The name 'ProtectedData' does not exist in the current context, and The name 'DataProtectionScope' does not exist in the current context

if the compile time error "The name 'ProtectedData' does not exist in the current context" and "The name 'DataProtectionScope' does not exist in the current context" are appearing even after providing the name space
using System.Security; and 
using System.Security.Cryptography; 
manually add the reference from solution explorer >> add reference >> select the .Net tab >> select System.Security and click Ok.