Enable Google Cloud IAP Authentication (OIDC) using ASP.NET Core

In this article, we will see how to Enable Google Cloud IAP Authentication – OIDC using ASP.NET Core and add an Identity-Aware Proxy to access Google Cloud-hosted protected resources and applications.

We shall see using a service account to authenticate when allowing accessing an application to access your IAP-secured resources. 

Today in this article, we will cover below aspects,

We shall be using HTTPClientFactory to call the API/Services after obtaining the OIDC Token programmatically.

Approaches to Authenticate a service account using OIDC Token,

• Using ClientID

• Using Secured Service Account File

• IAM credentials API

Today in this article, We shall follow an approach of using client ID to Authenticate a service account

Getting started

Please create an ASP.NET Core 3.1 or .NET 5 application,

Kindly add below “Google.Apis.Auth” NuGet package. This NuGet package gives a runtime client for working with Google services.

PM> Install-Package Google.Apis.Auth -Version 1.50.0

How to obtain ClientID in Google Cloud

• Go to the IAP page:

https://console.cloud.google.com/apis/credentials

• Copy the Client ID as below,

Please enable IAP if it is not already enabled.

Obtain an OIDC token for authentication – IAP request

You can obtain the OIDC token for authentication of an IAP request using the below-simplified class IAPClient.

public class IAPClient
{
    /// <summary>
    /// Makes a request to a IAP secured applicatio
    public async Task<string> InvokeRequestAsync(
        string iapClientId, CancellationToken cancellationToken = default)
    {
        
        OidcToken oidcToken = await GetOidcTokenAsync(iapClientId, cancellationToken);
        string token = await oidcToken.GetAccessTokenAsync(cancellationToken);
        return token;
    }

    /// <summary>
    /// 
    /// </summary>
    /// <param name="iapClientId"></param>
    /// <param name="cancellationToken"></param>
    /// <returns></returns>
    public async Task<OidcToken> GetOidcTokenAsync(string iapClientId, CancellationToken cancellationToken)
    {
        GoogleCredential credential = await GoogleCredential.GetApplicationDefaultAsync(cancellationToken);

        // Request an OIDC token for the Cloud IAP-secured client ID.
        return await credential.GetOidcTokenAsync(OidcTokenOptions.FromTargetAudience(iapClientId), cancellationToken);
    }
}

Below is a simplified Controller method for understanding purposes. Here we get the OpenID token and call the actual API.

        [HttpGet]
        public async Task<IActionResult> GetAsync()
        {
            IAPClient clientIAP = new IAPClient();
            var token = await clientIAP.InvokeRequestAsync(_clientID);
            var client = _httpClientFactory.CreateClient();
            client.DefaultRequestHeaders.Add("Authorization",$"Bearer {token}");
            var response = await client.GetAsync(_clientURL);
            var output = response.Content.ReadAsStringAsync().Result;
            WeatherForecast forcast = JsonSerializer.Deserialize<WeatherForecast>(output);
            return Ok(forcast);

        }

In the above code, we kept the logic simplified for demonstration purposes.

Ideally, tokens can be created once and re-used with the given session as per the use cases.

Above using OIDC, now you can access the secured resources, and API endpoints available in Google Cloud.

How to debug on LocalHost

If you already have Service Account created then it applies to an application instead of an individual user. You need to authenticate a service account when while accessing your IAP-secured resources.

Please make sure to set Environment Variable GOOGLE_APPLICATION_CREDENTIALS with the secured key JSON file path.

Example:

Environment.SetEnvironmentVariable("GOOGLE_APPLICATION_CREDENTIALS", "secured-key.json");

After adding the above user can debug locally from their machine and access or verify restricted or secured resources.

Your sample secured JSON will look like as below,

{
  "type": "service_account",
  "project_id": "***",
  "private_key_id": "3efae6*******",
  "private_key": "-----BEGIN PRIVATE KEY-----\n***************\n-----END PRIVATE KEY-----\n",
  "client_email": "*************gserviceaccount.com",
  "client_id": "******",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/*********"
}

Note: Do not store secrets related to authentication in the source code.

References :

• Upload, Download file Google Storage Bucket using gsutil

Do you have any comments or ideas or any better suggestions to share?

Please sound off your comments below.

Happy Coding !!

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