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/it.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
index 364c26c..f971320 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
@@ -650,6 +650,15 @@ export default {
   'chat.unread': 'Nuovi messaggi',
   'chat.jump_latest': 'Vai ai più recenti',
   'chat.jump_new': 'Nuovi messaggi',
+  'chat.unreadable': 'Impossibile leggere questo messaggio',
+  'chat.unreadable_signature': 'Messaggio non verificato: la firma non corrisponde al mittente',
+  'chat.unreadable_epoch': 'Scritto prima che questo dispositivo potesse leggere questa conversazione',
+  'chat.unreadable_keys': 'Chiavi della chat non disponibili: riconnettiti per leggerlo',
+  'chat.unreadable_decrypt': 'Impossibile decifrare questo messaggio',
+  'chat.unreadable_envelope': 'Questo messaggio è arrivato incompleto',
+  'chat.unreadable_format': 'Questo messaggio richiede una versione più recente di MeshBay',
+  'chat.encrypted_needs_newer': 'Questa conversazione è cifrata e questo client non può leggerla: aggiorna MeshBay',
+  'chat.encrypted_cannot_send': 'Questa conversazione è cifrata e questo dispositivo non può ancora scriverci',
   'group.leave': 'Esci dal gruppo',
   'group.leave_confirm': 'Uscire da «{name}»? Perderà l’accesso ai suoi file e alla chat. I file che ha caricato restano sul node, e il node mantiene l’identità che ha fissato per lei finché il suo operatore non la rimuove.',
   'group.mute': 'Mute notifications',
@@ -862,6 +871,9 @@ export default {
   'settings_app.chat_no_writable_root': 'Questo gruppo non ha directory scrivibili, quindi gli allegati sono disattivati.',
   'settings_app.chat_link_preview_label': 'Anteprime dei link',
   'settings_app.chat_link_preview_hint': 'Quando un membro pubblica un link, il nodo recupera titolo e immagine della pagina. È una richiesta dalla tua macchina a un sito scelto da qualcun altro.',
+  'settings_app.chat_encrypted_always': 'La chat di questo gruppo è sempre cifrata. Non può essere disattivata.',
+  'settings_app.chat_rotate_epoch': 'Ruota la chiave della chat',
+  'settings_app.chat_rotate_epoch_hint': 'I membri continuano a leggere tutta la cronologia. Chi è stato rimosso dal gruppo non potrà leggere ciò che verrà scritto d\'ora in poi. Rimuovere un membro o un dispositivo lo fa già da solo.',
   'settings_app.tmdb_token_prompt': 'Registrati su TMDB per generare la tua chiave API.',
   'settings_app.tmdb_token_link': 'Ottieni una chiave',
   'settings_node.roots_offline_hint': 'Non connesso al nodo: le modifiche passano dal nodo locale e hanno effetto al successivo ricaricamento.',