Device authentication in Dart/Flutter

Dart can call native APIs via message passing. Let's call a function called generateKey with the parameter {'alias': 'my_key_01'}:

Future<void> _generateKey() async {
setState(() => _isLoading = true);

try {
  // Calling the native method
  final String result = await platform.invokeMethod('generateKey', {
    'alias': 'my_key_01',
  });

  setState(() {
    _keyStoreResult = result;
    _isLoading = false;
  });
} on PlatformException catch (e) {
  setState(() {
    _keyStoreResult = "Failed to generate key: '${e.message}'.";
    _isLoading = false;
  });
}
}

Since the call might block, it is marked async and a loading indicator is shown in the UI via the _isLoading field.

Now to the platform code, for example for Android:

class MainActivity: FlutterActivity() {
    private val CHANNEL = "com.example.secure/keystore"

    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)

        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler { call, result ->
            if (call.method == "generateKey") {
                val alias = call.argument<String>("alias") ?: "default_alias"
                try {
                    val keyStoreResult = [..] // Call the KeyStore here.

                    // Send the result back to Flutter.
                    result.success(keyStoreResult)
                } catch (e: Exception) {
                    // If generation fails (e.g., hardware issues), send an error
                    result.error("KEY_GEN_FAIL", e.localizedMessage, null)
                }
            } else {
                result.notImplemented()
            }
        }
    }
}

And for iOS:

import UIKit
import Flutter

@main
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {

    // 1. Standard plugin registration for things like path_provider, etc.
    GeneratedPluginRegistrant.register(with: self)

    // 2. Create a registrar for our custom "inline" plugin
    // The name "SecureKeystorePlugin" can be anything unique.
    let registrar = self.registrar(forPlugin: "SecureKeystorePlugin")

    // 3. Setup the channel using the registrar's messenger
    let channel = FlutterMethodChannel(
        name: "com.example.secure/keystore",
        binaryMessenger: registrar!.messenger()
    )

    // 4. Handle the method calls
    channel.setMethodCallHandler({
      (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in

      if call.method == "generateResidentKey" {
        let alias = (call.arguments as? [String: Any])?["alias"] as? String ?? "unknown"

        // Just for the example, get the iOS version.
        result("iOS \(version)")
      } else {
        result(FlutterMethodNotImplemented)
      }
    })

    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

And the Flutter code gets this result back: iOS 26.2.1 (for example).