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, 12 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
index daa9f94..f470bf1 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
@@ -651,6 +651,15 @@ export default {
   'chat.unread': 'Nouveaux messages',
   'chat.jump_latest': 'Aller au plus récent',
   'chat.jump_new': 'Nouveaux messages',
+  'chat.unreadable': 'Ce message n\'a pas pu être lu',
+  'chat.unreadable_signature': 'Message non vérifié — la signature ne correspond pas à l\'expéditeur',
+  'chat.unreadable_epoch': 'Écrit avant que cet appareil puisse lire cette conversation',
+  'chat.unreadable_keys': 'Clés de discussion indisponibles — reconnectez-vous pour lire ceci',
+  'chat.unreadable_decrypt': 'Ce message n\'a pas pu être déchiffré',
+  'chat.unreadable_envelope': 'Ce message est arrivé incomplet',
+  'chat.unreadable_format': 'Ce message nécessite une version plus récente de MeshBay',
+  'chat.encrypted_needs_newer': 'Cette conversation est chiffrée et ce client ne peut pas la lire — mettez MeshBay à jour',
+  'chat.encrypted_cannot_send': 'Cette conversation est chiffrée et cet appareil ne peut pas encore y écrire',
   'group.leave': 'Quitter le groupe',
   'group.leave_confirm': 'Quitter « {name} » ? Vous perdrez l’accès à ses fichiers et à sa discussion. Les fichiers que vous avez envoyés restent sur le node, et celui-ci conserve l’identité qu’il a épinglée pour vous jusqu’à ce que son opérateur la retire.',
   'group.mute': 'Couper les notifications',
@@ -866,6 +875,9 @@ export default {
   'settings_app.chat_no_writable_root': 'Ce groupe n\'a aucun répertoire en écriture : les pièces jointes sont désactivées.',
   'settings_app.chat_link_preview_label': 'Aperçus des liens',
   'settings_app.chat_link_preview_hint': 'Quand un membre poste un lien, le nœud récupère le titre et l\'image de la page. C\'est une requête depuis votre machine vers un site choisi par quelqu\'un d\'autre.',
+  'settings_app.chat_encrypted_always': 'La discussion de ce groupe est toujours chiffrée. Cela ne peut pas être désactivé.',
+  'settings_app.chat_rotate_epoch': 'Faire tourner la clé de discussion',
+  'settings_app.chat_rotate_epoch_hint': 'Les membres continuent de lire tout l\'historique. Quiconque a été retiré du groupe ne peut plus lire ce qui sera écrit. Retirer un membre ou un appareil le fait déjà tout seul.',
   'settings_app.tmdb_token_prompt': 'Créez un compte TMDB pour générer votre propre clé d\'API.',
   'settings_app.tmdb_token_link': 'Obtenir une clé',
   'settings_node.roots_offline_hint': 'Non connecté au nœud — les changements passent par le nœud local et prennent effet à son prochain rechargement.',