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+# MSC4303: Disallowing non-compliant user IDs in rooms
+
+The specification presently allows for a "historical" grammar for user IDs, described
+[here](https://spec.matrix.org/v1.15/appendices/#historical-user-ids). The use of this historical
+character set has allowed for "weird" and generally not-great user IDs to appear in the wild,
+causing a variety of issues for software projects.
+
+The historical grammar can be split into two classes:
+
+1. printable ASCII (U+0021 to U+007E), aka. historical-but-compliant
+2. arbitrary unicode and blank localparts, aka. non-compliant
+
+This proposal uses a future room version to prohibit the second class, while still allowing the first.
+Users with non-ASCII user IDs will not be able to join or participate in those room versions.
+
+This proposal does not cover room ID grammar, as that will be handled by another MSC,
+such as [MSC4051](https://github.com/matrix-org/matrix-spec-proposals/pull/4051).
+
+## Proposal
+
+In a future room version, the historical-but-compliant grammar for user IDs is strictly enforced on
+all places a user ID can appear in an event.
+
+* The `sender` of all events.
+* The `state_key` of `m.room.member` events.
+* The `join_authorised_via_users_server` field in `m.room.member` events.
+* Keys of the `users` object in `m.room.power_levels`.
+
+User IDs in the new room version must only consist of ASCII characters between U+0021 and U+007E
+and the localpart must not be empty.
+
+By making this change in a room version, non-compliant user IDs are slowly removed from the public
+federation. Several rooms will naturally upgrade to a room version which includes this MSC's change
+after it becomes available in servers, and eventually the specification will update the
+[recommended default room version](https://spec.matrix.org/v1.15/rooms/#complete-list-of-room-versions)
+to include this MSC's change as well to affect new room creation.
+
+In short, this MSC starts a multi-year process to formally phase out non-compliant user IDs as new
+(and existing through upgrades) rooms use a room version which bans such user IDs.
+
+## Potential issues
+
+Unlike [MSC4044](https://github.com/matrix-org/matrix-spec-proposals/pull/4044), this MSC does
+not define all historical user IDs as non-compliant. Therefore, real users who registered historical
+user IDs should not be affected. Only accounts registered by modified servers will be prohibited
+from participating in new rooms. Additionally, the existing spec already recommends servers drop
+traffic from such non-compliant user IDs outside of rooms (e.g. in to-device events), which means
+such users may already not be able to participate in encrypted rooms.
+
+## Alternatives
+
+[MSC4044](https://github.com/matrix-org/matrix-spec-proposals/pull/4044)
+
+We don't do this, or hope that pseudo IDs fix the problem.
+
+## Security considerations
+
+This MSC improves the security posture of the protocol by reducing the likelihood of strange characters
+appearing in user IDs.
+
+## Unstable prefix
+
+This MSC can be implemented in a room version identified as `fi.mau.msc4303` building on top of
+room version 11. A future MSC will be required to incorporate this MSC into a stable room version.
+
+## Dependencies
+
+None.