SSL TLS

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls (for all .Net 4.0 and lower applications)

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 (for all .Net 4.5 and higher applications)

For reference: https://msdn.microsoft.com/en-us/library/system.net.securityprotocoltype(v=vs.110).aspx