If you don't need 100% reach, have a look at xauth source: http://github.com/xauth/xauth http://github.com/xauth/xauth They use a central domain and window.pos

If you don't need 100% reach, have a look at xauth source: http://github.com/xauth/xauth http://github.com/xauth/xauth They use a central domain and window.postMessage to make a secure but shared storage.