The anti-forgery token could not be decrypted

Solution 1:

I just received this error as well and, in my case, it was caused by the anti-forgery token being applied twice in the same form. The second instance was coming from a partial view so wasn't immediately obvious.

Solution 2:

validationKey="AutoGenerate"

This tells ASP.NET to generate a new encryption key for use in encrypting things like authentication tickets and antiforgery tokens every time the application starts up. If you received a request that used a different key (prior to a restart for instance) to encrypt items of the request (e.g. authenication cookies) that this exception can occur.

If you move away from "AutoGenerate" and specify it (the encryption key) specifically, requests that depend on that key to be decrypted correctly and validation will work from app restart to restart. For example:

<machineKey  
validationKey="21F090935F6E49C2C797F69BBAAD8402ABD2EE0B667A8B44EA7DD4374267A75D7
               AD972A119482D15A4127461DB1DC347C1A63AE5F1CCFAACFF1B72A7F0A281B"           
decryptionKey="ABAA84D7EC4BB56D75D217CECFFB9628809BDB8BF91CFCD64568A145BE59719F"
validation="SHA1"
decryption="AES"
/>

You can read to your heart's content at MSDN page: How To: Configure MachineKey in ASP.NET