From 7a1fd7f512ce06a472a2051ee63eae4a270eb71a Mon Sep 17 00:00:00 2001
From: devonh <devon.dmytro@gmail.com>
Date: Tue, 6 Jun 2023 20:55:18 +0000
Subject: [PATCH] PDU Sender split (#3100)

Initial cut of splitting PDU Sender into SenderID & looking up UserID where required.
---
 appservice/consumers/roomserver.go            | 12 ++++-
 clientapi/routing/directory.go                | 30 +++++++++++-
 clientapi/routing/redaction.go                |  2 +-
 clientapi/routing/sendevent.go                |  4 +-
 clientapi/routing/state.go                    | 21 +++++++--
 cmd/resolve-state/main.go                     |  5 +-
 federationapi/federationapi_test.go           |  4 ++
 federationapi/internal/perform.go             | 35 ++++++++------
 federationapi/routing/invite.go               |  6 +++
 federationapi/routing/join.go                 | 24 ++++++----
 federationapi/routing/leave.go                | 13 ++++--
 go.mod                                        |  4 +-
 go.sum                                        |  8 ++--
 internal/pushrules/evaluate.go                | 16 +++++--
 internal/pushrules/evaluate_test.go           | 17 ++++---
 internal/transactionrequest.go                |  4 +-
 internal/transactionrequest_test.go           |  8 ++++
 roomserver/api/alias.go                       |  2 +-
 roomserver/api/api.go                         | 12 +++++
 roomserver/api/query.go                       |  4 +-
 roomserver/internal/alias.go                  | 21 +++++----
 roomserver/internal/helpers/auth.go           |  4 +-
 roomserver/internal/input/input_events.go     | 32 +++++++++----
 .../internal/input/input_events_test.go       |  2 +-
 roomserver/internal/input/input_missing.go    | 24 +++++++---
 roomserver/internal/perform/perform_admin.go  |  8 +++-
 .../internal/perform/perform_backfill.go      | 12 +++--
 .../internal/perform/perform_create_room.go   |  4 +-
 roomserver/internal/perform/perform_invite.go | 12 +++--
 .../internal/perform/perform_upgrade.go       | 10 ++--
 roomserver/internal/query/query.go            | 30 ++++++++++--
 roomserver/producers/roomevent.go             |  2 +-
 roomserver/state/state.go                     |  9 +++-
 roomserver/storage/interface.go               |  5 ++
 .../storage/shared/membership_updater.go      |  2 +-
 roomserver/storage/shared/room_updater.go     |  5 ++
 roomserver/storage/shared/storage.go          | 28 +++++++++--
 setup/mscs/msc2836/msc2836.go                 |  8 ++--
 setup/mscs/msc2836/msc2836_test.go            |  4 ++
 setup/mscs/msc2946/msc2946.go                 |  2 +-
 syncapi/consumers/roomserver.go               |  2 +-
 syncapi/routing/context.go                    | 23 ++++++++--
 syncapi/routing/getevent.go                   |  7 ++-
 syncapi/routing/memberships.go                |  6 ++-
 syncapi/routing/messages.go                   | 12 +++--
 syncapi/routing/relations.go                  |  7 ++-
 syncapi/routing/routing.go                    |  2 +-
 syncapi/routing/search.go                     | 46 +++++++++++++------
 syncapi/routing/search_test.go                | 10 +++-
 .../postgres/current_room_state_table.go      |  2 +-
 .../postgres/output_room_events_table.go      |  2 +-
 syncapi/storage/shared/storage_consumer.go    | 21 ++++++++-
 .../sqlite3/current_room_state_table.go       |  2 +-
 .../sqlite3/output_room_events_table.go       |  2 +-
 syncapi/streams/stream_invite.go              | 12 ++++-
 syncapi/streams/stream_pdu.go                 | 38 ++++++++++-----
 syncapi/streams/streams.go                    |  1 +
 syncapi/syncapi_test.go                       |  4 ++
 syncapi/synctypes/clientevent.go              | 13 ++++--
 syncapi/synctypes/clientevent_test.go         | 17 +++++--
 syncapi/types/types.go                        |  4 +-
 syncapi/types/types_test.go                   | 12 ++++-
 test/room.go                                  |  6 ++-
 userapi/consumers/roomserver.go               | 43 ++++++++++++-----
 userapi/consumers/roomserver_test.go          | 11 ++++-
 userapi/util/notify_test.go                   |  9 +++-
 66 files changed, 580 insertions(+), 189 deletions(-)

diff --git a/appservice/consumers/roomserver.go b/appservice/consumers/roomserver.go
index c02d9040..06625ad7 100644
--- a/appservice/consumers/roomserver.go
+++ b/appservice/consumers/roomserver.go
@@ -181,7 +181,9 @@ func (s *OutputRoomEventConsumer) sendEvents(
 	// Create the transaction body.
 	transaction, err := json.Marshal(
 		ApplicationServiceTransaction{
-			Events: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatAll),
+			Events: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+				return s.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+			}),
 		},
 	)
 	if err != nil {
@@ -233,10 +235,16 @@ func (s *appserviceState) backoffAndPause(err error) error {
 //
 // TODO: This should be cached, see https://github.com/matrix-org/dendrite/issues/1682
 func (s *OutputRoomEventConsumer) appserviceIsInterestedInEvent(ctx context.Context, event *types.HeaderedEvent, appservice *config.ApplicationService) bool {
+	user := ""
+	userID, err := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+	if err == nil {
+		user = userID.String()
+	}
+
 	switch {
 	case appservice.URL == "":
 		return false
-	case appservice.IsInterestedInUserID(event.Sender()):
+	case appservice.IsInterestedInUserID(user):
 		return true
 	case appservice.IsInterestedInRoomID(event.RoomID()):
 		return true
diff --git a/clientapi/routing/directory.go b/clientapi/routing/directory.go
index c786f8cc..0c842e6a 100644
--- a/clientapi/routing/directory.go
+++ b/clientapi/routing/directory.go
@@ -215,9 +215,35 @@ func RemoveLocalAlias(
 	alias string,
 	rsAPI roomserverAPI.ClientRoomserverAPI,
 ) util.JSONResponse {
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: spec.InternalServerError{Err: "UserID for device is invalid"},
+		}
+	}
+
+	roomIDReq := roomserverAPI.GetRoomIDForAliasRequest{Alias: alias}
+	roomIDRes := roomserverAPI.GetRoomIDForAliasResponse{}
+	err = rsAPI.GetRoomIDForAlias(req.Context(), &roomIDReq, &roomIDRes)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: spec.NotFound("The alias does not exist."),
+		}
+	}
+
+	deviceSenderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomIDRes.RoomID, *userID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: spec.InternalServerError{Err: "Could not find SenderID for this device"},
+		}
+	}
+
 	queryReq := roomserverAPI.RemoveRoomAliasRequest{
-		Alias:  alias,
-		UserID: device.UserID,
+		Alias:    alias,
+		SenderID: deviceSenderID,
 	}
 	var queryRes roomserverAPI.RemoveRoomAliasResponse
 	if err := rsAPI.RemoveRoomAlias(req.Context(), &queryReq, &queryRes); err != nil {
diff --git a/clientapi/routing/redaction.go b/clientapi/routing/redaction.go
index 88312642..e94c7748 100644
--- a/clientapi/routing/redaction.go
+++ b/clientapi/routing/redaction.go
@@ -76,7 +76,7 @@ func SendRedaction(
 	// "Users may redact their own events, and any user with a power level greater than or equal
 	// to the redact power level of the room may redact events there"
 	// https://matrix.org/docs/spec/client_server/r0.6.1#put-matrix-client-r0-rooms-roomid-redact-eventid-txnid
-	allowedToRedact := ev.Sender() == device.UserID
+	allowedToRedact := ev.SenderID() == device.UserID // TODO: Should replace device.UserID with device...PerRoomKey
 	if !allowedToRedact {
 		plEvent := roomserverAPI.GetStateEvent(req.Context(), rsAPI, roomID, gomatrixserverlib.StateKeyTuple{
 			EventType: spec.MRoomPowerLevels,
diff --git a/clientapi/routing/sendevent.go b/clientapi/routing/sendevent.go
index 1a2e25c9..8b09f399 100644
--- a/clientapi/routing/sendevent.go
+++ b/clientapi/routing/sendevent.go
@@ -331,7 +331,9 @@ func generateSendEvent(
 		stateEvents[i] = queryRes.StateEvents[i].PDU
 	}
 	provider := gomatrixserverlib.NewAuthEvents(gomatrixserverlib.ToPDUs(stateEvents))
-	if err = gomatrixserverlib.Allowed(e.PDU, &provider); err != nil {
+	if err = gomatrixserverlib.Allowed(e.PDU, &provider, func(roomID, senderID string) (*spec.UserID, error) {
+		return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		return nil, &util.JSONResponse{
 			Code: http.StatusForbidden,
 			JSON: spec.Forbidden(err.Error()), // TODO: Is this error string comprehensible to the client?
diff --git a/clientapi/routing/state.go b/clientapi/routing/state.go
index 319f4eba..13f30899 100644
--- a/clientapi/routing/state.go
+++ b/clientapi/routing/state.go
@@ -140,9 +140,14 @@ func OnIncomingStateRequest(ctx context.Context, device *userapi.Device, rsAPI a
 		// use the result of the previous QueryLatestEventsAndState response
 		// to find the state event, if provided.
 		for _, ev := range stateRes.StateEvents {
+			sender := spec.UserID{}
+			userID, err := rsAPI.QueryUserIDForSender(ctx, ev.RoomID(), ev.SenderID())
+			if err == nil && userID != nil {
+				sender = *userID
+			}
 			stateEvents = append(
 				stateEvents,
-				synctypes.ToClientEvent(ev, synctypes.FormatAll),
+				synctypes.ToClientEvent(ev, synctypes.FormatAll, sender),
 			)
 		}
 	} else {
@@ -162,9 +167,14 @@ func OnIncomingStateRequest(ctx context.Context, device *userapi.Device, rsAPI a
 			}
 		}
 		for _, ev := range stateAfterRes.StateEvents {
+			sender := spec.UserID{}
+			userID, err := rsAPI.QueryUserIDForSender(ctx, ev.RoomID(), ev.SenderID())
+			if err == nil && userID != nil {
+				sender = *userID
+			}
 			stateEvents = append(
 				stateEvents,
-				synctypes.ToClientEvent(ev, synctypes.FormatAll),
+				synctypes.ToClientEvent(ev, synctypes.FormatAll, sender),
 			)
 		}
 	}
@@ -334,8 +344,13 @@ func OnIncomingStateTypeRequest(
 		}
 	}
 
+	sender := spec.UserID{}
+	userID, err := rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+	if err == nil && userID != nil {
+		sender = *userID
+	}
 	stateEvent := stateEventInStateResp{
-		ClientEvent: synctypes.ToClientEvent(event, synctypes.FormatAll),
+		ClientEvent: synctypes.ToClientEvent(event, synctypes.FormatAll, sender),
 	}
 
 	var res interface{}
diff --git a/cmd/resolve-state/main.go b/cmd/resolve-state/main.go
index 3a4255ba..36040309 100644
--- a/cmd/resolve-state/main.go
+++ b/cmd/resolve-state/main.go
@@ -18,6 +18,7 @@ import (
 	"github.com/matrix-org/dendrite/setup/config"
 	"github.com/matrix-org/dendrite/setup/process"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
 // This is a utility for inspecting state snapshots and running state resolution
@@ -182,7 +183,9 @@ func main() {
 	fmt.Println("Resolving state")
 	var resolved Events
 	resolved, err = gomatrixserverlib.ResolveConflicts(
-		gomatrixserverlib.RoomVersion(*roomVersion), events, authEvents,
+		gomatrixserverlib.RoomVersion(*roomVersion), events, authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+			return roomserverDB.GetUserIDForSender(ctx, roomID, senderID)
+		},
 	)
 	if err != nil {
 		panic(err)
diff --git a/federationapi/federationapi_test.go b/federationapi/federationapi_test.go
index beb648a4..a97bcdea 100644
--- a/federationapi/federationapi_test.go
+++ b/federationapi/federationapi_test.go
@@ -36,6 +36,10 @@ type fedRoomserverAPI struct {
 	queryRoomsForUser func(ctx context.Context, req *rsapi.QueryRoomsForUserRequest, res *rsapi.QueryRoomsForUserResponse) error
 }
 
+func (f *fedRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 // PerformJoin will call this function
 func (f *fedRoomserverAPI) InputRoomEvents(ctx context.Context, req *rsapi.InputRoomEventsRequest, res *rsapi.InputRoomEventsResponse) {
 	if f.inputRoomEvents == nil {
diff --git a/federationapi/internal/perform.go b/federationapi/internal/perform.go
index ed800d03..2d59d0f9 100644
--- a/federationapi/internal/perform.go
+++ b/federationapi/internal/perform.go
@@ -156,15 +156,20 @@ func (r *FederationInternalAPI) performJoinUsingServer(
 	}
 
 	joinInput := gomatrixserverlib.PerformJoinInput{
-		UserID:        user,
-		RoomID:        room,
-		ServerName:    serverName,
-		Content:       content,
-		Unsigned:      unsigned,
-		PrivateKey:    r.cfg.Matrix.PrivateKey,
-		KeyID:         r.cfg.Matrix.KeyID,
-		KeyRing:       r.keyRing,
-		EventProvider: federatedEventProvider(ctx, r.federation, r.keyRing, user.Domain(), serverName),
+		UserID:     user,
+		RoomID:     room,
+		ServerName: serverName,
+		Content:    content,
+		Unsigned:   unsigned,
+		PrivateKey: r.cfg.Matrix.PrivateKey,
+		KeyID:      r.cfg.Matrix.KeyID,
+		KeyRing:    r.keyRing,
+		EventProvider: federatedEventProvider(ctx, r.federation, r.keyRing, user.Domain(), serverName, func(roomID, senderID string) (*spec.UserID, error) {
+			return r.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}),
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return r.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		},
 	}
 	response, joinErr := gomatrixserverlib.PerformJoin(ctx, r, joinInput)
 
@@ -358,8 +363,11 @@ func (r *FederationInternalAPI) performOutboundPeekUsingServer(
 
