Identity of the Caller Between two Web Sites

[Sandeep Alur]

Brief Description:
2 Web sites, 1 Client, Website1 Redirects to Web Site 2;
Website 2 should know that the request came from Web site
1 without getting any info via Query String

In Detail:
I have two web sites. One in my website(WebSite A) and the
other is a third party website (WebSite B). Clients
connect to my website, 'WebSite A' to start with and
eventually I redirect the user to 'WebSite B'. There will
some sort of authentication that happens between WebSite A
and WebSite B in terms of encrypted userid and password
being sent across via query string. This is the protocol
by which these two websites interact.

Now, 'Website B' should know that the request is coming
from 'WebSite A'. This is because, the user can bookmark
the 'Website B' url and in future can go directly
to 'WebSite B'. So under such circumstances, 'Website B'
should not display the page as the request is not
originating from 'WebSite A'.

I have tried using HTTP-Referer, Host IP address etc.
Referer is something, someone can mimic and make requests.
Is there any way of finding out that the request is
originating from WebSite A.

Any help is much appreciated.

Thanks,
Sandeep Alur