fix(node): enforce private key file permissions

Co-Authored-By: Virgil <virgil@lethean.io>

node/identity.go

@@ -8,6 +8,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"os"
 	"path/filepath"
 	"sync"
 	"time"
@@ -208,10 +209,13 @@ func (n *NodeManager) savePrivateKey() error {
 		return coreerr.E("NodeManager.savePrivateKey", "failed to create key directory", err)
 	}
 
-	// Write private key
+	// Write private key and then tighten permissions explicitly.
 	if err := coreio.Local.Write(n.keyPath, string(n.privateKey)); err != nil {
 		return coreerr.E("NodeManager.savePrivateKey", "failed to write private key", err)
 	}
+	if err := os.Chmod(n.keyPath, 0600); err != nil {
+		return coreerr.E("NodeManager.savePrivateKey", "failed to set private key permissions", err)
+	}
 
 	return nil
 }

node/identity_test.go

@@ -74,6 +74,25 @@ func TestNodeIdentity(t *testing.T) {
 		}
 	})
 
+	t.Run("PrivateKeyPermissions", func(t *testing.T) {
+		nm, cleanup := setupTestNodeManager(t)
+		defer cleanup()
+
+		err := nm.GenerateIdentity("permission-test", RoleDual)
+		if err != nil {
+			t.Fatalf("failed to generate identity: %v", err)
+		}
+
+		info, err := os.Stat(nm.keyPath)
+		if err != nil {
+			t.Fatalf("failed to stat private key: %v", err)
+		}
+
+		if got := info.Mode().Perm(); got != 0600 {
+			t.Fatalf("expected private key permissions 0600, got %04o", got)
+		}
+	})
+
 	t.Run("LoadExistingIdentity", func(t *testing.T) {
 		tmpDir, err := os.MkdirTemp("", "node-load-test")
 		if err != nil {