 	// authenticate the state returned (check its auth events etc)
 	// the equivalent of CheckSendJoinResponse()
+	userIDProvider := func(roomID, senderID string) (*spec.UserID, error) {
+		return r.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	}
 	authEvents, stateEvents, err := gomatrixserverlib.CheckStateResponse(
-		ctx, &respPeek, respPeek.RoomVersion, r.keyRing, federatedEventProvider(ctx, r.federation, r.keyRing, r.cfg.Matrix.ServerName, serverName),
+		ctx, &respPeek, respPeek.RoomVersion, r.keyRing, federatedEventProvider(ctx, r.federation, r.keyRing, r.cfg.Matrix.ServerName, serverName, userIDProvider), userIDProvider,
 	)
 	if err != nil {
 		return fmt.Errorf("error checking state returned from peeking: %w", err)
@@ -509,7 +517,7 @@ func (r *FederationInternalAPI) SendInvite(
 	event gomatrixserverlib.PDU,
 	strippedState []gomatrixserverlib.InviteStrippedState,
 ) (gomatrixserverlib.PDU, error) {
-	_, origin, err := r.cfg.Matrix.SplitLocalID('@', event.Sender())
+	inviter, err := r.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
 	if err != nil {
 		return nil, err
 	}
@@ -542,7 +550,7 @@ func (r *FederationInternalAPI) SendInvite(
 		return nil, fmt.Errorf("gomatrixserverlib.NewInviteV2Request: %w", err)
 	}
 
-	inviteRes, err := r.federation.SendInviteV2(ctx, origin, destination, inviteReq)
+	inviteRes, err := r.federation.SendInviteV2(ctx, inviter.Domain(), destination, inviteReq)
 	if err != nil {
 		return nil, fmt.Errorf("r.federation.SendInviteV2: failed to send invite: %w", err)
 	}
@@ -635,6 +643,7 @@ func checkEventsContainCreateEvent(events []gomatrixserverlib.PDU) error {
 func federatedEventProvider(
 	ctx context.Context, federation fclient.FederationClient,
 	keyRing gomatrixserverlib.JSONVerifier, origin, server spec.ServerName,
+	userIDForSender spec.UserIDForSender,
 ) gomatrixserverlib.EventProvider {
 	// A list of events that we have retried, if they were not included in
 	// the auth events supplied in the send_join.
@@ -684,7 +693,7 @@ func federatedEventProvider(
 				}
 
 				// Check the signatures of the event.
-				if err := gomatrixserverlib.VerifyEventSignatures(ctx, ev, keyRing); err != nil {
+				if err := gomatrixserverlib.VerifyEventSignatures(ctx, ev, keyRing, userIDForSender); err != nil {
 					return nil, fmt.Errorf("missingAuth VerifyEventSignatures: %w", err)
 				}
 
diff --git a/federationapi/routing/invite.go b/federationapi/routing/invite.go
index 78a09d94..d792335b 100644
--- a/federationapi/routing/invite.go
+++ b/federationapi/routing/invite.go
@@ -95,6 +95,9 @@ func InviteV2(
 			StateQuerier:      rsAPI.StateQuerier(),
 			InviteEvent:       inviteReq.Event(),
 			StrippedState:     inviteReq.InviteRoomState(),
+			UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+				return rsAPI.QueryUserIDForSender(httpReq.Context(), roomID, senderID)
+			},
 		}
 		event, jsonErr := handleInvite(httpReq.Context(), input, rsAPI)
 		if jsonErr != nil {
@@ -185,6 +188,9 @@ func InviteV1(
 		StateQuerier:      rsAPI.StateQuerier(),
 		InviteEvent:       event,
 		StrippedState:     strippedState,
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(httpReq.Context(), roomID, senderID)
+		},
 	}
 	event, jsonErr := handleInvite(httpReq.Context(), input, rsAPI)
 	if jsonErr != nil {
diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index 2980c2af..9da05918 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -99,15 +99,18 @@ func MakeJoin(
 	}
 
 	input := gomatrixserverlib.HandleMakeJoinInput{
-		Context:            httpReq.Context(),
-		UserID:             userID,
-		RoomID:             roomID,
-		RoomVersion:        roomVersion,
-		RemoteVersions:     remoteVersions,
-		RequestOrigin:      request.Origin(),
-		LocalServerName:    cfg.Matrix.ServerName,
-		LocalServerInRoom:  res.RoomExists && res.IsInRoom,
-		RoomQuerier:        &roomQuerier,
+		Context:           httpReq.Context(),
+		UserID:            userID,
+		RoomID:            roomID,
+		RoomVersion:       roomVersion,
+		RemoteVersions:    remoteVersions,
+		RequestOrigin:     request.Origin(),
+		LocalServerName:   cfg.Matrix.ServerName,
+		LocalServerInRoom: res.RoomExists && res.IsInRoom,
+		RoomQuerier:       &roomQuerier,
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(httpReq.Context(), roomID, senderID)
+		},
 		BuildEventTemplate: createJoinTemplate,
 	}
 	response, internalErr := gomatrixserverlib.HandleMakeJoin(input)
@@ -202,6 +205,9 @@ func SendJoin(
 		PrivateKey:        cfg.Matrix.PrivateKey,
 		Verifier:          keys,
 		MembershipQuerier: &api.MembershipQuerier{Roomserver: rsAPI},
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(httpReq.Context(), roomID, senderID)
+		},
 	}
 	response, joinErr := gomatrixserverlib.HandleSendJoin(input)
 	switch e := joinErr.(type) {
diff --git a/federationapi/routing/leave.go b/federationapi/routing/leave.go
index d7d5b599..30e99c4f 100644
--- a/federationapi/routing/leave.go
+++ b/federationapi/routing/leave.go
@@ -95,6 +95,9 @@ func MakeLeave(
 		LocalServerName:    cfg.Matrix.ServerName,
 		LocalServerInRoom:  res.RoomExists && res.IsInRoom,
 		BuildEventTemplate: createLeaveTemplate,
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(httpReq.Context(), roomID, senderID)
+		},
 	}
 
 	response, internalErr := gomatrixserverlib.HandleMakeLeave(input)
@@ -213,7 +216,7 @@ func SendLeave(
 			JSON: spec.BadJSON("No state key was provided in the leave event."),
 		}
 	}
-	if !event.StateKeyEquals(event.Sender()) {
+	if !event.StateKeyEquals(event.SenderID()) {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: spec.BadJSON("Event state key must match the event sender."),
@@ -223,13 +226,13 @@ func SendLeave(
 	// Check that the sender belongs to the server that is sending us
 	// the request. By this point we've already asserted that the sender
 	// and the state key are equal so we don't need to check both.
-	var serverName spec.ServerName
-	if _, serverName, err = gomatrixserverlib.SplitID('@', event.Sender()); err != nil {
+	sender, err := rsAPI.QueryUserIDForSender(httpReq.Context(), event.RoomID(), event.SenderID())
+	if err != nil {
 		return util.JSONResponse{
 			Code: http.StatusForbidden,
 			JSON: spec.Forbidden("The sender of the join is invalid"),
 		}
-	} else if serverName != request.Origin() {
+	} else if sender.Domain() != request.Origin() {
 		return util.JSONResponse{
 			Code: http.StatusForbidden,
 			JSON: spec.Forbidden("The sender does not match the server that originated the request"),
@@ -291,7 +294,7 @@ func SendLeave(
 		}
 	}
 	verifyRequests := []gomatrixserverlib.VerifyJSONRequest{{
-		ServerName:           serverName,
+		ServerName:           sender.Domain(),
 		Message:              redacted,
 		AtTS:                 event.OriginServerTS(),
 		ValidityCheckingFunc: gomatrixserverlib.StrictValiditySignatureCheck,
diff --git a/go.mod b/go.mod
index a49dfa0c..10551f70 100644
--- a/go.mod
+++ b/go.mod
@@ -22,7 +22,7 @@ require (
 	github.com/matrix-org/dugong v0.0.0-20210921133753-66e6b1c67e2e
 	github.com/matrix-org/go-sqlite3-js v0.0.0-20220419092513-28aa791a1c91
 	github.com/matrix-org/gomatrix v0.0.0-20220926102614-ceba4d9f7530
-	github.com/matrix-org/gomatrixserverlib v0.0.0-20230606112941-1c41e92ddf9e
+	github.com/matrix-org/gomatrixserverlib v0.0.0-20230606202811-a644d5d8fb66
 	github.com/matrix-org/pinecone v0.11.1-0.20230210171230-8c3b24f2649a
 	github.com/matrix-org/util v0.0.0-20221111132719-399730281e66
 	github.com/mattn/go-sqlite3 v1.14.16
@@ -34,7 +34,7 @@ require (
 	github.com/patrickmn/go-cache v2.1.0+incompatible
 	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.13.0
-	github.com/sirupsen/logrus v1.9.2
+	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.8.2
 	github.com/tidwall/gjson v1.14.4
 	github.com/tidwall/sjson v1.2.5
diff --git a/go.sum b/go.sum
index 79154624..3ec1c115 100644
--- a/go.sum
+++ b/go.sum
@@ -323,8 +323,8 @@ github.com/matrix-org/go-sqlite3-js v0.0.0-20220419092513-28aa791a1c91 h1:s7fexw
 github.com/matrix-org/go-sqlite3-js v0.0.0-20220419092513-28aa791a1c91/go.mod h1:e+cg2q7C7yE5QnAXgzo512tgFh1RbQLC0+jozuegKgo=
 github.com/matrix-org/gomatrix v0.0.0-20220926102614-ceba4d9f7530 h1:kHKxCOLcHH8r4Fzarl4+Y3K5hjothkVW5z7T1dUM11U=
 github.com/matrix-org/gomatrix v0.0.0-20220926102614-ceba4d9f7530/go.mod h1:/gBX06Kw0exX1HrwmoBibFA98yBk/jxKpGVeyQbff+s=
-github.com/matrix-org/gomatrixserverlib v0.0.0-20230606112941-1c41e92ddf9e h1:I3Sfr8gZvVtLHOeI8lgc62kgLuzpMhBZ6EQOMyexXEA=
-github.com/matrix-org/gomatrixserverlib v0.0.0-20230606112941-1c41e92ddf9e/go.mod h1:H9V9N3Uqn1bBJqYJNGK1noqtgJTaCEhtTdcH/mp50uU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20230606202811-a644d5d8fb66 h1:6SixhMmB5Ir10xUJ6zh3A4NBxSaZCSz2s5U63Wg0eEU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20230606202811-a644d5d8fb66/go.mod h1:H9V9N3Uqn1bBJqYJNGK1noqtgJTaCEhtTdcH/mp50uU=
 github.com/matrix-org/pinecone v0.11.1-0.20230210171230-8c3b24f2649a h1:awrPDf9LEFySxTLKYBMCiObelNx/cBuv/wzllvCCH3A=
 github.com/matrix-org/pinecone v0.11.1-0.20230210171230-8c3b24f2649a/go.mod h1:HchJX9oKMXaT2xYFs0Ha/6Zs06mxLU8k6F1ODnrGkeQ=
 github.com/matrix-org/util v0.0.0-20221111132719-399730281e66 h1:6z4KxomXSIGWqhHcfzExgkH3Z3UkIXry4ibJS4Aqz2Y=
@@ -444,8 +444,8 @@ github.com/sirupsen/logrus v1.2.0/go.mod h1:LxeOpSwHxABJmUn/MG1IvRgCAasNZTLOkJPx
 github.com/sirupsen/logrus v1.4.2/go.mod h1:tLMulIdttU9McNUspp0xgXVQah82FyeX6MwdIuYE2rE=
 github.com/sirupsen/logrus v1.6.0/go.mod h1:7uNnSEd1DgxDLC74fIahvMZmmYsHGZGEOFrfsX/uA88=
 github.com/sirupsen/logrus v1.7.0/go.mod h1:yWOB1SBYBC5VeMP7gHvWumXLIWorT60ONWic61uBYv0=
-github.com/sirupsen/logrus v1.9.2 h1:oxx1eChJGI6Uks2ZC4W1zpLlVgqB8ner4EuQwV4Ik1Y=
-github.com/sirupsen/logrus v1.9.2/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
+github.com/sirupsen/logrus v1.9.3 h1:dueUQJ1C2q9oE3F7wvmSGAaVtTmUizReu6fjN8uqzbQ=
+github.com/sirupsen/logrus v1.9.3/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
 github.com/smartystreets/assertions v0.0.0-20180927180507-b2de0cb4f26d/go.mod h1:OnSkiWE9lh6wB0YB77sQom3nweQdgAjqCqsofrRNTgc=
 github.com/smartystreets/goconvey v0.0.0-20181108003508-044398e4856c/go.mod h1:XDJAKZRPZ1CvBcN2aX5YOUTYGHki24fSF0Iv48Ibg0s=
 github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
diff --git a/internal/pushrules/evaluate.go b/internal/pushrules/evaluate.go
index 7c98efd3..da33d386 100644
--- a/internal/pushrules/evaluate.go
+++ b/internal/pushrules/evaluate.go
@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
 // A RuleSetEvaluator encapsulates context to evaluate an event
@@ -53,7 +54,7 @@ func NewRuleSetEvaluator(ec EvaluationContext, ruleSet *RuleSet) *RuleSetEvaluat
 
 // MatchEvent returns the first matching rule. Returns nil if there
 // was no match rule.
-func (rse *RuleSetEvaluator) MatchEvent(event gomatrixserverlib.PDU) (*Rule, error) {
+func (rse *RuleSetEvaluator) MatchEvent(event gomatrixserverlib.PDU, userIDForSender spec.UserIDForSender) (*Rule, error) {
 	// TODO: server-default rules have lower priority than user rules,
 	// but they are stored together with the user rules. It's a bit
 	// unclear what the specification (11.14.1.4 Predefined rules)
@@ -68,7 +69,7 @@ func (rse *RuleSetEvaluator) MatchEvent(event gomatrixserverlib.PDU) (*Rule, err
 				if rule.Default != defRules {
 					continue
 				}
-				ok, err := ruleMatches(rule, rsat.Kind, event, rse.ec)
+				ok, err := ruleMatches(rule, rsat.Kind, event, rse.ec, userIDForSender)
 				if err != nil {
 					return nil, err
 				}
@@ -83,7 +84,7 @@ func (rse *RuleSetEvaluator) MatchEvent(event gomatrixserverlib.PDU) (*Rule, err
 	return nil, nil
 }
 
