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, 14 insertions, 3 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/__init__.py b/packages/meshbay-common/src/meshbay_common/__init__.py
index b3e24e3..ca4c6eb 100644
--- a/packages/meshbay-common/src/meshbay_common/__init__.py
+++ b/packages/meshbay-common/src/meshbay_common/__init__.py
@@ -90,8 +90,19 @@ __version__ = "0.11.0"
 # makes the *next* breaking change cost a refusal message instead of a second
 # flag day. `MNP_MIN_SUPPORTED` in `handshake.py` is the other half.
 #
+# **2.0 (2026-09-07): chat is encrypted, and there is no way to turn it off.**
+# A MAJOR bump because it is a real break: a 1.x peer cannot produce a sealed
+# chat message and cannot read one, so it is refused at the handshake with
+# `version_too_old` rather than connecting and then failing to speak. Expressing
+# the break in the version is what makes it a stated refusal instead of a
+# conversation that silently does not work — `MNP_MIN_SUPPORTED` moves with it.
+#
+# There is deliberately no per-group switch. Every node in existence is a test
+# node, so an opt-in flag would buy nothing and cost a compatibility path to
+# maintain; existing node data is migrated by `QE/migrate-chat-encryption.py`.
+#
 # The index at rest, `index_progress` (counters only, never a path — see
-# `groupbox.py` and daemon.py `_push_index_progress`), chat, and file content
-# on the operator's disk are all deliberately unchanged.
-MNP_VERSION = "1.1"
+# `groupbox.py` and daemon.py `_push_index_progress`), and file content on the
+# operator's disk are all deliberately unchanged.
+MNP_VERSION = "2.0"
 MHP_VERSION = "0.1"