ASP.MVC antiforgery token and cryptographic errors

I'm using ELMAH to handle errors in my MVC sites and I've noticed over the past couple of weeks that I'm getting some CryptographicExceptions thrown. The message is:

System.Security.Cryptography.CryptographicException: Padding is invalid and cannot be removed.

System.Web.Mvc.HttpAntiForgeryException: A required anti-forgery token was not supplied or was invalid. ---> System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. --->

The application is not running in a cluster and I can't seem to reproduce these errors. They look like valid requests -- not a hand-crafted post -- and do contain the __RequestVerificationToken cookie. I do have the required HTML helper on the page, inside the form (my login form).

I haven't had any user complaints, yet, so I'm assuming that eventually it works for whoever is trying to lo开发者_如何学Gogin, but I'm left wondering why this could be happening.

Anyone else seeing this behavior or have any ideas on how to diagnose the exception -- like I said, I can't get it to fail. Deleting the cookie in FF comes up with a different error. Modifying the cookie (changing or removing the contents) also results in a different error, as does modifying the contents of the hidden token input on the page.

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I'm not sure if there is a correlation, but after adding a robots.txt file that excludes my login actions, I am no longer seeing these errors. I suspect that it has to do with a crawler hitting the page and trying to invoke the login action.

EDIT: I've also see this issue when receiving old cookies after the application pool has recycled. I've resorted to setting the machineKey explicitly so that changes to the validation/decryption keys on application restarts don't affect old cookies that may be resent.

After updating the site and going to a fixed machineKey I found that I was still getting these errors from people who had cookies from the previous version. As a temporary work around I've added the following Application_Error handler:

    public void Application_Error()
    {
        var exception = Server.GetLastError().GetBaseException();
        if (exception is System.Security.Cryptography.CryptographicException)
        {
            Server.ClearError();
            if (Request.IsAuthenticated)
            {
                var form = new FormsAuthenticationWrapper();
                form.SignOut();
                Session.Clear();
            }
            Response.Cookies.Clear();
            Response.Redirect( "~" );
        }
    }

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I'm not so sure this has anything specifically to do with the antiforgery system, the inner exception states 'Validation of viewstate MAC failed.', from what I can tell, the default infrastructure for the antiforgery system has a dependency on the viewstate (actually if you take a look here you'll see see the dependency and horror (the CreateFormatterGenerator method at the bottom)).

As for why the viewstate mac is failing on the fake request, I'm not sure- but given the horror that exists in deserializing the antiforgery token (processing an entire fake request), it doesn't suprise me at all..