diff --git a/clientapi/routing/account_data.go b/clientapi/routing/account_data.go
index 7eacf9cc9..81afc3b13 100644
--- a/clientapi/routing/account_data.go
+++ b/clientapi/routing/account_data.go
@@ -145,8 +145,16 @@ func SaveReadMarker(
 	userAPI api.ClientUserAPI, rsAPI roomserverAPI.ClientRoomserverAPI,
 	syncProducer *producers.SyncAPIProducer, device *api.Device, roomID string,
 ) util.JSONResponse {
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.BadJSON("userID for this device is invalid"),
+		}
+	}
+
 	// Verify that the user is a member of this room
-	resErr := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	resErr := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if resErr != nil {
 		return *resErr
 	}
diff --git a/clientapi/routing/aliases.go b/clientapi/routing/aliases.go
index f6603be8b..2d6b72d3e 100644
--- a/clientapi/routing/aliases.go
+++ b/clientapi/routing/aliases.go
@@ -55,9 +55,16 @@ func GetAliases(
 		visibility = content.HistoryVisibility
 	}
 	if visibility != spec.WorldReadable {
+		deviceUserID, err := spec.NewUserID(device.UserID, true)
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: spec.Forbidden("userID doesn't have power level to change visibility"),
+			}
+		}
 		queryReq := api.QueryMembershipForUserRequest{
 			RoomID: roomID,
-			UserID: device.UserID,
+			UserID: *deviceUserID,
 		}
 		var queryRes api.QueryMembershipForUserResponse
 		if err := rsAPI.QueryMembershipForUser(req.Context(), &queryReq, &queryRes); err != nil {
diff --git a/clientapi/routing/createroom.go b/clientapi/routing/createroom.go
index 799fc7976..320f236cb 100644
--- a/clientapi/routing/createroom.go
+++ b/clientapi/routing/createroom.go
@@ -224,6 +224,7 @@ func createRoom(
 		PrivateKey:      privateKey,
 		EventTime:       evTime,
 	}
+
 	roomAlias, createRes := rsAPI.PerformCreateRoom(ctx, *userID, *roomID, &req)
 	if createRes != nil {
 		return *createRes
diff --git a/clientapi/routing/directory.go b/clientapi/routing/directory.go
index 034296f45..f01e24eca 100644
--- a/clientapi/routing/directory.go
+++ b/clientapi/routing/directory.go
@@ -314,7 +314,22 @@ func SetVisibility(
 	req *http.Request, rsAPI roomserverAPI.ClientRoomserverAPI, dev *userapi.Device,
 	roomID string,
 ) util.JSONResponse {
-	resErr := checkMemberInRoom(req.Context(), rsAPI, dev.UserID, roomID)
+	deviceUserID, err := spec.NewUserID(dev.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.BadJSON("userID for this device is invalid"),
+		}
+	}
+	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *deviceUserID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.Unknown("failed to find senderID for this user"),
+		}
+	}
+
+	resErr := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if resErr != nil {
 		return *resErr
 	}
@@ -327,7 +342,7 @@ func SetVisibility(
 		}},
 	}
 	var queryEventsRes roomserverAPI.QueryLatestEventsAndStateResponse
-	err := rsAPI.QueryLatestEventsAndState(req.Context(), &queryEventsReq, &queryEventsRes)
+	err = rsAPI.QueryLatestEventsAndState(req.Context(), &queryEventsReq, &queryEventsRes)
 	if err != nil || len(queryEventsRes.StateEvents) == 0 {
 		util.GetLogger(req.Context()).WithError(err).Error("could not query events from room")
 		return util.JSONResponse{
@@ -338,20 +353,6 @@ func SetVisibility(
 
 	// NOTSPEC: Check if the user's power is greater than power required to change m.room.canonical_alias event
 	power, _ := gomatrixserverlib.NewPowerLevelContentFromEvent(queryEventsRes.StateEvents[0].PDU)
-	fullUserID, err := spec.NewUserID(dev.UserID, true)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("userID doesn't have power level to change visibility"),
-		}
-	}
-	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *fullUserID)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("userID doesn't have power level to change visibility"),
-		}
-	}
 	if power.UserLevel(senderID) < power.EventLevel(spec.MRoomCanonicalAlias, true) {
 		return util.JSONResponse{
 			Code: http.StatusForbidden,
diff --git a/clientapi/routing/leaveroom.go b/clientapi/routing/leaveroom.go
index fbf148264..7e8c066eb 100644
--- a/clientapi/routing/leaveroom.go
+++ b/clientapi/routing/leaveroom.go
@@ -29,10 +29,18 @@ func LeaveRoomByID(
 	rsAPI roomserverAPI.ClientRoomserverAPI,
 	roomID string,
 ) util.JSONResponse {
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.Unknown("device userID is invalid"),
+		}
+	}
+
 	// Prepare to ask the roomserver to perform the room join.
 	leaveReq := roomserverAPI.PerformLeaveRequest{
 		RoomID: roomID,
-		UserID: device.UserID,
+		Leaver: *userID,
 	}
 	leaveRes := roomserverAPI.PerformLeaveResponse{}
 
diff --git a/clientapi/routing/membership.go b/clientapi/routing/membership.go
index 78829bec9..03e85edbf 100644
--- a/clientapi/routing/membership.go
+++ b/clientapi/routing/membership.go
@@ -57,7 +57,22 @@ func SendBan(
 		}
 	}
 
-	errRes := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to ban this user, bad userID"),
+		}
+	}
+	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *deviceUserID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to ban this user, unknown senderID"),
+		}
+	}
+
+	errRes := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if errRes != nil {
 		return *errRes
 	}
@@ -66,20 +81,6 @@ func SendBan(
 	if errRes != nil {
 		return *errRes
 	}
-	fullUserID, err := spec.NewUserID(device.UserID, true)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("You don't have permission to ban this user, bad userID"),
-		}
-	}
-	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *fullUserID)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("You don't have permission to ban this user, unknown senderID"),
-		}
-	}
 	allowedToBan := pl.UserLevel(senderID) >= pl.Ban
 	if !allowedToBan {
 		return util.JSONResponse{
@@ -147,7 +148,22 @@ func SendKick(
 		}
 	}
 
-	errRes := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, bad userID"),
+		}
+	}
+	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *deviceUserID)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, unknown senderID"),
+		}
+	}
+
+	errRes := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if errRes != nil {
 		return *errRes
 	}
@@ -156,20 +172,6 @@ func SendKick(
 	if errRes != nil {
 		return *errRes
 	}
-	fullUserID, err := spec.NewUserID(device.UserID, true)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("You don't have permission to kick this user, bad userID"),
-		}
-	}
-	senderID, err := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *fullUserID)
-	if err != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("You don't have permission to kick this user, unknown senderID"),
-		}
-	}
 	allowedToKick := pl.UserLevel(senderID) >= pl.Kick
 	if !allowedToKick {
 		return util.JSONResponse{
@@ -178,10 +180,17 @@ func SendKick(
 		}
 	}
 
+	bodyUserID, err := spec.NewUserID(body.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.BadJSON("body userID is invalid"),
+		}
+	}
 	var queryRes roomserverAPI.QueryMembershipForUserResponse
 	err = rsAPI.QueryMembershipForUser(req.Context(), &roomserverAPI.QueryMembershipForUserRequest{
 		RoomID: roomID,
-		UserID: body.UserID,
+		UserID: *bodyUserID,
 	}, &queryRes)
 	if err != nil {
 		return util.ErrorResponse(err)
@@ -213,15 +222,30 @@ func SendUnban(
 		}
 	}
 
-	errRes := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, bad userID"),
+		}
+	}
+
+	errRes := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if errRes != nil {
 		return *errRes
 	}
 
+	bodyUserID, err := spec.NewUserID(body.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.BadJSON("body userID is invalid"),
+		}
+	}
 	var queryRes roomserverAPI.QueryMembershipForUserResponse
-	err := rsAPI.QueryMembershipForUser(req.Context(), &roomserverAPI.QueryMembershipForUserRequest{
+	err = rsAPI.QueryMembershipForUser(req.Context(), &roomserverAPI.QueryMembershipForUserRequest{
 		RoomID: roomID,
-		UserID: body.UserID,
+		UserID: *bodyUserID,
 	}, &queryRes)
 	if err != nil {
 		return util.ErrorResponse(err)
@@ -272,7 +296,15 @@ func SendInvite(
 		}
 	}
 
-	errRes := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, bad userID"),
+		}
+	}
+
+	errRes := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if errRes != nil {
 		return *errRes
 	}
@@ -340,17 +372,18 @@ func sendInvite(
 
 func buildMembershipEventDirect(
 	ctx context.Context,
-	targetUserID, reason string, userDisplayName, userAvatarURL string,
-	sender string, senderDomain spec.ServerName,
+	targetSenderID spec.SenderID, reason string, userDisplayName, userAvatarURL string,
+	sender spec.SenderID, senderDomain spec.ServerName,
 	membership, roomID string, isDirect bool,
 	keyID gomatrixserverlib.KeyID, privateKey ed25519.PrivateKey, evTime time.Time,
 	rsAPI roomserverAPI.ClientRoomserverAPI,
 ) (*types.HeaderedEvent, error) {
+	targetSenderString := string(targetSenderID)
 	proto := gomatrixserverlib.ProtoEvent{
-		SenderID: sender,
+		SenderID: string(sender),
 		RoomID:   roomID,
 		Type:     "m.room.member",
-		StateKey: &targetUserID,
+		StateKey: &targetSenderString,
 	}
 
 	content := gomatrixserverlib.MemberContent{
@@ -391,8 +424,25 @@ func buildMembershipEvent(
 		return nil, err
 	}
 
-	return buildMembershipEventDirect(ctx, targetUserID, reason, profile.DisplayName, profile.AvatarURL,
-		device.UserID, device.UserDomain(), membership, roomID, isDirect, identity.KeyID, identity.PrivateKey, evTime, rsAPI)
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return nil, err
+	}
+	senderID, err := rsAPI.QuerySenderIDForUser(ctx, roomID, *userID)
+	if err != nil {
+		return nil, err
+	}
+
+	targetID, err := spec.NewUserID(targetUserID, true)
+	if err != nil {
+		return nil, err
+	}
+	targetSenderID, err := rsAPI.QuerySenderIDForUser(ctx, roomID, *targetID)
+	if err != nil {
+		return nil, err
+	}
+	return buildMembershipEventDirect(ctx, targetSenderID, reason, profile.DisplayName, profile.AvatarURL,
+		senderID, device.UserDomain(), membership, roomID, isDirect, identity.KeyID, identity.PrivateKey, evTime, rsAPI)
 }
 
 // loadProfile lookups the profile of a given user from the database and returns
@@ -490,7 +540,7 @@ func checkAndProcessThreepid(
 	return
 }
 
-func checkMemberInRoom(ctx context.Context, rsAPI roomserverAPI.ClientRoomserverAPI, userID, roomID string) *util.JSONResponse {
+func checkMemberInRoom(ctx context.Context, rsAPI roomserverAPI.ClientRoomserverAPI, userID spec.UserID, roomID string) *util.JSONResponse {
 	var membershipRes roomserverAPI.QueryMembershipForUserResponse
 	err := rsAPI.QueryMembershipForUser(ctx, &roomserverAPI.QueryMembershipForUserRequest{
 		RoomID: roomID,
@@ -518,12 +568,21 @@ func SendForget(
 ) util.JSONResponse {
 	ctx := req.Context()
 	logger := util.GetLogger(ctx).WithField("roomID", roomID).WithField("userID", device.UserID)
+
+	deviceUserID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, bad userID"),
+		}
+	}
+
 	var membershipRes roomserverAPI.QueryMembershipForUserResponse
 	membershipReq := roomserverAPI.QueryMembershipForUserRequest{
 		RoomID: roomID,
-		UserID: device.UserID,
+		UserID: *deviceUserID,
 	}
-	err := rsAPI.QueryMembershipForUser(ctx, &membershipReq, &membershipRes)
+	err = rsAPI.QueryMembershipForUser(ctx, &membershipReq, &membershipRes)
 	if err != nil {
 		logger.WithError(err).Error("QueryMembershipForUser: could not query membership for user")
 		return util.JSONResponse{
diff --git a/clientapi/routing/redaction.go b/clientapi/routing/redaction.go
index 22474fc08..da48e84de 100644
--- a/clientapi/routing/redaction.go
+++ b/clientapi/routing/redaction.go
@@ -47,7 +47,22 @@ func SendRedaction(
 	txnID *string,
 	txnCache *transactions.Cache,
 ) util.JSONResponse {
-	resErr := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
+	deviceUserID, userIDErr := spec.NewUserID(device.UserID, true)
+	if userIDErr != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("userID doesn't have power level to redact"),
+		}
+	}
+	senderID, queryErr := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *deviceUserID)
+	if queryErr != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("userID doesn't have power level to redact"),
+		}
+	}
+
+	resErr := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if resErr != nil {
 		return *resErr
 	}
@@ -73,25 +88,10 @@ func SendRedaction(
 		}
 	}
 
-	fullUserID, userIDErr := spec.NewUserID(device.UserID, true)
-	if userIDErr != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("userID doesn't have power level to redact"),
-		}
-	}
-	senderID, queryErr := rsAPI.QuerySenderIDForUser(req.Context(), roomID, *fullUserID)
-	if queryErr != nil {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("userID doesn't have power level to redact"),
-		}
-	}
-
 	// "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.SenderID() == senderID // TODO: Should replace device.UserID with device...PerRoomKey
+	allowedToRedact := ev.SenderID() == senderID
 	if !allowedToRedact {
 		plEvent := roomserverAPI.GetStateEvent(req.Context(), rsAPI, roomID, gomatrixserverlib.StateKeyTuple{
 			EventType: spec.MRoomPowerLevels,
diff --git a/clientapi/routing/sendtyping.go b/clientapi/routing/sendtyping.go
index c5b29297a..979bced3b 100644
--- a/clientapi/routing/sendtyping.go
+++ b/clientapi/routing/sendtyping.go
@@ -43,8 +43,16 @@ func SendTyping(
 		}
 	}
 
+	deviceUserID, err := spec.NewUserID(userID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("userID doesn't have power level to change visibility"),
+		}
+	}
+
 	// Verify that the user is a member of this room
-	resErr := checkMemberInRoom(req.Context(), rsAPI, userID, roomID)
+	resErr := checkMemberInRoom(req.Context(), rsAPI, *deviceUserID, roomID)
 	if resErr != nil {
 		return *resErr
 	}
diff --git a/clientapi/routing/server_notices.go b/clientapi/routing/server_notices.go
index 06714ed1f..7006ced46 100644
--- a/clientapi/routing/server_notices.go
+++ b/clientapi/routing/server_notices.go
@@ -52,6 +52,7 @@ type sendServerNoticeRequest struct {
 	StateKey string `json:"state_key,omitempty"`
 }
 
