Share session between domain and sub-domain

We're doing a whitelabelled version of our site, which will be hosted at foo.ourdomain.com.

However we need to ensure session is maintained between www.ourdomain.com and foo.ourdomain.com, as our SSL certificate only covers the main domain.

In practice this means we'll swap to the main domain on our payment pages, which run HTTPS, and then redirect back to the subdomain, after payment.

So the question is: How do we maintain the session when doing so ?

I've tried with <httpCookies domain=".ourdomain.开发者_如何学JAVAcom" /> in web.config to no avail :-(

Edit: Figured it out now, I lacked domain on my <forms /> tag to handle login properly.

Like I mentioned in my edit, I just lacked the domain attribute on my tag in web.config.