myapp://callback?code=…&state=…. This custom scheme to passed to windows native code from dart via the appCallbackUrl parameter

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New: Windows C++ plugin (auth0_flutter/windows/)

Component	Purpose
login_web_auth_request_handler.cpp	Orchestrates the full OAuth 2.0 + PKCE login flow
logout_web_auth_request_handler.cpp	Builds and opens the Auth0 logout URL
oauth_helpers.cpp	PKCE code verifier/challenge generation; auth0flutter:// callback polling
auth0_client.cpp	HTTP token exchange via cpprestsdk
id_token_validator.cpp	OpenID Connect ID token validation
id_token_signature_validator.cpp	RS256 signature verification via OpenSSL
jwt_util.cpp	JWT header/payload decoding
token_decoder.cpp	Maps token exchange response → Credentials struct
user_profile.cpp / user_identity.cpp	OIDC claims → UserProfile struct
time_util.cpp	ISO 8601 / RFC 3339 timestamp parsing
url_utils.cpp	RFC 3986 URL parsing and query parameter extraction
windows_utils.cpp	WideToUtf8, BringFlutterWindowToFront

Authentication flow:

1. Generate PKCE code_verifier (32 cryptographically random bytes via RAND_bytes) and code_challenge (SHA-256 via OpenSSL, base64-URL encoded)
2. Generate a random state value for CSRF protection
3. Build the Auth0 /authorize URL with all parameters RFC 3986-encoded
4. Open the URL in the system default browser via ShellExecuteA
5. Poll PLUGIN_STARTUP_URL environment variable (set by Windows when the app is launched via the auth0flutter:// custom scheme) every 200 ms until the callback arrives or the timeout expires
6. Validate state to prevent CSRF; extract code
7. Exchange code + code_verifier for tokens via POST to /oauth/token
8. Validate the ID token (issuer, audience, expiry, auth_time, nonce, RS256 signature)
9. Bring the Flutter window back to the foreground and return credentials

Key design decisions:

• The app always listens on auth0flutter://callback (kDefaultRedirectUri). The redirectUrl sent to Auth0 may differ (e.g. an HTTPS intermediary server URL); that server is responsible for forwarding to auth0flutter://callback?code=…&state=….
• Internal plugin parameters (authTimeoutSeconds) are consumed before building the authorize URL and are not appended to it.
• The authentication flow runs on a background std::thread to avoid blocking the Flutter UI thread.
• openid scope is always enforced even when not explicitly passed, as required by OpenID Connect.
• All URL parameters are RFC 3986 percent-encoded to prevent injection and handle special characters in values such as redirect URIs and scopes.

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New: vcpkg.json dependency manifest

Manages C++ dependencies via vcpkg, integrating automatically with CMake through the vcpkg toolchain file set by Flutter during flutter build windows:

Library	Purpose
cpprestsdk	Async HTTP client and JSON for token exchange
openssl	RAND_bytes (PKCE entropy), SHA-256 (code challenge), RS256 signature verification, TLS
boost-system / boost-date-time / boost-regex	Transitive cpprestsdk dependencies

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New: Unit tests (Google Test, auth0_flutter/windows/test/)

Test file	Coverage
oauth_helpers_test.cpp	Base64-URL encoding, code verifier/challenge generation, callback timeout behaviour
id_token_validator_test.cpp	Issuer, audience, expiry, auth_time, nonce, leeway validation
jwt_util_test.cpp	JWT splitting, header/payload decoding
time_util_test.cpp	ISO 8601 and RFC 3339 timestamp parsing
token_decoder_test.cpp	Token response → Credentials mapping
url_utils_test.cpp	URL parsing, query string extraction, RFC 3986 encoding
user_identity_test.cpp	Identity claims extraction
user_profile_test.cpp	User profile claims mapping
windows_utils_test.cpp	WideToUtf8 wide-to-UTF-8 conversion

Tests are compiled as a separate auth0_flutter_tests executable and registered with CTest, enabled via -DAUTH0_FLUTTER_ENABLE_TESTS=ON.

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New: CI pipeline (.github/workflows/main.yml)

Added a windows-tests job that installs vcpkg dependencies, builds the test executable with CMake, and runs all C++ unit tests via CTest on windows-latest.

📎 References

• RFC 7636 — Proof Key for Code Exchange (PKCE)
• RFC 8252 — OAuth 2.0 for Native Apps
• OpenID Connect Core 1.0 — ID Token Validation
• Auth0 docs: Authorization Code Flow with PKCE

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🎯 Testing

Automated — C++ unit tests (Windows)

cd auth0_flutter/windows
cmake -B build -S . \
  -DCMAKE_TOOLCHAIN_FILE=<vcpkg-root>/scripts/buildsystems/vcpkg.cmake \
  -DAUTH0_FLUTTER_ENABLE_TESTS=ON
cmake --build build
cd build && ctest --output-on-failure

All 9 test suites pass.

Automated — Flutter unit tests (any platform)

cd auth0_flutter
flutter test test/mobile/web_authentication_test.dart
# 34/34 tests pass

Manual — end-to-end on Windows

Prerequisites:

1. Register auth0flutter as a custom URL scheme pointing to your app executable (via installer or registry)
2. Add myapp://callback or 'https://mycustomhosteddomain.com/callback" to Allowed Callback URLs and **AllowedLogout URLs" in the Auth0 dashboard

cd auth0_flutter/example
flutter run -d windows

• Tap Log in → system default browser opens Auth0 Universal Login
• Authenticate; browser redirects to myApp://callback?code=…&state=…
• Windows launches the app via the registered custom scheme; the Flutter window comes to the foreground and displays the returned credentials

To test the intermediary server pattern, point redirectUrl at an HTTPS endpoint that reads the code and state query parameters and responds with a redirect to auth0flutter://callback?code=…&state=….