~4 minutes reading đź•‘

Wed, 16 September 2015

← Back to articles

How to encrypt data with Kinto.js

Lire la version française

A guest post this time, by Michiel de Jong from the Firefox OS team!

If you want to encrypt data that is uploaded to the cloud by an offline-first app, it makes sense to keep the local copy of the data in-the-clear, and encrypt the data only just before it is uploaded. That way, you can easily and efficiently display and update the local copy of the data, without the repeated cost of encrypting and decrypting everything all the time. Likewise, you only need to decrypt incoming data once, when it's downloaded, and your app can then display the data as often as needed without extra processing costs.

Syncing data to and from "the cloud" is a science in itself, and Kinto.js is a brilliant tool for this, developed by the equally brilliant Mozilla Services team. It's nearing its 1.0 release, and you'll hear a lot more about it on this blog in the future.

It so happens that Kinto.js now supports RemoteTransformers, which are superhandy for any type of transformation you may want to do on your data before it is saved into the locally cached copy (RemoteTransformer#decode), and before it is uploaded to the remote server (RemoteTransformer#encode).

This blogpost shows how to use a RemoteTransformer for encrypting your data just before it's sent to the cloud. It uses WebCrypto for the cryptography.

Note that I am not a security expert and this is just an example I wrote in an afternoon, so if you're going to copy-paste this code, please don't use it for anything valuable without doing your own proper security review first! :) For instance, it does not do any key stretching etcetera, it's just a very basic encrypt/decrypt proof-of-concept.

The main idea behind a RemoteTransformer is that you provide two functions, encode and decode, and Kinto.js makes sure to call the encode function on each record that gets uploaded, and to call the decode function on each record that gets downloaded. You can also read about this in the Kinto.js documentation.

The way we tell Kinto.js that we want to use our custom encode and decode procedures, is quite simple. When creating a Kinto collection, we pass a RemoteTransformer as a collection-level option. The full code example is here (note that it only works with latest Kinto.js master), but let's walk through the code step-by-step.

If you're unfamiliar with ES6, you may be surprised by some of this syntax; so let's take this opportunity to get used to it! Here's a little translation:

First, we need a few helper functions, which will be used later on to convert between byte arrays and ASCII strings.

// Helper functions:
const rawStringToByteArray = (str) => {
  const strLen = str.length;
  var byteArray = new Uint8Array(strLen);
  for (var i = 0; i < strLen; i++) {
    byteArray[i] = str.charCodeAt(i);
  return byteArray;
const base64StringToByteArray = (base64) => {
  return rawStringToByteArray(window.atob(base64));
const byteArrayToBase64String = (buffer) => {
  var bytes = new Uint8Array(buffer);
  var binary = '';
  for (var i=0; i<bytes.byteLength; i++) {
      binary += String.fromCharCode(bytes[i]);
  return window.btoa(binary);

Next, we need to create a RemoteTransformer that can encode and decode records. And for that, we need to generate an AES key. WebCrypto is a fairly new part of the web platform and it's awesome. We will use that key to encrypt and decrypt the data that is passed through the RemoteTransformer:

// RemoteTransformer:
const generateAesKey = () => {
  // See
  return window.crypto.subtle.generateKey({ name: 'AES-GCM', length: 128 },
       false, ['encrypt', 'decrypt']);

const createTransformer = (aesKey) => {
  const encode = (record) => {
    const cleartext = rawStringToByteArray(JSON.stringify(record));
    const IV = window.crypto.getRandomValues(new Uint8Array(16));

    return window.crypto.subtle.encrypt({ name: 'AES-GCM', iv: IV }, aesKey,
        cleartext).then(ciphertext => {
      return {
        ciphertext: byteArrayToBase64String(new Uint8Array(ciphertext)),
        IV: byteArrayToBase64String(IV)

  const decode = (record) => {
    const ciphertext = base64StringToByteArray(record.ciphertext);
    const IV = base64StringToByteArray(record.IV);

    return crypto.subtle.decrypt({ name: 'AES-GCM', iv: IV }, aesKey,
        ciphertext).then(recordArrayBuffer => {

      return JSON.parse(String.fromCharCode.apply(null,
          new Uint8Array(recordArrayBuffer)));
    }, () => {
      record.undecryptable = true;
      return record;

  return {

We create two Kinto instances, so that we can test syncing from one instance to the other, via the public Kinto demo instance on AWS. Creating multiple Kinto instances within the same origin is now possible with the recently added dbPrefix option:

// Kinto collection:
const createCollection = (transformer, testRun, instanceNo) => {
  const kinto = new Kinto({
    dbPrefix: `${testRun}-${instanceNo}`,
    remote: '',
    headers: {
      Authorization: 'Basic ' + btoa('public-demo:s3cr3t')

  return kinto.collection(`kinto-encryption-example-${testRun}`, {
    remoteTransformers: [ transformer ]

var coll1, coll2;
const prepare = () => {
  return generateAesKey().then(aesKey => {
    return createTransformer(aesKey);
  }).then(transformer => {
    // Create two fresh empty Kinto instances for testing:
    const testRun = new Date().getTime().toString();
    coll1 = createCollection(transformer, testRun, 1);
    coll2 = createCollection(transformer, testRun, 2);

Now, let's test if we can create an item in collection 1, sync it up (the data should be encrypted on the wire, we can check that on the Network tab of the browser console):

Network tab screenshot
const syncUp = () => {
  // Use first Kinto instance to demonstrate encryption:
  return coll1.create({
    URL: '',
    name: 'Web Cryptography API'
  }).then(() => {
    return coll1.sync();
  }).then(syncResults => {
    console.log('Sync up', syncResults);

And sync it down from the cloud again using collection 2. Again, the data coming in on the wire should be encrypted, but the decrypted results should show up in the sync results. And finally, a 'go()' method to put it all together.

The full source code is included in a script tag on this page you're reading right now, so go ahead and try opening your browser console on this page to try it out!

const syncDown = () => {
  // Use second Kinto instance to demonstrate decryption:
  return coll2.sync().then(syncResults => {
    console.log('Sync down', syncResults);

const go = () => {
  console.log('Watch the Network tab!');
  return prepare()
      .then(a => console.log('Success', a),
            b => console.error('Failure', b));
console.log('Type go(); to start!');

Hope you are as excited about Kinto.js as I am, comments below this blogpost and github issues on the example code very welcome! :)

Revenir au début