how to protect signup in devise using a security key in ruby on rails

In devise I want to authenticate signup for admin. only that person can signup who has a predefined random string. I want to do this for all devises except user. i have admin, faculty, student, branch and parent login. so i want to allow those to signup who has a key which provided by me and different for all login system. I want to do this by scaffold to create a new security key for those logins and edit any time. So Please help me how to do this task .