After exhausting the current spectrum of available options from Javascript I decided to simply implement certificate pinning natively it all seems so simple now that I’m done.

Skip to headers titled Android Solution and IOS Solution if you don’t want to read through the process of reaching the solution.

Android

Following Kudo’s recommendation I thought out to implement pinning using okhttp3.

client = new OkHttpClient.Builder()
        .certificatePinner(new CertificatePinner.Builder()
            .add("publicobject.com", "sha1/DmxUShsZuNiqPQsX2Oi9uv2sCnw=")
            .add("publicobject.com", "sha1/SXxoaOSEzPC6BgGmxAt/EAcsajw=")
            .add("publicobject.com", "sha1/blhOM3W9V/bVQhsWAcLYwPU6n24=")
            .add("publicobject.com", "sha1/T5x9IXmcrQ7YuQxXnxoCmeeQ84c=")
            .build())
        .build();

I first started by learning how to create a native android bridge with react nativecreating a toast module. I then extended it with a method for sending a simple request

@ReactMethod
public void showURL(String url, int duration) {
    try {
        Request request = new Request.Builder()
        .url(url)
        .build();
        Response response = client.newCall(request).execute();
        Toast.makeText(getReactApplicationContext(), response.body().string(), duration).show();
    } catch (IOException e) {
        Toast.makeText(getReactApplicationContext(), e.getMessage(), Toast.LENGTH_SHORT).show();
    }
}

Succeeding in sending a request I then turned to sending a request pinned.

I used these packages in my file

import com.facebook.react.bridge.NativeModule;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;
import com.facebook.react.bridge.ReactMethod;
import com.facebook.react.bridge.Callback;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.CertificatePinner;
import java.io.IOException;

import java.util.Map;
import java.util.HashMap;

Kudo’s approach wasn’t clear on where I would get the public keys or how to generate them. luckily okhttp3 docs in addition to providing a clear demonstration of how to use the CertificatePinner stated that to get the public keys all I would need to do is send a request with an incorrect pin, and the correct pins will appear in the error message.

After taking a moment to realise that OkHttpClent.Builder() can be chained and I can include the CertificatePinner before the build, unlike the misleading example in Kudo’s proposal (probably and older version) I came up with this method.

@ReactMethod
public void getKeyChainForHost(String hostname, Callback errorCallbackContainingCorrectKeys,
  Callback successCallback) {
    try {
        CertificatePinner certificatePinner = new CertificatePinner.Builder()
             .add(hostname, "sha256/AAAAAAAAAAAAAAAAAAAAAAAAAAA=")
             .build();
        OkHttpClient client = (new OkHttpClient.Builder()).certificatePinner(certificatePinner).build();

        Request request = new Request.Builder()
             .url("https://" + hostname)
             .build();
        Response response =client.newCall(request).execute();
        successCallback.invoke(response.body().string());
    } catch (Exception e) {
        errorCallbackContainingCorrectKeys.invoke(e.getMessage());
    }
}

Then replacing the public keychains I got in the error yielded back the page’s body, indicating I had made a successful request, I change one letter of the key to make sure it was working and I knew I was on track.

I finally had this method in my ToastModule.java file

@ReactMethod
public void getKeyChainForHost(String hostname, Callback errorCallbackContainingCorrectKeys,
  Callback successCallback) {
    try {
        CertificatePinner certificatePinner = new CertificatePinner.Builder()
             .add(hostname, "sha256/+Jg+cke8HLJNzDJB4qc1Aus14rNb6o+N3IrsZgZKXNQ=")
             .add(hostname, "sha256/aR6DUqN8qK4HQGhBpcDLVnkRAvOHH1behpQUU1Xl7fE=")
             .add(hostname, "sha256/HXXQgxueCIU5TTLHob/bPbwcKOKw6DkfsTWYHbxbqTY=")
             .build();
        OkHttpClient client = (new OkHttpClient.Builder()).certificatePinner(certificatePinner).build();

        Request request = new Request.Builder()
             .url("https://" + hostname)
             .build();
        Response response =client.newCall(request).execute();
        successCallback.invoke(response.body().string());
    } catch (Exception e) {
        errorCallbackContainingCorrectKeys.invoke(e.getMessage());
    }
}

Android Solution Extending React Native’s OkHttpClient

Having figured out how to send pinned http request was good, now I can use the method I created, but ideally I thought it would be best to extend the existing client, so as to immediately gain the benefit of implementing.

This solution is valid as of RN0.35 and I don’t know how it will fair in the future.

While looking into ways of extending the OkHttpClient for RN I came across this article explaining how to add TLS 1.2 support through replacing the SSLSocketFactory.

reading it I learned react uses an OkHttpClientProvider for creating the OkHttpClient instance used by the XMLHttpRequest Object and therefore if we replace that instance we would apply pinning to all the app.

