.NET IdentityServer4 for OAuth2.0 and OpenID Connect

A detailed guide to implementing IdentityServer4 for securing applications using OAuth2.0 and OpenID Connect.

Table of Contents

• Introduction
• What is IdentityServer4?
• Understanding OAuth2.0 and OpenID Connect
• Setting Up IdentityServer4
• Configuring Clients in IdentityServer4
• Implementing Secured APIs with IdentityServer4
• OpenID Connect Workflow in Action
• Example Implementation
• Best Practices
• Conclusion

Introduction

As modern applications rely heavily on secure authentication and authorization mechanisms, IdentityServer4 emerges as a robust solution for implementing OAuth2.0 and OpenID Connect. This blog explores how .NET developers can leverage IdentityServer4 to secure APIs, manage user identity, and enable Single Sign-On (SSO) across applications.

What is IdentityServer4?

IdentityServer4 is a popular open-source framework for implementing authentication and authorization solutions in .NET applications. It supports:

• OAuth2.0 for resource access authorization.
• OpenID Connect for user authentication and identity management.
• Token issuance for secure communication between clients and APIs.

Understanding OAuth2.0 and OpenID Connect

OAuth2.0 is an authorization framework that allows third-party applications to access resources on behalf of the user without sharing credentials. On the other hand, OpenID Connect (OIDC) builds on OAuth2.0 to provide authentication and user identity details.

Key components of OAuth2.0:

• Resource Owner: The user who owns the data.
• Client: The application requesting access.
• Authorization Server: Issues tokens.
• Resource Server: Provides access to protected resources.

Setting Up IdentityServer4

To start using IdentityServer4, follow these steps:

1. Install the IdentityServer4 NuGet package:

dotnet add package IdentityServer4

2. Configure IdentityServer in the Startup.cs file:

services.AddIdentityServer()
    .AddInMemoryClients(Clients.Get())
    .AddInMemoryApiResources(ApiResources.Get())
    .AddInMemoryApiScopes(ApiScopes.Get())
    .AddInMemoryIdentityResources(IdentityResources.Get())
    .AddDeveloperSigningCredential();
                

Configuring Clients in IdentityServer4

Clients represent applications that request tokens from the IdentityServer. Below is an example client configuration:

public static IEnumerable<Client> Get()
{
    return new List<Client>
    {
        new Client
        {
            ClientId = "client_id",
            ClientSecrets = { new Secret("client_secret".Sha256()) },
            AllowedGrantTypes = GrantTypes.ClientCredentials,
            AllowedScopes = { "api_scope" }
        }
    };
}
            

Implementing Secured APIs with IdentityServer4

Secure your API endpoints by validating tokens issued by IdentityServer4. Example middleware configuration:

services.AddAuthentication("Bearer")
    .AddJwtBearer("Bearer", options =>
    {
        options.Authority = "https://localhost:5001";
        options.RequireHttpsMetadata = false;
        options.Audience = "api_scope";
    });
            

OpenID Connect Workflow in Action

OpenID Connect extends OAuth2.0 by adding the following:

• ID Token: Contains user identity information.
• UserInfo Endpoint: Provides additional details about the user.

Example Implementation

Here’s a step-by-step example of integrating IdentityServer4 in an ASP.NET Core project:

1. Set up IdentityServer4 as described in the earlier section.
2. Secure your API endpoints using JWT authentication.
3. Create a client application to request tokens and call the API.

Best Practices

• Always use HTTPS to secure communication.
• Use strong secrets and rotate them periodically.
• Enable logging for troubleshooting authentication issues.
• Keep your IdentityServer4 packages updated.

Conclusion

IdentityServer4 provides a comprehensive solution for implementing OAuth2.0 and OpenID Connect in .NET applications. By leveraging its powerful features, developers can secure APIs, enable SSO, and build robust authentication and authorization systems.

© 2025 Sandeep Mhaske | ayodhyya