Wed 02 May 2018 By Skip
First experiences with React Native: bridging an Android native module for app authentication
My company, CriticalBlue, provides a remote mobile app authentication service called Approov. An Approov SDK is provided as a drop-in library to native iOS and Android app developers.
Overall, creating my first React Native native module was surprisingly straightforward. All code for my initial proof of concept is available in a github repository.
React Native Bridge
For react Native, Facebook provides a solid getting started guide which goes over the basics, including how to set up an application and how the underlying UI components differ from the usual React web elements.
Approov Native SDK
The Approov SDK is a drop-in native iOS or Android library. It interfaces with the cloud-based Approov authentication service which validates that the app is genuine, untampered, and not a bot. An app integrity token is returned from the authentication service, and that token is sent with each API call to ensure that the back-end API service is dealing with a known and genuine front-end request.
The basic operation we will expose in our native module proof of concept is
fetchApproovToken(), an asynchronous operation in the native SDK.
Before each back-end API call requiring app authentication, the client app makes a fetch token request. If a fresh token is needed, the SDK makes a remote attestation request, and the attestation service cryptographically authenticates the app and responds with an app integrity token.
The token has a short lifetime and is signed by a secret known only to the Approov service and the app’s back-end service. No secret is stored in the app, and in fact the app does not know whether the returned token is valid or not. The app simply adds the integrity token to the back-end API call, and the back-end server validates that the token has not expired and is properly signed before processing the request.
Approov Demo Service
Approov offers a downloadable demo which provides demonstration iOS and Android SDKs and a back-end service with two endpoints:
- https://demo-server.approovr.io/hello, which provides a publicly accessible test point.
- https://demo-server.approovr.io/shapes, which provides a random shape only if the request contains a valid integrity token.
We’ll use the Android SDK with the back-end service to demonstrate a simple React Native app using Approov. A version of this example using the iOS SDK with React native is also available.
I started my React Native project using create-react-native-app (CRNA). Follow the CRNA installation instructions to setup your React Native environment and then:
$ create-react-native-app rndemo
I will be adding native code to the app, so go ahead and eject now from create-react-native-app:
$ cd rndemo $ yarn eject ... Ejecting is permanent! Please be careful with your selection. ? How would you like to eject from create-react-native-app? React Native: I’d like a regular React Native project. We have a couple of questions to ask you about how you’d like to name your app: ? What should your app appear as on a user’s home screen? RN Demo ? What should your Android Studio and Xcode projects be called? rndemo Wrote to app.json, please update it manually in the future. Generating the iOS folder. Generating the Android folder. ...
Select a regular React Native project and name it as you wish. The iOS and Android projects are generated, and you will need Xcode and/or Android Studio build environments installed. Native code will be added to the native Android project later on.
We will experiment with a very simple proof of concept app that will use the demo server hello endpoint to validate our network connectivity.
React Native implements the fetch
API for networking.
We combine the connection check, UI rendering, and styling all in the
The top-level App component is registered in the
The main view is rendered by a stateless view component which displays a choice of image and a status message:
I am using Android for these examples, but this works similarly on iOS. Fire up
an Android emulator (you may need to launch Android Studio for this) or connect
a phone via
adb. In the
rndemo directory, launch the app:
$ cd rndemo $ yarn run android
You should see a screen like this:
TEST HELLO button and you should see a connected message if
everything went okay:
This validates network communication between our React Native app and the demo server. You can set airplane mode on your phone or emulator and push the test button again to verify there is then no longer a connection.
The Approov Native Module
The Approov demo package includes a README, the iOS and Android Approov demo libraries, sample clients, and app registration tools. Download the demo package, and save the app registration token which is included in your download email.
The Approov SDK includes the native code we want to expose to React Native. It
must be included in the iOS or Android native projects which were generated when
we ejected the create-react-native-app. For Android, this project is located at
rndemo/android. Import the Approov SDK into the Android project by following
from the Approov docs.
An Android native module is described in Java by extending the
ReactContextBaseJavaModule class. The
getName() method must be implemented
Methods exposed to React Native are adorned with
@ReactMethod and may provide
Facebook’s native module
to lifecycle events.
We initialize the default configuration for Approov authentication in the native
module constructor, and define ‘Approov’ in the
fetchApproovToken() call wraps the asynchronous native
fetchApproovToken() and settles a promise when the token fetch completes.
ReactPackage bundles and creates one or more native modules:
The project’s main application implements
ReactApplication and creates and
returns a list of module packages. We add the Approov package to the list in
NativeModules imported from
react-native. In our implementation,
When implementing natively with the Android SDK, most customers use interceptors
to fetch an Approov token and add it to each API request’s headers, so we’ll
want to fully implement this abstraction in our production module. For this
simple example though, we’ll hardwire the interception in a
fetchWithToken() method, when the native fetch token call completes, if
the promise is resolved, we add the token to the input request headers and make
fetch() call with the augmented input request. When completed, the fetch
returns a resolved promise holding the API server’s response.
For convenience, we create an
Approov object from the
object, adding a
fetch() method which is actually the
method, and then we export this as the
Now we are ready to use the
Approov object for authentication. We add a
getShape() method inside our
App which makes an
call to authenticate and request a random shape value. Once the fetch completes,
App component state updates, triggering a
render() call which causes the
ShapeView to display an updated shape and status message.
GET SHAPES button is added to the button bar to request new shapes.
Everything looks good, but when we request a new shape, we see a failure with a 400 status code, suggesting there is a problem with the client request.
The call fails because the integrity token added by Approov is invalid. Until
our example app is properly registered with the Approov service, the
fetchWithToken() call will always fail the authentication check.
Command line registration tools are included in the demo download. To register
the app, issue a registration request specifying the App’s APK bundle and the
app registration token you saved from the demo download email. As a courtesy to
other demo users, set your registration to expire after a few hours using the
$ cd <<approov-demo-package>>/registration-tools/Android/Mac/ $ ./registration -a <<rndemo-project>>/android/app/build/outputs/apk/app-debug.apk -t <registration-token> -e 2h Submitting data… Success: new app signature added to database.always be done.
Once the app is registered and can be properly authenticated, pressing the
SHAPES button should return one of these shapes:
Man in the Middle Attacks
The security of the communication channel is very important during API calls. If the channel is insecure, an API call could be intercepted and modified. An integrity token, although it has a short lifetime, could be observed in the insecure channel and used to make malicious API calls with impunity.
Despite using HTTPS/TLS when making API requests, an attacker who controls both the network and the mobile device can easily setup a Man in the Middle (MitM) attack to steal and quickly reuse Approov tokens before they expire.
To counter MitM attacks, mobile clients should use certificate or public key ‘pinning’ which checks that the certificate or public key presented by the back-end service is known specifically by the client app. Other certificates, though they might appear authentic, will be rejected by the client, and no API calls will be made.
Implementing pinning in React Native is a bit complicated and will be described in a separate article and integrated into this example’s code repository.
We’ve demonstrated a native module implementation in React Native with a hardwired interceptor successfully providing app authentication and API protection.
A production quality native module implementation for React Native would generalize the interceptor functionality, add convenience configuration methods, and provide full MitM protection. For comparison, a similar Approov plugin library already exists for Cordova and Ionic hybrid apps.
All code for this example is located on github.