📦 zetrix_vc_flutter Plugin

A Flutter plugin that enables Verifiable Credential (VC) and Verifiable Presentation (VP) generation using BBS+ signatures, built on top of Zetrix blockchain specifications.

This implementation uses platform-specific native libraries (Rust-compiled via JNI/FFI) and exposes functionality to Dart via MethodChannel.

---

✅ Features

• 🧠 BBS+ key generation (BLS12-381)
• ✍️ BBS+ signature creation
• 🔍 Selective disclosure proofs
• 🧱 Works across Android and iOS (iOS soon)
• 📦 Built as a Flutter plugin (no manual linking for consumers)

---

🧹 Project Structure

zetrix_vc_flutter/
├── android/
│   ├── src/main/java/.../MethodChannelHandler.java
│   ├── src/main/jniLibs/arm64-v8a/libbbs.so
│   └── ...
├── ios/
│   └── (pending FFI integration)
├── lib/
│   ├── bbs_bindings.dart
│   ├── bbs.dart
│   └── zetrix_vc_flutter.dart
├── example/
└── pubspec.yaml

---

🔧 How We Integrated Using MethodChannel

✅ Step-by-step:

1. Expose Native Methods via Java (Android)

We created a wrapper class in android/src/main/java/.../BbsMethodHandler.java that maps Dart calls to native Rust bindings via JNI.

Example:

methodChannel.setMethodCallHandler((call, result) -> {
    switch (call.method) {
        case "createBbsProof":
            // call Rust JNI wrapper
            byte[] proof = Bbs.createProof(...);
            result.success(proof);
            break;
        default:
            result.notImplemented();
    }
});

2. Implement Dart MethodChannel

In bbs.dart, we use Flutter's MethodChannel to call native methods:

const _channel = MethodChannel('zetrix_vc');

Future<Uint8List> createBbsProof(Map<String, dynamic> args) async {
  final result = await _channel.invokeMethod<Uint8List>('createBbsProof', args);
  return result!;
}

3. Link Native .so Library Automatically

We placed libbbs.so in android/src/main/jniLibs/ so it is automatically bundled into the APK:

android/
└── src/main/jniLibs/
    └── arm64-v8a/
        └── libbbs.so

No manual linking needed from consumers.

4. Generate and Use JNI Headers

To link Java and Rust, we generated bbs_signatures_Bbs.h using javac -h. This header defines all native functions that the Rust/C side must implement.

javac -h . Bbs.java

---

🔐 Why MethodChannel?

We chose MethodChannel over Dart FFI for Android because:

• JNI is well-documented and stable for native Rust ↔ Java bindings.
• Flutter Android's MethodChannel provides simple serialization and error propagation.
• No need to handle cross-platform memory management at Dart-level.
• Works well with .so libraries generated from Rust (cargo-ndk, jni crate).

---

🚀 Usage in Flutter

final proof = await createBbsProof({
  "publicKey": [...],
  "signature": [...],
  "nonce": [...],
  "messages": [...],
});

---

📋 TODOs

• iOS native integration (Obj-C/Swift + Rust static lib)
• Fallback to Dart FFI for cross-platform consistency

---

💪 Build Notes

To rebuild the plugin after modifying native libs:

flutter clean
flutter pub get
flutter build apk

If your app uses this plugin as a dependency:

dependencies:
  zetrix_vc_flutter: <VERSION>

---

🙌 Credits

• MATTR BBS+ Rust crate
• Flutter MethodChannel Docs