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);
    }