I have a client who has a C# app on windows. He wants to add LDAP authentication to it, replacing the bespoke user/password stuff he's got. I'm much more of a linux guy, not so knowledgable about what's current in windows/C#, but I want to help him out.

Is there a simple library we could use, or something which runs as a service and presents a REST API? I'd like to find the least intrusive way to help him with this integration.