No puzzle here. You can only log in if Google has flagged your search topics.
On google search, you have the variable window.location.search = "?gfe_rd=cr&ei=XXXGyZiVNHoFcuF8Qe7wYHACw&gws_rd=ssl"
That string is appended to the url of the iframe: src="https://foobar.withgoogle.com/"+window.location.search