feat(backup): import legacy password-encrypted backups

Try app-internal key first (new format); on failure, signal the
renderer to prompt for the old password and retry. Lets users import
.mhu files that were exported with a custom password in v2.7.6 or
earlier without downgrading.

lib/backup-crypto.js

@@ -14,8 +14,8 @@ const ALGO = 'aes-256-gcm';
 // (with random salt/iv) so each export is still distinct and authenticated.
 const APP_PASSPHRASE = 'multi-hoster-upload::backup::v1';
 
-function deriveKey(salt) {
-  return crypto.pbkdf2Sync(APP_PASSPHRASE, salt, ITERATIONS, KEY_LEN, DIGEST);
+function deriveKey(passphrase, salt) {
+  return crypto.pbkdf2Sync(passphrase, salt, ITERATIONS, KEY_LEN, DIGEST);
 }
 
 /**
@@ -26,7 +26,7 @@ function encrypt(config) {
   const plaintext = Buffer.from(JSON.stringify(config), 'utf-8');
   const salt = crypto.randomBytes(SALT_LEN);
   const iv = crypto.randomBytes(IV_LEN);
-  const key = deriveKey(salt);
+  const key = deriveKey(APP_PASSPHRASE, salt);
 
   const cipher = crypto.createCipheriv(ALGO, key, iv);
   const encrypted = Buffer.concat([cipher.update(plaintext), cipher.final()]);
@@ -39,9 +39,12 @@ function encrypt(config) {
 
 /**
  * Decrypt a .mhu buffer.
- * Returns the config object or throws on invalid data.
+ * Tries the app's built-in key first; if that fails and a user password is
+ * provided, falls back to legacy password-based decryption. Throws a special
+ * error with `needsPassword = true` if the app key fails and no password was
+ * given, so callers can prompt the user for the legacy password.
  */
-function decrypt(buffer) {
+function decrypt(buffer, userPassword) {
   if (buffer.length < MAGIC.length + SALT_LEN + IV_LEN + TAG_LEN + 1) {
     throw new Error('Ungültiges Backup-Format');
   }
@@ -57,25 +60,36 @@ function decrypt(buffer) {
   const tag = buffer.subarray(offset, offset += TAG_LEN);
   const ciphertext = buffer.subarray(offset);
 
-  const key = deriveKey(salt);
-  const decipher = crypto.createDecipheriv(ALGO, key, iv);
-  decipher.setAuthTag(tag);
+  const tryPassphrase = (passphrase) => {
+    const key = deriveKey(passphrase, salt);
+    const decipher = crypto.createDecipheriv(ALGO, key, iv);
+    decipher.setAuthTag(tag);
+    try {
+      const decrypted = Buffer.concat([decipher.update(ciphertext), decipher.final()]);
+      const result = JSON.parse(decrypted.toString('utf-8'));
+      decrypted.fill(0);
+      key.fill(0);
+      return result;
+    } catch {
+      key.fill(0);
+      return null;
+    }
+  };
 
-  let decrypted;
-  try {
-    decrypted = Buffer.concat([decipher.update(ciphertext), decipher.final()]);
-  } catch {
-    throw new Error('Backup-Datei beschädigt oder aus einer inkompatiblen Version');
+  // 1) Try the app-internal key (new format, no password required).
+  const fromApp = tryPassphrase(APP_PASSPHRASE);
+  if (fromApp) return fromApp;
+
+  // 2) Legacy format: user had set their own password.
+  if (userPassword) {
+    const fromUser = tryPassphrase(userPassword);
+    if (fromUser) return fromUser;
+    throw new Error('Falsches Passwort oder beschädigte Datei');
   }
 
