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 --- packages/meshbay-common/src/meshbay_common/groupbox.py | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'packages/meshbay-common/src/meshbay_common/groupbox.py') diff --git a/packages/meshbay-common/src/meshbay_common/groupbox.py b/packages/meshbay-common/src/meshbay_common/groupbox.py index f1091c3..ff60bba 100644 --- a/packages/meshbay-common/src/meshbay_common/groupbox.py +++ b/packages/meshbay-common/src/meshbay_common/groupbox.py @@ -41,6 +41,10 @@ from cryptography.hazmat.primitives.kdf.hkdf import HKDF PURPOSE_INDEX = "index" PURPOSE_ACK = "ack" +# The chat epoch keys themselves, on their way to a member. The keys are what +# the chat archive is encrypted under; this is only how they travel, which is +# why rotating the group key costs a re-wrap and not a re-encryption. +PURPOSE_CHAT_KEYS = "chat_keys" # `salt=None` here and `salt: new Uint8Array(0)` in crypto.js agree — RFC 5869 # extracts with a zero key either way. Already proven in production by @@ -48,6 +52,7 @@ PURPOSE_ACK = "ack" _INFO = { PURPOSE_INDEX: b"meshbay:index:v1", PURPOSE_ACK: b"meshbay:ack:v1", + PURPOSE_CHAT_KEYS: b"meshbay:chat_keys:v1", } NONCE_LEN = 12 # 96-bit, the WebCrypto AES-GCM standard -- cgit v1.2.3