Implement M7: auth middleware, rate limiting, CORS, and GUI login flow

Add SHA-256 API key authentication with constant-time comparison, configurable
token bucket rate limiter, CORS origin allowlist middleware, and React auth
context with login page. Auth info endpoint bootstraps GUI without credentials.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handler/health.go

@@ -5,11 +5,13 @@ import (
 )
 
 // HealthHandler handles health and readiness check endpoints.
-type HealthHandler struct{}
+type HealthHandler struct {
+	AuthType string // "api-key", "jwt", "none"
+}
 
 // NewHealthHandler creates a new HealthHandler.
-func NewHealthHandler() HealthHandler {
-	return HealthHandler{}
+func NewHealthHandler(authType string) HealthHandler {
+	return HealthHandler{AuthType: authType}
 }
 
 // Health responds with a simple health check indicating the service is alive.
@@ -41,3 +43,21 @@ func (h HealthHandler) Ready(w http.ResponseWriter, r *http.Request) {
 
 	JSON(w, http.StatusOK, response)
 }
+
+// AuthInfo responds with the server's authentication configuration.
+// This lets the GUI know whether to show a login screen.
+// GET /api/v1/auth/info (served without auth middleware)
+func (h HealthHandler) AuthInfo(w http.ResponseWriter, r *http.Request) {
+	response := map[string]interface{}{
+		"auth_type": h.AuthType,
+		"required":  h.AuthType != "none",
+	}
+	JSON(w, http.StatusOK, response)
+}
+
+// AuthCheck returns 200 if the request has valid auth credentials.
+// The auth middleware runs before this handler, so reaching here means auth passed.
+// GET /api/v1/auth/check
+func (h HealthHandler) AuthCheck(w http.ResponseWriter, r *http.Request) {
+	JSON(w, http.StatusOK, map[string]string{"status": "authenticated"})
+}

internal/api/middleware/middleware.go

@@ -2,8 +2,12 @@ package middleware
 
 import (
 	"context"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/hex"
 	"log"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/google/uuid"
@@ -48,36 +52,178 @@ func Recovery(next http.Handler) http.Handler {
 			if err := recover(); err != nil {
 				requestID := getRequestID(r.Context())
 				log.Printf("[%s] PANIC: %v", requestID, err)
-				http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+				http.Error(w, `{"error":"Internal Server Error"}`, http.StatusInternalServerError)
 			}
 		}()
 		next.ServeHTTP(w, r)
 	})
 }
 
-// Auth middleware is a placeholder that checks the Authorization header and extracts user information.
-// In production, this would validate tokens, verify signatures, etc.
-func Auth(next http.Handler) http.Handler {
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		authHeader := r.Header.Get("Authorization")
-		if authHeader == "" {
-			// For now, allow requests without auth (placeholder)
-			// In production, enforce auth on protected routes
-			next.ServeHTTP(w, r)
-			return
-		}
+// HashAPIKey computes the SHA-256 hash of an API key for secure storage.
+// We use SHA-256 rather than bcrypt because API keys are high-entropy
+// random strings (not user-chosen passwords), so rainbow tables and
+// brute-force attacks are not a practical concern.
+func HashAPIKey(key string) string {
+	h := sha256.Sum256([]byte(key))
+	return hex.EncodeToString(h[:])
+}
 
-		// Simple stub: just extract user ID from Bearer token (format: "Bearer <user_id>")
-		// This is NOT secure and for development only
-		if len(authHeader) > 7 && authHeader[:7] == "Bearer " {
-			userID := authHeader[7:]
-			ctx := context.WithValue(r.Context(), UserKey{}, userID)
+// AuthConfig holds configuration for the Auth middleware.
+type AuthConfig struct {
+	Type   string // "api-key", "jwt", "none"
+	Secret string // The raw API key (server compares against this)
+}
+
+// NewAuth creates an authentication middleware based on config.
+// When Type is "none", all requests pass through (demo/development mode).
+// When Type is "api-key", requests must include a valid Bearer token.
+func NewAuth(cfg AuthConfig) func(http.Handler) http.Handler {
+	if cfg.Type == "none" {
+		return func(next http.Handler) http.Handler {
+			return next
+		}
+	}
+
+	// Pre-compute hash of the expected key for constant-time comparison
+	expectedHash := HashAPIKey(cfg.Secret)
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			authHeader := r.Header.Get("Authorization")
+			if authHeader == "" {
+				w.Header().Set("Content-Type", "application/json; charset=utf-8")
+				w.Header().Set("WWW-Authenticate", `Bearer realm="certctl"`)
+				http.Error(w, `{"error":"Authorization header required"}`, http.StatusUnauthorized)
+				return
+			}
+
+			// Extract Bearer token
+			if len(authHeader) < 8 || authHeader[:7] != "Bearer " {
+				w.Header().Set("Content-Type", "application/json; charset=utf-8")
+				http.Error(w, `{"error":"Invalid Authorization header format, expected: Bearer <token>"}`, http.StatusUnauthorized)
+				return
+			}
+
+			token := authHeader[7:]
+			tokenHash := HashAPIKey(token)
+
+			// Constant-time comparison to prevent timing attacks
+			if subtle.ConstantTimeCompare([]byte(tokenHash), []byte(expectedHash)) != 1 {
+				w.Header().Set("Content-Type", "application/json; charset=utf-8")
+				http.Error(w, `{"error":"Invalid API key"}`, http.StatusUnauthorized)
+				return
+			}
+
+			// Store the authenticated identity in context
+			ctx := context.WithValue(r.Context(), UserKey{}, "api-key-user")
 			next.ServeHTTP(w, r.WithContext(ctx))
-			return
-		}
+		})
+	}
+}
 
