Authentication for Web Forms

Hi All,
     Consider I have 2 forms. one is login form, which contain login functionality. If the
login success means, the user can enter the 2nd form(userform). I don't have option of
using session. Currently i have used encrypted Query string to track the user.

 To elobrate this, if the user provide the username and pwd in login form, then the username
will be encrypted and it will be go to the next page that is UserForm as follows:

   http://www.mysite.com/Login.aspx?enc=USERID

If we copy the above url and put it in any browser the user bypass the LOGIN page and can enter DIRECTLY to the Userform page. I want to prevent this? How can i do this?

Thanks.
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