def get_security_error(node):
        """ return a SecurityError if the specified securityData element has one, else return None """
        assert node.Name == 'securityData' and not node.IsArray
        if node.HasElement('securityError'):
            secid = XmlHelper.get_child_value(node, 'security')
            err = XmlHelper.as_security_error(node.GetElement('securityError'), secid)
            return err
        else:
            return None