Accounts: re-encrypt all books on password change

The encryption key is PBKDF2-derived from the login password. Changing
the password without migrating the key would make all books undecryptable
on the next login.

- Add POST /books/<pk>/rekey/ endpoint to replace a book's ciphertexts
- Password change form is now JS-driven: before submitting to the server,
  derives the old key, verifies it matches localStorage, derives the new
  key, re-encrypts all books (data + meta) and their highlights/bookmarks,
  updates localStorage, then submits the Django form

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

books/urls.py

@@ -6,6 +6,7 @@ urlpatterns = [
     path('upload/', views.upload_book, name='upload_book'),
     path('<int:pk>/data/', views.get_book_data, name='get_book_data'),
     path('<int:pk>/delete/', views.delete_book, name='delete_book'),
+    path('<int:pk>/rekey/', views.rekey_book, name='rekey_book'),
     path('<int:pk>/progress/', views.save_progress, name='save_book_progress'),
     path('<int:pk>/highlights/', views.book_highlights, name='book_highlights'),
     path('<int:pk>/bookmarks/', views.book_bookmarks, name='book_bookmarks'),

books/views.py

@@ -94,6 +94,39 @@ def get_book_data(request, pk):
     return JsonResponse({'data_ct': book.data_ct, 'data_iv': book.data_iv})
 
 
+@csrf_exempt
+@require_http_methods(['POST'])
+def rekey_book(request, pk):
+    err = _require_auth(request)
+    if err:
+        return err
+
+    try:
+        book = EBook.objects.get(pk=pk, user=request.user)
+    except EBook.DoesNotExist:
+        return JsonResponse({'error': 'not found'}, status=404)
+
+    try:
+        body = json.loads(request.body)
+    except (json.JSONDecodeError, ValueError):
+        return JsonResponse({'error': 'invalid JSON'}, status=400)
+
+    meta_ct = body.get('meta_ct', '')
+    meta_iv = body.get('meta_iv', '')
+    data_ct = body.get('data_ct', '')
+    data_iv = body.get('data_iv', '')
+
+    if not all([meta_ct, meta_iv, data_ct, data_iv]):
+        return JsonResponse({'error': 'meta_ct, meta_iv, data_ct, data_iv required'}, status=400)
+
+    book.meta_ct = meta_ct
+    book.meta_iv = meta_iv
+    book.data_ct = data_ct
+    book.data_iv = data_iv
+    book.save(update_fields=['meta_ct', 'meta_iv', 'data_ct', 'data_iv'])
+    return JsonResponse({'ok': True})
+
+
 @csrf_exempt
 @require_http_methods(['POST'])
 def delete_book(request, pk):

templates/accounts/settings.html

@@ -75,7 +75,7 @@
 
     <details {% if password_form_open %}open{% endif %} style="margin-top:1rem;">
       <summary class="btn" style="display:inline-block;cursor:pointer;">Change password</summary>
-      <form method="post" action="{% url 'change_password' %}" style="margin-top:1rem; display:flex; flex-direction:column; gap:0.6rem; max-width:320px;">
+      <form id="pw-change-form" method="post" action="{% url 'change_password' %}" style="margin-top:1rem; display:flex; flex-direction:column; gap:0.6rem; max-width:320px;">
         {% csrf_token %}
         {% for field in password_form %}
           <div>
@@ -89,6 +89,7 @@
         {% if password_form.non_field_errors %}
           <div class="message message-error" style="padding:4px 8px; font-size:0.8rem;">{{ password_form.non_field_errors|join:", " }}</div>
         {% endif %}
+        <div id="pw-change-status" style="font-size:0.85rem; color:#888;"></div>
         <div><button type="submit" class="btn">Save</button></div>
       </form>
     </details>
@@ -142,6 +143,98 @@ async function _getOrCreateEncKey() {
   return key;
 }
 
