Refactor CSRF secret handling to validate key length and improve error messaging

cmd/start.go

@@ -2,7 +2,6 @@ package cmd
 
 import (
 	"context"
-	"crypto/rand"
 	"log/slog"
 	"net"
 	"net/http"
@@ -60,26 +59,14 @@ var startCmd = &cobra.Command{
 		// Create empty CSRF configuration with default values
 		var csrfConfig middleware.CSRFConfig
 		
-		// Set CSRF secret from config or generate a random one
+		// Get and validate CSRF secret from config
 		csrfSecret := viper.GetString("csrf-secret")
-		var csrfKey []byte
-		
-		if csrfSecret != "" {
-			// Use configured secret - must be at least 32 bytes
-			csrfKey = []byte(csrfSecret)
-			if len(csrfKey) < 32 {
-				logger.Error("csrf-secret must be at least 32 bytes")
-				return
-			}
-		} else {
-			// Generate a random secret
-			csrfKey = make([]byte, 32)
-			_, err = rand.Read(csrfKey)
-			if err != nil {
-				logger.Error("failed to generate CSRF key", slog.Any("error", err))
-				return
-			}
-			logger.Info("generated random CSRF key, consider setting csrf-secret for stability across restarts")
+		csrfKey, err := middleware.ParseCSRFKey(csrfSecret)
+		if err != nil {
+			logger.Error("invalid csrf-secret", 
+				slog.String("error", err.Error()),
+				slog.String("hint", "must be exactly 32 bytes and persist across restarts"))
+			return
 		}
 		
 		csrfConfig.Secret = csrfKey
@@ -138,7 +125,7 @@ func init() {
 	startCmd.Flags().String("issuer-url", "", "Identity Provider Issuer URL")
 	startCmd.Flags().String("hostname", "", "Address at which the server is exposed")
 	startCmd.Flags().String("session-secret", "", "Session encryption secret")
-	startCmd.Flags().String("csrf-secret", "", "Secret key for CSRF protection (min 32 bytes)")
+	startCmd.Flags().String("csrf-secret", "", "Secret key for CSRF protection (must be exactly 32 bytes)")
 	startCmd.Flags().String("env", "", "Environment (development/production)")
 
 	// Bind all flags to Viper

internal/middleware/csrf.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"fmt"
 	"html/template"
 	"net/http"
 
@@ -137,3 +138,18 @@ func CSRFToken(r *http.Request) string {
 func CSRFTemplateField(r *http.Request) template.HTML {
 	return csrf.TemplateField(r)
 }
+
+// ParseCSRFKey validates and converts a CSRF secret string to the required 32-byte key
+// It returns the key as a byte slice and an error if the key is invalid
+func ParseCSRFKey(secret string) ([]byte, error) {
+	if secret == "" {
+		return nil, fmt.Errorf("csrf secret is required and must be exactly 32 bytes")
+	}
+	
+	key := []byte(secret)
+	if len(key) != 32 {
+		return nil, fmt.Errorf("csrf secret must be exactly 32 bytes (got %d bytes)", len(key))
+	}
+	
+	return key, nil
+}