-func ruleMatches(rule *Rule, kind Kind, event gomatrixserverlib.PDU, ec EvaluationContext) (bool, error) {
+func ruleMatches(rule *Rule, kind Kind, event gomatrixserverlib.PDU, ec EvaluationContext, userIDForSender spec.UserIDForSender) (bool, error) {
 	if !rule.Enabled {
 		return false, nil
 	}
@@ -113,7 +114,12 @@ func ruleMatches(rule *Rule, kind Kind, event gomatrixserverlib.PDU, ec Evaluati
 		return rule.RuleID == event.RoomID(), nil
 
 	case SenderKind:
-		return rule.RuleID == event.Sender(), nil
+		userID := ""
+		sender, err := userIDForSender(event.RoomID(), event.SenderID())
+		if err == nil {
+			userID = sender.String()
+		}
+		return rule.RuleID == userID, nil
 
 	default:
 		return false, nil
@@ -143,7 +149,7 @@ func conditionMatches(cond *Condition, event gomatrixserverlib.PDU, ec Evaluatio
 		return cmp(n), nil
 
 	case SenderNotificationPermissionCondition:
-		return ec.HasPowerLevel(event.Sender(), cond.Key)
+		return ec.HasPowerLevel(event.SenderID(), cond.Key)
 
 	default:
 		return false, nil
diff --git a/internal/pushrules/evaluate_test.go b/internal/pushrules/evaluate_test.go
index 5045a864..34c1436f 100644
--- a/internal/pushrules/evaluate_test.go
+++ b/internal/pushrules/evaluate_test.go
@@ -5,8 +5,13 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
+func UserIDForSender(roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func TestRuleSetEvaluatorMatchEvent(t *testing.T) {
 	ev := mustEventFromJSON(t, `{}`)
 	defaultEnabled := &Rule{
@@ -45,7 +50,7 @@ func TestRuleSetEvaluatorMatchEvent(t *testing.T) {
 	for _, tst := range tsts {
 		t.Run(tst.Name, func(t *testing.T) {
 			rse := NewRuleSetEvaluator(fakeEvaluationContext{3}, &tst.RuleSet)
-			got, err := rse.MatchEvent(tst.Event)
+			got, err := rse.MatchEvent(tst.Event, UserIDForSender)
 			if err != nil {
 				t.Fatalf("MatchEvent failed: %v", err)
 			}
@@ -82,15 +87,15 @@ func TestRuleMatches(t *testing.T) {
 		{"contentMatch", ContentKind, Rule{Enabled: true, Pattern: pointer("b")}, `{"content":{"body":"abc"}}`, true},
 		{"contentNoMatch", ContentKind, Rule{Enabled: true, Pattern: pointer("d")}, `{"content":{"body":"abc"}}`, false},
 
-		{"roomMatch", RoomKind, Rule{Enabled: true, RuleID: "!room@example.com"}, `{"room_id":"!room@example.com"}`, true},
-		{"roomNoMatch", RoomKind, Rule{Enabled: true, RuleID: "!room@example.com"}, `{"room_id":"!otherroom@example.com"}`, false},
+		{"roomMatch", RoomKind, Rule{Enabled: true, RuleID: "!room:example.com"}, `{"room_id":"!room:example.com"}`, true},
+		{"roomNoMatch", RoomKind, Rule{Enabled: true, RuleID: "!room:example.com"}, `{"room_id":"!otherroom:example.com"}`, false},
 
-		{"senderMatch", SenderKind, Rule{Enabled: true, RuleID: "@user@example.com"}, `{"sender":"@user@example.com"}`, true},
-		{"senderNoMatch", SenderKind, Rule{Enabled: true, RuleID: "@user@example.com"}, `{"sender":"@otheruser@example.com"}`, false},
+		{"senderMatch", SenderKind, Rule{Enabled: true, RuleID: "@user:example.com"}, `{"sender":"@user:example.com"}`, true},
+		{"senderNoMatch", SenderKind, Rule{Enabled: true, RuleID: "@user:example.com"}, `{"sender":"@otheruser:example.com"}`, false},
 	}
 	for _, tst := range tsts {
 		t.Run(tst.Name, func(t *testing.T) {
-			got, err := ruleMatches(&tst.Rule, tst.Kind, mustEventFromJSON(t, tst.EventJSON), nil)
+			got, err := ruleMatches(&tst.Rule, tst.Kind, mustEventFromJSON(t, tst.EventJSON), nil, UserIDForSender)
 			if err != nil {
 				t.Fatalf("ruleMatches failed: %v", err)
 			}
diff --git a/internal/transactionrequest.go b/internal/transactionrequest.go
index c9d321f2..0bbe0720 100644
--- a/internal/transactionrequest.go
+++ b/internal/transactionrequest.go
@@ -167,7 +167,9 @@ func (t *TxnReq) ProcessTransaction(ctx context.Context) (*fclient.RespSend, *ut
 			}
 			continue
 		}
-		if err = gomatrixserverlib.VerifyEventSignatures(ctx, event, t.keys); err != nil {
+		if err = gomatrixserverlib.VerifyEventSignatures(ctx, event, t.keys, func(roomID, senderID string) (*spec.UserID, error) {
+			return t.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}); err != nil {
 			util.GetLogger(ctx).WithError(err).Debugf("Transaction: Couldn't validate signature of event %q", event.EventID())
 			results[event.EventID()] = fclient.PDUResult{
 				Error: err.Error(),
diff --git a/internal/transactionrequest_test.go b/internal/transactionrequest_test.go
index fb30d410..6f3ce0b3 100644
--- a/internal/transactionrequest_test.go
+++ b/internal/transactionrequest_test.go
@@ -70,6 +70,10 @@ type FakeRsAPI struct {
 	bannedFromRoom  bool
 }
 
+func (r *FakeRsAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func (r *FakeRsAPI) QueryRoomVersionForRoom(
 	ctx context.Context,
 	roomID string,
@@ -638,6 +642,10 @@ type testRoomserverAPI struct {
 	queryLatestEventsAndState func(*rsAPI.QueryLatestEventsAndStateRequest) rsAPI.QueryLatestEventsAndStateResponse
 }
 
+func (t *testRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func (t *testRoomserverAPI) InputRoomEvents(
 	ctx context.Context,
 	request *rsAPI.InputRoomEventsRequest,
diff --git a/roomserver/api/alias.go b/roomserver/api/alias.go
index 37892a44..1b947540 100644
--- a/roomserver/api/alias.go
+++ b/roomserver/api/alias.go
@@ -62,7 +62,7 @@ type GetAliasesForRoomIDResponse struct {
 // RemoveRoomAliasRequest is a request to RemoveRoomAlias
 type RemoveRoomAliasRequest struct {
 	// ID of the user removing the alias
-	UserID string `json:"user_id"`
+	SenderID string `json:"user_id"`
 	// The room alias to remove
 	Alias string `json:"alias"`
 }
diff --git a/roomserver/api/api.go b/roomserver/api/api.go
index a37ade3a..d61a0553 100644
--- a/roomserver/api/api.go
+++ b/roomserver/api/api.go
@@ -49,6 +49,7 @@ type RoomserverInternalAPI interface {
 	ClientRoomserverAPI
 	UserRoomserverAPI
 	FederationRoomserverAPI
+	QuerySenderIDAPI
 
 	// needed to avoid chicken and egg scenario when setting up the
 	// interdependencies between the roomserver and other input APIs
@@ -75,6 +76,11 @@ type InputRoomEventsAPI interface {
 	)
 }
 
+type QuerySenderIDAPI interface {
+	QuerySenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (string, error)
+	QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error)
+}
+
 // Query the latest events and state for a room from the room server.
 type QueryLatestEventsAndStateAPI interface {
 	QueryLatestEventsAndState(ctx context.Context, req *QueryLatestEventsAndStateRequest, res *QueryLatestEventsAndStateResponse) error
@@ -102,6 +108,7 @@ type QueryEventsAPI interface {
 type SyncRoomserverAPI interface {
 	QueryLatestEventsAndStateAPI
 	QueryBulkStateContentAPI
+	QuerySenderIDAPI
 	// QuerySharedUsers returns a list of users who share at least 1 room in common with the given user.
 	QuerySharedUsers(ctx context.Context, req *QuerySharedUsersRequest, res *QuerySharedUsersResponse) error
 	// QueryEventsByID queries a list of events by event ID for one room. If no room is specified, it will try to determine
@@ -142,6 +149,7 @@ type SyncRoomserverAPI interface {
 }
 
 type AppserviceRoomserverAPI interface {
+	QuerySenderIDAPI
 	// QueryEventsByID queries a list of events by event ID for one room. If no room is specified, it will try to determine
 	// which room to use by querying the first events roomID.
 	QueryEventsByID(
@@ -168,6 +176,7 @@ type ClientRoomserverAPI interface {
 	QueryLatestEventsAndStateAPI
 	QueryBulkStateContentAPI
 	QueryEventsAPI
+	QuerySenderIDAPI
 	QueryMembershipForUser(ctx context.Context, req *QueryMembershipForUserRequest, res *QueryMembershipForUserResponse) error
 	QueryMembershipsForRoom(ctx context.Context, req *QueryMembershipsForRoomRequest, res *QueryMembershipsForRoomResponse) error
 	QueryRoomsForUser(ctx context.Context, req *QueryRoomsForUserRequest, res *QueryRoomsForUserResponse) error
@@ -200,6 +209,7 @@ type ClientRoomserverAPI interface {
 }
 
 type UserRoomserverAPI interface {
+	QuerySenderIDAPI
 	QueryLatestEventsAndStateAPI
 	KeyserverRoomserverAPI
 	QueryCurrentState(ctx context.Context, req *QueryCurrentStateRequest, res *QueryCurrentStateResponse) error
@@ -213,6 +223,8 @@ type FederationRoomserverAPI interface {
 	InputRoomEventsAPI
 	QueryLatestEventsAndStateAPI
 	QueryBulkStateContentAPI
+	QuerySenderIDAPI
+
 	// QueryServerBannedFromRoom returns whether a server is banned from a room by server ACLs.
 	QueryServerBannedFromRoom(ctx context.Context, req *QueryServerBannedFromRoomRequest, res *QueryServerBannedFromRoomResponse) error
 	QueryMembershipForUser(ctx context.Context, req *QueryMembershipForUserRequest, res *QueryMembershipForUserResponse) error
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index e741c140..d79dcebb 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -491,10 +491,10 @@ type MembershipQuerier struct {
 	Roomserver FederationRoomserverAPI
 }
 
-func (mq *MembershipQuerier) CurrentMembership(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (string, error) {
+func (mq *MembershipQuerier) CurrentMembership(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (string, error) {
 	req := QueryMembershipForUserRequest{
 		RoomID: roomID.String(),
-		UserID: userID.String(),
+		UserID: string(senderID),
 	}
 	res := QueryMembershipForUserResponse{}
 	err := mq.Roomserver.QueryMembershipForUser(ctx, &req, &res)
diff --git a/roomserver/internal/alias.go b/roomserver/internal/alias.go
index 52b90cf4..dcfb26b8 100644
--- a/roomserver/internal/alias.go
+++ b/roomserver/internal/alias.go
@@ -119,11 +119,6 @@ func (r *RoomserverInternalAPI) RemoveRoomAlias(
 	request *api.RemoveRoomAliasRequest,
 	response *api.RemoveRoomAliasResponse,
 ) error {
-	_, virtualHost, err := r.Cfg.Global.SplitLocalID('@', request.UserID)
-	if err != nil {
-		return err
-	}
-
 	roomID, err := r.DB.GetRoomIDForAlias(ctx, request.Alias)
 	if err != nil {
 		return fmt.Errorf("r.DB.GetRoomIDForAlias: %w", err)
@@ -134,13 +129,19 @@ func (r *RoomserverInternalAPI) RemoveRoomAlias(
 		return nil
 	}
 
+	sender, err := r.QueryUserIDForSender(ctx, roomID, request.SenderID)
+	if err != nil {
+		return fmt.Errorf("r.QueryUserIDForSender: %w", err)
+	}
+	virtualHost := sender.Domain()
+
 	response.Found = true
 	creatorID, err := r.DB.GetCreatorIDForAlias(ctx, request.Alias)
 	if err != nil {
 		return fmt.Errorf("r.DB.GetCreatorIDForAlias: %w", err)
 	}
 
-	if creatorID != request.UserID {
+	if creatorID != request.SenderID {
 		var plEvent *types.HeaderedEvent
 		var pls *gomatrixserverlib.PowerLevelContent
 
@@ -154,7 +155,7 @@ func (r *RoomserverInternalAPI) RemoveRoomAlias(
 			return fmt.Errorf("plEvent.PowerLevels: %w", err)
 		}
 
-		if pls.UserLevel(request.UserID) < pls.EventLevel(spec.MRoomCanonicalAlias, true) {
+		if pls.UserLevel(request.SenderID) < pls.EventLevel(spec.MRoomCanonicalAlias, true) {
 			response.Removed = false
 			return nil
 		}
@@ -172,9 +173,9 @@ func (r *RoomserverInternalAPI) RemoveRoomAlias(
 				return err
 			}
 
-			sender := request.UserID
-			if request.UserID != ev.Sender() {
-				sender = ev.Sender()
+			sender := request.SenderID
+			if request.SenderID != ev.SenderID() {
+				sender = ev.SenderID()
 			}
 
 			_, senderDomain, err := r.Cfg.Global.SplitLocalID('@', sender)
diff --git a/roomserver/internal/helpers/auth.go b/roomserver/internal/helpers/auth.go
index 7ec0892e..932ce615 100644
--- a/roomserver/internal/helpers/auth.go
+++ b/roomserver/internal/helpers/auth.go
@@ -76,7 +76,9 @@ func CheckForSoftFail(
 	}
 
 	// Check if the event is allowed.
-	if err = gomatrixserverlib.Allowed(event.PDU, &authEvents); err != nil {
+	if err = gomatrixserverlib.Allowed(event.PDU, &authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+		return db.GetUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		// return true, nil
 		return true, err
 	}
diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 386083f6..764bdfe2 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -128,9 +128,13 @@ func (r *Inputer) processRoomEvent(
 	if roomInfo == nil && !isCreateEvent {
 		return fmt.Errorf("room %s does not exist for event %s", event.RoomID(), event.EventID())
 	}
-	_, senderDomain, err := gomatrixserverlib.SplitID('@', event.Sender())
+	sender, err := r.DB.GetUserIDForSender(ctx, event.RoomID(), event.SenderID())
 	if err != nil {
-		return fmt.Errorf("event has invalid sender %q", input.Event.Sender())
+		return fmt.Errorf("failed getting userID for sender %q. %w", event.SenderID(), err)
+	}
+	senderDomain := spec.ServerName("")
+	if sender != nil {
+		senderDomain = sender.Domain()
 	}
 
