The error is generic and there are many reasons why the SSL/TLS negotiation may fail. ServicePointManager.SecurityProtocol property selects the version of the Secure Sockets Layer (SSL) or Transport Layer Security (TLS) protocol to use for new connections; existing c# connections aren't changed. Make sure the ServicePointManager settings are made before the HttpWebRequest is created, else it will not work. Also, you have to enable other security protocol versions to resolve this issue: ServicePointManager.Expect100Continue = true; ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 SecurityProtocolType.Tls SecurityProtocolType.Tls11 SecurityProtocolType.Ssl3; //createing HttpWebRequest after ServicePointManager settings HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://google.com/api/") If you create HttpWebRequest before the ServicePointManager settings it will fail and shows the error message.

Please review KB#1161 for instructions on how to update this.

This is OS & .NET Framework dependent. Many users have reported that setting the follow registry will resolved this issue: Key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319 Value: SchUseStrongCrypto Data: 1 Note that this will involve a reboot. See this guide on TLS in .NET for more information