type APIKeyAuth struct {
    validKeys map[string]bool
}

func (a *APIKeyAuth) Authenticate(req *http.Request) bool {
    apiKey := req.Header.Get("X-API-Key")
    return a.validKeys[apiKey]
}

func validateJWT(tokenString string) (*jwt.Token, error) {
    return jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method")
        }
        return []byte(secretKey), nil
    })
}

func (h *Handler) validateEmailContent(content string) error {
    // Size validation
    if len(content) > int(h.security.MaxEmailSize) {
        return fmt.Errorf("email size exceeds limit of %d bytes", h.security.MaxEmailSize)
    }

    // Content validation
    if content == "" {
        return fmt.Errorf("email content cannot be empty")
    }

    // Format validation using Go's mail package
    if _, err := mail.ReadMessage(strings.NewReader(content)); err != nil {
        return fmt.Errorf("invalid email format: %w", err)
    }

    // Header sanitization
    content = sanitizeHeaders(content)
    
    return nil
}

func sanitizeHeaders(content string) string {
    // Remove potentially dangerous headers
    dangerousHeaders := []string{
        "X-Original-To:",
        "Return-Path:",
        "Envelope-To:",
    }
    
    lines := strings.Split(content, "\n")
    var sanitized []string
    
    for _, line := range lines {
        safe := true
        for _, header := range dangerousHeaders {
            if strings.HasPrefix(strings.ToLower(line), strings.ToLower(header)) {
                safe = false
                break
            }
        }
        if safe {
            sanitized = append(sanitized, line)
        }
    }
    
    return strings.Join(sanitized, "\n")
}

var (
    emailRegex = regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)
    ipRegex    = regexp.MustCompile(`^(\d{1,3}\.){3}\d{1,3}$`)
    domainRegex = regexp.MustCompile(`^[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)
)

func validateEmailAddress(email string) error {
    if !emailRegex.MatchString(email) {
        return fmt.Errorf("invalid email address format")
    }
    return nil
}

func validateIPAddress(ip string) error {
    if !ipRegex.MatchString(ip) {
        return fmt.Errorf("invalid IP address format")
    }
    
    // Additional validation for private/reserved ranges
    if net.ParseIP(ip) == nil {
        return fmt.Errorf("invalid IP address")
    }
    
    return nil
}

type RateLimiter struct {
    limiter *rate.Limiter
    clients map[string]*rate.Limiter
    mutex   sync.RWMutex
}

func NewRateLimiter(requestsPerMinute int, burstSize int) *RateLimiter {
    return &RateLimiter{
        limiter: rate.NewLimiter(
            rate.Every(time.Minute/time.Duration(requestsPerMinute)),
            burstSize,
        ),
        clients: make(map[string]*rate.Limiter),
    }
}

func (rl *RateLimiter) Allow(clientID string) bool {
    rl.mutex.RLock()
    clientLimiter, exists := rl.clients[clientID]
    rl.mutex.RUnlock()
    
    if !exists {
        rl.mutex.Lock()
        clientLimiter = rate.NewLimiter(rl.limiter.Limit(), rl.limiter.Burst())
        rl.clients[clientID] = clientLimiter
        rl.mutex.Unlock()
    }
    
    return clientLimiter.Allow()
}

type SecureEmailProcessor struct {
    maxSize int64
    timeout time.Duration
}

func (p *SecureEmailProcessor) ProcessEmail(content string) (*ScanResult, error) {
    // Input validation
    if err := p.validateInput(content); err != nil {
        return nil, err
    }
    
    // Process in memory only
    ctx, cancel := context.WithTimeout(context.Background(), p.timeout)
    defer cancel()
    
    result, err := p.scanWithTimeout(ctx, content)
    
    // Explicitly clear sensitive data
    content = ""
    runtime.GC()
    
    return result, err
}

func (p *SecureEmailProcessor) scanWithTimeout(ctx context.Context, content string) (*ScanResult, error) {
    done := make(chan *ScanResult, 1)
    errChan := make(chan error, 1)
    
    go func() {
        result, err := p.performScan(content)
        if err != nil {
            errChan <- err
            return
        }
        done <- result
    }()
    
    select {
    case result := <-done:
        return result, nil
    case err := <-errChan:
        return nil, err
    case <-ctx.Done():
        return nil, fmt.Errorf("scan timeout exceeded")
    }
}

type SecureLogger struct {
    logger *logrus.Logger
}

func (l *SecureLogger) LogScanRequest(userID string, emailSize int, scanType string) {
    l.logger.WithFields(logrus.Fields{
        "user_id":    hashUserID(userID),  // Hash for privacy
        "email_size": emailSize,
        "scan_type":  scanType,
        "timestamp":  time.Now().UTC(),
        "session_id": generateSessionID(),
    }).Info("Email scan requested")
}

func hashUserID(userID string) string {
    h := sha256.Sum256([]byte(userID + secretSalt))
    return hex.EncodeToString(h[:8]) // First 8 bytes for logging
}

type SecurityMonitor struct {
    alertThresholds map[string]float64
    alertManager    AlertManager
}

func (m *SecurityMonitor) MonitorRequest(req *Request) {
    // Monitor for suspicious patterns
    if m.detectAnomalousRequest(req) {
        m.alertManager.SendAlert(SecurityAlert{
            Type:      "anomalous_request",
            Severity:  "medium",
            Details:   req.Summary(),
            Timestamp: time.Now(),
        })
    }
    
    // Monitor rate limiting violations
    if req.RateLimited {
        m.alertManager.SendAlert(SecurityAlert{
            Type:      "rate_limit_violation",
            Severity:  "low",
            ClientID:  req.ClientID,
            Timestamp: time.Now(),
        })
    }
}

var (
    securityRequestsTotal = prometheus.NewCounterVec(
        prometheus.CounterOpts{
            Name: "security_requests_total",
            Help: "Total number of security-related requests",
        },
        []string{"type", "status"},
    )
    
    suspiciousActivityTotal = prometheus.NewCounterVec(
        prometheus.CounterOpts{
            Name: "suspicious_activity_total",
            Help: "Total number of suspicious activities detected",
        },
        []string{"type", "severity"},
    )
)

type AuditLogger struct {
    logger *logrus.Logger
}

func (a *AuditLogger) LogSecurityEvent(event SecurityEvent) {
    a.logger.WithFields(logrus.Fields{
        "event_type":    event.Type,
        "user_id":      hashString(event.UserID),
        "source_ip":    hashString(event.SourceIP),
        "action":       event.Action,
        "resource":     event.Resource,
        "result":       event.Result,
        "timestamp":    event.Timestamp.UTC(),
        "session_id":   event.SessionID,
        "risk_score":   event.RiskScore,
    }).Warn("Security event detected")
}

type SecurityEvent struct {
    Type      string    `json:"type"`
    UserID    string    `json:"user_id"`
    SourceIP  string    `json:"source_ip"`
    Action    string    `json:"action"`
    Resource  string    `json:"resource"`
    Result    string    `json:"result"`
    Timestamp time.Time `json:"timestamp"`
    SessionID string    `json:"session_id"`
    RiskScore float64   `json:"risk_score"`
}