 	// If we already know about this outlier and it hasn't been rejected
@@ -193,7 +197,9 @@ func (r *Inputer) processRoomEvent(
 			serverRes.ServerNames = append(serverRes.ServerNames, input.Origin)
 			delete(servers, input.Origin)
 		}
-		if senderDomain != input.Origin && senderDomain != r.Cfg.Matrix.ServerName {
+		// Only perform this check if the sender mxid_mapping can be resolved.
+		// Don't fail processing the event if we have no mxid_maping.
+		if sender != nil && senderDomain != input.Origin && senderDomain != r.Cfg.Matrix.ServerName {
 			serverRes.ServerNames = append(serverRes.ServerNames, senderDomain)
 			delete(servers, senderDomain)
 		}
@@ -276,7 +282,9 @@ func (r *Inputer) processRoomEvent(
 
 	// Check if the event is allowed by its auth events. If it isn't then
 	// we consider the event to be "rejected" — it will still be persisted.
-	if err = gomatrixserverlib.Allowed(event, &authEvents); err != nil {
+	if err = gomatrixserverlib.Allowed(event, &authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+		return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		isRejected = true
 		rejectionErr = err
 		logger.WithError(rejectionErr).Warnf("Event %s not allowed by auth events", event.EventID())
@@ -493,7 +501,7 @@ func (r *Inputer) processRoomEvent(
 func (r *Inputer) handleRemoteRoomUpgrade(ctx context.Context, event gomatrixserverlib.PDU) error {
 	oldRoomID := event.RoomID()
 	newRoomID := gjson.GetBytes(event.Content(), "replacement_room").Str
-	return r.DB.UpgradeRoom(ctx, oldRoomID, newRoomID, event.Sender())
+	return r.DB.UpgradeRoom(ctx, oldRoomID, newRoomID, event.SenderID())
 }
 
 // processStateBefore works out what the state is before the event and
@@ -579,7 +587,9 @@ func (r *Inputer) processStateBefore(
 	stateBeforeAuth := gomatrixserverlib.NewAuthEvents(
 		gomatrixserverlib.ToPDUs(stateBeforeEvent),
 	)
-	if rejectionErr = gomatrixserverlib.Allowed(event, &stateBeforeAuth); rejectionErr != nil {
+	if rejectionErr = gomatrixserverlib.Allowed(event, &stateBeforeAuth, func(roomID, senderID string) (*spec.UserID, error) {
+		return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+	}); rejectionErr != nil {
 		rejectionErr = fmt.Errorf("Allowed() failed for stateBeforeEvent: %w", rejectionErr)
 		return
 	}
@@ -690,7 +700,9 @@ nextAuthEvent:
 		// Check the signatures of the event. If this fails then we'll simply
 		// skip it, because gomatrixserverlib.Allowed() will notice a problem
 		// if a critical event is missing anyway.
-		if err := gomatrixserverlib.VerifyEventSignatures(ctx, authEvent, r.FSAPI.KeyRing()); err != nil {
+		if err := gomatrixserverlib.VerifyEventSignatures(ctx, authEvent, r.FSAPI.KeyRing(), func(roomID, senderID string) (*spec.UserID, error) {
+			return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+		}); err != nil {
 			continue nextAuthEvent
 		}
 
@@ -706,7 +718,9 @@ nextAuthEvent:
 		}
 
 		// Check if the auth event should be rejected.
-		err := gomatrixserverlib.Allowed(authEvent, auth)
+		err := gomatrixserverlib.Allowed(authEvent, auth, func(roomID, senderID string) (*spec.UserID, error) {
+			return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+		})
 		if isRejected = err != nil; isRejected {
 			logger.WithError(err).Warnf("Auth event %s rejected", authEvent.EventID())
 		}
@@ -828,11 +842,13 @@ func (r *Inputer) kickGuests(ctx context.Context, event gomatrixserverlib.PDU, r
 			continue
 		}
 
+		// TODO: pseudoIDs: get userID for room using state key (which is now senderID)
 		localpart, senderDomain, err := gomatrixserverlib.SplitID('@', *memberEvent.StateKey())
 		if err != nil {
 			continue
 		}
 
+		// TODO: pseudoIDs: query account by state key (which is now senderID)
 		accountRes := &userAPI.QueryAccountByLocalpartResponse{}
 		if err = r.UserAPI.QueryAccountByLocalpart(ctx, &userAPI.QueryAccountByLocalpartRequest{
 			Localpart:  localpart,
diff --git a/roomserver/internal/input/input_events_test.go b/roomserver/internal/input/input_events_test.go
index 56803813..0ba7d19f 100644
--- a/roomserver/internal/input/input_events_test.go
+++ b/roomserver/internal/input/input_events_test.go
@@ -58,7 +58,7 @@ func Test_EventAuth(t *testing.T) {
 	}
 
 	// Finally check that the event is NOT allowed
-	if err := gomatrixserverlib.Allowed(ev.PDU, &allower); err == nil {
+	if err := gomatrixserverlib.Allowed(ev.PDU, &allower, func(roomID, senderID string) (*spec.UserID, error) { return spec.NewUserID(senderID, true) }); err == nil {
 		t.Fatalf("event should not be allowed, but it was")
 	}
 }
diff --git a/roomserver/internal/input/input_missing.go b/roomserver/internal/input/input_missing.go
index 10486138..ac0670fc 100644
--- a/roomserver/internal/input/input_missing.go
+++ b/roomserver/internal/input/input_missing.go
@@ -473,14 +473,18 @@ func (t *missingStateReq) resolveStatesAndCheck(ctx context.Context, roomVersion
 		stateEventList = append(stateEventList, state.StateEvents...)
 	}
 	resolvedStateEvents, err := gomatrixserverlib.ResolveConflicts(
-		roomVersion, gomatrixserverlib.ToPDUs(stateEventList), gomatrixserverlib.ToPDUs(authEventList),
+		roomVersion, gomatrixserverlib.ToPDUs(stateEventList), gomatrixserverlib.ToPDUs(authEventList), func(roomID, senderID string) (*spec.UserID, error) {
+			return t.db.GetUserIDForSender(ctx, roomID, senderID)
+		},
 	)
 	if err != nil {
 		return nil, err
 	}
 	// apply the current event
 retryAllowedState:
-	if err = checkAllowedByState(backwardsExtremity, resolvedStateEvents); err != nil {
+	if err = checkAllowedByState(backwardsExtremity, resolvedStateEvents, func(roomID, senderID string) (*spec.UserID, error) {
+		return t.db.GetUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		switch missing := err.(type) {
 		case gomatrixserverlib.MissingAuthEventError:
 			h, err2 := t.lookupEvent(ctx, roomVersion, backwardsExtremity.RoomID(), missing.AuthEventID, true)
@@ -565,7 +569,9 @@ func (t *missingStateReq) getMissingEvents(ctx context.Context, e gomatrixserver
 	// will be added and duplicates will be removed.
 	missingEvents := make([]gomatrixserverlib.PDU, 0, len(missingResp.Events))
 	for _, ev := range missingResp.Events.UntrustedEvents(roomVersion) {
-		if err = gomatrixserverlib.VerifyEventSignatures(ctx, ev, t.keys); err != nil {
+		if err = gomatrixserverlib.VerifyEventSignatures(ctx, ev, t.keys, func(roomID, senderID string) (*spec.UserID, error) {
+			return t.db.GetUserIDForSender(ctx, roomID, senderID)
+		}); err != nil {
 			continue
 		}
 		missingEvents = append(missingEvents, t.cacheAndReturn(ev))
@@ -654,7 +660,9 @@ func (t *missingStateReq) lookupMissingStateViaState(
 	authEvents, stateEvents, err := gomatrixserverlib.CheckStateResponse(ctx, &fclient.RespState{
 		StateEvents: state.GetStateEvents(),
 		AuthEvents:  state.GetAuthEvents(),
-	}, roomVersion, t.keys, nil)
+	}, roomVersion, t.keys, nil, func(roomID, senderID string) (*spec.UserID, error) {
+		return t.db.GetUserIDForSender(ctx, roomID, senderID)
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -889,14 +897,16 @@ func (t *missingStateReq) lookupEvent(ctx context.Context, roomVersion gomatrixs
 		t.log.WithField("missing_event_id", missingEventID).Warnf("Failed to get missing /event for event ID from %d server(s)", len(t.servers))
 		return nil, fmt.Errorf("wasn't able to find event via %d server(s)", len(t.servers))
 	}
-	if err := gomatrixserverlib.VerifyEventSignatures(ctx, event, t.keys); err != nil {
+	if err := gomatrixserverlib.VerifyEventSignatures(ctx, event, t.keys, func(roomID, senderID string) (*spec.UserID, error) {
+		return t.db.GetUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		t.log.WithError(err).Warnf("Couldn't validate signature of event %q from /event", event.EventID())
 		return nil, verifySigError{event.EventID(), err}
 	}
 	return t.cacheAndReturn(event), nil
 }
 
-func checkAllowedByState(e gomatrixserverlib.PDU, stateEvents []gomatrixserverlib.PDU) error {
+func checkAllowedByState(e gomatrixserverlib.PDU, stateEvents []gomatrixserverlib.PDU, userIDForSender spec.UserIDForSender) error {
 	authUsingState := gomatrixserverlib.NewAuthEvents(nil)
 	for i := range stateEvents {
 		err := authUsingState.AddEvent(stateEvents[i])
@@ -904,7 +914,7 @@ func checkAllowedByState(e gomatrixserverlib.PDU, stateEvents []gomatrixserverli
 			return err
 		}
 	}
-	return gomatrixserverlib.Allowed(e, &authUsingState)
+	return gomatrixserverlib.Allowed(e, &authUsingState, userIDForSender)
 }
 
 func (t *missingStateReq) hadEvent(eventID string) {
diff --git a/roomserver/internal/perform/perform_admin.go b/roomserver/internal/perform/perform_admin.go
index 575525e2..ca736cb6 100644
--- a/roomserver/internal/perform/perform_admin.go
+++ b/roomserver/internal/perform/perform_admin.go
@@ -262,13 +262,17 @@ func (r *Admin) PerformAdminDownloadState(
 			return fmt.Errorf("r.Inputer.FSAPI.LookupState (%q): %s", fwdExtremity, err)
 		}
 		for _, authEvent := range state.GetAuthEvents().UntrustedEvents(roomInfo.RoomVersion) {
-			if err = gomatrixserverlib.VerifyEventSignatures(ctx, authEvent, r.Inputer.KeyRing); err != nil {
+			if err = gomatrixserverlib.VerifyEventSignatures(ctx, authEvent, r.Inputer.KeyRing, func(roomID, senderID string) (*spec.UserID, error) {
+				return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+			}); err != nil {
 				continue
 			}
 			authEventMap[authEvent.EventID()] = authEvent
 		}
 		for _, stateEvent := range state.GetStateEvents().UntrustedEvents(roomInfo.RoomVersion) {
-			if err = gomatrixserverlib.VerifyEventSignatures(ctx, stateEvent, r.Inputer.KeyRing); err != nil {
+			if err = gomatrixserverlib.VerifyEventSignatures(ctx, stateEvent, r.Inputer.KeyRing, func(roomID, senderID string) (*spec.UserID, error) {
+				return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+			}); err != nil {
 				continue
 			}
 			stateEventMap[stateEvent.EventID()] = stateEvent
diff --git a/roomserver/internal/perform/perform_backfill.go b/roomserver/internal/perform/perform_backfill.go
index fb579f03..0f743f4e 100644
--- a/roomserver/internal/perform/perform_backfill.go
+++ b/roomserver/internal/perform/perform_backfill.go
@@ -121,7 +121,9 @@ func (r *Backfiller) backfillViaFederation(ctx context.Context, req *api.Perform
 	// Specifically the test "Outbound federation can backfill events"
 	events, err := gomatrixserverlib.RequestBackfill(
 		ctx, req.VirtualHost, requester,
-		r.KeyRing, req.RoomID, info.RoomVersion, req.PrevEventIDs(), 100,
+		r.KeyRing, req.RoomID, info.RoomVersion, req.PrevEventIDs(), 100, func(roomID, senderID string) (*spec.UserID, error) {
+			return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+		},
 	)
 	// Only return an error if we really couldn't get any events.
 	if err != nil && len(events) == 0 {
@@ -210,7 +212,9 @@ func (r *Backfiller) fetchAndStoreMissingEvents(ctx context.Context, roomVer gom
 				continue
 			}
 			loader := gomatrixserverlib.NewEventsLoader(roomVer, r.KeyRing, backfillRequester, backfillRequester.ProvideEvents, false)
-			result, err := loader.LoadAndVerify(ctx, res.PDUs, gomatrixserverlib.TopologicalOrderByPrevEvents)
+			result, err := loader.LoadAndVerify(ctx, res.PDUs, gomatrixserverlib.TopologicalOrderByPrevEvents, func(roomID, senderID string) (*spec.UserID, error) {
+				return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+			})
 			if err != nil {
 				logger.WithError(err).Warn("failed to load and verify event")
 				continue
@@ -484,8 +488,8 @@ FindSuccessor:
 	// Store the server names in a temporary map to avoid duplicates.
 	serverSet := make(map[spec.ServerName]bool)
 	for _, event := range memberEvents {
-		if _, senderDomain, err := gomatrixserverlib.SplitID('@', event.Sender()); err == nil {
-			serverSet[senderDomain] = true
+		if sender, err := b.db.GetUserIDForSender(ctx, event.RoomID(), event.SenderID()); err == nil {
+			serverSet[sender.Domain()] = true
 		}
 	}
 	var servers []spec.ServerName
diff --git a/roomserver/internal/perform/perform_create_room.go b/roomserver/internal/perform/perform_create_room.go
index 41194832..897bd3a0 100644
--- a/roomserver/internal/perform/perform_create_room.go
+++ b/roomserver/internal/perform/perform_create_room.go
@@ -308,7 +308,9 @@ func (c *Creator) PerformCreateRoom(ctx context.Context, userID spec.UserID, roo
 			}
 		}
 
