Authentication

Is there a plug or a way in which you can provide an authentication key (password) which would then allow the person to create an account in Moodle?

I'd like to be able to provide an authentication key to someone I want to allow to create an account on my Moodle site, something simple, like an enrollment key but which allows them to create an account. 

I know there is the self registration with email confirmation plugin but that can have security issues with anyone being able to use it. Instead I want them to provide an authentication key before even being able to start that process.