Even if cookie are a nice solution i seriously wonder about the usage of your class to stock an entire session in cookie.
First a single cookie is limited to 4ko and second you only can have 20 cookie by server on a client (by server not by site).
So how can you stock more complicated stuff than ony login password saying for example a complete shoping cart in cookie?
This doesn't look really doable to me, in real life i mean.
Perhaps i'm wrong but i doubt about it so i think it's important to precise that your solution can only handle really simpe session with a few datas in it and certainly not a much larger dataset.