-		http.Error(w, "Invalid Authorization header", http.StatusUnauthorized)
-	})
+// RateLimitConfig holds configuration for the rate limiter.
+type RateLimitConfig struct {
+	RPS       float64 // Requests per second
+	BurstSize int     // Maximum burst size
+}
+
+// NewRateLimiter creates a token bucket rate limiting middleware.
+// Uses a simple token bucket: tokens refill at RPS rate, burst allows short spikes.
+func NewRateLimiter(cfg RateLimitConfig) func(http.Handler) http.Handler {
+	limiter := &tokenBucket{
+		rate:       cfg.RPS,
+		burstSize:  float64(cfg.BurstSize),
+		tokens:     float64(cfg.BurstSize),
+		lastRefill: time.Now(),
+	}
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if !limiter.allow() {
+				w.Header().Set("Content-Type", "application/json; charset=utf-8")
+				w.Header().Set("Retry-After", "1")
+				http.Error(w, `{"error":"Rate limit exceeded"}`, http.StatusTooManyRequests)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+// tokenBucket implements a simple thread-safe token bucket rate limiter.
+// This avoids importing golang.org/x/time/rate to keep dependencies minimal.
+type tokenBucket struct {
+	mu         sync.Mutex
+	rate       float64   // tokens per second
+	burstSize  float64   // max tokens
+	tokens     float64   // current tokens
+	lastRefill time.Time // last refill time
+}
+
+func (tb *tokenBucket) allow() bool {
+	tb.mu.Lock()
+	defer tb.mu.Unlock()
+
+	now := time.Now()
+	elapsed := now.Sub(tb.lastRefill).Seconds()
+	tb.tokens += elapsed * tb.rate
+	if tb.tokens > tb.burstSize {
+		tb.tokens = tb.burstSize
+	}
+	tb.lastRefill = now
+
+	if tb.tokens < 1 {
+		return false
+	}
+	tb.tokens--
+	return true
+}
+
+// CORSConfig holds configuration for the CORS middleware.
+type CORSConfig struct {
+	AllowedOrigins []string // Allowed origins; empty = same-origin only
+}
+
+// NewCORS creates a CORS middleware with configurable allowed origins.
+// If no origins are configured, same-origin requests are allowed by default.
+// If ["*"] is configured, all origins are allowed (development/demo mode).
+func NewCORS(cfg CORSConfig) func(http.Handler) http.Handler {
+	allowAll := false
+	originSet := make(map[string]bool)
+	for _, o := range cfg.AllowedOrigins {
+		if o == "*" {
+			allowAll = true
+		}
+		originSet[o] = true
+	}
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			origin := r.Header.Get("Origin")
+
+			if allowAll {
+				w.Header().Set("Access-Control-Allow-Origin", "*")
+			} else if origin != "" && originSet[origin] {
+				w.Header().Set("Access-Control-Allow-Origin", origin)
+				w.Header().Set("Vary", "Origin")
+			} else if len(cfg.AllowedOrigins) == 0 && origin != "" {
+				// No config = permissive same-origin default for single-host deployments
+				w.Header().Set("Access-Control-Allow-Origin", origin)
+				w.Header().Set("Vary", "Origin")
+			}
+
+			w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, PATCH, OPTIONS")
+			w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Authorization, X-Request-ID")
+			w.Header().Set("Access-Control-Max-Age", "86400")
+
+			if r.Method == http.MethodOptions {
+				w.WriteHeader(http.StatusNoContent)
+				return
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
 }
 
 // ContentType middleware sets the Content-Type header to application/json.
@@ -89,6 +235,7 @@ func ContentType(next http.Handler) http.Handler {
 }
 