-		if err = gomatrixserverlib.Allowed(ev, &authEvents); err != nil {
+		if err = gomatrixserverlib.Allowed(ev, &authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+			return c.DB.GetUserIDForSender(ctx, roomID, senderID)
+		}); err != nil {
 			util.GetLogger(ctx).WithError(err).Error("gomatrixserverlib.Allowed failed")
 			return "", &util.JSONResponse{
 				Code: http.StatusInternalServerError,
diff --git a/roomserver/internal/perform/perform_invite.go b/roomserver/internal/perform/perform_invite.go
index 1930b5ac..e8e20ede 100644
--- a/roomserver/internal/perform/perform_invite.go
+++ b/roomserver/internal/perform/perform_invite.go
@@ -97,11 +97,12 @@ func (r *Inviter) ProcessInviteMembership(
 ) ([]api.OutputEvent, error) {
 	var outputUpdates []api.OutputEvent
 	var updater *shared.MembershipUpdater
-	_, domain, err := gomatrixserverlib.SplitID('@', *inviteEvent.StateKey())
+
+	userID, err := r.RSAPI.QueryUserIDForSender(ctx, inviteEvent.RoomID(), *inviteEvent.StateKey())
 	if err != nil {
 		return nil, api.ErrInvalidID{Err: fmt.Errorf("the user ID %s is invalid", *inviteEvent.StateKey())}
 	}
-	isTargetLocal := r.Cfg.Matrix.IsLocalServerName(domain)
+	isTargetLocal := r.Cfg.Matrix.IsLocalServerName(userID.Domain())
 	if updater, err = r.DB.MembershipUpdater(ctx, inviteEvent.RoomID(), *inviteEvent.StateKey(), isTargetLocal, inviteEvent.Version()); err != nil {
 		return nil, fmt.Errorf("r.DB.MembershipUpdater: %w", err)
 	}
@@ -125,9 +126,9 @@ func (r *Inviter) PerformInvite(
 ) error {
 	event := req.Event
 
-	sender, err := spec.NewUserID(event.Sender(), true)
+	sender, err := r.DB.GetUserIDForSender(ctx, event.RoomID(), event.SenderID())
 	if err != nil {
-		return spec.InvalidParam("The user ID is invalid")
+		return spec.InvalidParam("The sender user ID is invalid")
 	}
 	if !r.Cfg.Matrix.IsLocalServerName(sender.Domain()) {
 		return api.ErrInvalidID{Err: fmt.Errorf("the invite must be from a local user")}
@@ -155,6 +156,9 @@ func (r *Inviter) PerformInvite(
 		StrippedState:     req.InviteRoomState,
 		MembershipQuerier: &api.MembershipQuerier{Roomserver: r.RSAPI},
 		StateQuerier:      &QueryState{r.DB},
+		UserIDQuerier: func(roomID, senderID string) (*spec.UserID, error) {
+			return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+		},
 	}
 	inviteEvent, err := gomatrixserverlib.PerformInvite(ctx, input, r.FSAPI)
 	if err != nil {
diff --git a/roomserver/internal/perform/perform_upgrade.go b/roomserver/internal/perform/perform_upgrade.go
index ff4a6a1d..8c0df1c4 100644
--- a/roomserver/internal/perform/perform_upgrade.go
+++ b/roomserver/internal/perform/perform_upgrade.go
@@ -176,7 +176,7 @@ func moveLocalAliases(ctx context.Context,
 	}
 
 	for _, alias := range aliasRes.Aliases {
-		removeAliasReq := api.RemoveRoomAliasRequest{UserID: userID, Alias: alias}
+		removeAliasReq := api.RemoveRoomAliasRequest{SenderID: userID, Alias: alias}
 		removeAliasRes := api.RemoveRoomAliasResponse{}
 		if err = URSAPI.RemoveRoomAlias(ctx, &removeAliasReq, &removeAliasRes); err != nil {
 			return fmt.Errorf("Failed to remove old room alias: %w", err)
@@ -484,7 +484,9 @@ func (r *Upgrader) sendInitialEvents(ctx context.Context, evTime time.Time, user
 
 		}
 
-		if err = gomatrixserverlib.Allowed(event, &authEvents); err != nil {
+		if err = gomatrixserverlib.Allowed(event, &authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+			return r.URSAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}); err != nil {
 			return fmt.Errorf("Failed to auth new %q event: %w", builder.Type, err)
 		}
 
@@ -567,7 +569,9 @@ func (r *Upgrader) makeHeaderedEvent(ctx context.Context, evTime time.Time, user
 		stateEvents[i] = queryRes.StateEvents[i].PDU
 	}
 	provider := gomatrixserverlib.NewAuthEvents(stateEvents)
-	if err = gomatrixserverlib.Allowed(headeredEvent.PDU, &provider); err != nil {
+	if err = gomatrixserverlib.Allowed(headeredEvent.PDU, &provider, func(roomID, senderID string) (*spec.UserID, error) {
+		return r.URSAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	}); err != nil {
 		return nil, api.ErrNotAllowed{Err: fmt.Errorf("failed to auth new %q event: %w", proto.Type, err)} // TODO: Is this error string comprehensible to the client?
 	}
 
diff --git a/roomserver/internal/query/query.go b/roomserver/internal/query/query.go
index 6d898e8a..707e95b2 100644
--- a/roomserver/internal/query/query.go
+++ b/roomserver/internal/query/query.go
@@ -159,7 +159,9 @@ func (r *Queryer) QueryStateAfterEvents(
 		}
 
 		stateEvents, err = gomatrixserverlib.ResolveConflicts(
-			info.RoomVersion, gomatrixserverlib.ToPDUs(stateEvents), gomatrixserverlib.ToPDUs(authEvents),
+			info.RoomVersion, gomatrixserverlib.ToPDUs(stateEvents), gomatrixserverlib.ToPDUs(authEvents), func(roomID, senderID string) (*spec.UserID, error) {
+				return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+			},
 		)
 		if err != nil {
 			return fmt.Errorf("state.ResolveConflictsAdhoc: %w", err)
@@ -386,7 +388,12 @@ func (r *Queryer) QueryMembershipsForRoom(
 			return fmt.Errorf("r.DB.Events: %w", err)
 		}
 		for _, event := range events {
-			clientEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatAll)
+			sender := spec.UserID{}
+			userID, queryErr := r.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+			if queryErr == nil && userID != nil {
+				sender = *userID
+			}
+			clientEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender)
 			response.JoinEvents = append(response.JoinEvents, clientEvent)
 		}
 		return nil
@@ -435,7 +442,12 @@ func (r *Queryer) QueryMembershipsForRoom(
 	}
 
 	for _, event := range events {
-		clientEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatAll)
+		sender := spec.UserID{}
+		userID, err := r.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+		if err == nil && userID != nil {
+			sender = *userID
+		}
+		clientEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender)
 		response.JoinEvents = append(response.JoinEvents, clientEvent)
 	}
 
@@ -625,7 +637,9 @@ func (r *Queryer) QueryStateAndAuthChain(
 
 	if request.ResolveState {
 		stateEvents, err = gomatrixserverlib.ResolveConflicts(
-			info.RoomVersion, gomatrixserverlib.ToPDUs(stateEvents), gomatrixserverlib.ToPDUs(authEvents),
+			info.RoomVersion, gomatrixserverlib.ToPDUs(stateEvents), gomatrixserverlib.ToPDUs(authEvents), func(roomID, senderID string) (*spec.UserID, error) {
+				return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+			},
 		)
 		if err != nil {
 			return err
@@ -960,3 +974,11 @@ func (r *Queryer) QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.Ro
 
 	return verImpl.CheckRestrictedJoin(ctx, r.Cfg.Global.ServerName, &api.JoinRoomQuerier{Roomserver: r}, roomID, userID)
 }
+
+func (r *Queryer) QuerySenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (string, error) {
+	return r.DB.GetSenderIDForUser(ctx, roomID, userID)
+}
+
+func (r *Queryer) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return r.DB.GetUserIDForSender(ctx, roomID, senderID)
+}
diff --git a/roomserver/producers/roomevent.go b/roomserver/producers/roomevent.go
index febe8ddf..165304d4 100644
--- a/roomserver/producers/roomevent.go
+++ b/roomserver/producers/roomevent.go
@@ -60,7 +60,7 @@ func (r *RoomEventProducer) ProduceRoomEvents(roomID string, updates []api.Outpu
 				"adds_state":     len(update.NewRoomEvent.AddsStateEventIDs),
 				"removes_state":  len(update.NewRoomEvent.RemovesStateEventIDs),
 				"send_as_server": update.NewRoomEvent.SendAsServer,
-				"sender":         update.NewRoomEvent.Event.Sender(),
+				"sender":         update.NewRoomEvent.Event.SenderID(),
 			})
 			if update.NewRoomEvent.Event.StateKey() != nil {
 				logger = logger.WithField("state_key", *update.NewRoomEvent.Event.StateKey())
diff --git a/roomserver/state/state.go b/roomserver/state/state.go
index f38d8f96..3131cbff 100644
--- a/roomserver/state/state.go
+++ b/roomserver/state/state.go
@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/matrix-org/util"
 	"github.com/prometheus/client_golang/prometheus"
 
@@ -43,6 +44,7 @@ type StateResolutionStorage interface {
 	AddState(ctx context.Context, roomNID types.RoomNID, stateBlockNIDs []types.StateBlockNID, state []types.StateEntry) (types.StateSnapshotNID, error)
 	Events(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, eventNIDs []types.EventNID) ([]types.Event, error)
 	EventsFromIDs(ctx context.Context, roomInfo *types.RoomInfo, eventIDs []string) ([]types.Event, error)
+	GetUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error)
 }
 
 type StateResolution struct {
@@ -945,7 +947,9 @@ func (v *StateResolution) resolveConflictsV1(
 	}
 
 	// Resolve the conflicts.
-	resolvedEvents := gomatrixserverlib.ResolveStateConflicts(conflictedEvents, authEvents)
+	resolvedEvents := gomatrixserverlib.ResolveStateConflicts(conflictedEvents, authEvents, func(roomID, senderID string) (*spec.UserID, error) {
+		return v.db.GetUserIDForSender(ctx, roomID, senderID)
+	})
 
 	// Map from the full events back to numeric state entries.
 	for _, resolvedEvent := range resolvedEvents {
@@ -1057,6 +1061,9 @@ func (v *StateResolution) resolveConflictsV2(
 			conflictedEvents,
 			nonConflictedEvents,
 			authEvents,
+			func(roomID, senderID string) (*spec.UserID, error) {
+				return v.db.GetUserIDForSender(ctx, roomID, senderID)
+			},
 		)
 	}()
 
diff --git a/roomserver/storage/interface.go b/roomserver/storage/interface.go
index 7d22df00..2d007bed 100644
--- a/roomserver/storage/interface.go
+++ b/roomserver/storage/interface.go
@@ -166,6 +166,10 @@ type Database interface {
 	GetServerInRoom(ctx context.Context, roomNID types.RoomNID, serverName spec.ServerName) (bool, error)
 	// GetKnownUsers searches all users that userID knows about.
 	GetKnownUsers(ctx context.Context, userID, searchString string, limit int) ([]string, error)
+	// GetKnownUsers tries to obtain the current mxid for a given user.
+	GetUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error)
+	// GetKnownUsers tries to obtain the current senderID for a given user.
+	GetSenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (string, error)
 	// GetKnownRooms returns a list of all rooms we know about.
 	GetKnownRooms(ctx context.Context) ([]string, error)
 	// ForgetRoom sets a flag in the membership table, that the user wishes to forget a specific room
@@ -211,6 +215,7 @@ type RoomDatabase interface {
 	GetOrCreateEventTypeNID(ctx context.Context, eventType string) (eventTypeNID types.EventTypeNID, err error)
 	GetOrCreateEventStateKeyNID(ctx context.Context, eventStateKey *string) (types.EventStateKeyNID, error)
 	GetStateEvent(ctx context.Context, roomID, evType, stateKey string) (*types.HeaderedEvent, error)
+	GetUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error)
 }
 
 type EventDatabase interface {
diff --git a/roomserver/storage/shared/membership_updater.go b/roomserver/storage/shared/membership_updater.go
index f9c889cb..105e61df 100644
--- a/roomserver/storage/shared/membership_updater.go
+++ b/roomserver/storage/shared/membership_updater.go
@@ -101,7 +101,7 @@ func (u *MembershipUpdater) Update(newMembership tables.MembershipState, event *
 	var inserted bool    // Did the query result in a membership change?
 	var retired []string // Did we retire any updates in the process?
 	return inserted, retired, u.d.Writer.Do(u.d.DB, u.txn, func(txn *sql.Tx) error {
-		senderUserNID, err := u.d.assignStateKeyNID(u.ctx, u.txn, event.Sender())
+		senderUserNID, err := u.d.assignStateKeyNID(u.ctx, u.txn, event.SenderID())
 		if err != nil {
 			return fmt.Errorf("u.d.AssignStateKeyNID: %w", err)
 		}
diff --git a/roomserver/storage/shared/room_updater.go b/roomserver/storage/shared/room_updater.go
index 70672a33..73500138 100644
--- a/roomserver/storage/shared/room_updater.go
+++ b/roomserver/storage/shared/room_updater.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 
 	"github.com/matrix-org/dendrite/roomserver/types"
 )
@@ -250,3 +251,7 @@ func (u *RoomUpdater) MarkEventAsSent(eventNID types.EventNID) error {
 func (u *RoomUpdater) MembershipUpdater(targetUserNID types.EventStateKeyNID, targetLocal bool) (*MembershipUpdater, error) {
 	return u.d.membershipUpdaterTxn(u.ctx, u.txn, u.roomInfo.RoomNID, targetUserNID, targetLocal)
 }
+
+func (u *RoomUpdater) GetUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return u.d.GetUserIDForSender(ctx, roomID, senderID)
+}
diff --git a/roomserver/storage/shared/storage.go b/roomserver/storage/shared/storage.go
index cefa58a3..406d7cf1 100644
--- a/roomserver/storage/shared/storage.go
+++ b/roomserver/storage/shared/storage.go
@@ -988,8 +988,18 @@ func (d *EventDatabase) MaybeRedactEvent(
 			return nil
 		}
 
