Showing RsaProtectedConfigurationProvider not found

I was trying to encrypt file using  RsaProtectedConfigurationProvider  and the wrote the following one

It is showing me RsaProtectedConfigurationProvider  not found error.

Can any one help me in this

using System.Configuration;
public static void Encrypt()
{

//Create a new ExeConfigurationFileMap
ConfigurationFileMap oFile = new ConfigurationFileMap();
//ExeConfigurationFileMap oFile = new ...