[Form Authentication] modify ReturnUrl value

Question

Hi,
i've a problem, and i know you can help me to solve it.
I'm using Asp Net and Form Authentication.
When i'm anonymous and i must show a protected page, i'm redirected into a login.aspx page and in the path of the page (in the address bar) is added "ReturnUrl=...ecc...eccc...". It's the page where i'm redirect when i logged in.

Ok.

Now, i'll want to change the value of ReturnUrl, and i'll want to crypt  it.

from "ReturnUrl=/home/home.aspx"
to "ReturnUrl=Xfdòlmremvemmpem"

My problem isn't the decrypt algorithm (i have it), my problem is how i can access to the ReturnUrl value.

Please help me.
I hope.

Bye! and thanx to all.