-		_, sender1, _ := gomatrixserverlib.SplitID('@', redactedEvent.Sender())
-		_, sender2, _ := gomatrixserverlib.SplitID('@', redactionEvent.Sender())
+		// TODO: Don't hack senderID into userID here (pseudoIDs)
+		sender1Domain := ""
+		sender1, err1 := spec.NewUserID(redactedEvent.SenderID(), true)
+		if err1 == nil {
+			sender1Domain = string(sender1.Domain())
+		}
+		// TODO: Don't hack senderID into userID here (pseudoIDs)
+		sender2Domain := ""
+		sender2, err2 := spec.NewUserID(redactionEvent.SenderID(), true)
+		if err2 == nil {
+			sender2Domain = string(sender2.Domain())
+		}
 		var powerlevels *gomatrixserverlib.PowerLevelContent
 		powerlevels, err = plResolver.Resolve(ctx, redactionEvent.EventID())
 		if err != nil {
@@ -997,9 +1007,9 @@ func (d *EventDatabase) MaybeRedactEvent(
 		}
 
 		switch {
-		case powerlevels.UserLevel(redactionEvent.Sender()) >= powerlevels.Redact:
+		case powerlevels.UserLevel(redactionEvent.SenderID()) >= powerlevels.Redact:
 			// 1. The power level of the redaction event’s sender is greater than or equal to the redact level.
-		case sender1 == sender2:
+		case sender1Domain == sender2Domain:
 			// 2. The domain of the redaction event’s sender matches that of the original event’s sender.
 		default:
 			ignoreRedaction = true
@@ -1514,6 +1524,16 @@ func (d *Database) GetKnownUsers(ctx context.Context, userID, searchString strin
 	return d.MembershipTable.SelectKnownUsers(ctx, nil, stateKeyNID, searchString, limit)
 }
 
+func (d *Database) GetUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	// TODO: Use real logic once DB for pseudoIDs is in place
+	return spec.NewUserID(senderID, true)
+}
+
+func (d *Database) GetSenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (string, error) {
+	// TODO: Use real logic once DB for pseudoIDs is in place
+	return userID.String(), nil
+}
+
 // GetKnownRooms returns a list of all rooms we know about.
 func (d *Database) GetKnownRooms(ctx context.Context) ([]string, error) {
 	return d.RoomsTable.SelectRoomIDsWithEvents(ctx, nil)
diff --git a/setup/mscs/msc2836/msc2836.go b/setup/mscs/msc2836/msc2836.go
index f468b048..5ce3b430 100644
--- a/setup/mscs/msc2836/msc2836.go
+++ b/setup/mscs/msc2836/msc2836.go
@@ -92,9 +92,11 @@ type MSC2836EventRelationshipsResponse struct {
 	ParsedAuthChain []gomatrixserverlib.PDU
 }
 
-func toClientResponse(res *MSC2836EventRelationshipsResponse) *EventRelationshipResponse {
+func toClientResponse(ctx context.Context, res *MSC2836EventRelationshipsResponse, rsAPI roomserver.RoomserverInternalAPI) *EventRelationshipResponse {
 	out := &EventRelationshipResponse{
-		Events:    synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(res.ParsedEvents), synctypes.FormatAll),
+		Events: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(res.ParsedEvents), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}),
 		Limited:   res.Limited,
 		NextBatch: res.NextBatch,
 	}
@@ -187,7 +189,7 @@ func eventRelationshipHandler(db Database, rsAPI roomserver.RoomserverInternalAP
 
 		return util.JSONResponse{
 			Code: 200,
-			JSON: toClientResponse(res),
+			JSON: toClientResponse(req.Context(), res, rsAPI),
 		}
 	}
 }
diff --git a/setup/mscs/msc2836/msc2836_test.go b/setup/mscs/msc2836/msc2836_test.go
index 2c6f63d4..c463fd72 100644
--- a/setup/mscs/msc2836/msc2836_test.go
+++ b/setup/mscs/msc2836/msc2836_test.go
@@ -525,6 +525,10 @@ type testRoomserverAPI struct {
 	events            map[string]*types.HeaderedEvent
 }
 
+func (r *testRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func (r *testRoomserverAPI) QueryEventsByID(ctx context.Context, req *roomserver.QueryEventsByIDRequest, res *roomserver.QueryEventsByIDResponse) error {
 	for _, eventID := range req.EventIDs {
 		ev := r.events[eventID]
diff --git a/setup/mscs/msc2946/msc2946.go b/setup/mscs/msc2946/msc2946.go
index 291e0f3b..f380d3d4 100644
--- a/setup/mscs/msc2946/msc2946.go
+++ b/setup/mscs/msc2946/msc2946.go
@@ -730,7 +730,7 @@ func stripped(ev gomatrixserverlib.PDU) *fclient.MSC2946StrippedEvent {
 		Type:           ev.Type(),
 		StateKey:       *ev.StateKey(),
 		Content:        ev.Content(),
-		Sender:         ev.Sender(),
+		Sender:         ev.SenderID(),
 		OriginServerTS: ev.OriginServerTS(),
 	}
 }
diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index 56285dbf..c0836465 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -523,7 +523,7 @@ func (s *OutputRoomEventConsumer) updateStateEvent(event *rstypes.HeaderedEvent)
 	prev := types.PrevEventRef{
 		PrevContent:   prevEvent.Content(),
 		ReplacesState: prevEvent.EventID(),
-		PrevSender:    prevEvent.Sender(),
+		PrevSender:    prevEvent.SenderID(),
 	}
 
 	event.PDU, err = event.SetUnsigned(prev)
diff --git a/syncapi/routing/context.go b/syncapi/routing/context.go
index ac17d39d..27e99a35 100644
--- a/syncapi/routing/context.go
+++ b/syncapi/routing/context.go
@@ -193,14 +193,20 @@ func Context(
 		}
 	}
 
-	eventsBeforeClient := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsBeforeFiltered), synctypes.FormatAll)
-	eventsAfterClient := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsAfterFiltered), synctypes.FormatAll)
+	eventsBeforeClient := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsBeforeFiltered), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+		return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	})
+	eventsAfterClient := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsAfterFiltered), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+		return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	})
 
 	newState := state
 	if filter.LazyLoadMembers {
 		allEvents := append(eventsBeforeFiltered, eventsAfterFiltered...)
 		allEvents = append(allEvents, &requestedEvent)
-		evs := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(allEvents), synctypes.FormatAll)
+		evs := synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(allEvents), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		newState, err = applyLazyLoadMembers(ctx, device, snapshot, roomID, evs, lazyLoadCache)
 		if err != nil {
 			logrus.WithError(err).Error("unable to load membership events")
@@ -211,12 +217,19 @@ func Context(
 		}
 	}
 
-	ev := synctypes.ToClientEvent(&requestedEvent, synctypes.FormatAll)
+	sender := spec.UserID{}
+	userID, err := rsAPI.QueryUserIDForSender(ctx, requestedEvent.RoomID(), requestedEvent.SenderID())
+	if err == nil && userID != nil {
+		sender = *userID
+	}
+	ev := synctypes.ToClientEvent(&requestedEvent, synctypes.FormatAll, sender)
 	response := ContextRespsonse{
 		Event:        &ev,
 		EventsAfter:  eventsAfterClient,
 		EventsBefore: eventsBeforeClient,
-		State:        synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(newState), synctypes.FormatAll),
+		State: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(newState), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}),
 	}
 
 	if len(response.State) > filter.Limit {
diff --git a/syncapi/routing/getevent.go b/syncapi/routing/getevent.go
index 0d3d412f..63df7e83 100644
--- a/syncapi/routing/getevent.go
+++ b/syncapi/routing/getevent.go
@@ -101,8 +101,13 @@ func GetEvent(
 		}
 	}
 
+	sender := spec.UserID{}
+	senderUserID, err := rsAPI.QueryUserIDForSender(req.Context(), roomID, events[0].SenderID())
+	if err == nil && senderUserID != nil {
+		sender = *senderUserID
+	}
 	return util.JSONResponse{
 		Code: http.StatusOK,
-		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll),
+		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll, sender),
 	}
 }
diff --git a/syncapi/routing/memberships.go b/syncapi/routing/memberships.go
index 7d2e137d..9c2319dd 100644
--- a/syncapi/routing/memberships.go
+++ b/syncapi/routing/memberships.go
@@ -144,7 +144,7 @@ func GetMemberships(
 					JSON: spec.InternalServerError{},
 				}
 			}
-			res.Joined[ev.Sender()] = joinedMember(content)
+			res.Joined[ev.SenderID()] = joinedMember(content)
 		}
 		return util.JSONResponse{
 			Code: http.StatusOK,
@@ -153,6 +153,8 @@ func GetMemberships(
 	}
 	return util.JSONResponse{
 		Code: http.StatusOK,
-		JSON: getMembershipResponse{synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(result), synctypes.FormatAll)},
+		JSON: getMembershipResponse{synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(result), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(req.Context(), roomID, senderID)
+		})},
 	}
 }
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index aeaec699..879739d0 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -241,7 +241,7 @@ func OnIncomingMessagesRequest(
 		device:           device,
 	}
 
-	clientEvents, start, end, err := mReq.retrieveEvents()
+	clientEvents, start, end, err := mReq.retrieveEvents(req.Context(), rsAPI)
 	if err != nil {
 		util.GetLogger(req.Context()).WithError(err).Error("mreq.retrieveEvents failed")
 		return util.JSONResponse{
@@ -273,7 +273,9 @@ func OnIncomingMessagesRequest(
 				JSON: spec.InternalServerError{},
 			}
 		}
-		res.State = append(res.State, synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(membershipEvents), synctypes.FormatAll)...)
+		res.State = append(res.State, synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(membershipEvents), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(req.Context(), roomID, senderID)
+		})...)
 	}
 
 	// If we didn't return any events, set the end to an empty string, so it will be omitted
@@ -310,7 +312,7 @@ func getMembershipForUser(ctx context.Context, roomID, userID string, rsAPI api.
 // homeserver in the room for older events.
 // Returns an error if there was an issue talking to the database or with the
 // remote homeserver.
-func (r *messagesReq) retrieveEvents() (
+func (r *messagesReq) retrieveEvents(ctx context.Context, rsAPI api.SyncRoomserverAPI) (
 	clientEvents []synctypes.ClientEvent, start,
 	end types.TopologyToken, err error,
 ) {
@@ -383,7 +385,9 @@ func (r *messagesReq) retrieveEvents() (
 		"events_before": len(events),
 		"events_after":  len(filteredEvents),
 	}).Debug("applied history visibility (messages)")
-	return synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(filteredEvents), synctypes.FormatAll), start, end, err
+	return synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(filteredEvents), synctypes.FormatAll, func(roomID, senderID string) (*spec.UserID, error) {
+		return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	}), start, end, err
 }
 
 func (r *messagesReq) getStartEnd(events []*rstypes.HeaderedEvent) (start, end types.TopologyToken, err error) {
diff --git a/syncapi/routing/relations.go b/syncapi/routing/relations.go
index 8374bf5b..f21c684c 100644
--- a/syncapi/routing/relations.go
+++ b/syncapi/routing/relations.go
@@ -114,9 +114,14 @@ func Relations(
 	// type if it was specified.
 	res.Chunk = make([]synctypes.ClientEvent, 0, len(filteredEvents))
 	for _, event := range filteredEvents {
+		sender := spec.UserID{}
+		userID, err := rsAPI.QueryUserIDForSender(req.Context(), event.RoomID(), event.SenderID())
+		if err == nil && userID != nil {
+			sender = *userID
+		}
 		res.Chunk = append(
 			res.Chunk,
-			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll),
+			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender),
 		)
 	}
 
diff --git a/syncapi/routing/routing.go b/syncapi/routing/routing.go
index 9ad0c047..8542c0b7 100644
--- a/syncapi/routing/routing.go
+++ b/syncapi/routing/routing.go
@@ -171,7 +171,7 @@ func Setup(
 				nb := req.FormValue("next_batch")
 				nextBatch = &nb
 			}
-			return Search(req, device, syncDB, fts, nextBatch)
+			return Search(req, device, syncDB, fts, nextBatch, rsAPI)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
 
diff --git a/syncapi/routing/search.go b/syncapi/routing/search.go
index b7191873..9cf3eabe 100644
--- a/syncapi/routing/search.go
+++ b/syncapi/routing/search.go
@@ -31,6 +31,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/httputil"
 	"github.com/matrix-org/dendrite/internal/fulltext"
 	"github.com/matrix-org/dendrite/internal/sqlutil"
+	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
@@ -38,7 +39,7 @@ import (
 )
 
 // nolint:gocyclo
-func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts fulltext.Indexer, from *string) util.JSONResponse {
+func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts fulltext.Indexer, from *string, rsAPI roomserverAPI.SyncRoomserverAPI) util.JSONResponse {
 	start := time.Now()
 	var (
 		searchReq SearchRequest
@@ -204,11 +205,17 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 
 		profileInfos := make(map[string]ProfileInfoResponse)
 		for _, ev := range append(eventsBefore, eventsAfter...) {
-			profile, ok := knownUsersProfiles[event.Sender()]
+			userID, queryErr := rsAPI.QueryUserIDForSender(req.Context(), ev.RoomID(), ev.SenderID())
+			if queryErr != nil {
+				logrus.WithError(queryErr).WithField("sender_id", event.SenderID()).Warn("failed to query userprofile")
+				continue
+			}
+
+			profile, ok := knownUsersProfiles[userID.String()]
 			if !ok {
-				stateEvent, err := snapshot.GetStateEvent(ctx, ev.RoomID(), spec.MRoomMember, ev.Sender())
-				if err != nil {
-					logrus.WithError(err).WithField("user_id", event.Sender()).Warn("failed to query userprofile")
+				stateEvent, stateErr := snapshot.GetStateEvent(ctx, ev.RoomID(), spec.MRoomMember, ev.SenderID())
+				if stateErr != nil {
+					logrus.WithError(stateErr).WithField("sender_id", event.SenderID()).Warn("failed to query userprofile")
 					continue
 				}
 				if stateEvent == nil {
@@ -218,21 +225,30 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 					AvatarURL:   gjson.GetBytes(stateEvent.Content(), "avatar_url").Str,
 					DisplayName: gjson.GetBytes(stateEvent.Content(), "displayname").Str,
 				}
-				knownUsersProfiles[event.Sender()] = profile
+				knownUsersProfiles[userID.String()] = profile
 			}
-			profileInfos[ev.Sender()] = profile
+			profileInfos[userID.String()] = profile
 		}
 
