Need a very specific login/password/code capability

I need a way to give certain people different codes so they can access exclusive pages. Ideally, I’d love a plugin that allows people to type in their secret code in the sidebar or on a page that will take them to a page just for them. Maybe a sidebar widget with a text area they can type the code in that will take them to website.com/code would work or a plugin to password protect pages but not require registration, login, or anything except one password. I do not want anyone to have to register or completely log in…just one access code to take them to the proper page and give them the ability to view it. Any ideas would be greatly appreciated.