EnableViewStateMac
Indicates that ASP.NET should run a machine authentication check (MAC) on the page's view state when the page is posted back from the client. true if view state should be MAC checked; otherwise, false. The default is true.
Note A view state MAC is an encrypted version the hidden variable that a page's view state is persisted to when sent to the browser. When you set this attribute to true, the encrypted view state is checked to verify that it has not been tampered with on the client.