ASP.NET: Get Value of Previous Page Controll's

My page consists of 4 tab(Using a Jquery Accordian).

Tab 1.) User selects Device Make and Model(Default.aspx)

Tab 2.)User Confirms Our Buyback price(GetPrice.aspx is loaded into Tab2)

Tab 3.)User Enters Contact information(Default.aspx)

Tab 4.)Confirmation Page Displaying Users Information(Conf.aspx is loaded into Tab3)

The problem arises because I need my CodeBehind(on Conf.aspx) to have access to the user's contact information. If I passed the value's through query string, it would enable users to tamper with it and insert erroneous information into my database. Keep in mind, all of this is done WITHOUT postback.

So I need:

1.)A way to tamper-proof the Query String(keep in my the query string would consist of a Phone number,email address,state,city,zip,etc)

2.)A simple way to grab the values of the previous field(Get the values of the contact page on Default.aspx, from the final page, conf.aspx)

Sorry for the long block of text, any help would be appreciated.

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