+		sender := spec.UserID{}
+		userID, err := rsAPI.QueryUserIDForSender(req.Context(), event.RoomID(), event.SenderID())
+		if err == nil && userID != nil {
+			sender = *userID
+		}
 		results = append(results, Result{
 			Context: SearchContextResponse{
-				Start:        startToken.String(),
-				End:          endToken.String(),
-				EventsAfter:  synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsAfter), synctypes.FormatSync),
-				EventsBefore: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsBefore), synctypes.FormatSync),
-				ProfileInfo:  profileInfos,
+				Start: startToken.String(),
+				End:   endToken.String(),
+				EventsAfter: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsAfter), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+					return rsAPI.QueryUserIDForSender(req.Context(), roomID, senderID)
+				}),
+				EventsBefore: synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(eventsBefore), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+					return rsAPI.QueryUserIDForSender(req.Context(), roomID, senderID)
+				}),
+				ProfileInfo: profileInfos,
 			},
 			Rank:   eventScore[event.EventID()].Score,
-			Result: synctypes.ToClientEvent(event, synctypes.FormatAll),
+			Result: synctypes.ToClientEvent(event, synctypes.FormatAll, sender),
 		})
 		roomGroup := groups[event.RoomID()]
 		roomGroup.Results = append(roomGroup.Results, event.EventID())
@@ -247,7 +263,9 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 					JSON: spec.InternalServerError{},
 				}
 			}
-			stateForRooms[event.RoomID()] = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(state), synctypes.FormatSync)
+			stateForRooms[event.RoomID()] = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(state), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+				return rsAPI.QueryUserIDForSender(req.Context(), roomID, senderID)
+			})
 		}
 	}
 
diff --git a/syncapi/routing/search_test.go b/syncapi/routing/search_test.go
index 1cc95a87..b36be823 100644
--- a/syncapi/routing/search_test.go
+++ b/syncapi/routing/search_test.go
@@ -2,6 +2,7 @@ package routing
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
@@ -9,6 +10,7 @@ import (
 
 	"github.com/matrix-org/dendrite/internal/fulltext"
 	"github.com/matrix-org/dendrite/internal/sqlutil"
+	rsapi "github.com/matrix-org/dendrite/roomserver/api"
 	rstypes "github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
@@ -21,6 +23,12 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+type FakeSyncRoomserverAPI struct{ rsapi.SyncRoomserverAPI }
+
+func (f *FakeSyncRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func TestSearch(t *testing.T) {
 	alice := test.NewUser(t)
 	aliceDevice := userapi.Device{UserID: alice.ID}
@@ -247,7 +255,7 @@ func TestSearch(t *testing.T) {
 				assert.NoError(t, err)
 				req := httptest.NewRequest(http.MethodPost, "/", reqBody)
 
-				res := Search(req, tc.device, db, fts, tc.from)
+				res := Search(req, tc.device, db, fts, tc.from, &FakeSyncRoomserverAPI{})
 				if !tc.wantOK && !res.Is2xx() {
 					return
 				}
diff --git a/syncapi/storage/postgres/current_room_state_table.go b/syncapi/storage/postgres/current_room_state_table.go
index 0cc96373..bfe5e9bd 100644
--- a/syncapi/storage/postgres/current_room_state_table.go
+++ b/syncapi/storage/postgres/current_room_state_table.go
@@ -343,7 +343,7 @@ func (s *currentRoomStateStatements) UpsertRoomState(
 		event.RoomID(),
 		event.EventID(),
 		event.Type(),
-		event.Sender(),
+		event.SenderID(),
 		containsURL,
 		*event.StateKey(),
 		headeredJSON,
diff --git a/syncapi/storage/postgres/output_room_events_table.go b/syncapi/storage/postgres/output_room_events_table.go
index 3aadbccf..e068afab 100644
--- a/syncapi/storage/postgres/output_room_events_table.go
+++ b/syncapi/storage/postgres/output_room_events_table.go
@@ -407,7 +407,7 @@ func (s *outputRoomEventsStatements) InsertEvent(
 		event.EventID(),
 		headeredJSON,
 		event.Type(),
-		event.Sender(),
+		event.SenderID(),
 		containsURL,
 		pq.StringArray(addState),
 		pq.StringArray(removeState),
diff --git a/syncapi/storage/shared/storage_consumer.go b/syncapi/storage/shared/storage_consumer.go
index ecfd418f..17a6a69c 100644
--- a/syncapi/storage/shared/storage_consumer.go
+++ b/syncapi/storage/shared/storage_consumer.go
@@ -195,7 +195,21 @@ func (d *Database) StreamEventsToEvents(device *userapi.Device, in []types.Strea
 	for i := 0; i < len(in); i++ {
 		out[i] = in[i].HeaderedEvent
 		if device != nil && in[i].TransactionID != nil {
-			if device.UserID == in[i].Sender() && device.SessionID == in[i].TransactionID.SessionID {
+			userID, err := spec.NewUserID(device.UserID, true)
+			if err != nil {
+				logrus.WithFields(logrus.Fields{
+					"event_id": out[i].EventID(),
+				}).WithError(err).Warnf("Failed to add transaction ID to event")
+				continue
+			}
+			deviceSenderID, err := d.getSenderIDForUser(in[i].RoomID(), *userID)
+			if err != nil {
+				logrus.WithFields(logrus.Fields{
+					"event_id": out[i].EventID(),
+				}).WithError(err).Warnf("Failed to add transaction ID to event")
+				continue
+			}
+			if deviceSenderID == in[i].SenderID() && device.SessionID == in[i].TransactionID.SessionID {
 				err := out[i].SetUnsignedField(
 					"transaction_id", in[i].TransactionID.TransactionID,
 				)
@@ -210,6 +224,11 @@ func (d *Database) StreamEventsToEvents(device *userapi.Device, in []types.Strea
 	return out
 }
 
+func (d *Database) getSenderIDForUser(roomID string, userID spec.UserID) (string, error) { // nolint
+	// TODO: Repalce with actual logic for pseudoIDs
+	return userID.String(), nil
+}
+
 // handleBackwardExtremities adds this event as a backwards extremity if and only if we do not have all of
 // the events listed in the event's 'prev_events'. This function also updates the backwards extremities table
 // to account for the fact that the given event is no longer a backwards extremity, but may be marked as such.
diff --git a/syncapi/storage/sqlite3/current_room_state_table.go b/syncapi/storage/sqlite3/current_room_state_table.go
index 1b8632eb..e432e483 100644
--- a/syncapi/storage/sqlite3/current_room_state_table.go
+++ b/syncapi/storage/sqlite3/current_room_state_table.go
@@ -342,7 +342,7 @@ func (s *currentRoomStateStatements) UpsertRoomState(
 		event.RoomID(),
 		event.EventID(),
 		event.Type(),
-		event.Sender(),
+		event.SenderID(),
 		containsURL,
 		*event.StateKey(),
 		headeredJSON,
diff --git a/syncapi/storage/sqlite3/output_room_events_table.go b/syncapi/storage/sqlite3/output_room_events_table.go
index d63e7606..5a47aec4 100644
--- a/syncapi/storage/sqlite3/output_room_events_table.go
+++ b/syncapi/storage/sqlite3/output_room_events_table.go
@@ -348,7 +348,7 @@ func (s *outputRoomEventsStatements) InsertEvent(
 		event.EventID(),
 		headeredJSON,
 		event.Type(),
-		event.Sender(),
+		event.SenderID(),
 		containsURL,
 		string(addStateJSON),
 		string(removeStateJSON),
diff --git a/syncapi/streams/stream_invite.go b/syncapi/streams/stream_invite.go
index becd863a..a8b0a7b6 100644
--- a/syncapi/streams/stream_invite.go
+++ b/syncapi/streams/stream_invite.go
@@ -10,6 +10,7 @@ import (
 
 	"github.com/matrix-org/gomatrixserverlib/spec"
 
+	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
 	"github.com/matrix-org/dendrite/syncapi/types"
@@ -17,6 +18,7 @@ import (
 
 type InviteStreamProvider struct {
 	DefaultStreamProvider
+	rsAPI api.SyncRoomserverAPI
 }
 
 func (p *InviteStreamProvider) Setup(
@@ -62,11 +64,17 @@ func (p *InviteStreamProvider) IncrementalSync(
 	}
 
 	for roomID, inviteEvent := range invites {
+		user := spec.UserID{}
+		sender, err := p.rsAPI.QueryUserIDForSender(ctx, inviteEvent.RoomID(), inviteEvent.SenderID())
+		if err == nil && sender != nil {
+			user = *sender
+		}
+
 		// skip ignored user events
-		if _, ok := req.IgnoredUsers.List[inviteEvent.Sender()]; ok {
+		if _, ok := req.IgnoredUsers.List[user.String()]; ok {
 			continue
 		}
-		ir := types.NewInviteResponse(inviteEvent)
+		ir := types.NewInviteResponse(inviteEvent, user)
 		req.Response.Rooms.Invite[roomID] = ir
 	}
 
diff --git a/syncapi/streams/stream_pdu.go b/syncapi/streams/stream_pdu.go
index 0ea48a9d..8f83a089 100644
--- a/syncapi/streams/stream_pdu.go
+++ b/syncapi/streams/stream_pdu.go
@@ -376,20 +376,28 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 			}
 		}
 		jr.Timeline.PrevBatch = &prevBatch
-		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync)
+		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		// If we are limited by the filter AND the history visibility filter
 		// didn't "remove" events, return that the response is limited.
 		jr.Timeline.Limited = (limited && len(events) == len(recentEvents)) || delta.NewlyJoined
-		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync)
+		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		req.Response.Rooms.Join[delta.RoomID] = jr
 
 	case spec.Peek:
 		jr := types.NewJoinResponse()
 		jr.Timeline.PrevBatch = &prevBatch
 		// TODO: Apply history visibility on peeked rooms
-		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(recentEvents), synctypes.FormatSync)
+		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(recentEvents), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		jr.Timeline.Limited = limited
-		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync)
+		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		req.Response.Rooms.Peek[delta.RoomID] = jr
 
 	case spec.Leave:
@@ -398,11 +406,15 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 	case spec.Ban:
 		lr := types.NewLeaveResponse()
 		lr.Timeline.PrevBatch = &prevBatch
-		lr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync)
+		lr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		// If we are limited by the filter AND the history visibility filter
 		// didn't "remove" events, return that the response is limited.
 		lr.Timeline.Limited = limited && len(events) == len(recentEvents)
-		lr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync)
+		lr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		})
 		req.Response.Rooms.Leave[delta.RoomID] = lr
 	}
 
@@ -425,7 +437,7 @@ func applyHistoryVisibilityFilter(
 	for _, ev := range recentEvents {
 		if ev.StateKey() != nil {
 			stateTypes = append(stateTypes, ev.Type())
-			senders = append(senders, ev.Sender())
+			senders = append(senders, ev.SenderID())
 		}
 	}
 
@@ -552,11 +564,15 @@ func (p *PDUStreamProvider) getJoinResponseForCompleteSync(
 	}
 
 	jr.Timeline.PrevBatch = prevBatch
-	jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync)
+	jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+		return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	})
 	// If we are limited by the filter AND the history visibility filter
 	// didn't "remove" events, return that the response is limited.
 	jr.Timeline.Limited = limited && len(events) == len(recentEvents)
-	jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(stateEvents), synctypes.FormatSync)
+	jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(stateEvents), synctypes.FormatSync, func(roomID, senderID string) (*spec.UserID, error) {
+		return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	})
 	return jr, nil
 }
 
@@ -577,8 +593,8 @@ func (p *PDUStreamProvider) lazyLoadMembers(
 	// Add all users the client doesn't know about yet to a list
 	for _, event := range timelineEvents {
 		// Membership is not yet cached, add it to the list
-		if _, ok := p.lazyLoadCache.IsLazyLoadedUserCached(device, roomID, event.Sender()); !ok {
-			timelineUsers[event.Sender()] = struct{}{}
+		if _, ok := p.lazyLoadCache.IsLazyLoadedUserCached(device, roomID, event.SenderID()); !ok {
+			timelineUsers[event.SenderID()] = struct{}{}
 		}
 	}
 	// Preallocate with the same amount, even if it will end up with fewer values
diff --git a/syncapi/streams/streams.go b/syncapi/streams/streams.go
index a35491ac..f25bc978 100644
--- a/syncapi/streams/streams.go
+++ b/syncapi/streams/streams.go
@@ -45,6 +45,7 @@ func NewSyncStreamProviders(
 		},
 		InviteStreamProvider: &InviteStreamProvider{
 			DefaultStreamProvider: DefaultStreamProvider{DB: d},
+			rsAPI:                 rsAPI,
 		},
 		SendToDeviceStreamProvider: &SendToDeviceStreamProvider{
 			DefaultStreamProvider: DefaultStreamProvider{DB: d},
diff --git a/syncapi/syncapi_test.go b/syncapi/syncapi_test.go
index bc766e66..78c857ab 100644
--- a/syncapi/syncapi_test.go
+++ b/syncapi/syncapi_test.go
@@ -40,6 +40,10 @@ type syncRoomserverAPI struct {
 	rooms []*test.Room
 }
 
+func (s *syncRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func (s *syncRoomserverAPI) QueryLatestEventsAndState(ctx context.Context, req *rsapi.QueryLatestEventsAndStateRequest, res *rsapi.QueryLatestEventsAndStateResponse) error {
 	var room *test.Room
 	for _, r := range s.rooms {
diff --git a/syncapi/synctypes/clientevent.go b/syncapi/synctypes/clientevent.go
index c722fe60..66fb1d01 100644
--- a/syncapi/synctypes/clientevent.go
+++ b/syncapi/synctypes/clientevent.go
@@ -44,22 +44,27 @@ type ClientEvent struct {
 }
 
 // ToClientEvents converts server events to client events.
-func ToClientEvents(serverEvs []gomatrixserverlib.PDU, format ClientEventFormat) []ClientEvent {
+func ToClientEvents(serverEvs []gomatrixserverlib.PDU, format ClientEventFormat, userIDForSender spec.UserIDForSender) []ClientEvent {
 	evs := make([]ClientEvent, 0, len(serverEvs))
 	for _, se := range serverEvs {
 		if se == nil {
 			continue // TODO: shouldn't happen?
 		}
-		evs = append(evs, ToClientEvent(se, format))
+		sender := spec.UserID{}
+		userID, err := userIDForSender(se.RoomID(), se.SenderID())
+		if err == nil && userID != nil {
+			sender = *userID
+		}
+		evs = append(evs, ToClientEvent(se, format, sender))
 	}
 	return evs
 }
 
