When you get this Error?

Two things to consider here.
1. ViewState
2. Message Authentication Code (MAC)

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1. What is ViewState

> When postback happens on webpages, the information of the
> webpage will be stored on the ViewState.
> After postback, the page controls would get the data from the ViewState.

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2. What is Message Authentication Code (MAC)

> When doing the page redirection or postback, there is a possibility that
> the data in the ViewState can be tampered.
> Can be called as HACKED ...!!!!!! ...
> In oreder to protect this tampering, the Viewstate data
> is protected by Message Authentication Code (MAC).
> This Error mainly occurs When the MAC validation for the Viewstate fails.
> When there is not provided any machine key on the web.config of each IIS application then
> Asp.Net automatically generate cryptographic key for each application and
> stores the key in the HKCU registry.
> When mismatch happens between the applications on the auto generated key, this error happens.

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How To Resolve this Issue

> On the web.config of your asp.net application, add the following code for machine key.



> For Creating the machine Key, Please follow this link. How To Generate Machine Key On IIS
> For Video of Generating Machine Key : Youtube - How To Generate Machine Key On IIS
> This would definitely resolve the issue.

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