Add API key authentication

src/main/java/com/litoralregas/vpnprovisioner/auth/ApiKeyAuthFilter.java

@@ -0,0 +1,68 @@
+package com.litoralregas.vpnprovisioner.auth;
+
+import com.litoralregas.vpnprovisioner.config.AppSecurityProperties;
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
+import org.springframework.security.core.context.SecurityContextHolder;
+import org.springframework.util.StringUtils;
+import org.springframework.web.filter.OncePerRequestFilter;
+
+import java.io.IOException;
+import java.util.List;
+
+public class ApiKeyAuthFilter extends OncePerRequestFilter {
+
+    private static final String API_KEY_HEADER = "X-API-Key";
+
+    private final AppSecurityProperties securityProperties;
+
+    public ApiKeyAuthFilter(AppSecurityProperties securityProperties) {
+        this.securityProperties = securityProperties;
+    }
+
+    @Override
+    protected boolean shouldNotFilter(HttpServletRequest request) {
+        return "/actuator/health".equals(request.getRequestURI());
+    }
+
+    @Override
+    protected void doFilterInternal(
+            HttpServletRequest request,
+            HttpServletResponse response,
+            FilterChain filterChain
+    ) throws ServletException, IOException {
+
+        String providedApiKey = request.getHeader(API_KEY_HEADER);
+        String expectedApiKey = securityProperties.getApiKey();
+
+        if (!StringUtils.hasText(expectedApiKey)) {
+            response.sendError(
+                    HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
+                    "API key is not configured"
+            );
+            return;
+        }
+
+        if (!expectedApiKey.equals(providedApiKey)) {
+            response.sendError(
+                    HttpServletResponse.SC_UNAUTHORIZED,
+                    "Invalid API key"
+            );
+            return;
+        }
+
+        UsernamePasswordAuthenticationToken authentication =
+                new UsernamePasswordAuthenticationToken(
+                        "api-key-client",
+                        null,
+                        List.of()
+                );
+
+        SecurityContextHolder.getContext().setAuthentication(authentication);
+
+        filterChain.doFilter(request, response);
+    }
+}

src/main/java/com/litoralregas/vpnprovisioner/config/AppSecurityProperties.java

@@ -0,0 +1,17 @@
+package com.litoralregas.vpnprovisioner.config;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
+@ConfigurationProperties(prefix = "app.security")
+public class AppSecurityProperties {
+
+    private String apiKey;
+
+    public String getApiKey() {
+        return apiKey;
+    }
+
+    public void setApiKey(String apiKey) {
+        this.apiKey = apiKey;
+    }
+}

src/main/java/com/litoralregas/vpnprovisioner/config/SecurityConfig.java

@@ -0,0 +1,34 @@
+package com.litoralregas.vpnprovisioner.config;
+
+import com.litoralregas.vpnprovisioner.auth.ApiKeyAuthFilter;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.web.SecurityFilterChain;
+import org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter;
+
+@Configuration
+@EnableConfigurationProperties(AppSecurityProperties.class)
+public class SecurityConfig {
+
+    private final AppSecurityProperties securityProperties;
+
+    public SecurityConfig(AppSecurityProperties securityProperties) {
+        this.securityProperties = securityProperties;
+    }
+
+    @Bean
+    public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
+        ApiKeyAuthFilter apiKeyAuthFilter = new ApiKeyAuthFilter(securityProperties);
+
+        return http
+                .csrf(csrf -> csrf.disable())
+                .authorizeHttpRequests(auth -> auth
+                        .requestMatchers("/actuator/health").permitAll()
+                        .anyRequest().authenticated()
+                )
+                .addFilterBefore(apiKeyAuthFilter, UsernamePasswordAuthenticationFilter.class)
+                .build();
+    }
+}