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/pt-BR.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
index 6eb2242..f23f237 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
@@ -649,6 +649,15 @@ export default {
   'chat.unread': 'Mensagens novas',
   'chat.jump_latest': 'Ir para a mais recente',
   'chat.jump_new': 'Mensagens novas',
+  'chat.unreadable': 'Não foi possível ler esta mensagem',
+  'chat.unreadable_signature': 'Mensagem não verificada — a assinatura não corresponde ao remetente',
+  'chat.unreadable_epoch': 'Escrita antes de este dispositivo poder ler esta conversa',
+  'chat.unreadable_keys': 'Chaves do chat indisponíveis — reconecte-se para ler isto',
+  'chat.unreadable_decrypt': 'Não foi possível descriptografar esta mensagem',
+  'chat.unreadable_envelope': 'Esta mensagem chegou incompleta',
+  'chat.unreadable_format': 'Esta mensagem exige uma versão mais recente do MeshBay',
+  'chat.encrypted_needs_newer': 'Esta conversa é criptografada e este cliente não consegue lê-la — atualize o MeshBay',
+  'chat.encrypted_cannot_send': 'Esta conversa é criptografada e este dispositivo ainda não pode escrever nela',
   'group.leave': 'Sair do grupo',
   'group.leave_confirm': 'Sair de “{name}”? Você perderá o acesso aos arquivos e à conversa. Os arquivos que você enviou permanecem no node, e ele mantém a identidade que fixou para você até que o operador dele a remova.',
   'group.mute': 'Mute notifications',
@@ -849,6 +858,9 @@ export default {
   'settings_app.chat_no_writable_root': 'Este grupo não tem diretório gravável, então os anexos estão desativados.',
   'settings_app.chat_link_preview_label': 'Prévias de links',
   'settings_app.chat_link_preview_hint': 'Quando alguém publica um link, o nó busca o título e a imagem da página. É uma requisição da sua máquina para um site escolhido por outra pessoa.',
+  'settings_app.chat_encrypted_always': 'O chat deste grupo é sempre criptografado. Não é possível desativar.',
+  'settings_app.chat_rotate_epoch': 'Girar a chave do chat',
+  'settings_app.chat_rotate_epoch_hint': 'Os membros continuam lendo todo o histórico. Quem foi removido do grupo não conseguirá ler o que for escrito daqui em diante. Remover um membro ou dispositivo já faz isso sozinho.',
   'settings_app.tmdb_token_prompt': 'Cadastre-se no TMDB para gerar sua própria chave de API.',
   'settings_app.tmdb_token_link': 'Obter uma chave',
   'settings_node.roots_offline_hint': 'Sem conexão com o nó — as alterações passam pelo nó local e entram em vigor no próximo recarregamento.',