Jquery error : too much recursion

I'm trying to create a file upload system implementing client side encryption using CryptoJS.

The problem I'm having is that execution of the script is stopped by the following error in Firebug's console : too much recursion

I have spent half of the day trying to resolve the problem, removing the var jqxhr = $.ajax part removes the error but removes posting functionality from my script. I have tried removing all the encryption lines, separating into different functions, but nothing seems to do it. Any jQuery pros know what's going wrong ?

Here's the code :

$("#successmsg").hide();
$("#errormsg").hide();

function randomString(n)
{
    var text = '';
    var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

    for(var i=0; i < n; i++)
    {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }

    return text;
}


$("#upload").submit(function(event) {
    event.preventDefault();

    input = document.getElementById('file');

    if(!input.files[0]) {
        $("#errormsg").html("No file selected.");
        $("#successmsg").hide();
        $("#errormsg").show();
    }

    fr = new FileReader();

    fr.onload = function() {
        var fname = input.files[0].name;
        var fkey = CryptoJS.SHA256(randomString(128));
        var skey = CryptoJS.SHA256(fkey);

        var fdata = CryptoJS.AES.encrypt(fr.result, "TestPassword");

        var jqxhr = $.ajax({
            url: "/api/files/upload",
            type: "POST",
            data: {
                'name': fname,
                'data': fdata,
                'key': skey
            },
            cache: false,
            dataType: 'json',
            processData: false
        });
    }

    fr.readAsText(input.files[0]);
});

Here is a JSFiddle : http://jsfiddle.net/wob66Lc0/

The issue is that CryptoJS functions return objects not strings, so you have to stringify it before you attempt to send it.

    var jqxhr = $.ajax({
        url: "/api/files/upload",
        type: "POST",
        data: {
            'name': fname,
            'data': fdata.toString(),
            'key': skey.toString()
        }
    });

http://jsfiddle.net/wob66Lc0/1/

Also encryption works on bytes not text so you should read the file as a binary string instead of text