Windows Store App and iFrame cookie

Question

I have a Javascript Windows Store Application that hosts an iframe. The webpage inside of my iframe is not aware of my app, so it doesn't have any PostMessage interface to my app.

The iframe will set a cookie when I 'log in' in the iframe. But in my app, I don't believe that there is a way that I can get the iframe cookie. Is this correct?

What if the web page knows about my application? (i.e. I can change the webpage in the iframe) Do I use PostMessage?