/// <summary>
        /// Imports the <see cref="AuthenticatedEncryptorDescriptor"/> from serialized XML.
        /// </summary>
        public IAuthenticatedEncryptorDescriptor ImportFromXml(XElement element)
        {
            if (element == null)
            {
                throw new ArgumentNullException(nameof(element));
            }

            // <descriptor>
            //   <encryption algorithm="..." />
            //   <validation algorithm="..." /> <!-- only if not GCM -->
            //   <masterKey requiresEncryption="true">...</masterKey>
            // </descriptor>

            var settings = new AuthenticatedEncryptionSettings();

            var encryptionElement = element.Element("encryption");

            settings.EncryptionAlgorithm = (EncryptionAlgorithm)Enum.Parse(typeof(EncryptionAlgorithm), (string)encryptionElement.Attribute("algorithm"));

            // only read <validation> if not GCM
            if (!AuthenticatedEncryptionSettings.IsGcmAlgorithm(settings.EncryptionAlgorithm))
            {
                var validationElement = element.Element("validation");
                settings.ValidationAlgorithm = (ValidationAlgorithm)Enum.Parse(typeof(ValidationAlgorithm), (string)validationElement.Attribute("algorithm"));
            }

            Secret masterKey = ((string)element.Elements("masterKey").Single()).ToSecret();

            return(new AuthenticatedEncryptorDescriptor(settings, masterKey, _services));
        }
        public XmlSerializedDescriptorInfo ExportToXml()
        {
            // <descriptor>
            //   <encryption algorithm="..." />
            //   <validation algorithm="..." /> <!-- only if not GCM -->
            //   <masterKey requiresEncryption="true">...</masterKey>
            // </descriptor>

            var encryptionElement = new XElement("encryption",
                                                 new XAttribute("algorithm", Settings.EncryptionAlgorithm));

            var validationElement = (AuthenticatedEncryptionSettings.IsGcmAlgorithm(Settings.EncryptionAlgorithm))
                ? (object)new XComment(" AES-GCM includes a 128-bit authentication tag, no extra validation algorithm required. ")
                : (object)new XElement("validation",
                                       new XAttribute("algorithm", Settings.ValidationAlgorithm));

            var outerElement = new XElement("descriptor",
                                            encryptionElement,
                                            validationElement,
                                            MasterKey.ToMasterKeyElement());

            return(new XmlSerializedDescriptorInfo(outerElement, typeof(AuthenticatedEncryptorDescriptorDeserializer)));
        }