In ASP.Net, ViewState is used to store the state of your web page controls between post backs. You can see this value by viewing the html source code and search for ViewState. This value is encrypted by default to prevent tampering and maintain its integrity. There are 2 encryption levels: base64 and 3DES.
The setting is control in this config: viewStateEncryptionMode
If it’s encrypted with base64 you can easily decrypt, if it’s 3DES, there is no way as i know so far.