+// nolint:gocyclo
 // SendServerNotice sends a message to a specific user. It can only be invoked by an admin.
 func SendServerNotice(
 	req *http.Request,
@@ -187,9 +188,17 @@ func SendServerNotice(
 		}
 	} else {
 		// we've found a room in common, check the membership
+		deviceUserID, err := spec.NewUserID(r.UserID, true)
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: spec.Forbidden("userID doesn't have power level to change visibility"),
+			}
+		}
+
 		roomID = commonRooms[0]
 		membershipRes := api.QueryMembershipForUserResponse{}
-		err := rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{UserID: r.UserID, RoomID: roomID}, &membershipRes)
+		err = rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{UserID: *deviceUserID, RoomID: roomID}, &membershipRes)
 		if err != nil {
 			util.GetLogger(ctx).WithError(err).Error("unable to query membership for user")
 			return util.JSONResponse{
@@ -234,7 +243,7 @@ func SendServerNotice(
 		ctx, rsAPI,
 		api.KindNew,
 		[]*types.HeaderedEvent{
-			&types.HeaderedEvent{PDU: e},
+			{PDU: e},
 		},
 		device.UserDomain(),
 		cfgClient.Matrix.ServerName,
diff --git a/clientapi/routing/state.go b/clientapi/routing/state.go
index 13f308998..e3a209b6e 100644
--- a/clientapi/routing/state.go
+++ b/clientapi/routing/state.go
@@ -99,9 +99,17 @@ func OnIncomingStateRequest(ctx context.Context, device *userapi.Device, rsAPI a
 	if !worldReadable {
 		// The room isn't world-readable so try to work out based on the
 		// user's membership if we want the latest state or not.
-		err := rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{
+		userID, err := spec.NewUserID(device.UserID, true)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Error("UserID is invalid")
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: spec.Unknown("Device UserID is invalid"),
+			}
+		}
+		err = rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{
 			RoomID: roomID,
-			UserID: device.UserID,
+			UserID: *userID,
 		}, &membershipRes)
 		if err != nil {
 			util.GetLogger(ctx).WithError(err).Error("Failed to QueryMembershipForUser")
@@ -140,14 +148,11 @@ 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, sender),
+				synctypes.ToClientEventDefault(func(roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+					return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+				}, ev),
 			)
 		}
 	} else {
@@ -172,9 +177,18 @@ func OnIncomingStateRequest(ctx context.Context, device *userapi.Device, rsAPI a
 			if err == nil && userID != nil {
 				sender = *userID
 			}
+
+			sk := ev.StateKey()
+			if sk != nil && *sk != "" {
+				skUserID, err := rsAPI.QueryUserIDForSender(ctx, ev.RoomID(), spec.SenderID(*ev.StateKey()))
+				if err == nil && skUserID != nil {
+					skString := skUserID.String()
+					sk = &skString
+				}
+			}
 			stateEvents = append(
 				stateEvents,
-				synctypes.ToClientEvent(ev, synctypes.FormatAll, sender),
+				synctypes.ToClientEvent(ev, synctypes.FormatAll, sender, sk),
 			)
 		}
 	}
@@ -259,11 +273,19 @@ func OnIncomingStateTypeRequest(
 	// membershipRes will only be populated if the room is not world-readable.
 	var membershipRes api.QueryMembershipForUserResponse
 	if !worldReadable {
+		userID, err := spec.NewUserID(device.UserID, true)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Error("UserID is invalid")
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: spec.Unknown("Device UserID is invalid"),
+			}
+		}
 		// The room isn't world-readable so try to work out based on the
 		// user's membership if we want the latest state or not.
-		err := rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{
+		err = rsAPI.QueryMembershipForUser(ctx, &api.QueryMembershipForUserRequest{
 			RoomID: roomID,
-			UserID: device.UserID,
+			UserID: *userID,
 		}, &membershipRes)
 		if err != nil {
 			util.GetLogger(ctx).WithError(err).Error("Failed to QueryMembershipForUser")
@@ -344,13 +366,10 @@ 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, sender),
+		ClientEvent: synctypes.ToClientEventDefault(func(roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+			return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+		}, event),
 	}
 
 	var res interface{}
diff --git a/clientapi/routing/upgrade_room.go b/clientapi/routing/upgrade_room.go
index a0b280789..03c0230e6 100644
--- a/clientapi/routing/upgrade_room.go
+++ b/clientapi/routing/upgrade_room.go
@@ -59,7 +59,15 @@ func UpgradeRoom(
 		}
 	}
 
-	newRoomID, err := rsAPI.PerformRoomUpgrade(req.Context(), roomID, device.UserID, gomatrixserverlib.RoomVersion(r.NewVersion))
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		util.GetLogger(req.Context()).WithError(err).Error("device UserID is invalid")
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: spec.InternalServerError{},
+		}
+	}
+	newRoomID, err := rsAPI.PerformRoomUpgrade(req.Context(), roomID, *userID, gomatrixserverlib.RoomVersion(r.NewVersion))
 	switch e := err.(type) {
 	case nil:
 	case roomserverAPI.ErrNotAllowed:
diff --git a/federationapi/routing/eventauth.go b/federationapi/routing/eventauth.go
index ca279ac22..c26aa2f15 100644
--- a/federationapi/routing/eventauth.go
+++ b/federationapi/routing/eventauth.go
@@ -45,7 +45,7 @@ func GetEventAuth(
 	if event.RoomID() != roomID {
 		return util.JSONResponse{Code: http.StatusNotFound, JSON: spec.NotFound("event does not belong to this room")}
 	}
-	resErr = allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID)
+	resErr = allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID, event.RoomID())
 	if resErr != nil {
 		return *resErr
 	}
diff --git a/federationapi/routing/events.go b/federationapi/routing/events.go
index 196a54db1..d3f0e81c3 100644
--- a/federationapi/routing/events.go
+++ b/federationapi/routing/events.go
@@ -35,10 +35,6 @@ func GetEvent(
 	eventID string,
 	origin spec.ServerName,
 ) util.JSONResponse {
-	err := allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID)
-	if err != nil {
-		return *err
-	}
 	// /_matrix/federation/v1/event/{eventId} doesn't have a roomID, we use an empty string,
 	// which results in `QueryEventsByID` to first get the event and use that to determine the roomID.
 	event, err := fetchEvent(ctx, rsAPI, "", eventID)
@@ -46,6 +42,11 @@ func GetEvent(
 		return *err
 	}
 
+	err = allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID, event.RoomID())
+	if err != nil {
+		return *err
+	}
+
 	return util.JSONResponse{Code: http.StatusOK, JSON: gomatrixserverlib.Transaction{
 		Origin:         origin,
 		OriginServerTS: spec.AsTimestamp(time.Now()),
@@ -62,8 +63,9 @@ func allowedToSeeEvent(
 	origin spec.ServerName,
 	rsAPI api.FederationRoomserverAPI,
 	eventID string,
+	roomID string,
 ) *util.JSONResponse {
-	allowed, err := rsAPI.QueryServerAllowedToSeeEvent(ctx, origin, eventID)
+	allowed, err := rsAPI.QueryServerAllowedToSeeEvent(ctx, origin, eventID, roomID)
 	if err != nil {
 		resErr := util.ErrorResponse(err)
 		return &resErr
diff --git a/federationapi/routing/state.go b/federationapi/routing/state.go
index fa0e9351e..11ad1ebfc 100644
--- a/federationapi/routing/state.go
+++ b/federationapi/routing/state.go
@@ -116,7 +116,7 @@ func getState(
 	if event.RoomID() != roomID {
 		return nil, nil, &util.JSONResponse{Code: http.StatusNotFound, JSON: spec.NotFound("event does not belong to this room")}
 	}
-	resErr = allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID)
+	resErr = allowedToSeeEvent(ctx, request.Origin(), rsAPI, eventID, event.RoomID())
 	if resErr != nil {
 		return nil, nil, resErr
 	}
diff --git a/go.mod b/go.mod
index 3621428c3..2fbae3148 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 v0.0.0-20230607161930-ea5ef168992d
+	github.com/matrix-org/gomatrixserverlib v0.0.0-20230612110349-8e7766804077
 	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
diff --git a/go.sum b/go.sum
index 1ee0261f6..ef8c298ab 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 v0.0.0-20230607161930-ea5ef168992d h1:MjL8SXRzhO61aXDFL+gA3Bx1SicqLGL9gCWXDv8jkD8=
-github.com/matrix-org/gomatrixserverlib v0.0.0-20230607161930-ea5ef168992d/go.mod h1:H9V9N3Uqn1bBJqYJNGK1noqtgJTaCEhtTdcH/mp50uU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20230612110349-8e7766804077 h1:AmKkAUjy9rZA2K+qHXm/O/dPEPnUYfRE2I6SL+Dj+LU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20230612110349-8e7766804077/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=
diff --git a/roomserver/api/api.go b/roomserver/api/api.go
index 8c2cbd6b2..bafde91c9 100644
--- a/roomserver/api/api.go
+++ b/roomserver/api/api.go
@@ -34,11 +34,11 @@ func (e ErrNotAllowed) Error() string {
 
 type RestrictedJoinAPI interface {
 	CurrentStateEvent(ctx context.Context, roomID spec.RoomID, eventType string, stateKey string) (gomatrixserverlib.PDU, error)
-	InvitePending(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (bool, error)
-	RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, userID spec.UserID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error)
+	InvitePending(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (bool, error)
+	RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error)
 	QueryRoomInfo(ctx context.Context, roomID spec.RoomID) (*types.RoomInfo, error)
 	QueryServerJoinedToRoom(ctx context.Context, req *QueryServerJoinedToRoomRequest, res *QueryServerJoinedToRoomResponse) error
-	UserJoinedToRoom(ctx context.Context, roomID types.RoomNID, userID spec.UserID) (bool, error)
+	UserJoinedToRoom(ctx context.Context, roomID types.RoomNID, senderID spec.SenderID) (bool, error)
 	LocallyJoinedUsers(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomNID types.RoomNID) ([]gomatrixserverlib.PDU, error)
 }
 
@@ -191,7 +191,7 @@ type ClientRoomserverAPI interface {
 
 	PerformCreateRoom(ctx context.Context, userID spec.UserID, roomID spec.RoomID, createRequest *PerformCreateRoomRequest) (string, *util.JSONResponse)
 	// PerformRoomUpgrade upgrades a room to a newer version
-	PerformRoomUpgrade(ctx context.Context, roomID, userID string, roomVersion gomatrixserverlib.RoomVersion) (newRoomID string, err error)
+	PerformRoomUpgrade(ctx context.Context, roomID string, userID spec.UserID, roomVersion gomatrixserverlib.RoomVersion) (newRoomID string, err error)
 	PerformAdminEvacuateRoom(ctx context.Context, roomID string) (affected []string, err error)
 	PerformAdminEvacuateUser(ctx context.Context, userID string) (affected []string, err error)
 	PerformAdminPurgeRoom(ctx context.Context, roomID string) error
@@ -228,6 +228,7 @@ type FederationRoomserverAPI interface {
 	// 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
+	QueryMembershipForSenderID(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID, res *QueryMembershipForUserResponse) error
 	QueryMembershipsForRoom(ctx context.Context, req *QueryMembershipsForRoomRequest, res *QueryMembershipsForRoomResponse) error
 	QueryRoomVersionForRoom(ctx context.Context, roomID string) (gomatrixserverlib.RoomVersion, error)
 	GetRoomIDForAlias(ctx context.Context, req *GetRoomIDForAliasRequest, res *GetRoomIDForAliasResponse) error
@@ -238,15 +239,13 @@ type FederationRoomserverAPI interface {
 	// Takes lists of PrevEventIDs and AuthEventsIDs and uses them to calculate
 	// the state and auth chain to return.
 	QueryStateAndAuthChain(ctx context.Context, req *QueryStateAndAuthChainRequest, res *QueryStateAndAuthChainResponse) error
-	// Query if we think we're still in a room.
-	QueryServerJoinedToRoom(ctx context.Context, req *QueryServerJoinedToRoomRequest, res *QueryServerJoinedToRoomResponse) error
 	QueryPublishedRooms(ctx context.Context, req *QueryPublishedRoomsRequest, res *QueryPublishedRoomsResponse) error
 	// Query missing events for a room from roomserver
 	QueryMissingEvents(ctx context.Context, req *QueryMissingEventsRequest, res *QueryMissingEventsResponse) error
 	// Query whether a server is allowed to see an event
-	QueryServerAllowedToSeeEvent(ctx context.Context, serverName spec.ServerName, eventID string) (allowed bool, err error)
+	QueryServerAllowedToSeeEvent(ctx context.Context, serverName spec.ServerName, eventID string, roomID string) (allowed bool, err error)
 	QueryRoomsForUser(ctx context.Context, req *QueryRoomsForUserRequest, res *QueryRoomsForUserResponse) error
-	QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (string, error)
+	QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (string, error)
 	PerformInboundPeek(ctx context.Context, req *PerformInboundPeekRequest, res *PerformInboundPeekResponse) error
 	HandleInvite(ctx context.Context, event *types.HeaderedEvent) error
 
@@ -254,12 +253,6 @@ type FederationRoomserverAPI interface {
 	// Query a given amount (or less) of events prior to a given set of events.
 	PerformBackfill(ctx context.Context, req *PerformBackfillRequest, res *PerformBackfillResponse) error
 
-	CurrentStateEvent(ctx context.Context, roomID spec.RoomID, eventType string, stateKey string) (gomatrixserverlib.PDU, error)
-	InvitePending(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (bool, error)
-	QueryRoomInfo(ctx context.Context, roomID spec.RoomID) (*types.RoomInfo, error)
-	UserJoinedToRoom(ctx context.Context, roomID types.RoomNID, userID spec.UserID) (bool, error)
-	LocallyJoinedUsers(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomNID types.RoomNID) ([]gomatrixserverlib.PDU, error)
-
 	IsKnownRoom(ctx context.Context, roomID spec.RoomID) (bool, error)
 	StateQuerier() gomatrixserverlib.StateQuerier
 }
diff --git a/roomserver/api/output.go b/roomserver/api/output.go
index 16b504957..852b64206 100644
--- a/roomserver/api/output.go
+++ b/roomserver/api/output.go
@@ -215,8 +215,10 @@ type OutputNewInviteEvent struct {
 type OutputRetireInviteEvent struct {
 	// The ID of the "m.room.member" invite event.
 	EventID string
-	// The target user ID of the "m.room.member" invite event that was retired.
-	TargetUserID string
+	// The room ID of the "m.room.member" invite event.
+	RoomID string
+	// The target sender ID of the "m.room.member" invite event that was retired.
+	TargetSenderID spec.SenderID
 	// Optional event ID of the event that replaced the invite.
 	// This can be empty if the invite was rejected locally and we were unable
 	// to reach the server that originally sent the invite.
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index 6cbaf5b19..b466b7ba8 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -41,8 +41,8 @@ type PerformJoinRequest struct {
 }
 
 type PerformLeaveRequest struct {
-	RoomID string `json:"room_id"`
-	UserID string `json:"user_id"`
+	RoomID string
+	Leaver spec.UserID
 }
 
 type PerformLeaveResponse struct {
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index d79dcebbb..684a5b0e3 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -113,9 +113,9 @@ type QueryEventsByIDResponse struct {
 // QueryMembershipForUserRequest is a request to QueryMembership
 type QueryMembershipForUserRequest struct {
 	// ID of the room to fetch membership from
-	RoomID string `json:"room_id"`
+	RoomID string
 	// ID of the user for whom membership is requested
-	UserID string `json:"user_id"`
+	UserID spec.UserID
 }
 
