How can I create a custom grant type in Keycloak?

Keycloak, an open-source identity and access management solution, provides a range of grant types to facilitate authentication and authorization. But what if you need a customized grant type to cater to your specific use case? Fear not, dear reader, for in this article, we’ll embark on a journey to create a custom grant type in Keycloak. Buckle up, and let’s dive in!

Why do I need a custom grant type?

Before we dive into the implementation, let’s understand why you might need a custom grant type. Perhaps you’re working on a project that requires a unique authentication flow, or you want to integrate Keycloak with a custom authentication system. Whatever the reason, a custom grant type can provide the flexibility and adaptability you need to meet your specific requirements.

The Anatomy of a Grant Type

In Keycloak, a grant type represents a way to obtain an access token. It consists of three primary components:

• Grant Type Provider: This is the core component that implements the grant type. It’s responsible for authenticating the user and issuing an access token.
• Grant Type Authenticator: This component is used to authenticate the user during the grant type flow.
• Grant Type Factory: This component is responsible for creating instances of the grant type provider and authenticator.

Creating a Custom Grant Type

Now that we’ve covered the basics, let’s create a custom grant type. We’ll create a simple grant type that authenticates users based on a custom authentication mechanism.

Step 1: Create a New Maven Project

First, create a new Maven project in your favorite IDE. We’ll use Java as our programming language.

<dependencies>
    <dependency>
        <groupId>org.keycloak</groupId>
        <artifactId>keycloak-core</artifactId>
        <version>${keycloak.version}</version>
    </dependency>
    <dependency>
        <groupId>org.keycloak</groupId>
        <artifactId>keycloak-server-spi</artifactId>
        <version>${keycloak.version}</version>
    </dependency>
</dependencies>

Make sure to update the `keycloak.version` property with the version of Keycloak you’re using.

Step 2: Create the Grant Type Provider

Create a new Java class that implements the `GrantTypeProvider` interface:

public class CustomGrantTypeProvider implements GrantTypeProvider {
    
    @Override
    public String getGrantType() {
        return "custom-grant-type";
    }
    
    @Override
    public AuthenticatedClientSessionModel authenticate(AuthenticationFlowContext context) {
        // Implement your custom authentication logic here
        // For demonstration purposes, we'll return a dummy authenticated client session
        AuthenticatedClientSessionModel clientSession = new AuthenticatedClientSessionModel();
        clientSession.setAuthenticated(true);
        return clientSession;
    }
    
    @Override
    public void close() {
        // No-op
    }
}

In the `authenticate` method, implement your custom authentication logic. For demonstration purposes, we’ve returned a dummy authenticated client session.

Step 3: Create the Grant Type Authenticator

Create a new Java class that implements the `Authenticator` interface:

public class CustomGrantTypeAuthenticator implements Authenticator {
    
    @Override
    public void authenticate(AuthenticationFlowContext context) {
        // Implement your custom authentication logic here
        // For demonstration purposes, we'll return an authentication result
        context.success();
    }
    
    @Override
    public void action(AuthenticationFlowContext context) {
        // No-op
    }
    
    @Override
    public void close() {
        // No-op
    }
}

In the `authenticate` method, implement your custom authentication logic. For demonstration purposes, we’ve returned an authentication result.

Step 4: Create the Grant Type Factory

Create a new Java class that implements the `GrantTypeFactory` interface:

public class CustomGrantTypeFactory implements GrantTypeFactory {
    
    @Override
    public GrantTypeProvider createGrantTypeProvider(KeycloakSessionFactory sessionFactory) {
        return new CustomGrantTypeProvider();
    }
    
    @Override
    public Authenticator createAuthenticator(KeycloakSessionFactory sessionFactory) {
        return new CustomGrantTypeAuthenticator();
    }
    
    @Override
    public String getGrantType() {
        return "custom-grant-type";
    }
}

The factory creates instances of the grant type provider and authenticator.

Step 5: Register the Custom Grant Type

Register the custom grant type factory in the `keycloak-server-spi` configuration file:

<spi>
    <provider>
        <id>custom-grant-type</id>
        <class>com.example.CustomGrantTypeFactory</class>
    </provider>
</spi>

Update the `com.example.CustomGrantTypeFactory` class name to match your implementation.

Step 6: Deploy the Custom Grant Type

Package your custom grant type implementation into a JAR file and deploy it to your Keycloak server.

Restart your Keycloak server to enable the custom grant type.

Using the Custom Grant Type

Now that we’ve created and deployed the custom grant type, let’s use it to authenticate a user.

Step 1: Create a New Client

Create a new client in Keycloak with the custom grant type enabled:

Client ID	Grant Type
my-client	custom-grant-type

Step 2: Authenticate a User

Use the custom grant type to authenticate a user:

curl -X POST \
  http://localhost:8080/auth/realms/my-realm/protocol/openid-connect/token \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -d 'grant_type=custom-grant-type&client_id=my-client&client_secret=my-client-secret'

Replace `my-realm`, `my-client`, and `my-client-secret` with your actual Keycloak realm, client ID, and client secret, respectively.

If everything is configured correctly, you should receive an access token in response.

Conclusion

And there you have it! You’ve successfully created and deployed a custom grant type in Keycloak. This opens up a world of possibilities for custom authentication flows and integrations with external systems. Remember to adapt this implementation to your specific use case, and don’t hesitate to reach out if you have any questions or need further clarification.

Happy coding, and may the authentication force be with you!

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