feat(chat): encrypt group chat under per-device epoch keys (MNP 2.0)

Chat messages are sealed with AES-256-GCM under a key derived per group, per
epoch, per *device*, and signed over the ciphertext with the device key the
node pinned. The node relays and archives; it cannot read a message.

There is no switch. MNP goes to 2.0 and MNP_MIN_SUPPORTED moves with it, so a
1.x peer is refused at the handshake with `version_too_old` rather than
admitted and then unable to speak. An opt-in flag was designed and rejected:
every node is a test node, so it would have bought nothing and left a plaintext
branch reachable — C6's lesson one feature later. A test reads the source and
refuses any code that consults a `chat_encrypted` setting.

Not Sender Keys, and `senderkeys.py` is now documented as unused. With
distribution under the group key and a node that serves history to devices
which were not present, the node must retain each chain's earliest key, and a
chain key at iteration i yields every message key from i on by pure HKDF —
forward secrecy is zero either way. What the ratchet was left buying was
stateful client code with silent failure modes, three of them reproduced: any
member could sign as any other, a second device dropped the first's chain, and
the skipped-key cache grew without bound. The reasoning is in
docs/chat-sender-keys.md, which is the specification and the decision record.

Epochs, not rotation: the epoch key is wrapped under the group key at delivery
and never stored under it, so `gek_rotate` is a re-wrap. A group-key-derived
archive key would have made every message ever sent unreadable on the first
`member unpin`, which is the documented step after removing a member. A new
epoch opens on member revoke/unpin, device revoke and `gek_rotate`; old epochs
are kept and still delivered, so history stays readable to everyone who could
already read it, and nothing anywhere deletes one.

Three prerequisites this needed, each a live defect on its own:

* The peer registry was keyed by user_id, so one account's second device
  evicted the first and the broadcast skipped recipients by account — a
  person's phone never saw what they typed on their laptop.
* The handshake authenticated an account, never a device. `device_hello`
  (additive, signed, refused unless the key is a live device of this account in
  the node's own roster) is what lets the node refuse a member claiming
  somebody else's key.
* `_admin_exec_file_delete` authorized against the exact uploading key, so
  device linking had already broken deleting your own file from your other
  device. It now authorizes against any non-revoked device of `uploader_id`.

Found by driving the real panel over the real transport, not by reading source:
`chat_keys_resp` was routed by arrival order and handed to an unanswered
`media_meta_req` — the original frozen-tab defect in a message type that did
not exist when that probe was written. And `_asText` had been deleted with an
unrelated helper beside it; its only caller sits inside a promise the panel
catches, so every conversation rendered empty with nothing in the console.

Existing node data is migrated by QE/migration/migrate_chat_encryption.py
(not versioned, per the QE rule), run with the node stopped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TZZxYjz8YeWRz13xDi8LJr

1 files changed, 15 insertions, 1 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/chat-app-settings.js b/packages/meshbay-hub/src/meshbay_hub/static/chat-app-settings.js
index 96fc52f..1eba59a 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/chat-app-settings.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/chat-app-settings.js
@@ -17,7 +17,8 @@ import { FolderPickerField } from './folder-tree.js';
  * has to be on a read-write root. The picker greys out the rest rather than
  * letting the node's refusal arrive after the fact.
  */
-function ChatSettings({ roots, dirs, settings, saveDirectories, transport, signFn }) {
+function ChatSettings({ roots, dirs, settings, saveDirectories, transport,
+                       signFn }) {
   const { busy, msg, run } = useSaver();
   const [directory, setDirectory] = useState(settings.chatDirectory || '');
   const [linkPreview, setLinkPreview] = useState(settings.chatLinkPreview !== false);
@@ -59,6 +60,19 @@ function ChatSettings({ roots, dirs, settings, saveDirectories, transport, signF
           label=${t('settings_app.chat_link_preview_label')} />
         <p class="settings-hint">${t('settings_app.chat_link_preview_hint')}</p>
       </div>
+      ${/* Not a toggle: chat is always encrypted (MNP 2.0), so there is
+             nothing here to turn on. What an operator may want is to move the
+             key on deliberately — the removals that matter already do it by
+             themselves. Stated rather than left invisible, because "is my chat
+             encrypted?" is a question people ask of a settings pane. */ ''}
+      <div class="settings-row" style="margin-top:12px">
+        <p class="settings-hint">${t('settings_app.chat_encrypted_always')}</p>
+        <button class="app-save" disabled=${busy}
+          onClick=${() => run(() => transport.rotateChatEpoch(signFn))}>
+          ${busy ? t('settings_app.saving') : t('settings_app.chat_rotate_epoch')}
+        </button>
+        <p class="settings-hint">${t('settings_app.chat_rotate_epoch_hint')}</p>
+      </div>
       ${msg && html`<p class="settings-hint">${msg}</p>`}
     </div>
   `;