The bookmark to login nobody but you. Simple and secure.
Before version 1.3.0, Login Dongle was site-wide available, and all users essentially shared the same bookmark as their login dongle. Starting from version 1.3.0, any user has her own login dongle. Upgrading from previous versions to 1.3.0 is done by automatically associating to each user a login dongle with the same challenge >> response as the one that was already site-wide set.
This makes it possible to upgrade hassle free, even if your blog had other users. Anyway, if that is not what you want, you can configure the login dongle of each user from the Users/All Users menu.