sharepoint login api

I am writing a proposal to a prospective client for a complete migration of their website from their classic ASP CMS to Drupal. For the most part, it will be a very straightforward migration using very common Drupal features. The only complicated part is the Sharepoint login. Currently, employees are able to use the website front-end to log into the Sharepoint portal. I only need to be able to replicate that feature and put it on the new site. So by no means does this need to be a complete Sharepoint integration.