-  try {
-    return JSON.parse(decrypted.toString('utf-8'));
-  } catch {
-    throw new Error('Entschlüsselte Daten sind kein gültiges JSON');
-  } finally {
-    decrypted.fill(0);
-    key.fill(0);
-  }
+  const err = new Error('Dieses Backup wurde mit einem Passwort verschlüsselt');
+  err.needsPassword = true;
+  throw err;
 }
 
 module.exports = { encrypt, decrypt };

main.js

@@ -15,6 +15,7 @@ const FolderMonitor = require('./lib/folder-monitor');
 const RemoteServer = require('./lib/remote-server');
 
 let mainWindow;
+let _lastImportPath = null;
 let dropTargetWindow = null;
 let tray = null;
 const configStore = new ConfigStore(app);
@@ -979,15 +980,33 @@ ipcMain.handle('export-backup', async () => {
   return { ok: true, path: filePath };
 });
 
-ipcMain.handle('import-backup', async () => {
-  const { canceled, filePaths } = await dialog.showOpenDialog(mainWindow, {
-    title: 'Backup importieren',
-    filters: [{ name: 'Multi-Hoster Backup', extensions: ['mhu'] }],
-    properties: ['openFile']
-  });
-  if (canceled || !filePaths.length) return { ok: false, canceled: true };
-  const buffer = fs.readFileSync(filePaths[0]);
-  const imported = backupCrypto.decrypt(buffer);
+ipcMain.handle('import-backup', async (_event, legacyPassword) => {
+  let buffer;
+  let sourcePath = _lastImportPath;
+  if (legacyPassword && sourcePath) {
+    buffer = fs.readFileSync(sourcePath);
+  } else {
+    const { canceled, filePaths } = await dialog.showOpenDialog(mainWindow, {
+      title: 'Backup importieren',
+      filters: [{ name: 'Multi-Hoster Backup', extensions: ['mhu'] }],
+      properties: ['openFile']
+    });
+    if (canceled || !filePaths.length) return { ok: false, canceled: true };
+    sourcePath = filePaths[0];
+    buffer = fs.readFileSync(sourcePath);
+    _lastImportPath = sourcePath;
+  }
+  let imported;
+  try {
+    imported = backupCrypto.decrypt(buffer, legacyPassword);
+  } catch (err) {
+    if (err && err.needsPassword) {
+      return { ok: false, needsPassword: true };
+    }
+    _lastImportPath = null;
+    throw err;
+  }
+  _lastImportPath = null;
   // Validate imported data has required structure
   if (!imported || typeof imported !== 'object' || !imported.hosters || !imported.hosterSettings || !imported.globalSettings) {
     return { ok: false, error: 'Backup-Datei hat ungültige Struktur (hosters, hosterSettings oder globalSettings fehlt).' };

preload.js

@@ -60,7 +60,7 @@ contextBridge.exposeInMainWorld('api', {
 
   // Backup
   exportBackup: () => ipcRenderer.invoke('export-backup'),
-  importBackup: () => ipcRenderer.invoke('import-backup'),
+  importBackup: (legacyPassword) => ipcRenderer.invoke('import-backup', legacyPassword),
 
   // Folder Monitor
   folderMonitorStart: (settings) => ipcRenderer.invoke('folder-monitor:start', settings),

