From 36cebf25d0e0f24cf63be4380ccb5d03da726a74 Mon Sep 17 00:00:00 2001 From: Christophe Besson Date: Mon, 7 Sep 2026 17:50:28 +0200 Subject: feat(chat): encrypt group chat under per-device epoch keys (MNP 2.0) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Claude-Session: https://claude.ai/code/session_01TZZxYjz8YeWRz13xDi8LJr --- .../meshbay-common/src/meshbay_common/handshake.py | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) (limited to 'packages/meshbay-common/src/meshbay_common/handshake.py') diff --git a/packages/meshbay-common/src/meshbay_common/handshake.py b/packages/meshbay-common/src/meshbay_common/handshake.py index f7d4911..188a8aa 100644 --- a/packages/meshbay-common/src/meshbay_common/handshake.py +++ b/packages/meshbay-common/src/meshbay_common/handshake.py @@ -66,11 +66,19 @@ from meshbay_common import MNP_VERSION HANDSHAKE_PREFIX = b"meshbay:mnp:handshake:v1" -# The oldest peer this build will talk to. MNP 1.0 sealed `index_sync`, -# `index_delta` and the `handshake_ack` payload under the group key, which no -# 0.x peer can open and which a 0.x peer's own messages do not carry — there is -# nothing to be compatible with, which is what makes it a MAJOR bump. -MNP_MIN_SUPPORTED = "1.0" +# The oldest peer this build will talk to. +# +# 2.0 (2026-09-07): chat messages are sealed under a per-device subkey of the +# group's chat epoch key, and the node refuses a plaintext one. A 1.x peer can +# neither produce nor read that, so there is nothing to be compatible with — +# the same reasoning that made 1.0 a MAJOR bump for the sealed index. +# +# Moving the floor with the version is the point: a 1.x client is refused here, +# with `version_too_old` and a sentence saying so, instead of completing a +# handshake and then discovering that every message it sends is rejected and +# every message it receives is unreadable. A stated refusal is a bug report; a +# chat that quietly does not work is a support case. +MNP_MIN_SUPPORTED = "2.0" ROLE_CLIENT = "client" ROLE_NODE = "node" -- cgit v1.2.3