Getting RsaProtectedConfigurationProvider not found error

public static void Encrypt()

//Create a new ExeConfigurationFileMap

ConfigurationFileMap oFile = new ConfigurationFileMap();

//ExeConfigurationFileMap oFile = new ExeConfigurationFileMap();

////Precise its path

oFile.MachineConfigFilename = @"C:\Encryption\web.config";

string provider = "RsaProtectedConfigurationProvider";

//Create a new configuration object related to the configuration file

Configuration Config = ConfigurationManager.OpenMappedMachineConfiguration(oFile);

//Create a section and set it as the targeted section

ConfigurationSection constrings = Config.Sections.Get("connectionStrings");

//oSection = oConfiguration.GetSection("connectionStrings") as ConnectionStringsSection;

//if the section is already encrypted dont ecrypt it again

if (constrings != null && !constrings.SectionInformation.IsProtected)

if (!constrings.ElementInformation.IsLocked)

//if (!oSection.SectionInformation.IsProtected)

//{

//The section ecryption

//Update the configuration file

MessageBox.Show("Connection string encrypted");

//}