Hiding the query string in ASP.NET

This may or may not be a .NET related question, but I really don't know
for sure, so...

In our application we pass some data from one page to another using
other user cookies (not browser cookies), and we also have been passing
some information via the querystring.  For example, a reimbursement
request id appearing in the querystring as 

ViewReimbursementRequest.aspx?request_id=324

Well, this works great except that if a user happens to know the
reimbursement request number of a request belonging to another user, he
could edit the query string to be able to look at that request.  We
would like to limit this capability, and I recall having heard there was
a way to suppress the querystring part after the page name, and still be
able to pass the querystring.  I'm not talking about using POST, or
about embedding the relevant data as a cookie, or in a session variable.
I remember hearing about this capability in a presentation I heard about
eight or nine years ago, as a new capability of Windows Server 2000 or
NT.  But I don't know if it was a server option or setting or something
that could be set within an application.

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