 // QueryMembershipForUserResponse is a response to QueryMembership
@@ -145,7 +145,7 @@ type QueryMembershipsForRoomRequest struct {
 	// Optional - ID of the user sending the request, for checking if the
 	// user is allowed to see the memberships. If not specified then all
 	// room memberships will be returned.
-	Sender string `json:"sender"`
+	SenderID spec.SenderID `json:"sender"`
 }
 
 // QueryMembershipsForRoomResponse is a response to QueryMembershipsForRoom
@@ -448,11 +448,11 @@ func (rq *JoinRoomQuerier) CurrentStateEvent(ctx context.Context, roomID spec.Ro
 	return rq.Roomserver.CurrentStateEvent(ctx, roomID, eventType, stateKey)
 }
 
-func (rq *JoinRoomQuerier) InvitePending(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (bool, error) {
-	return rq.Roomserver.InvitePending(ctx, roomID, userID)
+func (rq *JoinRoomQuerier) InvitePending(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (bool, error) {
+	return rq.Roomserver.InvitePending(ctx, roomID, senderID)
 }
 
-func (rq *JoinRoomQuerier) RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, userID spec.UserID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error) {
+func (rq *JoinRoomQuerier) RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error) {
 	roomInfo, err := rq.Roomserver.QueryRoomInfo(ctx, roomID)
 	if err != nil || roomInfo == nil || roomInfo.IsStub() {
 		return nil, err
@@ -468,7 +468,7 @@ func (rq *JoinRoomQuerier) RestrictedRoomJoinInfo(ctx context.Context, roomID sp
 		return nil, fmt.Errorf("InternalServerError: Failed to query room: %w", err)
 	}
 
-	userJoinedToRoom, err := rq.Roomserver.UserJoinedToRoom(ctx, types.RoomNID(roomInfo.RoomNID), userID)
+	userJoinedToRoom, err := rq.Roomserver.UserJoinedToRoom(ctx, types.RoomNID(roomInfo.RoomNID), senderID)
 	if err != nil {
 		util.GetLogger(ctx).WithError(err).Error("rsAPI.UserJoinedToRoom failed")
 		return nil, fmt.Errorf("InternalServerError: %w", err)
@@ -492,12 +492,8 @@ type MembershipQuerier struct {
 }
 
 func (mq *MembershipQuerier) CurrentMembership(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (string, error) {
-	req := QueryMembershipForUserRequest{
-		RoomID: roomID.String(),
-		UserID: string(senderID),
-	}
 	res := QueryMembershipForUserResponse{}
-	err := mq.Roomserver.QueryMembershipForUser(ctx, &req, &res)
+	err := mq.Roomserver.QueryMembershipForSenderID(ctx, roomID, senderID, &res)
 
 	membership := ""
 	if err == nil {
diff --git a/roomserver/auth/auth.go b/roomserver/auth/auth.go
index b6168d38b..ba10a4332 100644
--- a/roomserver/auth/auth.go
+++ b/roomserver/auth/auth.go
@@ -13,6 +13,9 @@
 package auth
 
 import (
+	"context"
+
+	"github.com/matrix-org/dendrite/roomserver/storage"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 )
@@ -22,6 +25,7 @@ import (
 // IsServerAllowed returns true if the server is allowed to see events in the room
 // at this particular state. This function implements https://matrix.org/docs/spec/client_server/r0.6.0#id87
 func IsServerAllowed(
+	ctx context.Context, db storage.RoomDatabase,
 	serverName spec.ServerName,
 	serverCurrentlyInRoom bool,
 	authEvents []gomatrixserverlib.PDU,
@@ -37,7 +41,7 @@ func IsServerAllowed(
 		return true
 	}
 	// 2. If the user's membership was join, allow.
-	joinedUserExists := IsAnyUserOnServerWithMembership(serverName, authEvents, spec.Join)
+	joinedUserExists := IsAnyUserOnServerWithMembership(ctx, db, serverName, authEvents, spec.Join)
 	if joinedUserExists {
 		return true
 	}
@@ -46,7 +50,7 @@ func IsServerAllowed(
 		return true
 	}
 	// 4. If the user's membership was invite, and the history_visibility was set to invited, allow.
-	invitedUserExists := IsAnyUserOnServerWithMembership(serverName, authEvents, spec.Invite)
+	invitedUserExists := IsAnyUserOnServerWithMembership(ctx, db, serverName, authEvents, spec.Invite)
 	if invitedUserExists && historyVisibility == gomatrixserverlib.HistoryVisibilityInvited {
 		return true
 	}
@@ -70,7 +74,7 @@ func HistoryVisibilityForRoom(authEvents []gomatrixserverlib.PDU) gomatrixserver
 	return visibility
 }
 
-func IsAnyUserOnServerWithMembership(serverName spec.ServerName, authEvents []gomatrixserverlib.PDU, wantMembership string) bool {
+func IsAnyUserOnServerWithMembership(ctx context.Context, db storage.RoomDatabase, serverName spec.ServerName, authEvents []gomatrixserverlib.PDU, wantMembership string) bool {
 	for _, ev := range authEvents {
 		if ev.Type() != spec.MRoomMember {
 			continue
@@ -85,12 +89,12 @@ func IsAnyUserOnServerWithMembership(serverName spec.ServerName, authEvents []go
 			continue
 		}
 
-		_, domain, err := gomatrixserverlib.SplitID('@', *stateKey)
+		userID, err := db.GetUserIDForSender(ctx, ev.RoomID(), spec.SenderID(*stateKey))
 		if err != nil {
 			continue
 		}
 
-		if domain == serverName {
+		if userID.Domain() == serverName {
 			return true
 		}
 	}
diff --git a/roomserver/auth/auth_test.go b/roomserver/auth/auth_test.go
index e3eea5d8b..192d9e5da 100644
--- a/roomserver/auth/auth_test.go
+++ b/roomserver/auth/auth_test.go
@@ -1,13 +1,23 @@
 package auth
 
 import (
+	"context"
 	"testing"
 
+	"github.com/matrix-org/dendrite/roomserver/storage"
 	"github.com/matrix-org/dendrite/test"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
+type FakeStorageDB struct {
+	storage.RoomDatabase
+}
+
+func (f *FakeStorageDB) GetUserIDForSender(ctx context.Context, roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+	return spec.NewUserID(string(senderID), true)
+}
+
 func TestIsServerAllowed(t *testing.T) {
 	alice := test.NewUser(t)
 
@@ -77,7 +87,7 @@ func TestIsServerAllowed(t *testing.T) {
 				authEvents = append(authEvents, ev.PDU)
 			}
 
-			if got := IsServerAllowed(tt.serverName, tt.serverCurrentlyInRoom, authEvents); got != tt.want {
+			if got := IsServerAllowed(context.Background(), &FakeStorageDB{}, tt.serverName, tt.serverCurrentlyInRoom, authEvents); got != tt.want {
 				t.Errorf("IsServerAllowed() = %v, want %v", got, tt.want)
 			}
 		})
diff --git a/roomserver/internal/helpers/helpers.go b/roomserver/internal/helpers/helpers.go
index 95397cd5e..263cb9f85 100644
--- a/roomserver/internal/helpers/helpers.go
+++ b/roomserver/internal/helpers/helpers.go
@@ -6,7 +6,6 @@ import (
 	"errors"
 	"fmt"
 	"sort"
-	"strings"
 
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
@@ -55,9 +54,10 @@ func UpdateToInviteMembership(
 			Type: api.OutputTypeRetireInviteEvent,
 			RetireInviteEvent: &api.OutputRetireInviteEvent{
 				EventID:          eventID,
+				RoomID:           add.RoomID(),
 				Membership:       spec.Join,
 				RetiredByEventID: add.EventID(),
-				TargetUserID:     *add.StateKey(),
+				TargetSenderID:   spec.SenderID(*add.StateKey()),
 			},
 		})
 	}
@@ -94,13 +94,13 @@ func IsServerCurrentlyInRoom(ctx context.Context, db storage.Database, serverNam
 	for i := range events {
 		gmslEvents[i] = events[i].PDU
 	}
-	return auth.IsAnyUserOnServerWithMembership(serverName, gmslEvents, spec.Join), nil
+	return auth.IsAnyUserOnServerWithMembership(ctx, db, serverName, gmslEvents, spec.Join), nil
 }
 
 func IsInvitePending(
 	ctx context.Context, db storage.Database,
-	roomID, userID string,
-) (bool, string, string, gomatrixserverlib.PDU, error) {
+	roomID string, senderID spec.SenderID,
+) (bool, spec.SenderID, string, gomatrixserverlib.PDU, error) {
 	// Look up the room NID for the supplied room ID.
 	info, err := db.RoomInfo(ctx, roomID)
 	if err != nil {
@@ -111,13 +111,13 @@ func IsInvitePending(
 	}
 
 	// Look up the state key NID for the supplied user ID.
-	targetUserNIDs, err := db.EventStateKeyNIDs(ctx, []string{userID})
+	targetUserNIDs, err := db.EventStateKeyNIDs(ctx, []string{string(senderID)})
 	if err != nil {
 		return false, "", "", nil, fmt.Errorf("r.DB.EventStateKeyNIDs: %w", err)
 	}
-	targetUserNID, targetUserFound := targetUserNIDs[userID]
+	targetUserNID, targetUserFound := targetUserNIDs[string(senderID)]
 	if !targetUserFound {
-		return false, "", "", nil, fmt.Errorf("missing NID for user %q (%+v)", userID, targetUserNIDs)
+		return false, "", "", nil, fmt.Errorf("missing NID for user %q (%+v)", senderID, targetUserNIDs)
 	}
 
 	// Let's see if we have an event active for the user in the room. If
@@ -156,7 +156,7 @@ func IsInvitePending(
 
 	event, err := verImpl.NewEventFromTrustedJSON(eventJSON, false)
 
-	return true, senderUser, userNIDToEventID[senderUserNIDs[0]], event, err
+	return true, spec.SenderID(senderUser), userNIDToEventID[senderUserNIDs[0]], event, err
 }
 
 // GetMembershipsAtState filters the state events to
@@ -264,7 +264,7 @@ func LoadStateEvents(
 }
 
 func CheckServerAllowedToSeeEvent(
-	ctx context.Context, db storage.Database, info *types.RoomInfo, eventID string, serverName spec.ServerName, isServerInRoom bool,
+	ctx context.Context, db storage.Database, info *types.RoomInfo, roomID string, eventID string, serverName spec.ServerName, isServerInRoom bool,
 ) (bool, error) {
 	stateAtEvent, err := db.GetHistoryVisibilityState(ctx, info, eventID, string(serverName))
 	switch err {
@@ -273,7 +273,7 @@ func CheckServerAllowedToSeeEvent(
 	case tables.OptimisationNotSupportedError:
 		// The database engine didn't support this optimisation, so fall back to using
 		// the old and slow method
-		stateAtEvent, err = slowGetHistoryVisibilityState(ctx, db, info, eventID, serverName)
+		stateAtEvent, err = slowGetHistoryVisibilityState(ctx, db, info, roomID, eventID, serverName)
 		if err != nil {
 			return false, err
 		}
@@ -288,11 +288,11 @@ func CheckServerAllowedToSeeEvent(
 			return false, err
 		}
 	}
-	return auth.IsServerAllowed(serverName, isServerInRoom, stateAtEvent), nil
+	return auth.IsServerAllowed(ctx, db, serverName, isServerInRoom, stateAtEvent), nil
 }
 
 func slowGetHistoryVisibilityState(
-	ctx context.Context, db storage.Database, info *types.RoomInfo, eventID string, serverName spec.ServerName,
+	ctx context.Context, db storage.Database, info *types.RoomInfo, roomID, eventID string, serverName spec.ServerName,
 ) ([]gomatrixserverlib.PDU, error) {
 	roomState := state.NewStateResolution(db, info)
 	stateEntries, err := roomState.LoadStateAtEvent(ctx, eventID)
@@ -319,8 +319,13 @@ func slowGetHistoryVisibilityState(
 	// then we'll filter it out. This does preserve state keys that
 	// are "" since these will contain history visibility etc.
 	for nid, key := range stateKeys {
-		if key != "" && !strings.HasSuffix(key, ":"+string(serverName)) {
-			delete(stateKeys, nid)
+		if key != "" {
+			userID, err := db.GetUserIDForSender(ctx, roomID, spec.SenderID(key))
+			if err == nil && userID != nil {
+				if userID.Domain() != serverName {
+					delete(stateKeys, nid)
+				}
+			}
 		}
 	}
 
@@ -410,7 +415,7 @@ BFSLoop:
 				// hasn't been seen before.
 				if !visited[pre] {
 					visited[pre] = true
-					allowed, err = CheckServerAllowedToSeeEvent(ctx, db, info, pre, serverName, isServerInRoom)
+					allowed, err = CheckServerAllowedToSeeEvent(ctx, db, info, ev.RoomID(), pre, serverName, isServerInRoom)
 					if err != nil {
 						util.GetLogger(ctx).WithField("server", serverName).WithField("event_id", pre).WithError(err).Error(
 							"Error checking if allowed to see event",
diff --git a/roomserver/internal/helpers/helpers_test.go b/roomserver/internal/helpers/helpers_test.go
index f1896277e..1cef83df7 100644
--- a/roomserver/internal/helpers/helpers_test.go
+++ b/roomserver/internal/helpers/helpers_test.go
@@ -8,6 +8,7 @@ import (
 	"github.com/matrix-org/dendrite/internal/caching"
 	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/stretchr/testify/assert"
 
 	"github.com/matrix-org/dendrite/roomserver/types"
@@ -58,12 +59,12 @@ func TestIsInvitePendingWithoutNID(t *testing.T) {
 		}
 
 		// Alice should have no pending invites and should have a NID
-		pendingInvite, _, _, _, err := IsInvitePending(context.Background(), db, room.ID, alice.ID)
+		pendingInvite, _, _, _, err := IsInvitePending(context.Background(), db, room.ID, spec.SenderID(alice.ID))
 		assert.NoError(t, err, "failed to get pending invites")
 		assert.False(t, pendingInvite, "unexpected pending invite")
 
