System.Net.Mail.SmtpNegotiateAuthenticationModule.Authenticate C# (CSharp) Method

        public Authorization Authenticate(string challenge, NetworkCredential credential, object sessionCookie, string spn, ChannelBinding channelBindingToken)
        {
            if (NetEventSource.IsEnabled) NetEventSource.Enter(this, "Authenticate");
            try
            {
                lock (_sessions)
                {
                    NTAuthentication clientContext;
                    if (!_sessions.TryGetValue(sessionCookie, out clientContext))
                    {
                        if (credential == null)
                        {
                            return null;
                        }

                        _sessions[sessionCookie] =
                            clientContext =
                            new NTAuthentication(false, "Negotiate", credential, spn,
                                                 ContextFlagsPal.Connection | ContextFlagsPal.InitIntegrity, channelBindingToken);
                    }

                    byte[] byteResp;
                    string resp = null;

                    if (!clientContext.IsCompleted)
                    {

                        // If auth is not yet completed keep producing
                        // challenge responses with GetOutgoingBlob

                        byte[] decodedChallenge = null;
                        if (challenge != null)
                        {
                            decodedChallenge =
                                Convert.FromBase64String(challenge);
                        }
                        byteResp = clientContext.GetOutgoingBlob(decodedChallenge, false);
                        if (clientContext.IsCompleted && byteResp == null)
                        {
                            resp = "\r\n";
                        }
                        if (byteResp != null)
                        {
                            resp = Convert.ToBase64String(byteResp);
                        }
                    }
                    else
                    {
                        // If auth completed and still have a challenge then
                        // server may be doing "correct" form of GSSAPI SASL.
                        // Validate incoming and produce outgoing SASL security 
                        // layer negotiate message.

                        resp = GetSecurityLayerOutgoingBlob(challenge, clientContext);
                    }

                    return new Authorization(resp, clientContext.IsCompleted);
                }
            }
            // From reflected type NTAuthentication in System.Net.Security.
            catch (NullReferenceException)
            {
                return null;
            }
            finally
            {
                if (NetEventSource.IsEnabled) NetEventSource.Exit(this, "Authenticate");
            }
        }