The best way / workaround for non-compliant protocols is the link between http / https

I looked through several methods and I'm not sure if this is the best approach.

I have 2 domains - exactly the same, with the exception of different protocols.

The parent page is on http, the child iframe is enabled https.

I need to send 3 variables from a child to a parent.

What makes this the easiest way? postMessagereturn IE7? For example: http://benalman.com/projects/jquery-postmessage-plugin/

Or jsonp?