renderer/app.js

@@ -1140,10 +1140,74 @@ async function doBackupExport() {
   }
 }
 
-async function doBackupImport() {
+function askLegacyBackupPassword() {
+  return new Promise((resolve) => {
+    const overlay = document.createElement('div');
+    overlay.className = 'modal-overlay';
+    overlay.style.display = 'flex';
+
+    const card = document.createElement('div');
+    card.className = 'modal-card';
+    card.style.width = 'min(380px,100%)';
+
+    const header = document.createElement('div');
+    header.className = 'modal-header';
+    const h3 = document.createElement('h3');
+    h3.textContent = 'Passwort erforderlich';
+    header.appendChild(h3);
+
+    const body = document.createElement('div');
+    body.className = 'modal-body';
+    const p = document.createElement('p');
+    p.style.margin = '0 0 10px';
+    p.style.fontSize = '13px';
+    p.textContent = 'Dieses Backup wurde mit einem Passwort verschlüsselt.';
+    const input = document.createElement('input');
+    input.type = 'password';
+    input.className = 'key-input';
+    input.placeholder = 'Passwort';
+    input.autocomplete = 'off';
+    input.style.width = '100%';
+    body.appendChild(p);
+    body.appendChild(input);
+
+    const footer = document.createElement('div');
+    footer.className = 'modal-footer';
+    const cancelBtn = document.createElement('button');
+    cancelBtn.className = 'btn btn-secondary';
+    cancelBtn.textContent = 'Abbrechen';
+    const okBtn = document.createElement('button');
+    okBtn.className = 'btn btn-primary';
+    okBtn.textContent = 'Importieren';
+    footer.appendChild(cancelBtn);
+    footer.appendChild(okBtn);
+
+    card.appendChild(header);
+    card.appendChild(body);
+    card.appendChild(footer);
+    overlay.appendChild(card);
+    document.body.appendChild(overlay);
+
+    const done = (val) => { overlay.remove(); resolve(val); };
+    okBtn.onclick = () => done(input.value || null);
+    cancelBtn.onclick = () => done(null);
+    input.addEventListener('keydown', (e) => {
+      if (e.key === 'Enter') done(input.value || null);
+      if (e.key === 'Escape') done(null);
+    });
+    input.focus();
+  });
+}
+
+async function doBackupImport(legacyPassword) {
   try {
-    const result = await window.api.importBackup();
+    const result = await window.api.importBackup(legacyPassword);
     if (!result || result.canceled) return;
+    if (result.needsPassword) {
+      const pw = await askLegacyBackupPassword();
+      if (pw) doBackupImport(pw);
+      return;
+    }
     if (result.ok) {
       config = result.config;
       hosterSettings = config.hosterSettings || {};

tests/backup-crypto.test.js

@@ -19,7 +19,10 @@ describe('backup-crypto', () => {
   it('decrypt with corrupted data throws', () => {
     const buf = encrypt(sampleConfig);
     buf[buf.length - 1] ^= 0xff; // flip last byte
-    assert.throws(() => decrypt(buf), /beschädigt|inkompatiblen/);
+    // With no password: app-key fails → needsPassword surfaces.
+    assert.throws(() => decrypt(buf), (err) => err.needsPassword === true);
+    // With a password: both app-key and password fail → Falsches Passwort.
+    assert.throws(() => decrypt(buf, 'anything'), /Falsches Passwort/);
   });
 
   it('decrypt with invalid magic throws', () => {
@@ -38,6 +41,28 @@ describe('backup-crypto', () => {
     assert.deepStrictEqual(decrypt(buf), empty);
   });
 
+  it('decrypts legacy password-encrypted buffer when password is provided', () => {
+    // Reproduce the old format: same envelope, but key derived from user password.
+    const crypto = require('crypto');
+    const plaintext = Buffer.from(JSON.stringify(sampleConfig), 'utf-8');
+    const salt = crypto.randomBytes(16);
+    const iv = crypto.randomBytes(12);
+    const key = crypto.pbkdf2Sync('oldUserPw', salt, 100_000, 32, 'sha512');
+    const cipher = crypto.createCipheriv('aes-256-gcm', key, iv);
+    const enc = Buffer.concat([cipher.update(plaintext), cipher.final()]);
+    const tag = cipher.getAuthTag();
+    const legacyBuf = Buffer.concat([Buffer.from('MHU1'), salt, iv, tag, enc]);
+
+    // Without password → should throw needsPassword
+    assert.throws(() => decrypt(legacyBuf), (err) => err.needsPassword === true);
+
+    // With correct password → should decrypt
+    assert.deepStrictEqual(decrypt(legacyBuf, 'oldUserPw'), sampleConfig);
+
+    // With wrong password → should throw (not needsPassword)
+    assert.throws(() => decrypt(legacyBuf, 'wrongPw'), /Falsches Passwort/);
+  });
+
   it('each encryption produces different output (random salt/iv)', () => {
     const a = encrypt(sampleConfig);
     const b = encrypt(sampleConfig);