I am trying to implement FTP over SSL to upload encrypted files. I've created a SSL certificate and send it to the vendor. But I couldn't make a FTPS connection to the server. When connecting via FTPS, I'm authenticating using my private key file. I have tried .NET FTPWebRequest, SmartFTp,CuteFTP Pro. I am getting the following error:- A call to SSPI failed. See inner exception. The inner exception is "The token supplied to the function is invalid" FtpWebRequest request = (FtpWebRequest)FtpWebRequest.Create("ftp://RemoteHost.Com"); request.Credentials = new NetworkCredential("UserName", "Password"); request.KeepAlive = false; request.EnableSsl = true; X509Certificate2 cert2 = new X509Certificate2("PrivateKeyFile.pfx", "password"); request.ClientCertificates.Add(cert2); FtpWebResponse response = (FtpWebResponse)request.GetResponse(); Any Help Appreciated. Thanks.