 		// Bob should have no pending invites and receive a new NID
-		pendingInvite, _, _, _, err = IsInvitePending(context.Background(), db, room.ID, bob.ID)
+		pendingInvite, _, _, _, err = IsInvitePending(context.Background(), db, room.ID, spec.SenderID(bob.ID))
 		assert.NoError(t, err, "failed to get pending invites")
 		assert.False(t, pendingInvite, "unexpected pending invite")
 	})
diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 1f273da01..7bb401632 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -842,17 +842,15 @@ 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())
+		memberUserID, err := r.Queryer.QueryUserIDForSender(ctx, memberEvent.RoomID(), spec.SenderID(*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,
-			ServerName: senderDomain,
+			Localpart:  memberUserID.Local(),
+			ServerName: memberUserID.Domain(),
 		}, accountRes); err != nil {
 			return err
 		}
@@ -896,8 +894,8 @@ func (r *Inputer) kickGuests(ctx context.Context, event gomatrixserverlib.PDU, r
 		inputEvents = append(inputEvents, api.InputRoomEvent{
 			Kind:         api.KindNew,
 			Event:        event,
-			Origin:       senderDomain,
-			SendAsServer: string(senderDomain),
+			Origin:       memberUserID.Domain(),
+			SendAsServer: string(memberUserID.Domain()),
 		})
 		prevEvents = []string{event.EventID()}
 	}
diff --git a/roomserver/internal/input/input_membership.go b/roomserver/internal/input/input_membership.go
index 98d7d13b1..09c65dfe9 100644
--- a/roomserver/internal/input/input_membership.go
+++ b/roomserver/internal/input/input_membership.go
@@ -18,7 +18,6 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 
 	"github.com/matrix-org/dendrite/internal"
@@ -72,7 +71,7 @@ func (r *Inputer) updateMemberships(
 		if change.addedEventNID != 0 {
 			ae, _ = helpers.EventMap(events).Lookup(change.addedEventNID)
 		}
-		if updates, err = r.updateMembership(updater, targetUserNID, re, ae, updates); err != nil {
+		if updates, err = r.updateMembership(ctx, updater, targetUserNID, re, ae, updates); err != nil {
 			return nil, err
 		}
 	}
@@ -80,6 +79,7 @@ func (r *Inputer) updateMemberships(
 }
 
 func (r *Inputer) updateMembership(
+	ctx context.Context,
 	updater *shared.RoomUpdater,
 	targetUserNID types.EventStateKeyNID,
 	remove, add *types.Event,
@@ -97,7 +97,7 @@ func (r *Inputer) updateMembership(
 
 	var targetLocal bool
 	if add != nil {
-		targetLocal = r.isLocalTarget(add)
+		targetLocal = r.isLocalTarget(ctx, add)
 	}
 
 	mu, err := updater.MembershipUpdater(targetUserNID, targetLocal)
@@ -136,11 +136,14 @@ func (r *Inputer) updateMembership(
 	}
 }
 
-func (r *Inputer) isLocalTarget(event *types.Event) bool {
+func (r *Inputer) isLocalTarget(ctx context.Context, event *types.Event) bool {
 	isTargetLocalUser := false
 	if statekey := event.StateKey(); statekey != nil {
-		_, domain, _ := gomatrixserverlib.SplitID('@', *statekey)
-		isTargetLocalUser = domain == r.ServerName
+		userID, err := r.Queryer.QueryUserIDForSender(ctx, event.RoomID(), spec.SenderID(*statekey))
+		if err != nil || userID == nil {
+			return isTargetLocalUser
+		}
+		isTargetLocalUser = userID.Domain() == r.ServerName
 	}
 	return isTargetLocalUser
 }
@@ -161,9 +164,10 @@ func updateToJoinMembership(
 			Type: api.OutputTypeRetireInviteEvent,
 			RetireInviteEvent: &api.OutputRetireInviteEvent{
 				EventID:          eventID,
+				RoomID:           add.RoomID(),
 				Membership:       spec.Join,
 				RetiredByEventID: add.EventID(),
-				TargetUserID:     *add.StateKey(),
+				TargetSenderID:   spec.SenderID(*add.StateKey()),
 			},
 		})
 	}
@@ -187,9 +191,10 @@ func updateToLeaveMembership(
 			Type: api.OutputTypeRetireInviteEvent,
 			RetireInviteEvent: &api.OutputRetireInviteEvent{
 				EventID:          eventID,
+				RoomID:           add.RoomID(),
 				Membership:       newMembership,
 				RetiredByEventID: add.EventID(),
-				TargetUserID:     *add.StateKey(),
+				TargetSenderID:   spec.SenderID(*add.StateKey()),
 			},
 		})
 	}
diff --git a/roomserver/internal/perform/perform_admin.go b/roomserver/internal/perform/perform_admin.go
index eeb1ac406..ec13bff87 100644
--- a/roomserver/internal/perform/perform_admin.go
+++ b/roomserver/internal/perform/perform_admin.go
@@ -149,11 +149,11 @@ func (r *Admin) PerformAdminEvacuateUser(
 	ctx context.Context,
 	userID string,
 ) (affected []string, err error) {
-	_, domain, err := gomatrixserverlib.SplitID('@', userID)
+	fullUserID, err := spec.NewUserID(userID, true)
 	if err != nil {
 		return nil, err
 	}
-	if !r.Cfg.Matrix.IsLocalServerName(domain) {
+	if !r.Cfg.Matrix.IsLocalServerName(fullUserID.Domain()) {
 		return nil, fmt.Errorf("can only evacuate local users using this endpoint")
 	}
 
@@ -172,7 +172,7 @@ func (r *Admin) PerformAdminEvacuateUser(
 	for _, roomID := range allRooms {
 		leaveReq := &api.PerformLeaveRequest{
 			RoomID: roomID,
-			UserID: userID,
+			Leaver: *fullUserID,
 		}
 		leaveRes := &api.PerformLeaveResponse{}
 		outputEvents, err := r.Leaver.PerformLeave(ctx, leaveReq, leaveRes)
diff --git a/roomserver/internal/perform/perform_backfill.go b/roomserver/internal/perform/perform_backfill.go
index 388150936..8e87359a3 100644
--- a/roomserver/internal/perform/perform_backfill.go
+++ b/roomserver/internal/perform/perform_backfill.go
@@ -582,7 +582,7 @@ func joinEventsFromHistoryVisibility(
 	}
 
 	// Can we see events in the room?
-	canSeeEvents := auth.IsServerAllowed(thisServer, true, events)
+	canSeeEvents := auth.IsServerAllowed(ctx, db, thisServer, true, events)
 	visibility := auth.HistoryVisibilityForRoom(events)
 	if !canSeeEvents {
 		logrus.Infof("ServersAtEvent history not visible to us: %s", visibility)
diff --git a/roomserver/internal/perform/perform_create_room.go b/roomserver/internal/perform/perform_create_room.go
index a3ba20f70..475418aa3 100644
--- a/roomserver/internal/perform/perform_create_room.go
+++ b/roomserver/internal/perform/perform_create_room.go
@@ -63,9 +63,17 @@ func (c *Creator) PerformCreateRoom(ctx context.Context, userID spec.UserID, roo
 			}
 		}
 	}
-	createContent["creator"] = userID.String()
+	senderID, err := c.DB.GetSenderIDForUser(ctx, roomID.String(), userID)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Error("Failed getting senderID for user")
+		return "", &util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: spec.InternalServerError{},
+		}
+	}
+	createContent["creator"] = senderID
 	createContent["room_version"] = createRequest.RoomVersion
-	powerLevelContent := eventutil.InitialPowerLevelsContent(userID.String())
+	powerLevelContent := eventutil.InitialPowerLevelsContent(string(senderID))
 	joinRuleContent := gomatrixserverlib.JoinRuleContent{
 		JoinRule: spec.Invite,
 	}
@@ -121,7 +129,7 @@ func (c *Creator) PerformCreateRoom(ctx context.Context, userID spec.UserID, roo
 	}
 	membershipEvent := gomatrixserverlib.FledglingEvent{
 		Type:     spec.MRoomMember,
-		StateKey: userID.String(),
+		StateKey: string(senderID),
 		Content: gomatrixserverlib.MemberContent{
 			Membership:  spec.Join,
 			DisplayName: createRequest.UserDisplayName,
@@ -270,7 +278,6 @@ func (c *Creator) PerformCreateRoom(ctx context.Context, userID spec.UserID, roo
 
 	var builtEvents []*types.HeaderedEvent
 	authEvents := gomatrixserverlib.NewAuthEvents(nil)
-	senderID, err := c.RSAPI.QuerySenderIDForUser(ctx, roomID.String(), userID)
 	if err != nil {
 		util.GetLogger(ctx).WithError(err).Error("rsapi.QuerySenderIDForUser failed")
 		return "", &util.JSONResponse{
diff --git a/roomserver/internal/perform/perform_invite.go b/roomserver/internal/perform/perform_invite.go
index 56ee16065..1440daad4 100644
--- a/roomserver/internal/perform/perform_invite.go
+++ b/roomserver/internal/perform/perform_invite.go
@@ -134,12 +134,12 @@ func (r *Inviter) PerformInvite(
 		return api.ErrInvalidID{Err: fmt.Errorf("the invite must be from a local user")}
 	}
 
-	if event.StateKey() == nil {
+	if event.StateKey() == nil || *event.StateKey() == "" {
 		return fmt.Errorf("invite must be a state event")
 	}
-	invitedUser, err := spec.NewUserID(*event.StateKey(), true)
-	if err != nil {
-		return spec.InvalidParam("The user ID is invalid")
+	invitedUser, err := r.RSAPI.QueryUserIDForSender(ctx, event.RoomID(), spec.SenderID(*event.StateKey()))
+	if err != nil || invitedUser == nil {
+		return spec.InvalidParam("Could not find the matching senderID for this user")
 	}
 	isTargetLocal := r.Cfg.Matrix.IsLocalServerName(invitedUser.Domain())
 
diff --git a/roomserver/internal/perform/perform_join.go b/roomserver/internal/perform/perform_join.go
index d41cc214b..83c3b7c3e 100644
--- a/roomserver/internal/perform/perform_join.go
+++ b/roomserver/internal/perform/perform_join.go
@@ -162,7 +162,7 @@ func (r *Joiner) performJoinRoomByID(
 	}
 
 	// Get the domain part of the room ID.
-	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
+	roomID, err := spec.NewRoomID(req.RoomIDOrAlias)
 	if err != nil {
 		return "", "", rsAPI.ErrInvalidID{Err: fmt.Errorf("room ID %q is invalid: %w", req.RoomIDOrAlias, err)}
 	}
@@ -170,8 +170,8 @@ func (r *Joiner) performJoinRoomByID(
 	// If the server name in the room ID isn't ours then it's a
 	// possible candidate for finding the room via federation. Add
 	// it to the list of servers to try.
-	if !r.Cfg.Matrix.IsLocalServerName(domain) {
-		req.ServerNames = append(req.ServerNames, domain)
+	if !r.Cfg.Matrix.IsLocalServerName(roomID.Domain()) {
+		req.ServerNames = append(req.ServerNames, roomID.Domain())
 	}
 
 	// Prepare the template for the join event.
@@ -203,7 +203,7 @@ func (r *Joiner) performJoinRoomByID(
 		req.Content = map[string]interface{}{}
 	}
 	req.Content["membership"] = spec.Join
-	if authorisedVia, aerr := r.populateAuthorisedViaUserForRestrictedJoin(ctx, req); aerr != nil {
+	if authorisedVia, aerr := r.populateAuthorisedViaUserForRestrictedJoin(ctx, req, senderID); aerr != nil {
 		return "", "", aerr
 	} else if authorisedVia != "" {
 		req.Content["join_authorised_via_users_server"] = authorisedVia
@@ -226,17 +226,17 @@ func (r *Joiner) performJoinRoomByID(
 
 	// Force a federated join if we're dealing with a pending invite
 	// and we aren't in the room.
-	isInvitePending, inviteSender, _, inviteEvent, err := helpers.IsInvitePending(ctx, r.DB, req.RoomIDOrAlias, req.UserID)
+	isInvitePending, inviteSender, _, inviteEvent, err := helpers.IsInvitePending(ctx, r.DB, req.RoomIDOrAlias, senderID)
 	if err == nil && !serverInRoom && isInvitePending {
-		_, inviterDomain, ierr := gomatrixserverlib.SplitID('@', inviteSender)
-		if ierr != nil {
-			return "", "", fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
+		inviter, queryErr := r.RSAPI.QueryUserIDForSender(ctx, req.RoomIDOrAlias, inviteSender)
+		if queryErr != nil {
+			return "", "", fmt.Errorf("r.RSAPI.QueryUserIDForSender: %w", queryErr)
 		}
 
 		// If we were invited by someone from another server then we can
 		// assume they are in the room so we can join via them.
-		if !r.Cfg.Matrix.IsLocalServerName(inviterDomain) {
-			req.ServerNames = append(req.ServerNames, inviterDomain)
+		if inviter != nil && !r.Cfg.Matrix.IsLocalServerName(inviter.Domain()) {
+			req.ServerNames = append(req.ServerNames, inviter.Domain())
 			forceFederatedJoin = true
 			memberEvent := gjson.Parse(string(inviteEvent.JSON()))
 			// only set unsigned if we've got a content.membership, which we _should_
@@ -298,12 +298,8 @@ func (r *Joiner) performJoinRoomByID(
 		// a member of the room. This is best-effort (as in we won't
 		// fail if we can't find the existing membership) because there
 		// is really no harm in just sending another membership event.
-		membershipReq := &api.QueryMembershipForUserRequest{
-			RoomID: req.RoomIDOrAlias,
-			UserID: userID.String(),
-		}
 		membershipRes := &api.QueryMembershipForUserResponse{}
-		_ = r.Queryer.QueryMembershipForUser(ctx, membershipReq, membershipRes)
+		_ = r.Queryer.QueryMembershipForSenderID(ctx, *roomID, senderID, membershipRes)
 
 		// If we haven't already joined the room then send an event
 		// into the room changing our membership status.
@@ -328,7 +324,7 @@ func (r *Joiner) performJoinRoomByID(
 		// The room doesn't exist locally. If the room ID looks like it should
 		// be ours then this probably means that we've nuked our database at
 		// some point.
-		if r.Cfg.Matrix.IsLocalServerName(domain) {
+		if r.Cfg.Matrix.IsLocalServerName(roomID.Domain()) {
 			// If there are no more server names to try then give up here.
 			// Otherwise we'll try a federated join as normal, since it's quite
 			// possible that the room still exists on other servers.
@@ -376,15 +372,12 @@ func (r *Joiner) performFederatedJoinRoomByID(
 func (r *Joiner) populateAuthorisedViaUserForRestrictedJoin(
 	ctx context.Context,
 	joinReq *rsAPI.PerformJoinRequest,
+	senderID spec.SenderID,
 ) (string, error) {
 	roomID, err := spec.NewRoomID(joinReq.RoomIDOrAlias)
 	if err != nil {
 		return "", err
 	}
-	userID, err := spec.NewUserID(joinReq.UserID, true)
-	if err != nil {
-		return "", err
-	}
 
