From 650b02ddcc20266acbb658b6ad669caf99f6aa74 Mon Sep 17 00:00:00 2001
From: Matthias Nott <mnott@mnsoft.org>
Date: Wed, 25 Mar 2026 14:50:59 +0100
Subject: [PATCH] feat: TOFU cert pinning - trust on first use with reset in settings
---
lib/services/mqtt_service.dart | 68 ++++++++++++++++++++++++++++++++-
1 files changed, 65 insertions(+), 3 deletions(-)
diff --git a/lib/services/mqtt_service.dart b/lib/services/mqtt_service.dart
index cb04d2f..5df0b7b 100644
--- a/lib/services/mqtt_service.dart
+++ b/lib/services/mqtt_service.dart
@@ -1,6 +1,8 @@
import 'dart:async';
import 'dart:convert';
import 'dart:io';
+import 'dart:typed_data';
+import 'package:crypto/crypto.dart';
import 'package:bonsoir/bonsoir.dart';
import 'package:flutter/widgets.dart';
@@ -95,6 +97,9 @@
_intentionalClose = false;
_setStatus(ConnectionStatus.connecting);
+
+ // Load trusted cert fingerprint for TOFU verification
+ if (_trustedFingerprint == null) await _loadTrustedFingerprint();
// Send Wake-on-LAN if MAC configured
if (config.macAddress != null && config.macAddress!.isNotEmpty) {
@@ -241,6 +246,60 @@
return null;
}
+ // --- TOFU (Trust On First Use) certificate pinning ---
+
+ String? _trustedFingerprint; // Loaded from SharedPreferences at startup
+
+ /// Load the trusted cert fingerprint from storage.
+ Future<void> _loadTrustedFingerprint() async {
+ final prefs = await SharedPreferences.getInstance();
+ _trustedFingerprint = prefs.getString('trustedCertFingerprint');
+ if (_trustedFingerprint != null) {
+ _mqttLog('TOFU: loaded trusted fingerprint: ${_trustedFingerprint!.substring(0, 16)}...');
+ }
+ }
+
+ /// Compute SHA-256 fingerprint of a certificate's DER bytes.
+ String _certFingerprint(X509Certificate cert) {
+ final der = cert.der;
+ final digest = sha256.convert(der);
+ return digest.toString();
+ }
+
+ /// TOFU verification: accept on first use, reject if fingerprint changes.
+ bool _verifyCertTofu(dynamic certificate) {
+ if (certificate is! X509Certificate) return true; // Can't verify, accept
+
+ final fingerprint = _certFingerprint(certificate);
+
+ if (_trustedFingerprint == null) {
+ // First connection — trust and save
+ _trustedFingerprint = fingerprint;
+ SharedPreferences.getInstance().then((prefs) {
+ prefs.setString('trustedCertFingerprint', fingerprint);
+ });
+ _mqttLog('TOFU: first connection, saved fingerprint: ${fingerprint.substring(0, 16)}...');
+ return true;
+ }
+
+ if (_trustedFingerprint == fingerprint) {
+ return true; // Known cert, trusted
+ }
+
+ // Fingerprint mismatch — possible MITM or server reinstall
+ _mqttLog('TOFU: CERT MISMATCH! Expected ${_trustedFingerprint!.substring(0, 16)}... got ${fingerprint.substring(0, 16)}...');
+ // Reject the connection. User must reset trust in settings.
+ return false;
+ }
+
+ /// Reset the trusted cert fingerprint (e.g., after server reinstall).
+ Future<void> resetTrustedCert() async {
+ _trustedFingerprint = null;
+ final prefs = await SharedPreferences.getInstance();
+ await prefs.remove('trustedCertFingerprint');
+ _mqttLog('TOFU: trust reset');
+ }
+
/// Probe a single host:port with a TLS connection attempt (1s timeout).
/// Uses SecureSocket since the broker now requires TLS.
Future<String?> _probeHost(String host, int port) async {
@@ -267,11 +326,14 @@
// client.maxConnectionAttempts is final — can't set it
client.logging(on: false);
- // TLS: broker uses a self-signed certificate.
- // TODO: pin the cert fingerprint once cert rotation story is defined.
+ // TLS with TOFU (Trust On First Use) cert pinning.
+ // First connection: accept cert, save its SHA-256 fingerprint.
+ // Future connections: only accept certs matching the saved fingerprint.
client.secure = true;
client.securityContext = SecurityContext(withTrustedRoots: true);
- client.onBadCertificate = (dynamic certificate) => true;
+ client.onBadCertificate = (dynamic certificate) {
+ return _verifyCertTofu(certificate);
+ };
client.onConnected = _onConnected;
client.onDisconnected = _onDisconnected;
--
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