OAuth 2.0 and JWT Authorization in Kotlin: In Practice

Introduction to OAuth 2.0 and JWT in Kotlin

Modern web applications and APIs require reliable authentication and authorization mechanisms. Two key standards — OAuth 2.0 (an access delegation protocol) and JWT (JSON Web Token, a compact token format) — are often used together to build secure systems. Kotlin, thanks to its compatibility with Java and concise syntax, is well-suited for implementing these protocols.

In this article, we will explore how to practically implement OAuth 2.0 and JWT authorization in Kotlin. You will learn how to configure a token issuance server, create protected endpoints, and handle various grant types such as Authorization Code and Client Credentials.

Basics: OAuth 2.0 and JWT

What is OAuth 2.0?

OAuth 2.0 is a protocol that allows third-party applications to obtain limited access to user resources without sharing login credentials. The main participants are:

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• Resource Owner — typically the user.
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• Client — the application requesting access.
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• Authorization Server — issues tokens.
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• Resource Server — stores protected data.
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What is JWT?

JWT is a compact, URL-safe token consisting of three parts (header, payload, signature) separated by dots. It is used to transmit claims between parties. Advantages of JWT:

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• Does not require server-side session storage (stateless).
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• Contains all necessary information (e.g., user ID, roles).
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• Digitally signed, ensuring its integrity.
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Implementing OAuth 2.0 with JWT in Kotlin (Spring Boot)

Let's look at a practical example using Spring Boot and the Spring Security library. We will create a simple authorization server and a protected resource.

Step 1: Setting Up Dependencies

Add the necessary dependencies to build.gradle.kts:

dependencies {\n    implementation("org.springframework.boot:spring-boot-starter-web")\n    implementation("org.springframework.boot:spring-boot-starter-security")\n    implementation("org.springframework.security:spring-security-oauth2-authorization-server")\n    implementation("io.jsonwebtoken:jjwt-api:0.12.3")\n    runtimeOnly("io.jsonwebtoken:jjwt-impl:0.12.3")\n    runtimeOnly("io.jsonwebtoken:jjwt-jackson:0.12.3")\n}

Step 2: Authorization Server Configuration

Create a configuration class for OAuth 2.0:

import org.springframework.context.annotation.Bean\nimport org.springframework.context.annotation.Configuration\nimport org.springframework.security.config.annotation.web.builders.HttpSecurity\nimport org.springframework.security.config.annotation.web.configuration.EnableWebSecurity\nimport org.springframework.security.core.userdetails.User\nimport org.springframework.security.core.userdetails.UserDetailsService\nimport org.springframework.security.oauth2.core.AuthorizationGrantType\nimport org.springframework.security.oauth2.core.ClientAuthenticationMethod\nimport org.springframework.security.oauth2.server.authorization.client.InMemoryRegisteredClientRepository\nimport org.springframework.security.oauth2.server.authorization.client.RegisteredClient\nimport org.springframework.security.oauth2.server.authorization.client.RegisteredClientRepository\nimport org.springframework.security.oauth2.server.authorization.config.annotation.web.configuration.OAuth2AuthorizationServerConfiguration\nimport org.springframework.security.provisioning.InMemoryUserDetailsManager\nimport org.springframework.security.web.SecurityFilterChain\nimport java.util.UUID\n\n@Configuration\n@EnableWebSecurity\nclass SecurityConfig {\n\n    @Bean\n