dvote_native 0.9.4

Flutter plugin to allow native mobile apps to interact with Vocdoni decentralized votes.

DVote Flutter Native

This project is a Flutter Plugin that provides access to native code written in Rust.

It provides out-of-the box support for cross-compiling native Rust code for all available iOS and Android architectures and call it from plain Dart using Foreign Function Interface.

Overview

DVote Flutter Native provides:

• ZK Snarks prove generation using ZA
• Poseidon hashes for Hex strings and plain strings

The following platforms and architectures are supported:

• Android
  • ARMv7
  • ARM 64
  • x86
• iOS
  • ARM 64
  • x86_64

Cross compilation for MacOS, Linux and Windows should not be difficult but is not available yet.

For a census of 1 million claims, ZK proof generation times are in the range of:

• 2 to 4 seconds on a desktop computer
• 3 seconds on a 2020 iPhone SE
• 7 seconds (Android ARM 64) to 52 seconds (older Android ARM v7)

Considerations

• Every census size needs the generation of a specific setup + proving key
  • The setup and the ceremony are out of the scope of this project
• A census of 1M claims takes 20Mb of space
  • Such data should be hosted elsewhere other than the repo itself
  • Standard proving keys can be placed on a CDN/IPFS and be fetched+cached when the app launches

Getting started

Import the native code

The Rust source code is located at https://gitlab.com/vocdoni/dvote-rs-ffi and mounted on the rust folder by git.

$ git submodule init
$ git submodule update

Compile the library

• Make sure that the Android NDK is installed
  • You might also need LLVM from the SDK manager
• Ensure that the env variable $ANDROID_NDK_HOME points to the NDK base folder
  • It may look like /Users/name/Library/Android/sdk/ndk-bundle on MacOS
  • And look like /home/name/dev/android/ndk-bundle on Linux
• On the rust folder:
  • Run make to see the available actions
  • Run make init to install the Rust targets
  • Run make all to build the libraries and the .h file
• Update the name of your library in Cargo.toml
  • You'll need to update the symlinks to target the new file names. See iOS and Android below.

Generated artifacts:

• Android libraries
  • target/aarch64-linux-android/release/libdvote.so
  • target/armv7-linux-androideabi/release/libdvote.so
  • target/i686-linux-android/release/libdvote.so
• iOS library
  • target/universal/release/libdvote.a
• Bindings header
  • target/bindings.h

Reference the shared objects

iOS

Ensure that ios/dvote_native.podspec includes the following directives:

...
   s.source           = { :path => '.' }
+  s.public_header_files = 'Classes**/*.h'
   s.source_files = 'Classes/**/*'
+  s.static_framework = true
+  s.vendored_libraries = "**/*.a"
   s.dependency 'Flutter'
   s.platform = :ios, '8.0'
...

On flutter/ios, place a symbolic link to the libdvote.a file

$ cd flutter/ios
$ ln -s ../rust/target/universal/release/libdvote.a .

Append the generated function signatures from rust/target/bindings.h into flutter/ios/Classes/DVotePlugin.h

$ cd flutter/ios
$ cat ../rust/target/bindings.h >> Classes/DVotePlugin.h

In our case, it will append char *rust_greeting(const char *to); and void rust_cstr_free(char *s);

NOTE: By default, XCode will skip bundling the libdvote.a library if it detects that it is not being used. To force its inclusion, add a dummy method in SwiftDVotePlugin.swift that uses at least one of the native functions:

...
  public func dummyMethodToEnforceBundling() {
    rust_greeting("");
  }
}

If you won't be using Flutter channels, the rest of methods can be left empty.

Android

Similarly as we did on iOS with libdvote.a, create symlinks pointing to the binary libraries on rust/target.

You should have the following structure on flutter/android for each architecture:

src
└── main
    └── jniLibs
        ├── arm64-v8a
        │   └── libdvote.so@ -> ../../../../../rust/target/aarch64-linux-android/release/libdvote.so
        ├── armeabi-v7a
        │   └── libdvote.so@ -> ../../../../../rust/target/armv7-linux-androideabi/release/libdvote.so
        └── x86
            └── libdvote.so@ -> ../../../../../rust/target/i686-linux-android/release/libdvote.so

As before, if you are not using Flutter channels, the methods within android/src/main/kotlin/com/dvote/dvote_native/DvoteNativePlugin.kt can be left empty.

Declare the bindings in Dart

In /lib/dvote_native.dart, initialize the function bindings from Dart and implement any additional logic that you need.

Load the library:

final DynamicLibrary nativeExampleLib = Platform.isAndroid
    ? DynamicLibrary.open("libdvote.so")
    : DynamicLibrary.process();

Find the symbols we want to use, with the appropriate Dart signatures:

final Pointer<Utf8> Function(Pointer<Utf8>) rustGreeting = nativeExampleLib
    .lookup<NativeFunction<Pointer<Utf8> Function(Pointer<Utf8>)>>("rust_greeting")
    .asFunction();

final void Function(Pointer<Utf8>) freeGreeting = nativeExampleLib
    .lookup<NativeFunction<Void Function(Pointer<Utf8>)>>("rust_cstr_free")
    .asFunction();

Call them:

// Prepare the parameters
final name = "John Smith";
final Pointer<Utf8> namePtr = Utf8.toUtf8(name);
print("- Calling rust_greeting with argument:  $namePtr");

// Call rust_greeting
final Pointer<Utf8> resultPtr = rustGreeting(namePtr);
print("- Result pointer:  $resultPtr");

final String greetingStr = Utf8.fromUtf8(resultPtr);
print("- Response string:  $greetingStr");

When we are done using greetingStr, tell Rust to free it, since the Rust implementation kept it alive for us to use it.

freeGreeting(resultPtr);

Publishing to pub.dev

• Run make init to start developing
• Run make publish to update the symbolic links and upload to pub.dev

More information

• https://dart.dev/guides/libraries/c-interop
• https://flutter.dev/docs/development/platform-integration/c-interop
• https://github.com/dart-lang/samples/blob/master/ffi/structs/structs.dart
• https://mozilla.github.io/firefox-browser-architecture/experiments/2017-09-06-rust-on-ios.html
• https://mozilla.github.io/firefox-browser-architecture/experiments/2017-09-21-rust-on-android.html