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, 11 insertions, 0 deletions

diff --git a/packages/meshbay-node/tests/test_cli_dispatch.py b/packages/meshbay-node/tests/test_cli_dispatch.py
index cf91564..6f43772 100644
--- a/packages/meshbay-node/tests/test_cli_dispatch.py
+++ b/packages/meshbay-node/tests/test_cli_dispatch.py
@@ -49,6 +49,11 @@ VERBS = [
     ["file", "list"],
     ["file", "rm", "abc", "--yes"],
     ["video", "rematch", "--yes"],
+    ["chat", "status"],
+    ["chat", "rotate"],
+    ["chat", "encrypt-history", "--yes"],
+    ["chat", "prune", "30"],
+    ["chat", "prune"],                     # missing days: usage, then exit
     ["denylist", "show"],
     ["denylist", "clear", "--yes"],
     ["stun", "list"],
@@ -78,6 +83,12 @@ def stub_daemon(monkeypatch, tmp_path):
             "user_id": "u", "authorized_members": 0, "errors": [],
             "name": "g", "group_id": "g", "shared_dir": str(tmp_path),
             "config": str(tmp_path / "node.toml"),
+            # Chat encryption: the switch's answer, the epoch a rotation
+            # opened, and what a history re-encryption reports.
+            "enabled": False, "epoch": 1, "converted": 0,
+            "backup": str(tmp_path / "chat.db.bak"),
+            "encrypted": False, "plaintext_messages": 0,
+            "encrypted_messages": 0, "max_age_days": 30,
         }
 
     monkeypatch.setattr(daemon_mod, "_daemon_api", fake_api)