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, 39 insertions, 0 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/device.py b/packages/meshbay-common/src/meshbay_common/device.py
index 8951dbb..cfa8dd6 100644
--- a/packages/meshbay-common/src/meshbay_common/device.py
+++ b/packages/meshbay-common/src/meshbay_common/device.py
@@ -36,6 +36,7 @@ import hashlib
 
 DEVICE_REQUEST_PREFIX = b"meshbay:device_req:v1"
 DEVICE_ADD_PREFIX = b"meshbay:device_add:v1"
+DEVICE_HELLO_PREFIX = b"meshbay:device_hello:v1"
 
 # Same as the join and admin transcripts: interactive exchanges that complete in
 # milliseconds, so anything older is a replay.
@@ -123,3 +124,41 @@ def device_add_transcript(
         nonce_node,
         str(ts).encode(),
     ])
+
+
+def device_hello_transcript(
+    node_pk_b64: str,
+    group_id: str,
+    user_id: str,
+    pk_ed25519_b64: str,
+    nonce_node: bytes,
+    ts: int,
+) -> bytes:
+    """
+    Signed by the device on an already-authenticated connection, saying **which
+    of the account's devices this connection is**.
+
+    The handshake proves membership of a group (a GEK-HMAC) and carries an
+    account from the hub's token; it proves nothing about *which* device is
+    talking. The node needed that the moment one account could hold several:
+    `_load_pinned_pk` was resolving "this account's oldest live device" and
+    recording it as the uploader of every file, so a phone's uploads were
+    attributed to a laptop.
+
+    Additive and optional. A client that does not send it leaves the node where
+    it was, which is why this could ship without a breaking protocol change —
+    but a node that *has* been told refuses a later claim to be a different
+    device on the same connection.
+
+    `nonce_node` is this connection's handshake nonce, so the signature cannot
+    be lifted onto another connection, and `node_pk` binds it to one node —
+    the same rule as every other transcript here.
+    """
+    return _pack(DEVICE_HELLO_PREFIX, [
+        node_pk_b64.encode(),
+        group_id.encode(),
+        user_id.encode(),
+        pk_ed25519_b64.encode(),
+        nonce_node,
+        str(ts).encode(),
+    ])