Introduction to Identity Server

What is Identity Server?

Identity Server (IdentityServer4) is an OpenID Connect and OAuth 2.0 framework for ASP.NET Core
- IdentityServer4 is redesigned for ASP.NET Core and .NET Core
- incorporates all the protocol implementations and extensibility points needed to integrate token-based authentication, SSO and API access control in your applications
It can be used to make your application an authentication / single sign on server
It can also issue access tokens for 3rd party clients
companies using .NET to build identity and access control solutions for modern applications, including single sign-on, identity management, authorization, and API security
IdentityServer is OpenID Certified and part of the .NET Foundation
Founded and maintained by Dominick Baier and Brock Allen

It enables the following features in your applications:

Authentication as a Service: Centralized login logic and workflow for all of your applications (web, native, mobile, services). IdentityServer is an officially certified implementation of OpenID Connect.
Single Sign-on / Sign-out: Single sign-on (and out) over multiple application types.
Access Control for APIs: Issue access tokens for APIs for various types of clients, e.g. server to server, web applications, SPAs and native/mobile apps.
Federation Gateway: Support for external identity providers like Azure Active Directory, Google, Facebook etc. This shields your applications from the details of how to connect to these external providers.
Focus on Customization: many aspects of IdentityServer can be customized to fit your needs. Since IdentityServer is a framework and not a boxed product or a SaaS, you can write code to adapt the system the way it makes sense for your scenarios.