+function _hexToBytes(hex) {
+  const buf = new Uint8Array(hex.length / 2);
+  for (let i = 0; i < buf.length; i++) buf[i] = parseInt(hex.slice(i*2, i*2+2), 16);
+  return buf;
+}
+
+async function _pbkdf2Key(password, username) {
+  const enc = new TextEncoder();
+  const mat = await crypto.subtle.importKey('raw', enc.encode(password), 'PBKDF2', false, ['deriveKey']);
+  return crypto.subtle.deriveKey(
+    {name: 'PBKDF2', salt: enc.encode('diora:' + username), iterations: 200000, hash: 'SHA-256'},
+    mat, {name: 'AES-GCM', length: 256}, true, ['encrypt', 'decrypt']
+  );
+}
+
+async function _reEncrypt(oldKey, newKey, iv, ct) {
+  const plain = await crypto.subtle.decrypt({name: 'AES-GCM', iv: _hexToBytes(iv)}, oldKey, _base64ToBytes(ct));
+  const newIv = crypto.getRandomValues(new Uint8Array(12));
+  const newCt = await crypto.subtle.encrypt({name: 'AES-GCM', iv: newIv}, newKey, plain);
+  return {iv: _bytesToHex(newIv), ct: _bytesToBase64(newCt)};
+}
+
+document.getElementById('pw-change-form')?.addEventListener('submit', async function(e) {
+  e.preventDefault();
+  const form = this;
+  const statusEl = document.getElementById('pw-change-status');
+  const oldPw = form.old_password.value;
+  const newPw = form.new_password1.value;
+  const userId = {{ request.user.id }};
+  const username = '{{ request.user.username }}';
+  const storageKey = `diora_enc_key_${userId}`;
+
+  statusEl.textContent = 'Schlüssel ableiten…';
+  try {
+    const oldKey = await _pbkdf2Key(oldPw, username);
+
+    // Verify old key matches localStorage
+    const stored = localStorage.getItem(storageKey);
+    if (stored) {
+      const storedBytes = _base64ToBytes(stored);
+      const derivedBytes = new Uint8Array(await crypto.subtle.exportKey('raw', oldKey));
+      if (!storedBytes.every((b, i) => b === derivedBytes[i])) {
+        statusEl.textContent = 'Fehler: Das alte Passwort stimmt nicht mit dem gespeicherten Schlüssel überein. Bitte altes Passwort prüfen.';
+        return;
+      }
+    }
+
+    const newKey = await _pbkdf2Key(newPw, username);
+
+    // Re-encrypt all books
+    statusEl.textContent = 'Bücherliste laden…';
+    const books = await fetch('/books/').then(r => r.json());
+
+    for (let i = 0; i < books.length; i++) {
+      const book = books[i];
+      statusEl.textContent = `Buch ${i + 1} / ${books.length} neu verschlüsseln…`;
+
+      const newMeta = await _reEncrypt(oldKey, newKey, book.meta_iv, book.meta_ct);
+      const data    = await fetch(`/books/${book.id}/data/`).then(r => r.json());
+      const newData = await _reEncrypt(oldKey, newKey, data.data_iv, data.data_ct);
+
+      await fetch(`/books/${book.id}/rekey/`, {
+        method: 'POST',
+        headers: {'Content-Type': 'application/json', 'X-CSRFToken': getCsrfToken()},
+        body: JSON.stringify({meta_ct: newMeta.ct, meta_iv: newMeta.iv, data_ct: newData.ct, data_iv: newData.iv}),
+      });
+
+      // Re-encrypt highlights and bookmarks if they exist
+      for (const endpoint of ['highlights', 'bookmarks']) {
+        const resp = await fetch(`/books/${book.id}/${endpoint}/`).then(r => r.json());
+        if (resp.ct) {
+          const reenc = await _reEncrypt(oldKey, newKey, resp.iv, resp.ct);
+          await fetch(`/books/${book.id}/${endpoint}/`, {
+            method: 'POST',
+            headers: {'Content-Type': 'application/json', 'X-CSRFToken': getCsrfToken()},
+            body: JSON.stringify({ct: reenc.ct, iv: reenc.iv}),
+          });
+        }
+      }
+    }
+
+    // Update localStorage key
+    const newRaw = new Uint8Array(await crypto.subtle.exportKey('raw', newKey));
+    localStorage.setItem(storageKey, _bytesToBase64(newRaw));
+
+    statusEl.textContent = 'Passwort ändern…';
+    form.submit();
+  } catch (err) {
+    statusEl.textContent = 'Fehler: ' + err.message;
+  }
+});
+
 async function uploadBackground(input) {
   const file = input.files[0];
   if (!file) return;