# File lib/openid/message.rb, line 357
    def get_key(namespace, ns_key)
      namespace = _fix_ns(namespace)
      return ns_key if namespace == BARE_NS

      ns_alias = @namespaces.get_alias(namespace)

      # no alias is defined, so no key can exist
      return nil if ns_alias.nil?

      if ns_alias == NULL_NAMESPACE
        tail = ns_key
      else
        tail = "#{ns_alias}.#{ns_key}"
      end

      return 'openid.' + tail
    end