ASP.NET runs a message authentication check (MAC) on the view state of the page when the page is posted back from the client. This check determines if the view state of the page was modified on the client. For security purposes, it is recommended that you keep this attribute set to true.
For configuring the Machine Keys properly you can refer to:
How To: Configure MachineKey in ASP.NET 2.0[
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Hope it helps.