Verify AD or LDAP crendentials via REST API

Active Directory is a database based system that provides authentication, directory, policy, and other services in a Windows environment

LDAP (Lightweight Directory Access Protocol) is an application protocol for querying and modifying items in directory service providers like Active Directory, which supports a form of LDAP.

Short answer: AD is a directory services database, and LDAP is one of the protocols you can use to talk to it.

There are several options. If you really mean LDAP, as opposed to just Active Directory, I would probably look at using System.DirectoryServices.Protocols to perform an LDAP bind using the supplied credentials via a secure channel.

Strictly, this isn't Single Sign-On. SSO means only having to submit your creds once when you first log on. This is simply reducing complexity for the users by only having a single ID. Usually, for Windows clients in an enterprise environment with a mixture of platforms and technologies, SSO is achieved by a client added to the desktop which manages authentication to various systems. In an MS-only environment, you might achieve SSO if all of your web apps are on IIS, you use IE and use Integrated Windows Authentication, impersonation and all of that stuff.

You could consider auto-enrolling an authenticated user into your system, unless you require profile-type data to be preconfigured. If you do require pre-configuration of users, you could consider regularly importing (all, or a filtered subset of) users from the LDAP directory and having them in a not-configured state, such that the admins select from an existing list of not-configured users rather than typing in IDs. Otherwise, you risk your admins typing in the wrong user ID and having mismatches.

You could provide an API such that Identity and Access Management solutions (given your Microsoft slant, see ILM2 007 as one example) can integrate with your system and do all of the user account management for you.