A reference credential wallet built on Flutter and DIDKit

Flutterappworld

3 years ago

Check out the Credible documentation here.

Credible

Credible is a native mobile wallet that supports W3C Verifiable Credentials and Decentralized Identifiers built on DIDKit and Flutter. We packaged the DIDKit library written in Rust into a Flutter application running on both Android and iOS, using C bindings and Dart’s FFI capabilities. This is the wallet counterpart to the rich, growing toolkit supplied by DIDKit, the two pillars of a reference architecture for creating trusted interactions at scale.

Maturity disclaimer

In the v0.1 release on February 10, 2021, Credible has not yet undergone a formal security audit and to desired levels of confidence for suitable use in production systems. This implementation is currently suitable for exploratory work and experimentation only. We welcome feedback on the usability, architecture, and security of this implementation and are committed to a conducting a formal audit with a reputable security firm before the v1.0 release. We are also working on large stability improvements through a continuous integration testing process across Android and iOS.

We are setting up a process to accept contributions. Please feel free to open issues or PRs in the interim, but we cannot merge external changes until this process is in place.

We are also in the process of listing Credible on the iOS TestFlight and Android Play Beta programs, and eventually their respective app marketplaces plus F-Droid.

Common Dependencies

To manually build Credible for either Android or iOS, you will need to install the following dependencies:

Rust
Java 7 or higher
Flutter (dev channel)
DIDKit/SSI
wasm-pack (WEB)
binaryen (WEB and targeting ASM.js)

Rust

It is recommended to use rustup to manage your Rust installation.

Java

On Ubuntu you could run:

# apt update
# apt install openjdk-8-jdk

For more information, please refer to the documentation of your favorite flavour of Java and your operating system/package manager.

Flutter

Please follow the official instalation instructions available here to install Flutter, don’t forget to also install the build dependencies for the platform you will be building (Android SDK/NDK, Xcode, etc).

We currently only support build this project using the dev channel of Flutter.

To change your installation to the dev channel, please execute the following command:

$ flutter channel dev
$ flutter upgrade

To confirm that everything is setup correctly, please run the following command and resolve any issues that arise before proceeding to the next steps.

$ flutter doctor

`wasm-pack` (Required for both WEB targets)

To build the WASM target you will need wasm-pack, it can be obtained running:

$ curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh

`binaryen`

To build Credible for WEB using ASM.js you will need binaryen, which allows the conversion of DIDKit WASM to ASM.js. This is necessary when you don’t have WASM support and need to run your page in pure Javascript. More detailed instructions on how to build binaryen can be found here.

If you are in a UNIX-like distribution you just have to clone the repo and build, we recommend cloning into your ${HOME}, to avoid having to specify the ${BINARYEN_ROOT} variable:

$ git clone https://github.com/WebAssembly/binaryen ~/binaryen
$ cd ~/binaryen
$ cmake . && make

DIDKit and SSI

This project also depends on two other Spruce projects, DIDKit and SSI.

These projects are all configured to work with relative paths by default, so it is recommended to clone them all under the same root directory, for example $HOME/spruceid/{didkit,ssi,credible}.

Target-Specific Dependencies

Android Dependencies

To build Credible for Android, you will require both the Android SDK and NDK.

These two dependencies can be easily obtained with Android Studio, which install further dependencies upon first being opened after installation. Installing the appropriate Android NDK (often not the newest) in Android Studio can be accomplished by going to Settings > Appearance & Behavior > System Settings > Android SDK and selecting to install the “NDK (Side by Side)”. An alternative method of installing SDK and NDK without Android Studio can be found in the optional install_android_dependencies.sh script included here.

If your Android SDK doesn’t live at $HOME/Android/Sdk you will need to set ANDROID_SDK_ROOT like so:

$ export ANDROID_SDK_ROOT=/path/to/Android/Sdk

:::note Some users have experienced difficulties with cross-compilation artefacts missing from the newest NDK, which is downloaded by default in the installation process. If you experience errors of this kind, you may have to manually downgrade or install multiple NDK versions as [shown here])(img/ndk_downgrade.png) in the Android Studio installer (screengrabbed from an Ubuntu installation). Alternately, running all or parts of the install_android_dependencies.sh script may be helpful.

If your build-tools and/or NDK live in different locations than the default ones inside /SDK/, or if you want to specify a specific NDK or build-tools version, you can manually configure the following two environment variables:

$ export ANDROID_TOOLS=/path/to/SDK/build-tools/XX.X.X/
$ export ANDROID_NDK_HOME=/path/to/SDK/ndk/XX.X.XXXXX/

iOS Dependencies

To build Credible for iOS you will need to install CocoaPods, which can be done with Homebrew on MacOS.

$ brew install cocoapods

Building DIDKit for different targets

Android

To build DIDKit for the Android targets, you will go to the root of DIDKit and run:

$ make -C lib install-rustup-android
$ make -C lib ../target/test/java.stamp
$ make -C lib ../target/test/aar.stamp
$ make -C lib ../target/test/flutter.stamp
$ cargo build

This may take some time as it compiles the entire project for multiple targets

Android APK

$ flutter build apk

Android App Bundle

$ flutter build appbundle

iOS

To build DIDKit for the iOS targets, you will go to the root of DIDKit and run :

$ make -C lib install-rustup-ios 
$ make -C lib ../target/test/ios.stamp
$ cargo build

Web using WASM

$ make -C lib ../target/test/wasm.stamp

Web using ASM.js

If you have installed bynarien somewhere other than $HOME, you will have to set BYNARIEN_ROOT as shown below, otherwise, just run the make command.

