Protecting an API using Client Credentials

This quickstart presents the most basic scenario for protecting APIs using IdentityServer. We will define an API and a Client that wants to access it. The client will request an access token at IdentityServer by providing a ClientCredentials which acts as a secret known to both the client and IdentityServer and it will use the token to gain access to the API.

Setting up the ASP.NET Core application

First create a directory for the application - then use our template to create an ASP.NET Core application that includes a basic IdentityServer setup, e.g.:

md quickstart
cd quickstart

md src
cd src

dotnet new is4empty -n IdentityServer

This will create the following files:

  • IdentityServer.csproj - the project file and a Properties\launchSettings.json file
  • Program.cs and Startup.cs - the main application entry point
  • Config.cs - IdentityServer resources and clients configuration file

You can now use your favourite text editor to edit or view the files. If you want to have Visual Studio support, you can add a solution file like this:

cd ..
dotnet new sln -n Quickstart

and let it add your IdentityServer project (keep this command in mind as we will create other projects below):

dotnet sln add .\src\IdentityServer\IdentityServer.csproj


The protocol used in this Template is http and the port is set to 5000 when running on Kestrel or a random one on IISExpress. You can change that in the Properties\launchSettings.json file. However, all of the quickstart instructions will assume you use the default port on Kestrel as well as the http protocol, which is sufficient for local development.

Defining an API Resource

An API is a resource in your system that you want to protect.

Resource definitions can be loaded in many ways, the template uses a “code as configuration” appproach. In the [Config.cs]( file you can find a method called GetApis, define the API as follows:

public static IEnumerable<ApiResource> GetApis()
    return new List<ApiResource>
        new ApiResource("api1", "My API")

Defining the client

The next step is to define a client that can access this API.

For this scenario, the client will not have an interactive user, and will authenticate using the so called client secret with IdentityServer. Add the following code to your [Config.cs]( file:

public static IEnumerable<Client> GetClients()
    return new List<Client>
        new Client
            ClientId = "client",

            // no interactive user, use the clientid/secret for authentication
            AllowedGrantTypes = GrantTypes.ClientCredentials,

            // secret for authentication
            ClientSecrets =
                new Secret("secret".Sha256())

            // scopes that client has access to
            AllowedScopes = { "api1" }

Configuring IdentityServer

Loading the resource and client definitions happens in Startup.cs - the template already does this for you:

public void ConfigureServices(IServiceCollection services)
    var builder = services.AddIdentityServer()

    // rest omitted

That’s it - if you run the server and navigate the browser to http://localhost:5000/.well-known/openid-configuration, you should see the so-called discovery document. This will be used by your clients and APIs to download the necessary configuration data.


At first startup, IdentityServer will create a developer signing key for you, it’s a file called tempkey.rsa. You don’t have to check that file into your source control, it will be re-created if it is not present.

Adding an API

Next, add an API to your solution.

You can either use the ASP.NET Core Web API (or empty) template from Visual Studio or use the .NET CLI to create the API project as we do here. Run from within the src folder the following command:

dotnet new web -n Api

Configure the API application to run on http://localhost:5001 only.

The controller

Add a new folder Controllers and a new controller IdentityController to your API project:

public class IdentityController : ControllerBase
    public IActionResult Get()
        return new JsonResult(from c in User.Claims select new { c.Type, c.Value });

This controller will be used later to test the authorization requirement, as well as visualize the claims identity through the eyes of the API.


The last step is to add the authentication services to DI and the authentication middleware to the pipeline. These will:

  • validate the incoming token to make sure it is coming from a trusted issuer
  • validate that the token is valid to be used with this api (aka audience)

Update Startup to look like this:

public class Startup
    public void ConfigureServices(IServiceCollection services)

            .AddJwtBearer("Bearer", options =>
                options.Authority = "http://localhost:5000";
                options.RequireHttpsMetadata = false;

                options.Audience = "api1";

    public void Configure(IApplicationBuilder app)


AddAuthentication adds the authentication services to DI and configures "Bearer" as the default scheme. UseAuthentication adds the authentication middleware to the pipeline so authentication will be performed automatically on every call into the host.

Navigating to the controller http://localhost:5001/identity on a browser should return a 401 status code. This means your API requires a credential and is now protected by IdentityServer.

Creating the client

The last step is to write a client that requests an access token, and then uses this token to access the API. For that, add a console project to your solution:

dotnet new console -n Client

and copy the content from Program.cs here.

The client program invokes the Main method asynchronously in order to run asynchronous http calls. This feature is possible since C# 7.1 and will be available once you edit Client.csproj to add the following line as a PropertyGroup:


The token endpoint at IdentityServer implements the OAuth 2.0 protocol, and you could use raw HTTP to access it. However, we have a client library called IdentityModel, that encapsulates the protocol interaction in an easy to use API.

Add the IdentityModel NuGet package to your client. This can be done either via Visual Studio’s nuget dialog, by adding it manually to the Client.csproj file, or by using the CLI:

dotnet add package IdentityModel

IdentityModel includes a client library to use with the discovery endpoint. This way you only need to know the base-address of IdentityServer - the actual endpoint addresses can be read from the metadata:

// discover endpoints from metadata
var client = new HttpClient();
var disco = await client.GetDiscoveryDocumentAsync("http://localhost:5000");
if (disco.IsError)

Next you can use the information from the discovery document to request a token to IdentityServer to access api1:

// request token
var tokenResponse = await client.RequestClientCredentialsTokenAsync(new ClientCredentialsTokenRequest
    Address = disco.TokenEndpoint,

    ClientId = "client",
    ClientSecret = "secret",
    Scope = "api1"

if (tokenResponse.IsError)



Copy and paste the access token from the console to to inspect the raw token.

Calling the API

To send the access token to the API you typically use the HTTP Authorization header. This is done using the SetBearerToken extension method:

// call api
var client = new HttpClient();

var response = await client.GetAsync("http://localhost:5001/identity");
if (!response.IsSuccessStatusCode)
    var content = await response.Content.ReadAsStringAsync();

The output should look like this:



By default an access token will contain claims about the scope, lifetime (nbf and exp), the client ID (client_id) and the issuer name (iss).

Further experiments

This walkthrough focused on the success path so far

  • client was able to request token
  • client could use the token to access the API

You can now try to provoke errors to learn how the system behaves, e.g.

  • try to connect to IdentityServer when it is not running (unavailable)
  • try to use an invalid client id or secret to request the token
  • try to ask for an invalid scope during the token request
  • try to call the API when it is not running (unavailable)
  • don’t send the token to the API
  • configure the API to require a different scope than the one in the token