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Kegan DougalandClaude Fable 5.1
5161187a50
Rust: `RoomVersion.msc4430_member_keys` and `MSC4430V12`
("org.matrix.msc4430.12" = `MSC4242V12` + the flag); redaction of
`m.room.member` keeps `content.domain` and drops `content.localpart` in such
rooms, so the reference hash, the event ID and the member key signature cover
the claimed domain but not the claimed localpart;
`membership_field::{DOMAIN,LOCALPART}`; `org.matrix.msc4430` in `/versions`;
`.pyi` updated. The version is defined but *not* registered in
`KNOWN_ROOM_VERSIONS`, not even when the flag is on.
Python: `MemberKey` in `synapse.types` next to the other identifier types, with
the grammar `^@[A-Za-z0-9_-]{43}$` (anything with a `:` is a user ID),
`is_member_key` (syntactic, regex only, for hot paths) and
`MemberKey.from_string` (strict: rejects non-canonical base64, so each public
key has exactly one member key string). `MemberKeys` constants
(`SIGNATURE_ENTITY = "sender"`, key ID `ed25519:1`) and
`EventContentFields.MSC4430_{LOCALPART,DOMAIN}`. `supports_msc4430_member_keys`
is a `TypeIs[MSC4430Event]` modelled on `supports_msc4242_state_dag`;
`MSC4430Event` subclasses `MSC4242Event`, since every member key room is a state
DAG room, so one predicate narrows to both.
Signing: `add_hashes_and_signatures_from_signers(room_version, event_dict,
signers, *, server_name)` hashes once and then applies `(entity, key)` signers
in order; `compute_member_key_room_signature(room_version, event_dict, signer,
*, server_name)` adds one signature to an already hashed event (the authoriser's
signature on `/send_join`, our co-signature on an inbound `/invite`) without
rehashing a PDU we have not checked. The three signer kinds are: the sender's
member key under `sender`; our own server name co-signing linkage information;
the authorising member key of a restricted join under the member key string
itself. Together these are the one choke point for signing in member key rooms,
and they refuse with `MemberKeySigningRefusedError` (500) any signer the MSC
does not allow: a `sender` signature by a key other than the one `sender` names,
a key ID other than `ed25519:1`, a server co-signature under any name but ours
or on anything but an `m.room.member` whose `content.domain` is us, or an
authorising signature on anything but a join that names that key in
`join_authorised_via_users_server`. A bug in choosing signers therefore cannot
make us sign as a key we do not hold or vouch for linkage information that is
not ours. An empty signer list hashes only (`/make_join` templates).
Loudness: `get_domain_from_id` and `get_localpart_from_id` raise
`MemberKeyHasNoDomainError` (a `SynapseError`, 500: it is a server bug) when
given a member key, on the no-colon branch only, so user IDs and aliases are
untouched. Because the HTTP layer logs every `SynapseError` at INFO without a
traceback, and the error may be caught before it gets there, both MSC4430 errors
log themselves at ERROR with the stack of the raise site: the point of them is
to name the unadapted code path. `hs.is_mine_id` is left alone: it is called on
raw client strings (state keys, sliding-sync `required_state`,
`/users/{id}/report`, module API) and on remote-controlled room content
(`/upgrade` moving a canonical alias), where "False if malformed" is relied upon
and a member-key-shaped string in a legacy room is just malformed.
`hs.is_mine_identity(room_id, identity)` is the sanctioned question for room
identities; it answers for user IDs and raises `MemberKeyHasNoDomainError` for
member keys. The Prometheus counter
`synapse_msc4430_unsupported_path_total{server_name,path}` and
`record_unsupported_path(server_name, path)` (counts always, warns once per path
per process) are for branches that deliberately skip member keys.
The identity module is `synapse/handlers/member_keys.py` (`MemberKeyHandler`,
`hs.get_member_key_handler()`). The `MemberKey` type and `is_member_key` live in
`synapse.types` because `get_domain_from_id` needs them and the module cannot
import handlers.
Config: `msc4430_enabled` implies `msc4428_enabled`; the Complement image sets
it.
None of this is reachable: no identity in any existing room can satisfy
`is_member_key`, and the room version cannot be used. Not gated, by design: the
`is_member_key`-guarded raises in `get_domain_from_id`/`get_localpart_from_id`,
which only fire for member-key-shaped strings and turn a 400 "Invalid ID" into a
500. The one remote-reachable path that was a clean 400 before,
`/make_join|leave|knock/{roomId}/{userId}` with a member-key-shaped path
parameter (the handlers compare `get_domain_from_id(user_id)` with the origin),
now validates the parameter as a user ID at the servlet (400 `M_INVALID_PARAM`),
so a malformed request cannot make the Synapse federation client fail over
across every resident on a 5xx. A PDU whose sender is member-key-shaped in a
legacy room already fails at `_check_sigs_on_pdu` and is logged at ERROR with a
traceback on `/send`; only the status code changes.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>