Techniques to Understand Spring Authorization Server Authentication—from Browser Redirects to Token Signing

// build.gradle (or use Maven equivalents)
// dependencies {
//   implementation("org.springframework.boot:spring-boot-starter-web")
//   implementation("org.springframework.boot:spring-boot-starter-oauth2-authorization-server")
//   implementation("org.springframework.boot:spring-boot-starter-security")
//   implementation("org.springframework.boot:spring-boot-starter-thymeleaf") // for default login
//   implementation("com.nimbusds:nimbus-jose-jwt")
// }

// src/main/java/com/example/authserver/SecurityConfig.java
package com.example.authserver;

import com.nimbusds.jose.jwk.;
import com.nimbusds.jose.jwk.source.ImmutableJWKSet;
import com.nimbusds.jose.jwk.source.JWKSource;
import com.nimbusds.jose.proc.SecurityContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.config.Customizer;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.OAuth2AuthorizationServerConfiguration;
import org.springframework.security.config.annotation.web.configurers.oauth2.server.resource.OAuth2ResourceServerConfigurer;
import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.crypto.factory.PasswordEncoderFactories;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.security.oauth2.core.AuthorizationGrantType;
import org.springframework.security.oauth2.core.oidc.OidcScopes;
import org.springframework.security.oauth2.jwt.JwtDecoder;
import org.springframework.security.oauth2.server.authorization.InMemoryRegisteredClientRepository;
import org.springframework.security.oauth2.server.authorization.client.RegisteredClient;
import org.springframework.security.oauth2.server.authorization.client.RegisteredClientRepository;
import org.springframework.security.web.SecurityFilterChain;

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.interfaces.RSAPublicKey;
import java.util.UUID;

@Configuration
public class SecurityConfig {

    // 1) The dedicated chain that serves /oauth2/ endpoints
    @Bean
    SecurityFilterChain authServerSecurityFilterChain(HttpSecurity http) throws Exception {
        OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(http);
        // Optional: enable OpenID Connect endpoints
        http.getConfigurer(OAuth2AuthorizationServerConfiguration.class)
                .oidc(Customizer.withDefaults());
        // JWT resource server support for UserInfo endpoint if needed
        http.oauth2ResourceServer(OAuth2ResourceServerConfigurer::jwt);
        return http.build();
    }

    // 2) The app's own login form / session for resource owner authentication
    @Bean
    SecurityFilterChain appSecurity(HttpSecurity http) throws Exception {
        http.authorizeHttpRequests(auth -> auth
                    .requestMatchers("/assets/**").permitAll()
                    .anyRequest().authenticated())
            .formLogin(Customizer.withDefaults());
        return http.build();
    }

    // 3) Two example OAuth clients (public PKCE + confidential secret-based)
    @Bean
    RegisteredClientRepository registeredClientRepository(PasswordEncoder encoder) {
        RegisteredClient publicPkceClient = RegisteredClient.withId(UUID.randomUUID().toString())
                .clientId("toolbox-pkce")
                .clientAuthenticationMethod(auth -> {}) // public; no client secret
                .authorizationGrantType(AuthorizationGrantType.AUTHORIZATION_CODE)
                .authorizationGrantType(AuthorizationGrantType.REFRESH_TOKEN)
                .redirectUri("http://127.0.0.1:8081/login/oauth2/code/toolbox")
                .scope(OidcScopes.OPENID)
                .scope("profile.read")
                .clientSettings(settings -> settings.requireProofKey(true).requireAuthorizationConsent(true))
                .build();

        RegisteredClient confidentialClient = RegisteredClient.withId(UUID.randomUUID().toString())
                .clientId("service-client")
                .clientSecret(encoder.encode("service-secret-123"))
                .authorizationGrantType(AuthorizationGrantType.CLIENT_CREDENTIALS)
                .scope("reports.read")
                .build();

        return new InMemoryRegisteredClientRepository(publicPkceClient, confidentialClient);
    }

    // 4) Local in-memory users for the login screen
    @Bean
    UserDetailsService userDetailsService(PasswordEncoder encoder) {
        return username -> User.withUsername("anh")
                .password(encoder.encode("password"))
                .roles("USER")
                .build();
    }

    @Bean
    PasswordEncoder passwordEncoder() {
        return PasswordEncoderFactories.createDelegatingPasswordEncoder();
    }

    // 5) JWK source for signing JWT access tokens and ID tokens
    @Bean
    JWKSource<securitycontext> jwkSource() {
        RSAKey rsaKey = generateRsa();
        JWKSet jwkSet = new JWKSet(rsaKey);
        return new ImmutableJWKSet<>(jwkSet);
    }

    // 6) Decoder for resource server validations (UserInfo, etc.)
    @Bean
    JwtDecoder jwtDecoder(JWKSource<securitycontext> jwkSource) {
        return OAuth2AuthorizationServerConfiguration.jwtDecoder(jwkSource);
    }

    private static RSAKey generateRsa() {
        KeyPair keyPair = generateRsaKey();
        RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
        return new RSAKey.Builder(publicKey)
                .privateKey(keyPair.getPrivate())
                .keyID(UUID.randomUUID().toString())
                .build();
    }

    private static KeyPair generateRsaKey() {
        try {
            KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
            kpg.initialize(2048);
            return kpg.generateKeyPair();
        } catch (Exception ex) {
            throw new IllegalStateException(ex);
        }
    }
}

// src/main/java/com/example/authserver/JwtCustomizerConfig.java
package com.example.authserver;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.oauth2.server.authorization.token.JwtEncodingContext;
import org.springframework.security.oauth2.server.authorization.token.OAuth2TokenCustomizer;

@Configuration
public class JwtCustomizerConfig {

    @Bean
    OAuth2TokenCustomizer<jwtencodingcontext> tokenCustomizer() {
        return context -> {
            if (context.getTokenType().getValue().equals("access_token")) {
                context.getClaims().claim("tenant_id", "klara-tech")
                        .claim("roles", new String[]{"USER", "REPORT_VIEWER"});
            }
            if (context.getTokenType().getValue().equals("id_token")) {
                context.getClaims().claim("ui_theme", "dark");
            }
        };
    }
}

// In a downstream API
@EnableWebSecurity
class ApiSecurity {

    @Bean
    SecurityFilterChain api(HttpSecurity http) throws Exception {
        http.authorizeHttpRequests(auth -> auth
                    .requestMatchers("/public/**").permitAll()
                    .anyRequest().hasAuthority("SCOPE_profile.read"))
            .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()));
        return http.build();
    }
}

@Bean
RegisteredClientRepository jdbcClients(JdbcTemplate jdbc) {
    JdbcRegisteredClientRepository repo = new JdbcRegisteredClientRepository(jdbc);
    // bootstrap or migrate clients here...
    return repo;
}

@Bean
OAuth2AuthorizationService jdbcAuthorizationService(JdbcTemplate jdbc, RegisteredClientRepository clients) {
    return new JdbcOAuth2AuthorizationService(jdbc, clients);
}

# application.properties
logging.level.org.springframework.security=DEBUG
logging.level.org.springframework.security.oauth2=DEBUG
logging.level.org.springframework.security.oauth2.server.authorization=DEBUG

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