-	return r.Queryer.QueryRestrictedJoinAllowed(ctx, *roomID, *userID)
+	return r.Queryer.QueryRestrictedJoinAllowed(ctx, *roomID, senderID)
 }
diff --git a/roomserver/internal/perform/perform_leave.go b/roomserver/internal/perform/perform_leave.go
index 094537f8b..1b23cc1ff 100644
--- a/roomserver/internal/perform/perform_leave.go
+++ b/roomserver/internal/perform/perform_leave.go
@@ -53,16 +53,12 @@ func (r *Leaver) PerformLeave(
 	req *api.PerformLeaveRequest,
 	res *api.PerformLeaveResponse,
 ) ([]api.OutputEvent, error) {
-	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
-	if err != nil {
-		return nil, fmt.Errorf("supplied user ID %q in incorrect format", req.UserID)
-	}
-	if !r.Cfg.Matrix.IsLocalServerName(domain) {
-		return nil, fmt.Errorf("user %q does not belong to this homeserver", req.UserID)
+	if !r.Cfg.Matrix.IsLocalServerName(req.Leaver.Domain()) {
+		return nil, fmt.Errorf("user %q does not belong to this homeserver", req.Leaver.String())
 	}
 	logger := logrus.WithContext(ctx).WithFields(logrus.Fields{
 		"room_id": req.RoomID,
-		"user_id": req.UserID,
+		"user_id": req.Leaver.String(),
 	})
 	logger.Info("User requested to leave join")
 	if strings.HasPrefix(req.RoomID, "!") {
@@ -82,21 +78,26 @@ func (r *Leaver) performLeaveRoomByID(
 	req *api.PerformLeaveRequest,
 	res *api.PerformLeaveResponse, // nolint:unparam
 ) ([]api.OutputEvent, error) {
+	leaver, err := r.RSAPI.QuerySenderIDForUser(ctx, req.RoomID, req.Leaver)
+	if err != nil {
+		return nil, fmt.Errorf("leaver %s has no matching senderID in this room", req.Leaver.String())
+	}
+
 	// If there's an invite outstanding for the room then respond to
 	// that.
-	isInvitePending, senderUser, eventID, _, err := helpers.IsInvitePending(ctx, r.DB, req.RoomID, req.UserID)
+	isInvitePending, senderUser, eventID, _, err := helpers.IsInvitePending(ctx, r.DB, req.RoomID, leaver)
 	if err == nil && isInvitePending {
-		_, senderDomain, serr := gomatrixserverlib.SplitID('@', senderUser)
-		if serr != nil {
-			return nil, fmt.Errorf("sender %q is invalid", senderUser)
+		sender, serr := r.RSAPI.QueryUserIDForSender(ctx, req.RoomID, senderUser)
+		if serr != nil || sender == nil {
+			return nil, fmt.Errorf("sender %q has no matching userID", senderUser)
 		}
-		if !r.Cfg.Matrix.IsLocalServerName(senderDomain) {
-			return r.performFederatedRejectInvite(ctx, req, res, senderUser, eventID)
+		if !r.Cfg.Matrix.IsLocalServerName(sender.Domain()) {
+			return r.performFederatedRejectInvite(ctx, req, res, *sender, eventID, leaver)
 		}
 		// check that this is not a "server notice room"
 		accData := &userapi.QueryAccountDataResponse{}
 		if err = r.UserAPI.QueryAccountData(ctx, &userapi.QueryAccountDataRequest{
-			UserID:   req.UserID,
+			UserID:   req.Leaver.String(),
 			RoomID:   req.RoomID,
 			DataType: "m.tag",
 		}, accData); err != nil {
@@ -127,7 +128,7 @@ func (r *Leaver) performLeaveRoomByID(
 		StateToFetch: []gomatrixserverlib.StateKeyTuple{
 			{
 				EventType: spec.MRoomMember,
-				StateKey:  req.UserID,
+				StateKey:  string(leaver),
 			},
 		},
 	}
@@ -141,26 +142,18 @@ func (r *Leaver) performLeaveRoomByID(
 
 	// Now let's see if the user is in the room.
 	if len(latestRes.StateEvents) == 0 {
-		return nil, fmt.Errorf("user %q is not a member of room %q", req.UserID, req.RoomID)
+		return nil, fmt.Errorf("user %q is not a member of room %q", req.Leaver.String(), req.RoomID)
 	}
 	membership, err := latestRes.StateEvents[0].Membership()
 	if err != nil {
 		return nil, fmt.Errorf("error getting membership: %w", err)
 	}
 	if membership != spec.Join && membership != spec.Invite {
-		return nil, fmt.Errorf("user %q is not joined to the room (membership is %q)", req.UserID, membership)
+		return nil, fmt.Errorf("user %q is not joined to the room (membership is %q)", req.Leaver.String(), membership)
 	}
 
 	// Prepare the template for the leave event.
-	fullUserID, err := spec.NewUserID(req.UserID, true)
-	if err != nil {
-		return nil, err
-	}
-	senderID, err := r.RSAPI.QuerySenderIDForUser(ctx, req.RoomID, *fullUserID)
-	if err != nil {
-		return nil, err
-	}
-	senderIDString := string(senderID)
+	senderIDString := string(leaver)
 	proto := gomatrixserverlib.ProtoEvent{
 		Type:     spec.MRoomMember,
 		SenderID: senderIDString,
@@ -175,16 +168,13 @@ func (r *Leaver) performLeaveRoomByID(
 		return nil, fmt.Errorf("eb.SetUnsigned: %w", err)
 	}
 
-	// Get the sender domain.
-	senderDomain := fullUserID.Domain()
-
 	// We know that the user is in the room at this point so let's build
 	// a leave event.
 	// TODO: Check what happens if the room exists on the server
 	// but everyone has since left. I suspect it does the wrong thing.
 
 	var buildRes rsAPI.QueryLatestEventsAndStateResponse
-	identity, err := r.Cfg.Matrix.SigningIdentityFor(senderDomain)
+	identity, err := r.Cfg.Matrix.SigningIdentityFor(req.Leaver.Domain())
 	if err != nil {
 		return nil, fmt.Errorf("SigningIdentityFor: %w", err)
 	}
@@ -201,8 +191,8 @@ func (r *Leaver) performLeaveRoomByID(
 			{
 				Kind:         api.KindNew,
 				Event:        event,
-				Origin:       senderDomain,
-				SendAsServer: string(senderDomain),
+				Origin:       req.Leaver.Domain(),
+				SendAsServer: string(req.Leaver.Domain()),
 			},
 		},
 	}
@@ -219,21 +209,17 @@ func (r *Leaver) performFederatedRejectInvite(
 	ctx context.Context,
 	req *api.PerformLeaveRequest,
 	res *api.PerformLeaveResponse, // nolint:unparam
-	senderUser, eventID string,
+	inviteSender spec.UserID, eventID string,
+	leaver spec.SenderID,
 ) ([]api.OutputEvent, error) {
-	_, domain, err := gomatrixserverlib.SplitID('@', senderUser)
-	if err != nil {
-		return nil, fmt.Errorf("user ID %q invalid: %w", senderUser, err)
-	}
-
 	// Ask the federation sender to perform a federated leave for us.
 	leaveReq := fsAPI.PerformLeaveRequest{
 		RoomID:      req.RoomID,
-		UserID:      req.UserID,
-		ServerNames: []spec.ServerName{domain},
+		UserID:      req.Leaver.String(),
+		ServerNames: []spec.ServerName{inviteSender.Domain()},
 	}
 	leaveRes := fsAPI.PerformLeaveResponse{}
-	if err = r.FSAPI.PerformLeave(ctx, &leaveReq, &leaveRes); err != nil {
+	if err := r.FSAPI.PerformLeave(ctx, &leaveReq, &leaveRes); err != nil {
 		// failures in PerformLeave should NEVER stop us from telling other components like the
 		// sync API that the invite was withdrawn. Otherwise we can end up with stuck invites.
 		util.GetLogger(ctx).WithError(err).Errorf("failed to PerformLeave, still retiring invite event")
@@ -244,7 +230,7 @@ func (r *Leaver) performFederatedRejectInvite(
 		util.GetLogger(ctx).WithError(err).Errorf("failed to get RoomInfo, still retiring invite event")
 	}
 
-	updater, err := r.DB.MembershipUpdater(ctx, req.RoomID, req.UserID, true, info.RoomVersion)
+	updater, err := r.DB.MembershipUpdater(ctx, req.RoomID, string(leaver), true, info.RoomVersion)
 	if err != nil {
 		util.GetLogger(ctx).WithError(err).Errorf("failed to get MembershipUpdater, still retiring invite event")
 	}
@@ -267,9 +253,10 @@ func (r *Leaver) performFederatedRejectInvite(
 		{
 			Type: api.OutputTypeRetireInviteEvent,
 			RetireInviteEvent: &api.OutputRetireInviteEvent{
-				EventID:      eventID,
-				Membership:   "leave",
-				TargetUserID: req.UserID,
+				EventID:        eventID,
+				RoomID:         req.RoomID,
+				Membership:     "leave",
+				TargetSenderID: leaver,
 			},
 		},
 	}, nil
diff --git a/roomserver/internal/perform/perform_upgrade.go b/roomserver/internal/perform/perform_upgrade.go
index 5710352bb..1aaa42c94 100644
--- a/roomserver/internal/perform/perform_upgrade.go
+++ b/roomserver/internal/perform/perform_upgrade.go
@@ -38,19 +38,15 @@ type Upgrader struct {
 // PerformRoomUpgrade upgrades a room from one version to another
 func (r *Upgrader) PerformRoomUpgrade(
 	ctx context.Context,
-	roomID, userID string, roomVersion gomatrixserverlib.RoomVersion,
+	roomID string, userID spec.UserID, roomVersion gomatrixserverlib.RoomVersion,
 ) (newRoomID string, err error) {
 	return r.performRoomUpgrade(ctx, roomID, userID, roomVersion)
 }
 
 func (r *Upgrader) performRoomUpgrade(
 	ctx context.Context,
-	roomID, userID string, roomVersion gomatrixserverlib.RoomVersion,
+	roomID string, userID spec.UserID, roomVersion gomatrixserverlib.RoomVersion,
 ) (string, error) {
-	_, userDomain, err := r.Cfg.Matrix.SplitLocalID('@', userID)
-	if err != nil {
-		return "", api.ErrNotAllowed{Err: fmt.Errorf("error validating the user ID")}
-	}
 	evTime := time.Now()
 
 	// Return an immediate error if the room does not exist
@@ -58,14 +54,20 @@ func (r *Upgrader) performRoomUpgrade(
 		return "", err
 	}
 
+	senderID, err := r.URSAPI.QuerySenderIDForUser(ctx, roomID, userID)
+	if err != nil {
+		util.GetLogger(ctx).WithError(err).Error("Failed getting senderID for user")
+		return "", err
+	}
+
 	// 1. Check if the user is authorized to actually perform the upgrade (can send m.room.tombstone)
-	if !r.userIsAuthorized(ctx, userID, roomID) {
+	if !r.userIsAuthorized(ctx, senderID, roomID) {
 		return "", api.ErrNotAllowed{Err: fmt.Errorf("You don't have permission to upgrade the room, power level too low.")}
 	}
 
 	// TODO (#267): Check room ID doesn't clash with an existing one, and we
 	//              probably shouldn't be using pseudo-random strings, maybe GUIDs?
-	newRoomID := fmt.Sprintf("!%s:%s", util.RandomString(16), userDomain)
+	newRoomID := fmt.Sprintf("!%s:%s", util.RandomString(16), userID.Domain())
 
 	// Get the existing room state for the old room.
 	oldRoomReq := &api.QueryLatestEventsAndStateRequest{
@@ -77,25 +79,25 @@ func (r *Upgrader) performRoomUpgrade(
 	}
 
 	// Make the tombstone event
-	tombstoneEvent, pErr := r.makeTombstoneEvent(ctx, evTime, userID, roomID, newRoomID)
+	tombstoneEvent, pErr := r.makeTombstoneEvent(ctx, evTime, senderID, userID.Domain(), roomID, newRoomID)
 	if pErr != nil {
 		return "", pErr
 	}
 
 	// Generate the initial events we need to send into the new room. This includes copied state events and bans
 	// as well as the power level events needed to set up the room
-	eventsToMake, pErr := r.generateInitialEvents(ctx, oldRoomRes, userID, roomID, roomVersion, tombstoneEvent)
+	eventsToMake, pErr := r.generateInitialEvents(ctx, oldRoomRes, senderID, roomID, roomVersion, tombstoneEvent)
 	if pErr != nil {
 		return "", pErr
 	}
 
 	// Send the setup events to the new room
-	if pErr = r.sendInitialEvents(ctx, evTime, userID, userDomain, newRoomID, roomVersion, eventsToMake); pErr != nil {
+	if pErr = r.sendInitialEvents(ctx, evTime, senderID, userID.Domain(), newRoomID, roomVersion, eventsToMake); pErr != nil {
 		return "", pErr
 	}
 
 	// 5. Send the tombstone event to the old room
-	if pErr = r.sendHeaderedEvent(ctx, userDomain, tombstoneEvent, string(userDomain)); pErr != nil {
+	if pErr = r.sendHeaderedEvent(ctx, userID.Domain(), tombstoneEvent, string(userID.Domain())); pErr != nil {
 		return "", pErr
 	}
 
@@ -105,17 +107,17 @@ func (r *Upgrader) performRoomUpgrade(
 	}
 
 	// If the old room had a canonical alias event, it should be deleted in the old room
-	if pErr = r.clearOldCanonicalAliasEvent(ctx, oldRoomRes, evTime, userID, userDomain, roomID); pErr != nil {
+	if pErr = r.clearOldCanonicalAliasEvent(ctx, oldRoomRes, evTime, senderID, userID.Domain(), roomID); pErr != nil {
 		return "", pErr
 	}
 
 	// 4. Move local aliases to the new room
-	if pErr = moveLocalAliases(ctx, roomID, newRoomID, userID, r.URSAPI); pErr != nil {
+	if pErr = moveLocalAliases(ctx, roomID, newRoomID, senderID, userID, r.URSAPI); pErr != nil {
 		return "", pErr
 	}
 
 	// 6. Restrict power levels in the old room
-	if pErr = r.restrictOldRoomPowerLevels(ctx, evTime, userID, userDomain, roomID); pErr != nil {
+	if pErr = r.restrictOldRoomPowerLevels(ctx, evTime, senderID, userID.Domain(), roomID); pErr != nil {
 		return "", pErr
 	}
 
@@ -130,7 +132,7 @@ func (r *Upgrader) getRoomPowerLevels(ctx context.Context, roomID string) (*goma
 	return oldPowerLevelsEvent.PowerLevels()
 }
 
-func (r *Upgrader) restrictOldRoomPowerLevels(ctx context.Context, evTime time.Time, userID string, userDomain spec.ServerName, roomID string) error {
+func (r *Upgrader) restrictOldRoomPowerLevels(ctx context.Context, evTime time.Time, senderID spec.SenderID, userDomain spec.ServerName, roomID string) error {
 	restrictedPowerLevelContent, pErr := r.getRoomPowerLevels(ctx, roomID)
 	if pErr != nil {
 		return pErr
@@ -147,7 +149,7 @@ func (r *Upgrader) restrictOldRoomPowerLevels(ctx context.Context, evTime time.T
 	restrictedPowerLevelContent.EventsDefault = restrictedDefaultPowerLevel
 	restrictedPowerLevelContent.Invite = restrictedDefaultPowerLevel
 
-	restrictedPowerLevelsHeadered, resErr := r.makeHeaderedEvent(ctx, evTime, userID, roomID, gomatrixserverlib.FledglingEvent{
+	restrictedPowerLevelsHeadered, resErr := r.makeHeaderedEvent(ctx, evTime, senderID, userDomain, roomID, gomatrixserverlib.FledglingEvent{
 		Type:     spec.MRoomPowerLevels,
 		StateKey: "",
 		Content:  restrictedPowerLevelContent,
@@ -165,7 +167,7 @@ func (r *Upgrader) restrictOldRoomPowerLevels(ctx context.Context, evTime time.T
 }
 
 func moveLocalAliases(ctx context.Context,
-	roomID, newRoomID, userID string,
+	roomID, newRoomID string, senderID spec.SenderID, userID spec.UserID,
 	URSAPI api.RoomserverInternalAPI,
 ) (err error) {
 
@@ -175,14 +177,6 @@ func moveLocalAliases(ctx context.Context,
 		return fmt.Errorf("Failed to get old room aliases: %w", err)
 	}
 
-	fullUserID, err := spec.NewUserID(userID, true)
-	if err != nil {
-		return fmt.Errorf("Failed to get userID: %w", err)
-	}
-	senderID, err := URSAPI.QuerySenderIDForUser(ctx, roomID, *fullUserID)
-	if err != nil {
-		return fmt.Errorf("Failed to get senderID: %w", err)
-	}
 	for _, alias := range aliasRes.Aliases {
 		removeAliasReq := api.RemoveRoomAliasRequest{SenderID: senderID, Alias: alias}
 		removeAliasRes := api.RemoveRoomAliasResponse{}
@@ -190,7 +184,7 @@ func moveLocalAliases(ctx context.Context,
 			return fmt.Errorf("Failed to remove old room alias: %w", err)
 		}
 
