Problem with WriteSessionToCookie method

Hi, I use method like to write session token to cookie:

System.IdentityModel.Services.FederatedAuthentication.SessionAuthenticationModule.WriteSessionTokenToCookie(sessionToken);

but in this line I've got error like this:

ID1074: A CryptographicException occurred when attempting to encrypt the cookie using the ProtectedData API (see inner exception for details). If you are using IIS 7.5, this could be due to the loadUserProfile setting on the Application Pool being set to false.

Application Pool for sharepoint is NetworkService. How can I resolve this?