A very basic plugin that when a player joins, if they haven't registered before they'll be required to register. If a player has already registered they'll be required to log in. It's very safe because it teleports them back to the world spawn, makes them immune to damage, they can't talk, or move until they log in. They have the ability to change there password too.
register (password) (password again)
changepass (current password) (new password) (new password again)
don't put your password as your minecraft password, as it is viewable in the config in plain text.
- if the spawn at block 68 isn't air, when they register for the first time, they'll be suffocating
--they won't take damage if they just have to log in because they'll be teleported back to where they were
---they'll be fine after the first death