-		setAliasReq := api.SetRoomAliasRequest{UserID: userID, Alias: alias, RoomID: newRoomID}
+		setAliasReq := api.SetRoomAliasRequest{UserID: userID.String(), Alias: alias, RoomID: newRoomID}
 		setAliasRes := api.SetRoomAliasResponse{}
 		if err = URSAPI.SetRoomAlias(ctx, &setAliasReq, &setAliasRes); err != nil {
 			return fmt.Errorf("Failed to set new room alias: %w", err)
@@ -199,7 +193,7 @@ func moveLocalAliases(ctx context.Context,
 	return nil
 }
 
-func (r *Upgrader) clearOldCanonicalAliasEvent(ctx context.Context, oldRoom *api.QueryLatestEventsAndStateResponse, evTime time.Time, userID string, userDomain spec.ServerName, roomID string) error {
+func (r *Upgrader) clearOldCanonicalAliasEvent(ctx context.Context, oldRoom *api.QueryLatestEventsAndStateResponse, evTime time.Time, senderID spec.SenderID, userDomain spec.ServerName, roomID string) error {
 	for _, event := range oldRoom.StateEvents {
 		if event.Type() != spec.MRoomCanonicalAlias || !event.StateKeyEquals("") {
 			continue
@@ -217,7 +211,7 @@ func (r *Upgrader) clearOldCanonicalAliasEvent(ctx context.Context, oldRoom *api
 		}
 	}
 
-	emptyCanonicalAliasEvent, resErr := r.makeHeaderedEvent(ctx, evTime, userID, roomID, gomatrixserverlib.FledglingEvent{
+	emptyCanonicalAliasEvent, resErr := r.makeHeaderedEvent(ctx, evTime, senderID, userDomain, roomID, gomatrixserverlib.FledglingEvent{
 		Type:    spec.MRoomCanonicalAlias,
 		Content: map[string]interface{}{},
 	})
@@ -280,7 +274,7 @@ func (r *Upgrader) validateRoomExists(ctx context.Context, roomID string) error
 	return nil
 }
 
-func (r *Upgrader) userIsAuthorized(ctx context.Context, userID, roomID string,
+func (r *Upgrader) userIsAuthorized(ctx context.Context, senderID spec.SenderID, roomID string,
 ) bool {
 	plEvent := api.GetStateEvent(ctx, r.URSAPI, roomID, gomatrixserverlib.StateKeyTuple{
 		EventType: spec.MRoomPowerLevels,
@@ -295,26 +289,18 @@ func (r *Upgrader) userIsAuthorized(ctx context.Context, userID, roomID string,
 	}
 	// Check for power level required to send tombstone event (marks the current room as obsolete),
 	// if not found, use the StateDefault power level
-	fullUserID, err := spec.NewUserID(userID, true)
-	if err != nil {
-		return false
-	}
-	senderID, err := r.URSAPI.QuerySenderIDForUser(ctx, roomID, *fullUserID)
-	if err != nil {
-		return false
-	}
 	return pl.UserLevel(senderID) >= pl.EventLevel("m.room.tombstone", true)
 }
 
 // nolint:gocyclo
-func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.QueryLatestEventsAndStateResponse, userID, roomID string, newVersion gomatrixserverlib.RoomVersion, tombstoneEvent *types.HeaderedEvent) ([]gomatrixserverlib.FledglingEvent, error) {
+func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.QueryLatestEventsAndStateResponse, senderID spec.SenderID, roomID string, newVersion gomatrixserverlib.RoomVersion, tombstoneEvent *types.HeaderedEvent) ([]gomatrixserverlib.FledglingEvent, error) {
 	state := make(map[gomatrixserverlib.StateKeyTuple]*types.HeaderedEvent, len(oldRoom.StateEvents))
 	for _, event := range oldRoom.StateEvents {
 		if event.StateKey() == nil {
 			// This shouldn't ever happen, but better to be safe than sorry.
 			continue
 		}
-		if event.Type() == spec.MRoomMember && !event.StateKeyEquals(userID) {
+		if event.Type() == spec.MRoomMember && !event.StateKeyEquals(string(senderID)) {
 			// With the exception of bans which we do want to copy, we
 			// should ignore membership events that aren't our own, as event auth will
 			// prevent us from being able to create membership events on behalf of other
@@ -330,6 +316,7 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 			}
 		}
 		// skip events that rely on a specific user being present
+		// TODO: What to do here for pseudoIDs? It's checking non-member events for state keys with userIDs.
 		sKey := *event.StateKey()
 		if event.Type() != spec.MRoomMember && len(sKey) > 0 && sKey[:1] == "@" {
 			continue
@@ -340,10 +327,10 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 	// The following events are ones that we are going to override manually
 	// in the following section.
 	override := map[gomatrixserverlib.StateKeyTuple]struct{}{
-		{EventType: spec.MRoomCreate, StateKey: ""}:      {},
-		{EventType: spec.MRoomMember, StateKey: userID}:  {},
-		{EventType: spec.MRoomPowerLevels, StateKey: ""}: {},
-		{EventType: spec.MRoomJoinRules, StateKey: ""}:   {},
+		{EventType: spec.MRoomCreate, StateKey: ""}:               {},
+		{EventType: spec.MRoomMember, StateKey: string(senderID)}: {},
+		{EventType: spec.MRoomPowerLevels, StateKey: ""}:          {},
+		{EventType: spec.MRoomJoinRules, StateKey: ""}:            {},
 	}
 
 	// The overridden events are essential events that must be present in the
@@ -355,7 +342,7 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 	}
 
 	oldCreateEvent := state[gomatrixserverlib.StateKeyTuple{EventType: spec.MRoomCreate, StateKey: ""}]
-	oldMembershipEvent := state[gomatrixserverlib.StateKeyTuple{EventType: spec.MRoomMember, StateKey: userID}]
+	oldMembershipEvent := state[gomatrixserverlib.StateKeyTuple{EventType: spec.MRoomMember, StateKey: string(senderID)}]
 	oldPowerLevelsEvent := state[gomatrixserverlib.StateKeyTuple{EventType: spec.MRoomPowerLevels, StateKey: ""}]
 	oldJoinRulesEvent := state[gomatrixserverlib.StateKeyTuple{EventType: spec.MRoomJoinRules, StateKey: ""}]
 
@@ -364,7 +351,7 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 	// in the create event (such as for the room types MSC).
 	newCreateContent := map[string]interface{}{}
 	_ = json.Unmarshal(oldCreateEvent.Content(), &newCreateContent)
-	newCreateContent["creator"] = userID
+	newCreateContent["creator"] = string(senderID)
 	newCreateContent["room_version"] = newVersion
 	newCreateContent["predecessor"] = gomatrixserverlib.PreviousRoom{
 		EventID: tombstoneEvent.EventID(),
@@ -385,7 +372,7 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 	newMembershipContent["membership"] = spec.Join
 	newMembershipEvent := gomatrixserverlib.FledglingEvent{
 		Type:     spec.MRoomMember,
-		StateKey: userID,
+		StateKey: string(senderID),
 		Content:  newMembershipContent,
 	}
 
@@ -400,14 +387,6 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 		return nil, fmt.Errorf("Power level event content was invalid")
 	}
 
-	fullUserID, err := spec.NewUserID(userID, true)
-	if err != nil {
-		return nil, err
-	}
-	senderID, err := r.URSAPI.QuerySenderIDForUser(ctx, roomID, *fullUserID)
-	if err != nil {
-		return nil, err
-	}
 	tempPowerLevelsEvent, powerLevelsOverridden := createTemporaryPowerLevels(powerLevelContent, senderID)
 
 	// Now do the join rules event, same as the create and membership
@@ -470,21 +449,13 @@ func (r *Upgrader) generateInitialEvents(ctx context.Context, oldRoom *api.Query
 	return eventsToMake, nil
 }
 
-func (r *Upgrader) sendInitialEvents(ctx context.Context, evTime time.Time, userID string, userDomain spec.ServerName, newRoomID string, newVersion gomatrixserverlib.RoomVersion, eventsToMake []gomatrixserverlib.FledglingEvent) error {
+func (r *Upgrader) sendInitialEvents(ctx context.Context, evTime time.Time, senderID spec.SenderID, userDomain spec.ServerName, newRoomID string, newVersion gomatrixserverlib.RoomVersion, eventsToMake []gomatrixserverlib.FledglingEvent) error {
 	var err error
 	var builtEvents []*types.HeaderedEvent
 	authEvents := gomatrixserverlib.NewAuthEvents(nil)
 	for i, e := range eventsToMake {
 		depth := i + 1 // depth starts at 1
 
-		fullUserID, userIDErr := spec.NewUserID(userID, true)
-		if userIDErr != nil {
-			return userIDErr
-		}
-		senderID, queryErr := r.URSAPI.QuerySenderIDForUser(ctx, newRoomID, *fullUserID)
-		if queryErr != nil {
-			return queryErr
-		}
 		proto := gomatrixserverlib.ProtoEvent{
 			SenderID: string(senderID),
 			RoomID:   newRoomID,
@@ -549,7 +520,7 @@ func (r *Upgrader) sendInitialEvents(ctx context.Context, evTime time.Time, user
 func (r *Upgrader) makeTombstoneEvent(
 	ctx context.Context,
 	evTime time.Time,
-	userID, roomID, newRoomID string,
+	senderID spec.SenderID, senderDomain spec.ServerName, roomID, newRoomID string,
 ) (*types.HeaderedEvent, error) {
 	content := map[string]interface{}{
 		"body":             "This room has been replaced",
@@ -559,30 +530,21 @@ func (r *Upgrader) makeTombstoneEvent(
 		Type:    "m.room.tombstone",
 		Content: content,
 	}
-	return r.makeHeaderedEvent(ctx, evTime, userID, roomID, event)
+	return r.makeHeaderedEvent(ctx, evTime, senderID, senderDomain, roomID, event)
 }
 
-func (r *Upgrader) makeHeaderedEvent(ctx context.Context, evTime time.Time, userID, roomID string, event gomatrixserverlib.FledglingEvent) (*types.HeaderedEvent, error) {
-	fullUserID, err := spec.NewUserID(userID, true)
-	if err != nil {
-		return nil, err
-	}
-	senderID, err := r.URSAPI.QuerySenderIDForUser(ctx, roomID, *fullUserID)
-	if err != nil {
-		return nil, err
-	}
+func (r *Upgrader) makeHeaderedEvent(ctx context.Context, evTime time.Time, senderID spec.SenderID, senderDomain spec.ServerName, roomID string, event gomatrixserverlib.FledglingEvent) (*types.HeaderedEvent, error) {
 	proto := gomatrixserverlib.ProtoEvent{
 		SenderID: string(senderID),
 		RoomID:   roomID,
 		Type:     event.Type,
 		StateKey: &event.StateKey,
 	}
-	err = proto.SetContent(event.Content)
+	err := proto.SetContent(event.Content)
 	if err != nil {
 		return nil, fmt.Errorf("failed to set new %q event content: %w", proto.Type, err)
 	}
 	// Get the sender domain.
-	senderDomain := fullUserID.Domain()
 	identity, err := r.Cfg.Matrix.SigningIdentityFor(senderDomain)
 	if err != nil {
 		return nil, fmt.Errorf("failed to get signing identity for %q: %w", senderDomain, err)
diff --git a/roomserver/internal/query/query.go b/roomserver/internal/query/query.go
index ae2b7cf57..caea6b526 100644
--- a/roomserver/internal/query/query.go
+++ b/roomserver/internal/query/query.go
@@ -48,7 +48,7 @@ type Queryer struct {
 	Cfg               *config.Dendrite
 }
 
-func (r *Queryer) RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, userID spec.UserID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error) {
+func (r *Queryer) RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID, localServerName spec.ServerName) (*gomatrixserverlib.RestrictedRoomJoinInfo, error) {
 	roomInfo, err := r.QueryRoomInfo(ctx, roomID)
 	if err != nil || roomInfo == nil || roomInfo.IsStub() {
 		return nil, err
@@ -64,7 +64,7 @@ func (r *Queryer) RestrictedRoomJoinInfo(ctx context.Context, roomID spec.RoomID
 		return nil, fmt.Errorf("InternalServerError: Failed to query room: %w", err)
 	}
 
-	userJoinedToRoom, err := r.UserJoinedToRoom(ctx, types.RoomNID(roomInfo.RoomNID), userID)
+	userJoinedToRoom, err := r.UserJoinedToRoom(ctx, types.RoomNID(roomInfo.RoomNID), senderID)
 	if err != nil {
 		util.GetLogger(ctx).WithError(err).Error("rsAPI.UserJoinedToRoom failed")
 		return nil, fmt.Errorf("InternalServerError: %w", err)
@@ -220,13 +220,14 @@ func (r *Queryer) QueryEventsByID(
 	return nil
 }
 
-// QueryMembershipForUser implements api.RoomserverInternalAPI
-func (r *Queryer) QueryMembershipForUser(
+// QueryMembershipForSenderID implements api.RoomserverInternalAPI
+func (r *Queryer) QueryMembershipForSenderID(
 	ctx context.Context,
-	request *api.QueryMembershipForUserRequest,
+	roomID spec.RoomID,
+	senderID spec.SenderID,
 	response *api.QueryMembershipForUserResponse,
 ) error {
-	info, err := r.DB.RoomInfo(ctx, request.RoomID)
+	info, err := r.DB.RoomInfo(ctx, roomID.String())
 	if err != nil {
 		return err
 	}
@@ -236,7 +237,7 @@ func (r *Queryer) QueryMembershipForUser(
 	}
 	response.RoomExists = true
 
-	membershipEventNID, stillInRoom, isRoomforgotten, err := r.DB.GetMembership(ctx, info.RoomNID, request.UserID)
+	membershipEventNID, stillInRoom, isRoomforgotten, err := r.DB.GetMembership(ctx, info.RoomNID, senderID)
 	if err != nil {
 		return err
 	}
@@ -264,6 +265,24 @@ func (r *Queryer) QueryMembershipForUser(
 	return err
 }
 
+// QueryMembershipForUser implements api.RoomserverInternalAPI
+func (r *Queryer) QueryMembershipForUser(
+	ctx context.Context,
+	request *api.QueryMembershipForUserRequest,
+	response *api.QueryMembershipForUserResponse,
+) error {
+	senderID, err := r.DB.GetSenderIDForUser(ctx, request.RoomID, request.UserID)
+	if err != nil {
+		return err
+	}
+
+	roomID, err := spec.NewRoomID(request.RoomID)
+	if err != nil {
+		return err
+	}
+	return r.QueryMembershipForSenderID(ctx, *roomID, senderID, response)
+}
+
 // QueryMembershipAtEvent returns the known memberships at a given event.
 // If the state before an event is not known, an empty list will be returned
 // for that event instead.
@@ -373,7 +392,7 @@ func (r *Queryer) QueryMembershipsForRoom(
 	// If no sender is specified then we will just return the entire
 	// set of memberships for the room, regardless of whether a specific
 	// user is allowed to see them or not.
-	if request.Sender == "" {
+	if request.SenderID == "" {
 		var events []types.Event
 		var eventNIDs []types.EventNID
 		eventNIDs, err = r.DB.GetMembershipEventNIDsForRoom(ctx, info.RoomNID, request.JoinedOnly, request.LocalOnly)
@@ -388,18 +407,15 @@ func (r *Queryer) QueryMembershipsForRoom(
 			return fmt.Errorf("r.DB.Events: %w", err)
 		}
 		for _, event := range events {
-			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)
+			clientEvent := synctypes.ToClientEventDefault(func(roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+				return r.QueryUserIDForSender(ctx, roomID, senderID)
+			}, event)
 			response.JoinEvents = append(response.JoinEvents, clientEvent)
 		}
 		return nil
 	}
 
