Implementing Custom Authentication in .NET with Azure AD B2C

A .NET application needs to support custom authentication scenarios for users, including social logins (Facebook, Google, etc.). Walk through the process of using Azure AD B2C for custom authentication.

Article focused on custom authentication .NET, Azure AD B2C for .NET, secure .NET authentication, Azure AD authentication

Authentication is a critical aspect of any modern application, ensuring that only authorized users can access your system. For .NET developers, integrating Azure Active Directory Business to Consumer (Azure AD B2C) for custom authentication provides a secure, flexible, and scalable solution. This article will guide you through implementing custom authentication in your .NET application using Azure AD B2C, enabling you to offer a seamless login experience for your users while leveraging Azure’s enterprise-grade security.

Why Use Azure AD B2C for Custom Authentication?

Azure AD B2C is a cloud identity service that allows you to customize and control how customers sign up, sign-in, and manage their profiles. It supports a wide variety of authentication options, from social accounts (like Facebook and Google) to local accounts and even custom policies.

Key Benefits of Using Azure AD B2C for Custom Authentication:

  • Customizable User Experience: Azure AD B2C allows you to customize the login and sign-up experience with your branding and user flows.

  • Security and Compliance: Leverage Azure’s enterprise-grade security and compliance features for managing user identities.

  • Scalability: Azure AD B2C is designed to handle millions of users and scale as your application grows.

  • Multi-Platform Support: It supports web, mobile, and desktop applications, making it versatile for various development scenarios.

Prerequisites for Setting Up Custom Authentication with Azure AD B2C

Before diving into the implementation of custom authentication, ensure you meet the following prerequisites:

  1. Azure Subscription: You need an active Azure subscription.

  2. Azure AD B2C Tenant: An Azure AD B2C tenant must be created for managing users and configuring authentication flows.

  3. .NET Core Application: A running .NET application, either ASP.NET Core MVC or Web API, that will leverage Azure AD B2C for authentication.

  4. Azure AD B2C Policies: You should configure user flows (or custom policies) in Azure AD B2C for sign-up, sign-in, and profile editing.

Step 1: Create an Azure AD B2C Tenant

  1. Navigate to the Azure Portal and search for Azure Active Directory.

  2. Select Create a resource, search for Azure AD B2C, and follow the prompts to create a new Azure AD B2C tenant.

  3. Once your tenant is created, go to the Azure AD B2C settings page and configure the necessary policies like:

    • Sign-up and sign-in: Define how users will authenticate (email, social accounts, etc.).

    • Profile editing: Allow users to update their information.

    • Password reset: Enable users to reset their password.

Step 2: Register Your .NET Application with Azure AD B2C

  1. In the Azure Portal, go to the Azure AD B2C tenant.

  2. Under App registrations, click New registration and enter a name for your application.

  3. Set the Redirect URI for your application (e.g., https://localhost:5001/signin-oidc for a local development environment).

  4. Note the Application (client) ID and Directory (tenant) ID, as you will need them later.

Step 3: Configure Authentication in the .NET Application

To integrate Azure AD B2C with your .NET Core application, you will use the Microsoft.Identity.Web library, which simplifies authentication for Azure AD and Azure AD B2C.

  1. Add the Microsoft.Identity.Web NuGet package to your .NET application:

    dotnet add package Microsoft.Identity.Web

  2. In your Startup.cs, configure authentication middleware to integrate Azure AD B2C:

    public void ConfigureServices(IServiceCollection services)
    {
    services.AddAuthentication(options =>
    {
    options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
    })
    .AddMicrosoftIdentityWebApp(Configuration.GetSection("AzureAdB2C"))
    .EnableTokenAcquisitionToCallDownstreamApi()
    .AddInMemoryTokenCaches();    services.AddControllersWithViews();
    }

  3. In your appsettings.json, add your Azure AD B2C configuration settings:

    "AzureAdB2C": {
    "Instance": "https://login.microsoftonline.com/tfp/",
    "Domain": "yourb2ctenant.onmicrosoft.com",
    "ClientId": "your-client-id",
    "ClientSecret": "your-client-secret",
    "SignUpSignInPolicyId": "B2C_1_signupsignin",
    "CallbackPath": "/signin-oidc",
    "Authority": "https://yourb2ctenant.b2clogin.com/yourb2ctenant.onmicrosoft.com/B2C_1_signupsignin"
    }

  4. Finally, in Configure method, add:

    app.UseAuthentication();
    app.UseAuthorization();

Now, your application is set up to handle authentication via Azure AD B2C. When a user accesses a page requiring authentication, they will be redirected to the Azure AD B2C login page.

Step 4: Customize the Authentication Experience

One of the key features of Azure AD B2C is the ability to customize the authentication experience (like sign-up and sign-in pages) to match your application’s branding.

  1. In the Azure Portal, go to your Azure AD B2C tenant.

  2. Under User flows, you can customize your login, sign-up, and password reset pages. You can upload your own HTML, CSS, and branding to make these pages look like your application.

  3. You can also configure social accounts such as Facebook, Google, and LinkedIn as alternative ways for users to sign in, in addition to traditional local accounts.

Step 5: Implement Custom Policies (Optional)

If you need more control over the authentication experience, you can create Custom Policies instead of using pre-configured user flows. Custom policies allow you to create complex authentication scenarios and user journeys that go beyond what is available in default user flows.

  1. In your Azure AD B2C tenant, go to Identity Experience Framework and create a new custom policy.

  2. Customize the policy by editing the XML files, where you can define different user journeys, validate custom attributes, or even trigger API calls during the sign-in process.

Step 6: Test Authentication and User Flows

Once the integration is complete, test your authentication flow by navigating to your application and triggering the login process. Azure AD B2C should redirect users to the sign-in page, handle authentication, and then return an ID token, which can be used to access your application.

Step 7: Handle Tokens and API Access

After authentication, your .NET application will receive an ID Token and an Access Token. These tokens can be used to secure APIs, identify users, and manage user-specific resources.

  1. Use the Microsoft.Identity.Web library to acquire tokens and securely call APIs:

    var token = await _tokenAcquisition.GetAccessTokenForUserAsync(new[] { "api://your-api-id/.default" });

  2. Secure your API by validating the incoming tokens using middleware in Startup.cs:

    services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApi(options =>
    {
    options.Metrics.EnableJwtBearerTokenValidation();
    });

By implementing custom authentication in .NET with Azure AD B2C, you ensure that your application benefits from robust security, scalability, and flexibility. Azure AD B2C’s ability to support multiple identity providers, including social logins and local accounts, allows you to customize the user authentication experience to fit your needs. By following the steps outlined in this guide, you can integrate Azure AD B2C with your .NET application and offer a seamless, secure authentication process for your users.

FAQs:

  1. What is Azure AD B2C? Azure AD B2C is a cloud-based identity service that enables you to authenticate users and manage their profiles for your applications.

  2. How can I customize the login page in Azure AD B2C? You can customize the login, sign-up, and profile editing pages by uploading custom HTML, CSS, and branding in the Azure portal under User Flows.

  3. Can I integrate social logins with Azure AD B2C? Yes, Azure AD B2C supports social accounts like Facebook, Google, LinkedIn, and more, along with local accounts.

  4. What are Custom Policies in Azure AD B2C? Custom Policies allow for advanced customization of authentication processes, such as multi-step user journeys or integrating external APIs during sign-in.

  5. How do I secure APIs with Azure AD B2C? After user authentication, use the ID Token and Access Token to secure and authorize access to your APIs in your .NET application.

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