chill-bundles/Resources/public/module/async_upload/uploader.js

var algo = 'AES-CBC';

var keyDefinition = {
    name: algo,
    length: 256
};

var searchForZones = function(root) {
    var zones = root.querySelectorAll('div[data-stored-object]');
    
    for(let i=0; i < zones.length; i++) {
        initialize(zones[i]);
    }
};


var initialize = function(zone) {
    var 
        dropZone = document.createElement('div'),
        input = document.createElement('input')
        ;
        
    input.type = 'file';
    input.addEventListener('change', function(e) {
        handleInputFile(zone);
    });
    
    dropZone.classList.add('chill-doc__dropzone__drop');
    
    zone.insertBefore(input, zone.firstChild);
    zone.insertBefore(dropZone, zone.firstChild);
};

var handleInputFile = function (zone) {
    var 
        file = zone.querySelector('input[type="file"]').files[0],
        type = file.type,
        reader = new FileReader()
        ;
    
    reader.onload = e => {
        transmitArrayBuffer(zone, e.target.result, type);
    };
    
    reader.readAsArrayBuffer(file);
};

var createFilename = () => {
    var text = "";
    var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    for (let i = 0; i < 7; i++)
    text += possible.charAt(Math.floor(Math.random() * possible.length));

    return text;
};

var transmitArrayBuffer = (zone, data, type) => {
    var 
        iv = crypto.getRandomValues(new Uint8Array(16)),
        generateTempUrlPost  = zone.querySelector('input[data-async-file-upload]').dataset.generateTempUrlPost,
        suffix = createFilename(),
        jsKey, rawKey, encryptedData, uploadData
        ;
    
    window.crypto.subtle.generateKey(keyDefinition, true, [ "encrypt", "decrypt" ])
        .then(key => { 
            jsKey = key;
            
            // we register the key somwhere
            return window.crypto.subtle.exportKey('jwk', key);
        }).then(exportedKey => {
            rawKey = exportedKey;
            
            // we start encryption
            return window.crypto.subtle.encrypt({ name: algo, iv: iv}, jsKey, data);
        })
        .then(encrypted => {
            
            encryptedData = encrypted;
            
            // we get the url and parameters to upload document
            return window.fetch(generateTempUrlPost);
        })
        .then(response => response.json())
        .then(data => {
            var 
                formData = new FormData();
            
            uploadData = data;
            
            formData.append("redirect", data.redirect);
            formData.append("max_file_size", data.max_file_size);
            formData.append("max_file_count", data.max_file_count);
            formData.append("expires", data.expires);
            formData.append("signature", data.signature);
            formData.append("file", new Blob([ encryptedData ]), suffix);
            
            return window.fetch(data.url, {
                method: 'POST',
                mode: 'cors',
                body: formData
            });
        })
        .then(response => {
            if (response.ok) {
                storeDataInForm(zone, suffix, rawKey, iv, uploadData, type);
                
            } else {
                throw new Error("error while sending data");
            }
        })
        .catch(error => {
            window.alert("Error while sending document.");
            console.log(error);
        })
        ;
};

var storeDataInForm = (zone, suffix, jskey, iv, uploaddata, type) => {
    var 
        inputKey = zone.querySelector('input[data-stored-object-key]'),
        inputIv  = zone.querySelector('input[data-stored-object-iv]'),
        inputObject = zone.querySelector('input[data-async-file-upload]'),
        inputType = zone.querySelector('input[data-async-file-type]')
        ;
    
    inputKey.value = JSON.stringify(jskey);
    inputIv.value = JSON.stringify(iv);
    inputType.value = type;
    inputObject.value = uploaddata.prefix + suffix;
};

window.addEventListener('load', function(e) {
    searchForZones(document);
});