-	membershipEventNID, stillInRoom, isRoomforgotten, err := r.DB.GetMembership(ctx, info.RoomNID, request.Sender)
+	membershipEventNID, stillInRoom, isRoomforgotten, err := r.DB.GetMembership(ctx, info.RoomNID, request.SenderID)
 	if err != nil {
 		return err
 	}
@@ -442,12 +458,9 @@ func (r *Queryer) QueryMembershipsForRoom(
 	}
 
 	for _, event := range events {
-		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)
+		clientEvent := synctypes.ToClientEventDefault(func(roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+			return r.QueryUserIDForSender(ctx, roomID, senderID)
+		}, event)
 		response.JoinEvents = append(response.JoinEvents, clientEvent)
 	}
 
@@ -489,6 +502,7 @@ func (r *Queryer) QueryServerAllowedToSeeEvent(
 	ctx context.Context,
 	serverName spec.ServerName,
 	eventID string,
+	roomID string,
 ) (allowed bool, err error) {
 	events, err := r.DB.EventNIDs(ctx, []string{eventID})
 	if err != nil {
@@ -518,7 +532,7 @@ func (r *Queryer) QueryServerAllowedToSeeEvent(
 	}
 
 	return helpers.CheckServerAllowedToSeeEvent(
-		ctx, r.DB, info, eventID, serverName, isInRoom,
+		ctx, r.DB, info, roomID, eventID, serverName, isInRoom,
 	)
 }
 
@@ -909,8 +923,8 @@ func (r *Queryer) QueryAuthChain(ctx context.Context, req *api.QueryAuthChainReq
 	return nil
 }
 
-func (r *Queryer) InvitePending(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (bool, error) {
-	pending, _, _, _, err := helpers.IsInvitePending(ctx, r.DB, roomID.String(), userID.String())
+func (r *Queryer) InvitePending(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (bool, error) {
+	pending, _, _, _, err := helpers.IsInvitePending(ctx, r.DB, roomID.String(), senderID)
 	return pending, err
 }
 
@@ -926,8 +940,8 @@ func (r *Queryer) CurrentStateEvent(ctx context.Context, roomID spec.RoomID, eve
 	return res, err
 }
 
-func (r *Queryer) UserJoinedToRoom(ctx context.Context, roomNID types.RoomNID, userID spec.UserID) (bool, error) {
-	_, isIn, _, err := r.DB.GetMembership(ctx, roomNID, userID.String())
+func (r *Queryer) UserJoinedToRoom(ctx context.Context, roomNID types.RoomNID, senderID spec.SenderID) (bool, error) {
+	_, isIn, _, err := r.DB.GetMembership(ctx, roomNID, senderID)
 	return isIn, err
 }
 
@@ -957,7 +971,7 @@ func (r *Queryer) LocallyJoinedUsers(ctx context.Context, roomVersion gomatrixse
 }
 
 // nolint:gocyclo
-func (r *Queryer) QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.RoomID, userID spec.UserID) (string, error) {
+func (r *Queryer) QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.RoomID, senderID spec.SenderID) (string, error) {
 	// Look up if we know anything about the room. If it doesn't exist
 	// or is a stub entry then we can't do anything.
 	roomInfo, err := r.DB.RoomInfo(ctx, roomID.String())
@@ -972,7 +986,7 @@ func (r *Queryer) QueryRestrictedJoinAllowed(ctx context.Context, roomID spec.Ro
 		return "", err
 	}
 
-	return verImpl.CheckRestrictedJoin(ctx, r.Cfg.Global.ServerName, &api.JoinRoomQuerier{Roomserver: r}, roomID, userID)
+	return verImpl.CheckRestrictedJoin(ctx, r.Cfg.Global.ServerName, &api.JoinRoomQuerier{Roomserver: r}, roomID, senderID)
 }
 
 func (r *Queryer) QuerySenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (spec.SenderID, error) {
diff --git a/roomserver/roomserver_test.go b/roomserver/roomserver_test.go
index 5e6ba7d4e..90c94bbce 100644
--- a/roomserver/roomserver_test.go
+++ b/roomserver/roomserver_test.go
@@ -722,7 +722,7 @@ func TestQueryRestrictedJoinAllowed(t *testing.T) {
 
 				roomID, _ := spec.NewRoomID(testRoom.ID)
 				userID, _ := spec.NewUserID(bob.ID, true)
-				got, err := rsAPI.QueryRestrictedJoinAllowed(processCtx.Context(), *roomID, *userID)
+				got, err := rsAPI.QueryRestrictedJoinAllowed(processCtx.Context(), *roomID, spec.SenderID(userID.String()))
 				if tc.wantError && err == nil {
 					t.Fatal("expected error, got none")
 				}
@@ -821,17 +821,6 @@ func TestUpgrade(t *testing.T) {
 		validateFunc func(t *testing.T, oldRoomID, newRoomID string, rsAPI api.RoomserverInternalAPI)
 		wantNewRoom  bool
 	}{
-		{
-			name:        "invalid userID",
-			upgradeUser: "!notvalid:test",
-			roomFunc: func(rsAPI api.RoomserverInternalAPI) string {
-				room := test.NewRoom(t, alice)
-				if err := api.SendEvents(ctx, rsAPI, api.KindNew, room.Events(), "test", "test", "test", nil, false); err != nil {
-					t.Errorf("failed to send events: %v", err)
-				}
-				return room.ID
-			},
-		},
 		{
 			name:        "invalid roomID",
 			upgradeUser: alice.ID,
@@ -1049,7 +1038,11 @@ func TestUpgrade(t *testing.T) {
 				}
 				roomID := tc.roomFunc(rsAPI)
 
-				newRoomID, err := rsAPI.PerformRoomUpgrade(processCtx.Context(), roomID, tc.upgradeUser, version.DefaultRoomVersion())
+				userID, err := spec.NewUserID(tc.upgradeUser, true)
+				if err != nil {
+					t.Fatalf("upgrade userID is invalid")
+				}
+				newRoomID, err := rsAPI.PerformRoomUpgrade(processCtx.Context(), roomID, *userID, version.DefaultRoomVersion())
 				if err != nil && tc.wantNewRoom {
 					t.Fatal(err)
 				}
diff --git a/roomserver/storage/interface.go b/roomserver/storage/interface.go
index 2d27d7999..ef4463781 100644
--- a/roomserver/storage/interface.go
+++ b/roomserver/storage/interface.go
@@ -131,7 +131,7 @@ type Database interface {
 	// in this room, along a boolean set to true if the user is still in this room,
 	// false if not.
 	// Returns an error if there was a problem talking to the database.
-	GetMembership(ctx context.Context, roomNID types.RoomNID, requestSenderUserID string) (membershipEventNID types.EventNID, stillInRoom, isRoomForgotten bool, err error)
+	GetMembership(ctx context.Context, roomNID types.RoomNID, requestSenderID spec.SenderID) (membershipEventNID types.EventNID, stillInRoom, isRoomForgotten bool, err error)
 	// Lookup the membership event numeric IDs for all user that are or have
 	// been members of a given room. Only lookup events of "join" membership if
 	// joinOnly is set to true.
diff --git a/roomserver/storage/shared/storage.go b/roomserver/storage/shared/storage.go
index cb12b3f57..85a1ba7a1 100644
--- a/roomserver/storage/shared/storage.go
+++ b/roomserver/storage/shared/storage.go
@@ -490,10 +490,10 @@ func (d *Database) RemoveRoomAlias(ctx context.Context, alias string) error {
 	})
 }
 
-func (d *Database) GetMembership(ctx context.Context, roomNID types.RoomNID, requestSenderUserID string) (membershipEventNID types.EventNID, stillInRoom, isRoomforgotten bool, err error) {
+func (d *Database) GetMembership(ctx context.Context, roomNID types.RoomNID, requestSenderID spec.SenderID) (membershipEventNID types.EventNID, stillInRoom, isRoomforgotten bool, err error) {
 	var requestSenderUserNID types.EventStateKeyNID
 	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
-		requestSenderUserNID, err = d.assignStateKeyNID(ctx, txn, requestSenderUserID)
+		requestSenderUserNID, err = d.assignStateKeyNID(ctx, txn, string(requestSenderID))
 		return err
 	})
 	if err != nil {
@@ -936,6 +936,7 @@ func extractRoomVersionFromCreateEvent(event gomatrixserverlib.PDU) (
 	return roomVersion, err
 }
 
+// nolint:gocyclo
 // MaybeRedactEvent manages the redacted status of events. There's two cases to consider in order to comply with the spec:
 // "servers should not apply or send redactions to clients until both the redaction event and original event have been seen, and are valid."
 // https://matrix.org/docs/spec/rooms/v3#authorization-rules-for-events
@@ -1014,7 +1015,7 @@ func (d *EventDatabase) MaybeRedactEvent(
 		switch {
 		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 sender1Domain == sender2Domain:
+		case sender1Domain != "" && sender2Domain != "" && sender1Domain == sender2Domain:
 			// 2. The domain of the redaction event’s sender matches that of the original event’s sender.
 		default:
 			ignoreRedaction = true
diff --git a/setup/mscs/msc2836/msc2836.go b/setup/mscs/msc2836/msc2836.go
index 47eb544ea..d3f1c9dd2 100644
--- a/setup/mscs/msc2836/msc2836.go
+++ b/setup/mscs/msc2836/msc2836.go
@@ -154,7 +154,7 @@ type reqCtx struct {
 	rsAPI       roomserver.RoomserverInternalAPI
 	db          Database
 	req         *EventRelationshipRequest
-	userID      string
+	userID      spec.UserID
 	roomVersion gomatrixserverlib.RoomVersion
 
 	// federated request args
@@ -173,10 +173,17 @@ func eventRelationshipHandler(db Database, rsAPI roomserver.RoomserverInternalAP
 				JSON: spec.BadJSON(fmt.Sprintf("invalid json: %s", err)),
 			}
 		}
+		userID, err := spec.NewUserID(device.UserID, true)
+		if err != nil {
+			return util.JSONResponse{
+				Code: 400,
+				JSON: spec.BadJSON(fmt.Sprintf("invalid json: %s", err)),
+			}
+		}
 		rc := reqCtx{
 			ctx:                req.Context(),
 			req:                relation,
-			userID:             device.UserID,
+			userID:             *userID,
 			rsAPI:              rsAPI,
 			fsAPI:              fsAPI,
 			isFederatedRequest: false,
diff --git a/setup/mscs/msc2836/msc2836_test.go b/setup/mscs/msc2836/msc2836_test.go
index 551d7ad45..e32d6a9f2 100644
--- a/setup/mscs/msc2836/msc2836_test.go
+++ b/setup/mscs/msc2836/msc2836_test.go
@@ -529,6 +529,10 @@ func (r *testRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID str
 	return spec.NewUserID(string(senderID), true)
 }
 
+func (r *testRoomserverAPI) QuerySenderIDForUser(ctx context.Context, roomID string, userID spec.UserID) (spec.SenderID, error) {
+	return spec.SenderID(userID.String()), nil
+}
+
 func (r *testRoomserverAPI) QueryEventsByID(ctx context.Context, req *roomserver.QueryEventsByIDRequest, res *roomserver.QueryEventsByIDResponse) error {
 	for _, eventID := range req.EventIDs {
 		ev := r.events[eventID]
@@ -540,7 +544,7 @@ func (r *testRoomserverAPI) QueryEventsByID(ctx context.Context, req *roomserver
 }
 
 func (r *testRoomserverAPI) QueryMembershipForUser(ctx context.Context, req *roomserver.QueryMembershipForUserRequest, res *roomserver.QueryMembershipForUserResponse) error {
-	rooms := r.userToJoinedRooms[req.UserID]
+	rooms := r.userToJoinedRooms[req.UserID.String()]
 	for _, roomID := range rooms {
 		if roomID == req.RoomID {
 			res.IsInRoom = true
diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index 8a2a0b1f6..c5f2db9c8 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -373,7 +373,15 @@ func (s *OutputRoomEventConsumer) notifyJoinedPeeks(ctx context.Context, ev *rst
 	// TODO: check that it's a join and not a profile change (means unmarshalling prev_content)
 	if membership == spec.Join {
 		// check it's a local join
-		if _, _, err := s.cfg.Matrix.SplitLocalID('@', *ev.StateKey()); err != nil {
+		if ev.StateKey() == nil {
+			return sp, fmt.Errorf("unexpected nil state_key")
+		}
+
+		userID, err := s.rsAPI.QueryUserIDForSender(ctx, ev.RoomID(), spec.SenderID(*ev.StateKey()))
+		if err != nil || userID == nil {
+			return sp, fmt.Errorf("failed getting userID for sender: %w", err)
+		}
+		if !s.cfg.Matrix.IsLocalServerName(userID.Domain()) {
 			return sp, nil
 		}
 
@@ -395,9 +403,15 @@ func (s *OutputRoomEventConsumer) onNewInviteEvent(
 	if msg.Event.StateKey() == nil {
 		return
 	}
-	if _, _, err := s.cfg.Matrix.SplitLocalID('@', *msg.Event.StateKey()); err != nil {
+
+	userID, err := s.rsAPI.QueryUserIDForSender(ctx, msg.Event.RoomID(), spec.SenderID(*msg.Event.StateKey()))
+	if err != nil || userID == nil {
 		return
 	}
+	if !s.cfg.Matrix.IsLocalServerName(userID.Domain()) {
+		return
+	}
+
 	pduPos, err := s.db.AddInviteEvent(ctx, msg.Event)
 	if err != nil {
 		sentry.CaptureException(err)
@@ -440,7 +454,16 @@ func (s *OutputRoomEventConsumer) onRetireInviteEvent(
 
