[WebMethod]
    public string HelloWorld() {
        String Res = "", MyOID = "1.2.3.4.5.6.7.8";
        Boolean Exist = false;
        X509Certificate2 x509 = new X509Certificate2();
        x509.Import(HttpContext.Current.Request.ClientCertificate.Certificate);

        foreach (X509Extension extension in x509.Extensions)
        {
            if (extension.Oid.FriendlyName == "Enhanced Key Usage")
            {
                X509EnhancedKeyUsageExtension ext = (X509EnhancedKeyUsageExtension)extension;
                OidCollection oids = ext.EnhancedKeyUsages;
                foreach (Oid oid in oids)
                {
                    if (oid.Value == MyOID) Exist = true;
                }

                if (Exist)
                {
                    Res = "О, ДА! ТЫ правильный чувак! У тебя есть OID: "+MyOID;


                    ... тут обработка метода ...
  
                }
                else
                {
                    Res = "А ну брысь отсюда, шпана голопузая!";
                }

            }
        }

        return Res;
    }