I know questions of this kind [have](https://stackoverflow.com/questions/24298/best-solution-for-authentication-in-ruby-on-rails) [been](https://stackoverflow.com/questions/13254/open-id-authentication-in-ruby-on-rails) [asked](https://stackoverflow.com/questions/38901/strange-rails-authentication-issue) before, but my situation differs a little.

On my rails app I have to validate the user login against an existing repository and then control authorization to given modules. So, I don't want the solution I go for to generate a model for my users and rely on that. The authetication per se needs to be customized.

Under that scenario, what would be the best plugin to use?