I added a file called OkHttpCertPin.java to my android/app/src/main/java/com/dreidev folder

package com.dreidev;

import android.util.Log;

import com.facebook.react.modules.network.OkHttpClientProvider;
import com.facebook.react.modules.network.ReactCookieJarContainer;


import java.util.concurrent.TimeUnit;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.CertificatePinner;

public class OkHttpCertPin {
    private static String hostname = "*.efghermes.com";
    private static final String TAG = "OkHttpCertPin";

    public static OkHttpClient extend(OkHttpClient currentClient){
      try {
        CertificatePinner certificatePinner = new CertificatePinner.Builder()
             .add(hostname, "sha256/+Jg+cke8HLJNzDJB4qc1Aus14rNb6o+N3IrsZgZKXNQ=")
             .add(hostname, "sha256/aR6DUqN8qK4HQGhBpcDLVnkRAvOHH1behpQUU1Xl7fE=")
             .add(hostname, "sha256/HXXQgxueCIU5TTLHob/bPbwcKOKw6DkfsTWYHbxbqTY=")
             .build();
        Log.d(TAG, "extending client");
        return currentClient.newBuilder().certificatePinner(certificatePinner).build();
      } catch (Exception e) {
        Log.e(TAG, e.getMessage());
      }
     return currentClient;
   }
}

This package has a method extend which takes an existing OkHttpClient and rebuilds it adding the certificatePinner and returns the newly built instance.

I then modified my MainActivity.java file following this answer’s advice by adding the following methods

.
.
.
import com.facebook.react.ReactActivity;
import android.os.Bundle;

import com.dreidev.OkHttpCertPin;
import com.facebook.react.modules.network.OkHttpClientProvider;
import okhttp3.OkHttpClient;

public class MainActivity extends ReactActivity {

  @Override
  public void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     rebuildOkHtttp();
  }

  private void rebuildOkHtttp() {
      OkHttpClient currentClient = OkHttpClientProvider.getOkHttpClient();
      OkHttpClient replacementClient = OkHttpCertPin.extend(currentClient);
      OkHttpClientProvider.replaceOkHttpClient(replacementClient);
  }
.
.
.

This solution was carried out in favor of completely reimplementing the OkHttpClientProvider createClient method, as inspecting the provider I realized that the master version had implemented TLS 1.2 support but was not yet an available option for me to use, and so rebuilding was found to be the best means of extending the client. I’m wondering how this approach will fair as I upgrade but for now it works well.

Update It seems that starting 0.43 this trick no longer works. For timebound reasons I will freeze my project at 0.42 for now, until the reason for why rebuilding stopped working is clear.

Solution IOS

For IOS I had thought I would need to follow a similar method, again starting with Kudo’s proposal as my lead.

Inspecting the RCTNetwork module I learned that NSURLConnection was used, so instead of trying to create a completely new module with AFNetworking as suggested in the proposal I discovered TrustKit

following its Getting Started Guide I simply added

pod 'TrustKit'

to my podfile and ran pod install

the GettingStartedGuide explained how I can configure this pod from my pList.file but preferring to use code than configuration files I added the following lines to my AppDelegate.m file

.
.
.
#import <TrustKit/TrustKit.h>
.
.
.
@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{


  // Initialize TrustKit
  NSDictionary *trustKitConfig =
    @{
    // Auto-swizzle NSURLSession delegates to add pinning validation
    kTSKSwizzleNetworkDelegates: @YES,

    kTSKPinnedDomains: @{

       // Pin invalid SPKI hashes to *.yahoo.com to demonstrate pinning failures
       @"efghermes.com" : @{
           kTSKEnforcePinning:@YES,
           kTSKIncludeSubdomains:@YES,
           kTSKPublicKeyAlgorithms : @[kTSKAlgorithmRsa2048],

           // Wrong SPKI hashes to demonstrate pinning failure
           kTSKPublicKeyHashes : @[
              @"+Jg+cke8HLJNzDJB4qc1Aus14rNb6o+N3IrsZgZKXNQ=",
              @"aR6DUqN8qK4HQGhBpcDLVnkRAvOHH1behpQUU1Xl7fE=",
              @"HXXQgxueCIU5TTLHob/bPbwcKOKw6DkfsTWYHbxbqTY="
              ],

          // Send reports for pinning failures
          // Email [email protected] if you need a free dashboard to see your App's reports
          kTSKReportUris: @[@"https://overmind.datatheorem.com/trustkit/report"]
          },

     }
  };

  [TrustKit initializeWithConfiguration:trustKitConfig];
.
.
.

I got the public key hashes from my android implementation and it just worked (the version of TrustKit I received in my pods is 1.3.2)

I was glad IOS turned out to be a breath

As a side note TrustKit warned that it’s Auto-swizzle won’t work if the NSURLSession and Connection are already swizzled. that said it seems to be working well so far.

Conclusion

This answer presents the solution for both Android and IOS, given I was able to implement this in native code.

One possible improvement may be to implement a common platform module where setting public keys and configuring the Network providers of both android and IOS can be managed in javascript.

Kudo’s proposal mentioned simply adding the public keys to the js bundle may however expose a vulnerability, where somehow the bundle file can be replaced.

I don’t know how that attack vector can function, but certainly the extra step of signing the bundle.js as proposed may protect the js bundle.

Another approach may be to simply encode the js bundle into a 64 bit string and include it in the native code directly as mentioned in this issue’s conversation. This approach has the benefit of obfuscating as well hardwiring the js bundle into the app, making it inaccessible for attackers or so I think.

If you read this far I hope I enlightened you on your quest for fixing your bug and wish you enjoy a sunny day.