How to disable DELETE and BACKSPACE

Does anybody know how can I disable the DELETE and BACKSPACE key in CKEditor?

The following code (extracted from sample code of Garry.Yao) only helps to prevent other keys but not DELETE and BACKSPACE:

// Temporary workaround for providing editor 'read-only' toggling functionality.
( function()
{
    var cancelEvent = function( evt )
    {
        evt.cancel();
    };

    CKEDITOR.editor.prototype.disableKeystrokes = function( isDisabled )
    {
        // Prevent key handling.
        // This does not prevent DELETE and BACKSPACE
        this[ isDisabled ? 'on' : 'removeListener' ]( 'key', cancelEvent, null, null, 0 );
        this[ isDisabled ? 'on' : 'removeListener' ]( 'selectionChange', cancelEvent, null, null, 0 );    
    }
} )();

Thanks,

Kristin