skybridge/internal/middleware/csrf.go

package middleware

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"strconv"
	"strings"
	"time"

	"go.uber.org/zap"

	"github.com/kms/api-key-service/internal/config"
)

// CSRFMiddleware provides CSRF protection
type CSRFMiddleware struct {
	config config.ConfigProvider
	logger *zap.Logger
}

// NewCSRFMiddleware creates a new CSRF middleware
func NewCSRFMiddleware(config config.ConfigProvider, logger *zap.Logger) *CSRFMiddleware {
	return &CSRFMiddleware{
		config: config,
		logger: logger,
	}
}

// CSRFProtection implements CSRF protection for state-changing operations
func (cm *CSRFMiddleware) CSRFProtection(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Skip CSRF protection for safe methods
		if r.Method == "GET" || r.Method == "HEAD" || r.Method == "OPTIONS" {
			next.ServeHTTP(w, r)
			return
		}

		// Skip CSRF protection for specific endpoints that use other authentication
		if cm.shouldSkipCSRF(r) {
			next.ServeHTTP(w, r)
			return
		}

		// Get CSRF token from header
		csrfToken := r.Header.Get("X-CSRF-Token")
		if csrfToken == "" {
			cm.logger.Warn("Missing CSRF token", 
				zap.String("path", r.URL.Path),
				zap.String("method", r.Method),
				zap.String("remote_addr", r.RemoteAddr))
			
			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusForbidden)
			w.Write([]byte(`{"error":"csrf_token_missing","message":"CSRF token required"}`))
			return
		}

		// Validate CSRF token
		if !cm.validateCSRFToken(csrfToken, r) {
			cm.logger.Warn("Invalid CSRF token", 
				zap.String("path", r.URL.Path),
				zap.String("method", r.Method),
				zap.String("remote_addr", r.RemoteAddr))
			
			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusForbidden)
			w.Write([]byte(`{"error":"csrf_token_invalid","message":"Invalid CSRF token"}`))
			return
		}

		cm.logger.Debug("CSRF token validated successfully",
			zap.String("path", r.URL.Path))

		next.ServeHTTP(w, r)
	})
}

// GenerateCSRFToken generates a new CSRF token for a user session
func (cm *CSRFMiddleware) GenerateCSRFToken(userID string) (string, error) {
	// Generate random bytes for token
	tokenBytes := make([]byte, 32)
	if _, err := rand.Read(tokenBytes); err != nil {
		cm.logger.Error("Failed to generate CSRF token", zap.Error(err))
		return "", err
	}

	// Create timestamp
	timestamp := time.Now().Unix()
	
	// Create token data
	tokenData := hex.EncodeToString(tokenBytes)
	
	// Create signing string: userID:timestamp:tokenData
	timestampStr := strconv.FormatInt(timestamp, 10)
	signingString := userID + ":" + timestampStr + ":" + tokenData
	
	// Sign the token with HMAC
	signature := cm.signData(signingString)
	
	// Return encoded token: tokenData.timestamp.signature
	token := tokenData + "." + timestampStr + "." + signature
	
	return token, nil
}

// validateCSRFToken validates a CSRF token
func (cm *CSRFMiddleware) validateCSRFToken(token string, r *http.Request) bool {
	// Parse token parts
	parts := strings.Split(token, ".")
	if len(parts) != 3 {
		cm.logger.Debug("Invalid CSRF token format")
		return false
	}

	tokenData, timestampStr, signature := parts[0], parts[1], parts[2]
	
	// Get user ID from request context or headers
	userID := cm.getUserIDFromRequest(r)
	if userID == "" {
		cm.logger.Debug("No user ID found for CSRF validation")
		return false
	}

	// Recreate signing string
	signingString := userID + ":" + timestampStr + ":" + tokenData
	
	// Verify signature
	expectedSignature := cm.signData(signingString)
	if !hmac.Equal([]byte(signature), []byte(expectedSignature)) {
		cm.logger.Debug("CSRF token signature verification failed")
		return false
	}

	// Parse timestamp
	timestampInt, err := strconv.ParseInt(timestampStr, 10, 64)
	if err != nil {
		cm.logger.Debug("Invalid timestamp in CSRF token", zap.Error(err))
		return false
	}
	timestamp := time.Unix(timestampInt, 0)

	// Check if token is expired (valid for 1 hour by default)
	maxAge := cm.config.GetDuration("CSRF_TOKEN_MAX_AGE")
	if maxAge <= 0 {
		maxAge = 1 * time.Hour
	}

	if time.Since(timestamp) > maxAge {
		cm.logger.Debug("CSRF token expired",
			zap.Time("timestamp", timestamp),
			zap.Duration("age", time.Since(timestamp)),
			zap.Duration("max_age", maxAge))
		return false
	}

	return true
}

// signData signs data with HMAC
func (cm *CSRFMiddleware) signData(data string) string {
	// Use the same signing key as for authentication
	signingKey := cm.config.GetString("AUTH_SIGNING_KEY")
	if signingKey == "" {
		cm.logger.Error("AUTH_SIGNING_KEY not configured for CSRF protection")
		return ""
	}

	mac := hmac.New(sha256.New, []byte(signingKey))
	mac.Write([]byte(data))
	return hex.EncodeToString(mac.Sum(nil))
}

// getUserIDFromRequest extracts user ID from request
func (cm *CSRFMiddleware) getUserIDFromRequest(r *http.Request) string {
	// Try to get from X-User-Email header
	userEmail := r.Header.Get(cm.config.GetString("AUTH_HEADER_USER_EMAIL"))
	if userEmail != "" {
		return userEmail
	}

	// Try to get from context (if set by authentication middleware)
	if userID := r.Context().Value("user_id"); userID != nil {
		if id, ok := userID.(string); ok {
			return id
		}
	}

	return ""
}

// shouldSkipCSRF determines if CSRF protection should be skipped for a request
func (cm *CSRFMiddleware) shouldSkipCSRF(r *http.Request) bool {
	// Skip for API endpoints that use API key authentication
	if strings.HasPrefix(r.URL.Path, "/api/verify") {
		return true
	}

	// Skip for health check endpoints
	if r.URL.Path == "/health" || r.URL.Path == "/ready" {
		return true
	}

	// Skip for webhook endpoints (if any)
	if strings.HasPrefix(r.URL.Path, "/webhook/") {
		return true
	}

	return false
}

// SetCSRFCookie sets a secure CSRF token cookie
func (cm *CSRFMiddleware) SetCSRFCookie(w http.ResponseWriter, token string) {
	cookie := &http.Cookie{
		Name:     "csrf_token",
		Value:    token,
		Path:     "/",
		MaxAge:   3600, // 1 hour
		HttpOnly: false, // JavaScript needs to read this for AJAX requests
		Secure:   true,  // HTTPS only
		SameSite: http.SameSiteStrictMode,
	}
	http.SetCookie(w, cookie)
}

// GetCSRFTokenFromCookie gets CSRF token from cookie
func (cm *CSRFMiddleware) GetCSRFTokenFromCookie(r *http.Request) string {
	cookie, err := r.Cookie("csrf_token")
	if err != nil {
		return ""
	}
	return cookie.Value
}