$ export BINARYEN_ROOT=/path/to/binaryen
$ make -C lib ../target/test/asmjs.stamp

Building Credible

You are now ready to build or run Credible.

Run on emulator

If you want to run the project on your connected device, you can use:

$ flutter run

Run on browser

If you want to run the project on your browser, you can use:

$ flutter run -d chrome --csp --release

Otherwise, Flutter allows us to build many artifacts for Android, iOS and WEB, below you can find the most common and useful commands, all of which you should run from the root of Credible.

iOS .app for Simulator

$ flutter build ios --simulator

iOS .app for Devices

$ flutter build ios --no-codesign

iOS IPA

$ flutter build ipa

Web

$ flutter build web \
  --csp \
  --release

If you don’t have support for WASM, you’ll probably need to provide your own canvaskit dependency without WASM as well as DIDKit, to do that you need to specify the FLUTTER_WEB_CANVASKIT_URL in the build command like below.

$ flutter build web \
  --csp \
  --dart-define=FLUTTER_WEB_CANVASKIT_URL=vendor/ \
  --release

For more details about any of these commands you can run

$ flutter build $SUBCOMMAND --help

Note about `canvaskit`

Since by default canvaskit comes in a WASM build, in order to the ASM.js be fully supported canvaskit was manually built for this target.

A prebuilt canvaskit is already included in the Credible web folder. If you want to build it by yourself, however, follow these steps:

Install emscripten
Clone Skia repository and pull its dependencies

git clone https://skia.googlesource.com/skia.git --depth 1 --branch canvaskit/0.22.0
cd skia
python2 tools/git-sync-deps

Modify build script modules/canvaskit/compile.sh

diff --git a/modules/canvaskit/compile.sh b/modules/canvaskit/compile.sh
index 6ba58bfae9..51f0297eb6 100755
--- a/modules/canvaskit/compile.sh
+++ b/modules/canvaskit/compile.sh
@@ -397,6 +397,7 @@ EMCC_DEBUG=1 ${EMCXX} \
     -s MODULARIZE=1 \
     -s NO_EXIT_RUNTIME=1 \
     -s INITIAL_MEMORY=128MB \
-    -s WASM=1 \
+    -s WASM=0 \
+    -s NO_DYNAMIC_EXECUTION=1 \
     $STRICTNESS \
     -o $BUILD_DIR/canvaskit.js

Build canvaskit

$ cd modules/canvaskit
$ make debug

Replace this line on $SKIA/modules/canvaskit/canvaskit/bin/canvaskit.js

618c618
< var isNode = !(new Function('try {return this===window;}catch(e){ return false;}')());
---
> var isNode = false;

Copy $SKIA/modules/canvaskit/canvaskit/bin/canvaskit.js to $CREDIBLE/web/vendor/
Build Credible as described above.

Troubleshooting

If you encounter any errors in the build process described here, please first try clean builds of the projects listed.

For instance, on Flutter, you can delete build files to start over by running:

$ flutter clean

Also, reviewing the install_android_dependencies.sh script may be helpful.

Supported Protocols

All QRCode interactions start as listed below:

User scans a QRCode containing a URL;
User is presented the choice to trust the domain in the URL;
App makes a GET request to the URL;

Then, depending on the type of message, one of the following protocols will be executed.

CredentialOffer

After receiving a CredentialOffer from a trusted host, the app calls the API with subject_id in the form body, that value is the didKey obtained from the private key stored in the FlutterSecureStorage, which is backed by KeyStore on Android and Keychain on iOS.

The flow of events and actions is listed below:

User is presented a credential preview to review and make a decision;
App generates didKey from the stored private key using DIDKit.keyToDIDKey;
App makes a POST request to the initial URL with the subject set to the didKey;
App receives and stores the new credential;
User is redirect back to the wallet.

And below is another version of the step-by-step:

Wallet	¹		Server
Scan QRCode ²
Trust Host	○ / ×
HTTP GET		→	https://domain.tld/endpoint
		←	CredentialOffer
Preview Credential
Choose DID	○ / ×
HTTP POST ³		→	https://domain.tld/endpoint
		←	VerifiableCredential
Verify Credential
Store Credential

¹ Whether this action requires user confirmation, exiting the flow early when the user denies.
² The QRCode should contain the HTTP endpoint where the requests will be made.
³ The body of the request contains a field subject_id set to the chosen DID.

VerifiablePresentationRequest

After receiving a VerifiablePresentationRequest from a trusted host, the app calls the API with presentation the form body, that value is a JSON encoded string with the presentation obtained from the selected credential and signed with the credential’s private key using DIDKit.issuePresentation.

Here are some of the parameters used to generate a presentation:

presentation
- id is set to a random UUID.v4 string;
- holder is set to the selected credential’s didKey;
- verifiableCredential is set to the credential value;
options
- verificationMethod is set to the DID’s verificationMethod id;
- proofPurpose is set to 'authentication';
- challenge is set to the request’s `challenge’;
- domain is set to the request’s `domain’;
key is the credential’s stored private key;