 // CORS middleware adds CORS headers to allow cross-origin requests.
+// Deprecated: Use NewCORS for configurable origins. Kept for health endpoints.
 func CORS(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Access-Control-Allow-Origin", "*")

internal/api/router/router.go

@@ -57,7 +57,7 @@ func (r *Router) RegisterHandlers(
 	notifications handler.NotificationHandler,
 	health handler.HealthHandler,
 ) {
-	// Health endpoints (no middleware)
+	// Health endpoints (no auth middleware — must always be accessible)
 	r.mux.Handle("GET /health", middleware.Chain(
 		http.HandlerFunc(health.Health),
 		middleware.CORS,
@@ -68,6 +68,14 @@ func (r *Router) RegisterHandlers(
 		middleware.CORS,
 		middleware.ContentType,
 	))
+	// Auth info endpoint (no auth middleware — GUI needs this before login)
+	r.mux.Handle("GET /api/v1/auth/info", middleware.Chain(
+		http.HandlerFunc(health.AuthInfo),
+		middleware.CORS,
+		middleware.ContentType,
+	))
+	// Auth check endpoint (uses full middleware chain via r.Register)
+	r.Register("GET /api/v1/auth/check", http.HandlerFunc(health.AuthCheck))
 
 	// Certificates routes: /api/v1/certificates
 	r.Register("GET /api/v1/certificates", http.HandlerFunc(certificates.ListCertificates))

internal/config/config.go

@@ -16,6 +16,8 @@ type Config struct {
 	Scheduler SchedulerConfig
 	Log       LogConfig
 	Auth      AuthConfig
+	RateLimit RateLimitConfig
+	CORS      CORSConfig
 }
 
 // ServerConfig contains HTTP server configuration.
@@ -51,6 +53,18 @@ type AuthConfig struct {
 	Secret string // Secret key for signing (if applicable)
 }
 
+// RateLimitConfig contains rate limiting configuration.
+type RateLimitConfig struct {
+	Enabled    bool
+	RPS        float64 // Requests per second
+	BurstSize  int     // Maximum burst size
+}
+
+// CORSConfig contains CORS configuration.
+type CORSConfig struct {
+	AllowedOrigins []string // Allowed origins; empty = same-origin only; ["*"] = all
+}
+
 // Load reads configuration from environment variables and returns a Config.
 // Environment variables must have the CERTCTL_ prefix.
 // Example: CERTCTL_SERVER_HOST, CERTCTL_DATABASE_URL, etc.
@@ -79,6 +93,14 @@ func Load() (*Config, error) {
 			Type:   getEnv("CERTCTL_AUTH_TYPE", "api-key"),
 			Secret: getEnv("CERTCTL_AUTH_SECRET", ""),
 		},
+		RateLimit: RateLimitConfig{
+			Enabled:   getEnvBool("CERTCTL_RATE_LIMIT_ENABLED", true),
+			RPS:       getEnvFloat("CERTCTL_RATE_LIMIT_RPS", 50),
+			BurstSize: getEnvInt("CERTCTL_RATE_LIMIT_BURST", 100),
+		},
+		CORS: CORSConfig{
+			AllowedOrigins: getEnvList("CERTCTL_CORS_ORIGINS", nil),
+		},
 	}
 
 	if err := cfg.Validate(); err != nil {
@@ -192,6 +214,67 @@ func getEnvDuration(key string, defaultValue time.Duration) time.Duration {
 	return defaultValue
 }
 
+// getEnvBool reads a boolean environment variable.
+func getEnvBool(key string, defaultValue bool) bool {
+	if value := os.Getenv(key); value != "" {
+		return value == "true" || value == "1" || value == "yes"
+	}
+	return defaultValue
+}
+
+// getEnvFloat reads a float64 environment variable.
+func getEnvFloat(key string, defaultValue float64) float64 {
+	if value := os.Getenv(key); value != "" {
+		f, err := strconv.ParseFloat(value, 64)
+		if err != nil {
+			return defaultValue
+		}
+		return f
+	}
+	return defaultValue
+}
+
+// getEnvList reads a comma-separated list environment variable.
+func getEnvList(key string, defaultValue []string) []string {
+	if value := os.Getenv(key); value != "" {
+		var result []string
+		for _, s := range splitComma(value) {
+			s = trimSpace(s)
+			if s != "" {
+				result = append(result, s)
+			}
+		}
+		return result
+	}
+	return defaultValue
+}
+
+// splitComma splits a string by commas (no strings import needed).
+func splitComma(s string) []string {
+	var parts []string
+	start := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == ',' {
+			parts = append(parts, s[start:i])
+			start = i + 1
+		}
+	}
+	parts = append(parts, s[start:])
+	return parts
+}
+
+// trimSpace trims leading/trailing whitespace.
+func trimSpace(s string) string {
+	start, end := 0, len(s)
+	for start < end && (s[start] == ' ' || s[start] == '\t') {
+		start++
+	}
+	for end > start && (s[end-1] == ' ' || s[end-1] == '\t') {
+		end--
+	}
+	return s[start:end]
+}
+
 // GetLogLevel returns the appropriate slog.Level from the configured log level.
 func (c *Config) GetLogLevel() slog.Level {
 	switch c.Log.Level {