What is OpenIddict?

At some point in every .NET developer's journey, the same question comes up: how do I handle authentication properly? Not just a login form — but token issuance, authorization flows, secure API access, and all the moving parts that modern applications require.

If you have been researching this topic, you have probably come across OpenIddict. This article explains what it is, what it actually does, and why so many developers in the .NET ecosystem rely on it.

What is OpenIddict?

OpenIddict is an open-source framework for ASP.NET Core that allows you to add a fully compliant OAuth 2.0 and OpenID Connect authorization server to your application. In plain terms: it is the component responsible for issuing and validating the tokens that your applications and APIs use to authenticate users and authorize requests.

It is not a hosted service, a SaaS product, or a cloud platform. It is a library you add to your .NET project, giving you complete control over where it runs and how it is configured.

OpenIddict is maintained by Kévin Chalet and has been actively developed for years. It is certified by the OpenID Foundation, which means it has passed formal conformance tests against the OpenID Connect specification — not a minor detail when security is involved.

How it fits into the .NET ecosystem

One of the reasons OpenIddict is popular in the .NET world is that it does not try to replace what already exists — it complements it.

OpenIddict integrates naturally with ASP.NET Core Identity, Microsoft's user management system. Identity handles the user store: creating accounts, hashing passwords, managing roles and claims. OpenIddict sits on top of it and handles the protocol layer: deciding when to issue a token, what claims to include, and how to respond to authorization requests.

For persistence, OpenIddict uses Entity Framework Core, which means it works with the same database you are probably already using — SQL Server, PostgreSQL, or MySQL. Applications, tokens, and authorizations are stored as regular EF Core entities, queryable and manageable like any other data in your system.

This tight integration with the existing .NET stack is intentional. Rather than introducing new abstractions and paradigms, OpenIddict speaks the same language as the rest of your application.

What OpenIddict actually handles

To understand what OpenIddict does, it helps to look at what an authorization server is responsible for. When a user logs into your application, or when one service needs to call another securely, a series of standardized exchanges takes place. OpenIddict implements all of them.

Concretely, it exposes and manages the following:

• Authorization endpoint — where users are redirected to authenticate and consent
• Token endpoint — where authorization codes or credentials are exchanged for tokens
• UserInfo endpoint — where applications retrieve profile information about the authenticated user
• Introspection endpoint — where resource servers validate tokens without parsing them directly
• Revocation endpoint — where tokens can be explicitly invalidated
• Discovery endpoint — the /.well-known/openid-configuration document that client applications use to configure themselves automatically

It also supports all the major OAuth 2.0 grant types: Authorization Code (with and without PKCE), Client Credentials, Refresh Token, and Device Authorization.

To give you a concrete sense of what this looks like in code, here is a minimal example of how OpenIddict is registered in an ASP.NET Core application:

services.AddOpenIddict()
    .AddCore(options =>
    {
        options.UseEntityFrameworkCore()
               .UseDbContext<ApplicationDbContext>();
    })
    .AddServer(options =>
    {
        options.SetAuthorizationEndpointUris("/connect/authorize")
               .SetTokenEndpointUris("/connect/token")
               .SetUserinfoEndpointUris("/connect/userinfo");

        options.AllowAuthorizationCodeFlow()
               .AllowRefreshTokenFlow()
               .RequireProofKeyForCodeExchange();

        options.AddDevelopmentEncryptionCertificate()
               .AddDevelopmentSigningCertificate();

        options.UseAspNetCore()
               .EnableAuthorizationEndpointPassthrough()
               .EnableTokenEndpointPassthrough();
    })
    .AddValidation(options =>
    {
        options.UseLocalServer();
        options.UseAspNetCore();
    });
        

This is a working starting point — but it is also just the beginning. A production-ready configuration requires handling certificates properly, configuring token lifetimes, securing endpoints, registering client applications, and more. The flexibility of OpenIddict means all of this is possible, but it also means the responsibility falls on the developer to get it right.

Why developers choose it

Several factors explain why OpenIddict has established itself as a reference choice in the .NET space:

Open source with a permissive license. OpenIddict is released under the Apache 2.0 license, which means you can use it freely in commercial projects without licensing fees or usage restrictions.

Active and transparent development. The project is actively maintained, with a public roadmap, regular releases, and a responsive issue tracker. When a security vulnerability is discovered or a new specification is published, updates follow.

Standards compliance, verified. As mentioned, OpenIddict holds official OpenID Connect certification. This is not self-declared — it is the result of passing a formal test suite administered by the OpenID Foundation.

No external dependencies beyond the .NET stack. Unlike some alternatives that require specific infrastructure or cloud services, OpenIddict runs wherever .NET runs. On-premises, in Docker, on Azure, on a Raspberry Pi — the deployment model is entirely yours to decide.

A strong and growing community. Stack Overflow, GitHub Discussions, and various .NET communities have built up a solid body of knowledge around OpenIddict. Finding answers to common configuration questions is increasingly straightforward.

Wrapping up

OpenIddict is a mature, certified, and well-maintained framework that brings OAuth 2.0 and OpenID Connect to the .NET ecosystem without forcing you outside the tools you already know. It handles the protocol complexity so you can focus on your application — provided you configure it correctly.

And that last part — configuring it correctly — is where things get interesting. In the next article in this series, we will look at the most common mistakes developers make when setting up OpenIddict from scratch, and why getting those details wrong can have real security consequences.

About IdentitySuite

IdentitySuite simplifies enterprise authentication for .NET developers. Built on proven technologies like ASP.NET Core Identity and Openiddict, we eliminate the complexity of OAuth 2.0 and OpenID Connect implementation while maintaining enterprise-grade security standards.