capturing an action when button is clicked

What is a good way:
 
I have page1.aspx contains (Next button), it redirects to Page2.aspx. Page2.aspx has a back button that redirects to page1.aspx
 
I want to capture the action if page2.aspx (back button was clicked) and it made it redirect to page1.aspx page.
 
Reason is because if the back button is clicked at   if (!IsPostBack) {...} I will reload the data that was entered when going from page1 to page2.
 
I know I can use a querystring, but dont want it cause it can be modified by the user. and dont want to use session, unless its the only way.
 
Thanks
 
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