I am quite new to OPC UA protocol; I recently wrote a C interface to connect to some machines using the libraries from OPCFoundation, and so far I did not have to deal with certificates, since all the machines allowed unsecured connections.

Having now to connect to a machine that uses certificates, how should I modify my code in order to simulate the "trust server certificate" action I can take on UAExpert software, which warns about the certificate, but allows me to trust the certificate from the machine (saving it to a local folder) and continue with the session?

In my code, I have "AutoAcceptUntrustedCertificates = true", but when I try to open the session I get a

Opc.Ua.ServiceResultException "Certificate is not trusted.\r\nSubjectName: CN=NxOpcUaServer@192.168.1.10 ..."

Thanks in advance

    using Opc.Ua;
    using Opc.Ua.Client;
    using Opc.Ua.Configuration;

    private static String CallOpcUA(string indirizzoMacchina, string displayNome, string idNodo)
    {
        var config = new ApplicationConfiguration()
        {
            ApplicationName = "MyApp",
            ApplicationUri = Utils.Format(@"urn:{0}:MyApp", System.Net.Dns.GetHostName()),
            ApplicationType = ApplicationType.Client,
            SecurityConfiguration = new SecurityConfiguration
            {
                ApplicationCertificate = new CertificateIdentifier { StoreType = @"Directory", StorePath = @"%CommonApplicationData%\OPC Foundation\CertificateStores\MachineDefault", SubjectName = "MyApp" },
                TrustedIssuerCertificates = new CertificateTrustList { StoreType = @"Directory", StorePath = @"%CommonApplicationData%\OPC Foundation\CertificateStores\UA Certificate Authorities" },
                TrustedPeerCertificates = new CertificateTrustList { StoreType = @"Directory", StorePath = @"%CommonApplicationData%\OPC Foundation\CertificateStores\UA Applications" },
                RejectedCertificateStore = new CertificateTrustList { StoreType = @"Directory", StorePath = @"%CommonApplicationData%\OPC Foundation\CertificateStores\RejectedCertificates" },
                AutoAcceptUntrustedCertificates = true
            },
            TransportConfigurations = new TransportConfigurationCollection(),
            TransportQuotas = new TransportQuotas { OperationTimeout = 15000 },
            ClientConfiguration = new ClientConfiguration { DefaultSessionTimeout = 60000 },
            TraceConfiguration = new TraceConfiguration()
        };
        config.Validate(ApplicationType.Client).GetAwaiter().GetResult();
        if (config.SecurityConfiguration.AutoAcceptUntrustedCertificates)
        {
            config.CertificateValidator.CertificateValidation += (s, e) => { e.Accept = (e.Error.StatusCode == StatusCodes.BadCertificateUntrusted); };
            config.CertificateValidator.CertificateValidation += (s, e) => { e.Accept = (e.Error.StatusCode == StatusCodes.BadCertificateTimeInvalid); };
        }

        var application = new ApplicationInstance
        {
            ApplicationName = "MyApplication",
            ApplicationType = ApplicationType.Client,
            ApplicationConfiguration = config
        };

        try
        {
            application.CheckApplicationInstanceCertificate(false, 2048).GetAwaiter().GetResult();
        }
        catch (System.IO.FileLoadException e)
        {
        }

        try
        {
            identity = new UserIdentity();

            var selectedEndpoint = CoreClientUtils.SelectEndpoint(indirizzoMacchina, useSecurity: false);

            using (var session = Session.Create(config, new ConfiguredEndpoint(null, selectedEndpoint, EndpointConfiguration.Create(config)), false, "", 60000, identity, null).GetAwaiter().GetResult())
            {
               
                [read value from the node]

            }

        }
        catch (Opc.Ua.ServiceResultException e)
        {
        }
        
    }