How can I encrypt/decrypt arbitrary binary files using Javascript in the browser?

Question

I need to let users load files from their system, encrypt them on-the-fly and upload to the server and do the opposite thing (download files from the server, decrypt on the fly

Answer 1

Note: I won't explain how to decrypt the data, but that should be rather easy to figure out using the code for encryption and the documentation-links provided.

First of all, the user has to be able to select a file via an input element.

You can then load the content of the file using the HTML5 FileReader API

function processFile(evt) {
    var file = evt.target.files[0],
        reader = new FileReader();

    reader.onload = function(e) {
        var data = e.target.result;

        // to be continued...
    }

    reader.readAsArrayBuffer(file);   
}

Encrypt the acquired data using the WebCrypto API.
If you don't want to randomly generate the key use crypto.subtle.deriveKey to create a key, for example, from a password that the user entered.

// [...]
var iv = crypto.getRandomValues(new Uint8Array(16)); // Generate a 16 byte long initialization vector

crypto.subtle.generateKey({ 'name': 'AES-CBC', 'length': 256 ]}, false, [ 'encrypt', 'decrypt' ])
    .then(key => crypto.subtle.encrypt({ 'name': 'AES-CBC', iv }, key, data))
    .then(encrypted => { /* ... */ });

Now you can send your encrypted data to the server (e.g. with AJAX). Obviously you will also have to somehow store the Initialization Vector to later successfully decrypt everything.

---

Here is a little example which alerts the length of the encrypted data.

Note: If it says Only secure origins are allowed, reload the page with https and try the sample again (This is a restriction of the WebCrypto API): HTTPS-Link

function processFile(evt) {
    var file = evt.target.files[0],
        reader = new FileReader();

    reader.onload = function(e) {
        var data = e.target.result,
            iv = crypto.getRandomValues(new Uint8Array(16));
      
        crypto.subtle.generateKey({ 'name': 'AES-CBC', 'length': 256 }, false, ['encrypt', 'decrypt'])
            .then(key => crypto.subtle.encrypt({ 'name': 'AES-CBC', iv }, key, data) )
            .then(encrypted => {
                console.log(encrypted);
                alert('The encrypted data is ' + encrypted.byteLength + ' bytes long'); // encrypted is an ArrayBuffer
            })
            .catch(console.error);
    }

    reader.readAsArrayBuffer(file);   
}