Whenever you use Session, ViewState, Forms authentication or any other encrypted or secured values, ASP.NET does the encryption and decryption. ASP.Net uses a set of special keys for this encryption and decryption process. Generally, these keys are hidden from you and generated automatically by ASP.NET every time your application recycles or your application pool is restarted. However, there are circumstances when you want to specify a particular, persistent key for a machine. please visit helpprogrammers.com for more info.