Using Core Node.js Modules in React Native Apps

Sometimes, while creating advanced apps in React Native, you may want to encrypt the data you are sending to the web for secured web communication. You may also use this module to encrypt the payment-related data that is sent to the payment gateway.

Let’s say you want to use the crypto module to create some encrypted data. It might seem natural to do something like this:

var crypto = require('crypto');

var mykey = crypto.createCipher('aes-128-cbc', 'mypassword');
var mystr = mykey.update('abc', 'utf8', 'hex')
mystr += mykey.final('hex');

console.log(mystr);//34feb914c099df25794bf9ccb85bea72

Just import a module and create an encrypted string, right? Right? Nope!

This doesn’t work, because crypto is a core Node.js module, which means it’s probably C++ code bundled with the Node.js binary, not JavaScript. The React Native packager can’t package it[1] along with your app’s JavaScript bundle, so you get a runtime error:

Unable to resolve module 'crypto' 

This makes core modules like crypto, stream, etc, and the thousands of npm modules that depend on them unusable for React Native. Fortunately, there’s a solution to this problem, but it takes some work.

Solution

If you’re familiar with the module bundler Browserify, you might know that it allows you to use core Node.js modules like crypto in the browser using polyfills[2]. So let’s try to create a standalone JavaScript file that contains a polyfill for the cryptomodule, and use it within our app:

1. First, install browserify : Here we are installing browserify globally. If you have installed it before globally, then the below command need not be executed.

npm install -g browserify

2. Create a file crypto-in.js in your root directory ;

In the file crypto-in.js, import the module crypto and export it in the same file. Make sure that you do not create crypto-in.js in the node_modules directory. If you create it in the node_modules directory, this file will be lost every time npm install is executed.

var crypto = require("crypto");
module.exports=crypto

3. Create a standalone JavaScript bundle crypto-custom.js using browserify.

browserify crypto-in.js -o crypto-custom.js

This creates a file crypto-custom.js, which contains some 26,660 lines of code (containing the polyfills). The last few lines contain our code from crypto-in.js.

// //26,656 lines above..
},{"indexof":100}],153:[function(require,module,exports){
  // Our code from crypto-in.js
  var crypto = require("crypto");
  module.exports = crypto;
},{"crypto":56}]},{},[153]);

At this point, it might seem like we’re done, but we’re not! If you replace the line

const crypto = require('crypto');

in the app code with

const crypto = require('./crypto-custom');
//here iam in root directory and crypto-custom.js file is also in 
//rootdirectory

and try to run the app, you get the following error:

Unable to resolve module bn.js

Turns out the word require is given special treatment by the React Native packager, and you can’t redefine it to mean something else, which is what Browserify tries to do within the bundle. If you look carefully inside crypto-custom.js, you will notice that it always passes in a custom require function as a parameter and never actually uses the global require. There is a hacky but simple way to fix this problem, explained below.

4. Open the file crypto-custom.js in a text editor, and replace all instances of the word require with something else e.g. req. Make sure it’s unique, so it doesn’t conflict with any existing code and mess things up.

If you savecrypto-custom.js and reload the app now, the previous error will go away, but a new error will appear:

The packager isn’t able to find the method createCipher, because crypto isn’t exported properly from crypto-custom.js. If you look at the last few lines of the bundle, you’ll find that this is because we’re setting module.exports not on the global modules object, but on something else that’s passed in as a function argument:

//26,656 lines above..
},{}],187:[function(req,module,exports){
var crypto = req("crypto");
module.exports = crypto;
},{"crypto":71}]},{},[187]);

5. To export crypto properly from crypto-custom.js, we need to make a few more changes to the file:

• Add the statement var crypto at the top of the file crypto-custom.js
• Replace the statement var crypto = require('crypto'); with crypto = require('crypto'); to set the outer variable, instead of creating a local variable inside the function body.
• Move the line module.exports = crypto; outside the function body to the bottom of the file, so that it references the global module object.

After the changes, crypto-custom.js should look something like this:

var crypto;  //at top of the file at line 1
// Some 26,656 lines of polyfills..
},{"indexof":100}],153:[function(req,module,exports){
  crypto = req("crypto");
},{"crypto":56}]},{},[153]);

module.exports = crypto; 

That’s it! If you run the app now, it should work perfectly:

There is a alternative module expo-crypto available which can also be used for encrypting the data , but it has only few methods when compared to core crypto module

There is one more module react-native-crypto which can also be used but it does not have createCipheriv method.

Thanks for reading!

References

• [1] — https://github.com/facebook/react-native/issues/1871
• [2] — https://github.com/substack/browserify-handbook#builtins
• [3] — https://github.com/facebook/react-native/issues/6253

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