 // ToClientEvent converts a single server event to a client event.
-func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat) ClientEvent {
+func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender spec.UserID) ClientEvent {
 	ce := ClientEvent{
 		Content:        spec.RawJSON(se.Content()),
-		Sender:         se.Sender(),
+		Sender:         sender.String(),
 		Type:           se.Type(),
 		StateKey:       se.StateKey(),
 		Unsigned:       spec.RawJSON(se.Unsigned()),
diff --git a/syncapi/synctypes/clientevent_test.go b/syncapi/synctypes/clientevent_test.go
index b914e64f..34179508 100644
--- a/syncapi/synctypes/clientevent_test.go
+++ b/syncapi/synctypes/clientevent_test.go
@@ -21,6 +21,7 @@ import (
 	"testing"
 
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
 func TestToClientEvent(t *testing.T) { // nolint: gocyclo
@@ -43,7 +44,11 @@ func TestToClientEvent(t *testing.T) { // nolint: gocyclo
 	if err != nil {
 		t.Fatalf("failed to create Event: %s", err)
 	}
-	ce := ToClientEvent(ev, FormatAll)
+	userID, err := spec.NewUserID("@test:localhost", true)
+	if err != nil {
+		t.Fatalf("failed to create userID: %s", err)
+	}
+	ce := ToClientEvent(ev, FormatAll, *userID)
 	if ce.EventID != ev.EventID() {
 		t.Errorf("ClientEvent.EventID: wanted %s, got %s", ev.EventID(), ce.EventID)
 	}
@@ -62,8 +67,8 @@ func TestToClientEvent(t *testing.T) { // nolint: gocyclo
 	if !bytes.Equal(ce.Unsigned, ev.Unsigned()) {
 		t.Errorf("ClientEvent.Unsigned: wanted %s, got %s", string(ev.Unsigned()), string(ce.Unsigned))
 	}
-	if ce.Sender != ev.Sender() {
-		t.Errorf("ClientEvent.Sender: wanted %s, got %s", ev.Sender(), ce.Sender)
+	if ce.Sender != userID.String() {
+		t.Errorf("ClientEvent.Sender: wanted %s, got %s", userID.String(), ce.Sender)
 	}
 	j, err := json.Marshal(ce)
 	if err != nil {
@@ -98,7 +103,11 @@ func TestToClientFormatSync(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create Event: %s", err)
 	}
-	ce := ToClientEvent(ev, FormatSync)
+	userID, err := spec.NewUserID("@test:localhost", true)
+	if err != nil {
+		t.Fatalf("failed to create userID: %s", err)
+	}
+	ce := ToClientEvent(ev, FormatSync, *userID)
 	if ce.RoomID != "" {
 		t.Errorf("ClientEvent.RoomID: wanted '', got %s", ce.RoomID)
 	}
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index 22c27fea..526a120d 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -539,7 +539,7 @@ type InviteResponse struct {
 }
 
 // NewInviteResponse creates an empty response with initialised arrays.
-func NewInviteResponse(event *types.HeaderedEvent) *InviteResponse {
+func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID) *InviteResponse {
 	res := InviteResponse{}
 	res.InviteState.Events = []json.RawMessage{}
 
@@ -552,7 +552,7 @@ func NewInviteResponse(event *types.HeaderedEvent) *InviteResponse {
 
 	// Then we'll see if we can create a partial of the invite event itself.
 	// This is needed for clients to work out *who* sent the invite.
-	inviteEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatSync)
+	inviteEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatSync, userID)
 	inviteEvent.Unsigned = nil
 	if ev, err := json.Marshal(inviteEvent); err == nil {
 		res.InviteState.Events = append(res.InviteState.Events, ev)
diff --git a/syncapi/types/types_test.go b/syncapi/types/types_test.go
index 8e0448fe..a79ce541 100644
--- a/syncapi/types/types_test.go
+++ b/syncapi/types/types_test.go
@@ -8,8 +8,13 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
+func UserIDForSender(roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func TestSyncTokens(t *testing.T) {
 	shouldPass := map[string]string{
 		"s4_0_0_0_0_0_0_0_3": StreamingToken{4, 0, 0, 0, 0, 0, 0, 0, 3}.String(),
@@ -56,7 +61,12 @@ func TestNewInviteResponse(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev})
+	sender, err := spec.NewUserID("@neilalexander:matrix.org", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev}, *sender)
 	j, err := json.Marshal(res)
 	if err != nil {
 		t.Fatal(err)
diff --git a/test/room.go b/test/room.go
index 852e3153..4cdb73aa 100644
--- a/test/room.go
+++ b/test/room.go
@@ -39,6 +39,10 @@ var (
 	roomIDCounter = int64(0)
 )
 
+func UserIDForSender(roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 type Room struct {
 	ID           string
 	Version      gomatrixserverlib.RoomVersion
@@ -195,7 +199,7 @@ func (r *Room) CreateEvent(t *testing.T, creator *User, eventType string, conten
 	if err != nil {
 		t.Fatalf("CreateEvent[%s]: failed to build event: %s", eventType, err)
 	}
-	if err = gomatrixserverlib.Allowed(ev, &r.authEvents); err != nil {
+	if err = gomatrixserverlib.Allowed(ev, &r.authEvents, UserIDForSender); err != nil {
 		t.Fatalf("CreateEvent[%s]: failed to verify event was allowed: %s", eventType, err)
 	}
 	headeredEvent := &rstypes.HeaderedEvent{PDU: ev}
diff --git a/userapi/consumers/roomserver.go b/userapi/consumers/roomserver.go
index 3cfdc0ce..c025deee 100644
--- a/userapi/consumers/roomserver.go
+++ b/userapi/consumers/roomserver.go
@@ -108,7 +108,7 @@ func (s *OutputRoomEventConsumer) onMessage(ctx context.Context, msgs []*nats.Ms
 	}
 
 	if s.cfg.Matrix.ReportStats.Enabled {
-		go s.storeMessageStats(ctx, event.Type(), event.Sender(), event.RoomID())
+		go s.storeMessageStats(ctx, event.Type(), event.SenderID(), event.RoomID())
 	}
 
 	log.WithFields(log.Fields{
@@ -301,7 +301,12 @@ func (s *OutputRoomEventConsumer) processMessage(ctx context.Context, event *rst
 
 	switch {
 	case event.Type() == spec.MRoomMember:
-		cevent := synctypes.ToClientEvent(event, synctypes.FormatAll)
+		sender := spec.UserID{}
+		userID, queryErr := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+		if queryErr == nil && userID != nil {
+			sender = *userID
+		}
+		cevent := synctypes.ToClientEvent(event, synctypes.FormatAll, sender)
 		var member *localMembership
 		member, err = newLocalMembership(&cevent)
 		if err != nil {
@@ -529,12 +534,17 @@ func (s *OutputRoomEventConsumer) notifyLocal(ctx context.Context, event *rstype
 		return fmt.Errorf("s.localPushDevices: %w", err)
 	}
 
+	sender := spec.UserID{}
+	userID, err := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+	if err == nil && userID != nil {
+		sender = *userID
+	}
 	n := &api.Notification{
 		Actions: actions,
 		// UNSPEC: the spec doesn't say this is a ClientEvent, but the
 		// fields seem to match. room_id should be missing, which
 		// matches the behaviour of FormatSync.
-		Event: synctypes.ToClientEvent(event, synctypes.FormatSync),
+		Event: synctypes.ToClientEvent(event, synctypes.FormatSync, sender),
 		// TODO: this is per-device, but it's not part of the primary
 		// key. So inserting one notification per profile tag doesn't
 		// make sense. What is this supposed to be? Sytests require it
@@ -615,7 +625,12 @@ func (s *OutputRoomEventConsumer) notifyLocal(ctx context.Context, event *rstype
 // evaluatePushRules fetches and evaluates the push rules of a local
 // user. Returns actions (including dont_notify).
 func (s *OutputRoomEventConsumer) evaluatePushRules(ctx context.Context, event *rstypes.HeaderedEvent, mem *localMembership, roomSize int) ([]*pushrules.Action, error) {
-	if event.Sender() == mem.UserID {
+	user := ""
+	sender, err := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+	if err == nil {
+		user = sender.String()
+	}
+	if user == mem.UserID {
 		// SPEC: Homeservers MUST NOT notify the Push Gateway for
 		// events that the user has sent themselves.
 		return nil, nil
@@ -632,9 +647,8 @@ func (s *OutputRoomEventConsumer) evaluatePushRules(ctx context.Context, event *
 		if err != nil {
 			return nil, err
 		}
-		sender := event.Sender()
-		if _, ok := ignored.List[sender]; ok {
-			return nil, fmt.Errorf("user %s is ignored", sender)
+		if _, ok := ignored.List[sender.String()]; ok {
+			return nil, fmt.Errorf("user %s is ignored", sender.String())
 		}
 	}
 	ruleSets, err := s.db.QueryPushRules(ctx, mem.Localpart, mem.Domain)
@@ -650,7 +664,9 @@ func (s *OutputRoomEventConsumer) evaluatePushRules(ctx context.Context, event *
 		roomSize: roomSize,
 	}
 	eval := pushrules.NewRuleSetEvaluator(ec, &ruleSets.Global)
-	rule, err := eval.MatchEvent(event.PDU)
+	rule, err := eval.MatchEvent(event.PDU, func(roomID, senderID string) (*spec.UserID, error) {
+		return s.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -682,7 +698,7 @@ func (rse *ruleSetEvalContext) UserDisplayName() string { return rse.mem.Display
 
 func (rse *ruleSetEvalContext) RoomMemberCount() (int, error) { return rse.roomSize, nil }
 
-func (rse *ruleSetEvalContext) HasPowerLevel(userID, levelKey string) (bool, error) {
+func (rse *ruleSetEvalContext) HasPowerLevel(senderID, levelKey string) (bool, error) {
 	req := &rsapi.QueryLatestEventsAndStateRequest{
 		RoomID: rse.roomID,
 		StateToFetch: []gomatrixserverlib.StateKeyTuple{
@@ -702,7 +718,7 @@ func (rse *ruleSetEvalContext) HasPowerLevel(userID, levelKey string) (bool, err
 		if err != nil {
 			return false, err
 		}
-		return plc.UserLevel(userID) >= plc.NotificationLevel(levelKey), nil
+		return plc.UserLevel(senderID) >= plc.NotificationLevel(levelKey), nil
 	}
 	return true, nil
 }
@@ -756,6 +772,11 @@ func (s *OutputRoomEventConsumer) notifyHTTP(ctx context.Context, event *rstypes
 		}
 
 	default:
+		sender, err := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), event.SenderID())
+		if err != nil {
+			logger.WithError(err).Errorf("Failed to get userID for sender %s", event.SenderID())
+			return nil, err
+		}
 		req = pushgateway.NotifyRequest{
 			Notification: pushgateway.Notification{
 				Content: event.Content(),
@@ -767,7 +788,7 @@ func (s *OutputRoomEventConsumer) notifyHTTP(ctx context.Context, event *rstypes
 				ID:       event.EventID(),
 				RoomID:   event.RoomID(),
 				RoomName: roomName,
-				Sender:   event.Sender(),
+				Sender:   sender.String(),
 				Type:     event.Type(),
 			},
 		}
diff --git a/userapi/consumers/roomserver_test.go b/userapi/consumers/roomserver_test.go
index 53977206..899a5aaf 100644
--- a/userapi/consumers/roomserver_test.go
+++ b/userapi/consumers/roomserver_test.go
@@ -14,6 +14,7 @@ import (
 	"github.com/stretchr/testify/assert"
 
 	"github.com/matrix-org/dendrite/internal/pushrules"
+	rsapi "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/setup/config"
 	"github.com/matrix-org/dendrite/test"
 	"github.com/matrix-org/dendrite/userapi/storage"
@@ -44,13 +45,19 @@ func mustCreateEvent(t *testing.T, content string) *types.HeaderedEvent {
 	return &types.HeaderedEvent{PDU: ev}
 }
 
+type FakeUserRoomserverAPI struct{ rsapi.UserRoomserverAPI }
+
+func (f *FakeUserRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID string) (*spec.UserID, error) {
+	return spec.NewUserID(senderID, true)
+}
+
 func Test_evaluatePushRules(t *testing.T) {
 	ctx := context.Background()
 
 	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
 		db, close := mustCreateDatabase(t, dbType)
 		defer close()
-		consumer := OutputRoomEventConsumer{db: db}
+		consumer := OutputRoomEventConsumer{db: db, rsAPI: &FakeUserRoomserverAPI{}}
 
 		testCases := []struct {
 			name         string
@@ -86,7 +93,7 @@ func Test_evaluatePushRules(t *testing.T) {
 			},
 			{
 				name:         "m.room.message highlights",
-				eventContent: `{"type":"m.room.message", "content": {"body": "test"} }`,
+				eventContent: `{"type":"m.room.message", "content": {"body": "test"}}`,
 				wantNotify:   true,
 				wantAction:   pushrules.NotifyAction,
 				wantActions: []*pushrules.Action{
diff --git a/userapi/util/notify_test.go b/userapi/util/notify_test.go
index e1c88d47..27dd373c 100644
--- a/userapi/util/notify_test.go
+++ b/userapi/util/notify_test.go
@@ -11,6 +11,7 @@ import (
 	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/matrix-org/dendrite/syncapi/synctypes"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/matrix-org/util"
 	"golang.org/x/crypto/bcrypt"
 
@@ -87,7 +88,7 @@ func TestNotifyUserCountsAsync(t *testing.T) {
 		}
 
 		// Prepare pusher with our test server URL
-		if err := db.UpsertPusher(ctx, api.Pusher{
+		if err = db.UpsertPusher(ctx, api.Pusher{
 			Kind:    api.HTTPKind,
 			AppID:   appID,
 			PushKey: pushKey,
@@ -99,8 +100,12 @@ func TestNotifyUserCountsAsync(t *testing.T) {
 		}
 
 		// Insert a dummy event
+		sender, err := spec.NewUserID(alice.ID, true)
+		if err != nil {
+			t.Error(err)
+		}
 		if err := db.InsertNotification(ctx, aliceLocalpart, serverName, dummyEvent.EventID(), 0, nil, &api.Notification{
-			Event: synctypes.ToClientEvent(dummyEvent, synctypes.FormatAll),
+			Event: synctypes.ToClientEvent(dummyEvent, synctypes.FormatAll, *sender),
 		}); err != nil {
 			t.Error(err)
 		}