Authentication with IdentityServer4 in .NET Core

14 January 2019

When building secure applications, one of the most important aspects to consider is authentication. In modern web applications, OAuth 2.0 and OpenID Connect have become the industry standards for handling user authentication and authorization. IdentityServer4 is a powerful library that simplifies the integration of these standards in .NET Core applications.

In this post, we’ll explore how to integrate IdentityServer4 into a .NET Core application, how to configure OAuth 2.0 and OpenID Connect, and how to secure microservices using IdentityServer4.

What is IdentityServer4?

IdentityServer4 is an open-source framework for implementing OAuth 2.0 and OpenID Connect in .NET Core applications. It allows you to create a central authentication server (also known as an Authorization Server) that can handle authentication for your clients, including web applications, mobile applications, and microservices.

By using IdentityServer4, you can:

Implement Single Sign-On (SSO) across multiple applications.
Secure APIs and microservices using OAuth 2.0.
Enable OpenID Connect for user authentication, allowing users to sign in with various identity providers.

With IdentityServer4, you have full control over the authentication flow and can easily integrate with external identity providers, such as Google, Facebook, or Azure Active Directory.

Setting Up IdentityServer4 in .NET Core

Let’s get started by setting up IdentityServer4 in a .NET Core application.

Install the NuGet package:

First, you need to install the IdentityServer4 package into your project. You can do this by running the following command in your terminal:

dotnet add package IdentityServer4

Configure IdentityServer:

Next, you’ll need to configure IdentityServer4 in the Startup.cs file. In the ConfigureServices method, add IdentityServer and configure your authentication resources.

Here’s a basic setup:

public void ConfigureServices(IServiceCollection services)
{
    services.AddIdentityServer()
        .AddInMemoryClients(Configuration.GetSection("IdentityServer:Clients"))
        .AddInMemoryApiResources(Configuration.GetSection("IdentityServer:ApiResources"))
        .AddInMemoryIdentityResources(Configuration.GetSection("IdentityServer:IdentityResources"))
        .AddTestUsers(TestUsers.Users)
        .AddDeveloperSigningCredential();
}

In this configuration:

AddInMemoryClients: You can configure client applications that will authenticate with IdentityServer.
AddInMemoryApiResources: Defines the APIs that are being protected by IdentityServer.
AddInMemoryIdentityResources: Configures the identity resources (such as user information) that IdentityServer will expose.
AddTestUsers: A simple way to add users for testing purposes.
AddDeveloperSigningCredential: This is for development purposes. In production, you’ll need to provide a proper signing certificate.

OAuth 2.0 and OpenID Connect Configuration

After setting up IdentityServer, the next step is to configure OAuth 2.0 and OpenID Connect to secure your APIs and web applications.

OAuth 2.0 Flow

OAuth 2.0 is used for authorization, allowing users to grant limited access to their resources (such as APIs) without sharing their credentials. The flow works as follows:

The client application redirects the user to IdentityServer to log in.
IdentityServer authenticates the user and asks for consent to grant access to the requested resources.
Upon successful authentication and consent, IdentityServer sends an authorization code back to the client application.
The client application exchanges the authorization code for an access token, which is used to access the protected API.

OpenID Connect Flow

OpenID Connect is an identity layer on top of OAuth 2.0. It allows you to authenticate users and retrieve information about their identity. The flow is similar to OAuth 2.0, but with the addition of an ID token, which contains user authentication data.

The client application redirects the user to IdentityServer for authentication.
IdentityServer authenticates the user and returns an ID token and an access token.
The client application can use the ID token to obtain information about the authenticated user.

In your appsettings.json, configure your IdentityServer clients and resources:

{
  "IdentityServer": {
    "Clients": [
      {
        "ClientId": "myclient",
        "ClientSecret": "mysecret",
        "AllowedGrantTypes": ["authorization_code"],
        "RedirectUris": ["http://localhost:5002/signin-oidc"],
        "PostLogoutRedirectUris": ["http://localhost:5002/signout-callback-oidc"],
        "AllowedScopes": ["openid", "profile", "api1"]
      }
    ],
    "ApiResources": [
      {
        "Name": "api1",
        "DisplayName": "My API"
      }
    ],
    "IdentityResources": [
      {
        "Name": "openid",
        "DisplayName": "OpenID Connect"
      },
      {
        "Name": "profile",
        "DisplayName": "Profile"
      }
    ]
  }
}

Securing Microservices with IdentityServer

IdentityServer4 is particularly useful in microservices architectures because it can serve as a centralized identity and access management solution. Each microservice can delegate authentication and authorization to IdentityServer.

Token-Based Authentication for Microservices

In a microservices environment, each service can secure its endpoints using OAuth 2.0 access tokens issued by IdentityServer.

API Gateway: The API Gateway authenticates incoming requests, validates access tokens, and forwards them to the appropriate microservice.
Microservices: Each microservice validates the token issued by IdentityServer to ensure the request is authorized.

To integrate token-based authentication in a microservice, you can add authentication middleware to the Startup.cs of each service:

public void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication("Bearer")
        .AddJwtBearer(options =>
        {
            options.Authority = "https://localhost:5000"; // IdentityServer URL
            options.Audience = "api1"; // API resource
            options.RequireHttpsMetadata = false;
        });
}

The AddJwtBearer middleware ensures that the incoming requests are authenticated using the access tokens issued by IdentityServer.

Conclusion

Integrating IdentityServer4 into your .NET Core application allows you to implement robust authentication and authorization mechanisms with OAuth 2.0 and OpenID Connect. Whether you’re building a simple web application or a complex microservices architecture, IdentityServer can secure your resources while giving you full control over the authentication flow.

By leveraging OAuth 2.0 for authorization and OpenID Connect for user authentication, you can ensure that your applications and microservices are secure and scalable.