 	// Notify any active sync requests that the invite has been retired.
 	s.inviteStream.Advance(pduPos)
-	s.notifier.OnNewInvite(types.StreamingToken{InvitePosition: pduPos}, msg.TargetUserID)
+	userID, err := s.rsAPI.QueryUserIDForSender(ctx, msg.RoomID, msg.TargetSenderID)
+	if err != nil || userID == nil {
+		log.WithFields(log.Fields{
+			"event_id":   msg.EventID,
+			"sender_id":  msg.TargetSenderID,
+			log.ErrorKey: err,
+		}).Errorf("failed to find userID for sender")
+		return
+	}
+	s.notifier.OnNewInvite(types.StreamingToken{InvitePosition: pduPos}, userID.String())
 }
 
 func (s *OutputRoomEventConsumer) onNewPeek(
diff --git a/syncapi/internal/history_visibility.go b/syncapi/internal/history_visibility.go
index 7449b4647..ab1a7f83d 100644
--- a/syncapi/internal/history_visibility.go
+++ b/syncapi/internal/history_visibility.go
@@ -134,9 +134,17 @@ func ApplyHistoryVisibilityFilter(
 			}
 		}
 		// NOTSPEC: Always allow user to see their own membership events (spec contains more "rules")
-		if ev.Type() == spec.MRoomMember && ev.StateKeyEquals(userID) {
-			eventsFiltered = append(eventsFiltered, ev)
-			continue
+
+		user, err := spec.NewUserID(userID, true)
+		if err != nil {
+			return nil, err
+		}
+		senderID, err := rsAPI.QuerySenderIDForUser(ctx, ev.RoomID(), *user)
+		if err == nil {
+			if ev.Type() == spec.MRoomMember && ev.StateKeyEquals(string(senderID)) {
+				eventsFiltered = append(eventsFiltered, ev)
+				continue
+			}
 		}
 		// Always allow history evVis events on boundaries. This is done
 		// by setting the effective evVis to the least restrictive
diff --git a/syncapi/internal/keychange.go b/syncapi/internal/keychange.go
index ad5935cdc..f4b6ace59 100644
--- a/syncapi/internal/keychange.go
+++ b/syncapi/internal/keychange.go
@@ -169,12 +169,16 @@ func TrackChangedUsers(
 		if err != nil {
 			return nil, nil, err
 		}
-		for _, state := range stateRes.Rooms {
+		for roomID, state := range stateRes.Rooms {
 			for tuple, membership := range state {
 				if membership != spec.Join {
 					continue
 				}
-				queryRes.UserIDsToCount[tuple.StateKey]--
+				user, queryErr := rsAPI.QueryUserIDForSender(ctx, roomID, spec.SenderID(tuple.StateKey))
+				if queryErr != nil || user == nil {
+					continue
+				}
+				queryRes.UserIDsToCount[user.String()]--
 			}
 		}
 
@@ -211,14 +215,18 @@ func TrackChangedUsers(
 	if err != nil {
 		return nil, left, err
 	}
-	for _, state := range stateRes.Rooms {
+	for roomID, state := range stateRes.Rooms {
 		for tuple, membership := range state {
 			if membership != spec.Join {
 				continue
 			}
 			// new user who we weren't previously sharing rooms with
 			if _, ok := queryRes.UserIDsToCount[tuple.StateKey]; !ok {
-				changed = append(changed, tuple.StateKey) // changed is returned
+				user, err := rsAPI.QueryUserIDForSender(ctx, roomID, spec.SenderID(tuple.StateKey))
+				if err != nil || user == nil {
+					continue
+				}
+				changed = append(changed, user.String()) // changed is returned
 			}
 		}
 	}
diff --git a/syncapi/internal/keychange_test.go b/syncapi/internal/keychange_test.go
index 23c2ecbaa..efa641475 100644
--- a/syncapi/internal/keychange_test.go
+++ b/syncapi/internal/keychange_test.go
@@ -64,6 +64,10 @@ type mockRoomserverAPI struct {
 	roomIDToJoinedMembers map[string][]string
 }
 
+func (s *mockRoomserverAPI) QueryUserIDForSender(ctx context.Context, roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+	return spec.NewUserID(string(senderID), true)
+}
+
 // QueryRoomsForUser retrieves a list of room IDs matching the given query.
 func (s *mockRoomserverAPI) QueryRoomsForUser(ctx context.Context, req *api.QueryRoomsForUserRequest, res *api.QueryRoomsForUserResponse) error {
 	return nil
diff --git a/syncapi/notifier/notifier.go b/syncapi/notifier/notifier.go
index f76456859..4ee7c8605 100644
--- a/syncapi/notifier/notifier.go
+++ b/syncapi/notifier/notifier.go
@@ -20,6 +20,7 @@ import (
 	"time"
 
 	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"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/types"
@@ -36,7 +37,8 @@ import (
 // the event, but the token has already advanced by the time they fetch it, resulting
 // in missed events.
 type Notifier struct {
-	lock *sync.RWMutex
+	lock  *sync.RWMutex
+	rsAPI api.SyncRoomserverAPI
 	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
 	roomIDToJoinedUsers map[string]*userIDSet
 	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
@@ -55,8 +57,9 @@ type Notifier struct {
 // NewNotifier creates a new notifier set to the given sync position.
 // In order for this to be of any use, the Notifier needs to be told all rooms and
 // the joined users within each of them by calling Notifier.Load(*storage.SyncServerDatabase).
-func NewNotifier() *Notifier {
+func NewNotifier(rsAPI api.SyncRoomserverAPI) *Notifier {
 	return &Notifier{
+		rsAPI:                  rsAPI,
 		roomIDToJoinedUsers:    make(map[string]*userIDSet),
 		roomIDToPeekingDevices: make(map[string]peekingDeviceSet),
 		userDeviceStreams:      make(map[string]map[string]*UserDeviceStream),
@@ -104,26 +107,32 @@ func (n *Notifier) OnNewEvent(
 		peekingDevicesToNotify := n._peekingDevices(ev.RoomID())
 		// If this is an invite, also add in the invitee to this list.
 		if ev.Type() == "m.room.member" && ev.StateKey() != nil {
-			targetUserID := *ev.StateKey()
-			membership, err := ev.Membership()
+			targetUserID, err := n.rsAPI.QueryUserIDForSender(context.Background(), ev.RoomID(), spec.SenderID(*ev.StateKey()))
 			if err != nil {
 				log.WithError(err).WithField("event_id", ev.EventID()).Errorf(
-					"Notifier.OnNewEvent: Failed to unmarshal member event",
+					"Notifier.OnNewEvent: Failed to find the userID for this event",
 				)
 			} else {
-				// Keep the joined user map up-to-date
-				switch membership {
-				case spec.Invite:
-					usersToNotify = append(usersToNotify, targetUserID)
-				case spec.Join:
-					// Manually append the new user's ID so they get notified
-					// along all members in the room
-					usersToNotify = append(usersToNotify, targetUserID)
-					n._addJoinedUser(ev.RoomID(), targetUserID)
-				case spec.Leave:
-					fallthrough
-				case spec.Ban:
-					n._removeJoinedUser(ev.RoomID(), targetUserID)
+				membership, err := ev.Membership()
+				if err != nil {
+					log.WithError(err).WithField("event_id", ev.EventID()).Errorf(
+						"Notifier.OnNewEvent: Failed to unmarshal member event",
+					)
+				} else {
+					// Keep the joined user map up-to-date
+					switch membership {
+					case spec.Invite:
+						usersToNotify = append(usersToNotify, targetUserID.String())
+					case spec.Join:
+						// Manually append the new user's ID so they get notified
+						// along all members in the room
+						usersToNotify = append(usersToNotify, targetUserID.String())
+						n._addJoinedUser(ev.RoomID(), targetUserID.String())
+					case spec.Leave:
+						fallthrough
+					case spec.Ban:
+						n._removeJoinedUser(ev.RoomID(), targetUserID.String())
+					}
 				}
 			}
 		}
diff --git a/syncapi/notifier/notifier_test.go b/syncapi/notifier/notifier_test.go
index 36577a0ee..7076f7134 100644
--- a/syncapi/notifier/notifier_test.go
+++ b/syncapi/notifier/notifier_test.go
@@ -22,9 +22,11 @@ import (
 	"testing"
 	"time"
 
+	"github.com/matrix-org/dendrite/roomserver/api"
 	rstypes "github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/matrix-org/util"
 )
 
@@ -105,9 +107,15 @@ func mustEqualPositions(t *testing.T, got, want types.StreamingToken) {
 	}
 }
 
+type TestRoomServer struct{ api.SyncRoomserverAPI }
+
+func (t *TestRoomServer) QueryUserIDForSender(ctx context.Context, roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+	return spec.NewUserID(string(senderID), true)
+}
+
 // Test that the current position is returned if a request is already behind.
 func TestImmediateNotification(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	pos, err := waitForEvents(n, newTestSyncRequest(alice, aliceDev, syncPositionVeryOld))
 	if err != nil {
@@ -118,7 +126,7 @@ func TestImmediateNotification(t *testing.T) {
 
 // Test that new events to a joined room unblocks the request.
 func TestNewEventAndJoinedToRoom(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	n.setUsersJoinedToRooms(map[string][]string{
 		roomID: {alice, bob},
@@ -144,7 +152,7 @@ func TestNewEventAndJoinedToRoom(t *testing.T) {
 }
 
 func TestCorrectStream(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	stream := lockedFetchUserStream(n, bob, bobDev)
 	if stream.UserID != bob {
@@ -156,7 +164,7 @@ func TestCorrectStream(t *testing.T) {
 }
 
 func TestCorrectStreamWakeup(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	awoken := make(chan string)
 
@@ -184,7 +192,7 @@ func TestCorrectStreamWakeup(t *testing.T) {
 
 // Test that an invite unblocks the request
 func TestNewInviteEventForUser(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	n.setUsersJoinedToRooms(map[string][]string{
 		roomID: {alice, bob},
@@ -241,7 +249,7 @@ func TestEDUWakeup(t *testing.T) {
 
 // Test that all blocked requests get woken up on a new event.
 func TestMultipleRequestWakeup(t *testing.T) {
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	n.setUsersJoinedToRooms(map[string][]string{
 		roomID: {alice, bob},
@@ -278,7 +286,7 @@ func TestMultipleRequestWakeup(t *testing.T) {
 func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 	// listen as bob. Make bob leave room. Make alice send event to room.
 	// Make sure alice gets woken up only and not bob as well.
-	n := NewNotifier()
+	n := NewNotifier(&TestRoomServer{})
 	n.SetCurrentPosition(syncPositionBefore)
 	n.setUsersJoinedToRooms(map[string][]string{
 		roomID: {alice, bob},
diff --git a/syncapi/routing/context.go b/syncapi/routing/context.go
index 7fb88faaa..55fd3c5a2 100644
--- a/syncapi/routing/context.go
+++ b/syncapi/routing/context.go
@@ -85,9 +85,16 @@ func Context(
 		*filter.Rooms = append(*filter.Rooms, roomID)
 	}
 
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.InvalidParam("Device UserID is invalid"),
+		}
+	}
 	ctx := req.Context()
 	membershipRes := roomserver.QueryMembershipForUserResponse{}
-	membershipReq := roomserver.QueryMembershipForUserRequest{UserID: device.UserID, RoomID: roomID}
+	membershipReq := roomserver.QueryMembershipForUserRequest{UserID: *userID, RoomID: roomID}
 	if err = rsAPI.QueryMembershipForUser(ctx, &membershipReq, &membershipRes); err != nil {
 		logrus.WithError(err).Error("unable to query membership")
 		return util.JSONResponse{
@@ -217,12 +224,9 @@ func Context(
 		}
 	}
 
-	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)
+	ev := synctypes.ToClientEventDefault(func(roomID string, senderID spec.SenderID) (*spec.UserID, error) {
+		return rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
+	}, requestedEvent)
 	response := ContextRespsonse{
 		Event:        &ev,
 		EventsAfter:  eventsAfterClient,
diff --git a/syncapi/routing/getevent.go b/syncapi/routing/getevent.go
index 63df7e837..de790e5cd 100644
--- a/syncapi/routing/getevent.go
+++ b/syncapi/routing/getevent.go
@@ -106,8 +106,17 @@ func GetEvent(
 	if err == nil && senderUserID != nil {
 		sender = *senderUserID
 	}
+
+	sk := events[0].StateKey()
+	if sk != nil && *sk != "" {
+		skUserID, err := rsAPI.QueryUserIDForSender(ctx, events[0].RoomID(), spec.SenderID(*events[0].StateKey()))
+		if err == nil && skUserID != nil {
+			skString := skUserID.String()
+			sk = &skString
+		}
+	}
 	return util.JSONResponse{
 		Code: http.StatusOK,
-		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll, sender),
+		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll, sender, sk),
 	}
 }
diff --git a/syncapi/routing/memberships.go b/syncapi/routing/memberships.go
index 813167a5e..cf6769ba4 100644
--- a/syncapi/routing/memberships.go
+++ b/syncapi/routing/memberships.go
@@ -59,14 +59,21 @@ func GetMemberships(
 	syncDB storage.Database, rsAPI api.SyncRoomserverAPI,
 	joinedOnly bool, membership, notMembership *string, at string,
 ) util.JSONResponse {
+	userID, err := spec.NewUserID(device.UserID, true)
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: spec.InvalidParam("Device UserID is invalid"),
+		}
+	}
 	queryReq := api.QueryMembershipForUserRequest{
 		RoomID: roomID,
-		UserID: device.UserID,
+		UserID: *userID,
 	}
 
 	var queryRes api.QueryMembershipForUserResponse
-	if err := rsAPI.QueryMembershipForUser(req.Context(), &queryReq, &queryRes); err != nil {
-		util.GetLogger(req.Context()).WithError(err).Error("rsAPI.QueryMembershipsForRoom failed")
+	if queryErr := rsAPI.QueryMembershipForUser(req.Context(), &queryReq, &queryRes); queryErr != nil {
+		util.GetLogger(req.Context()).WithError(queryErr).Error("rsAPI.QueryMembershipsForRoom failed")
 		return util.JSONResponse{
 			Code: http.StatusInternalServerError,
 			JSON: spec.InternalServerError{},
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index 781fd53e7..6784a27bd 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -296,9 +296,13 @@ func OnIncomingMessagesRequest(
 }
 
 func getMembershipForUser(ctx context.Context, roomID, userID string, rsAPI api.SyncRoomserverAPI) (resp api.QueryMembershipForUserResponse, err error) {
+	fullUserID, err := spec.NewUserID(userID, true)
+	if err != nil {
+		return resp, err
+	}
 	req := api.QueryMembershipForUserRequest{
 		RoomID: roomID,
-		UserID: userID,
+		UserID: *fullUserID,
 	}
 	if err := rsAPI.QueryMembershipForUser(ctx, &req, &resp); err != nil {
 		return api.QueryMembershipForUserResponse{}, err
diff --git a/syncapi/routing/relations.go b/syncapi/routing/relations.go
index f21c684c8..6efa065a9 100644
--- a/syncapi/routing/relations.go
+++ b/syncapi/routing/relations.go
@@ -119,9 +119,18 @@ func Relations(
 		if err == nil && userID != nil {
 			sender = *userID
 		}
+
+		sk := event.StateKey()
+		if sk != nil && *sk != "" {
+			skUserID, err := rsAPI.QueryUserIDForSender(req.Context(), event.RoomID(), spec.SenderID(*event.StateKey()))
+			if err == nil && skUserID != nil {
+				skString := skUserID.String()
+				sk = &skString
+			}
+		}
 		res.Chunk = append(
 			res.Chunk,
-			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender),
+			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender, sk),
 		)
 	}
 
diff --git a/syncapi/routing/search.go b/syncapi/routing/search.go
index add50b181..7d9182f47 100644
--- a/syncapi/routing/search.go
+++ b/syncapi/routing/search.go
@@ -235,6 +235,15 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 		if err == nil && userID != nil {
 			sender = *userID
 		}
+
+		sk := event.StateKey()
+		if sk != nil && *sk != "" {
+			skUserID, err := rsAPI.QueryUserIDForSender(req.Context(), event.RoomID(), spec.SenderID(*event.StateKey()))
+			if err == nil && skUserID != nil {
+				skString := skUserID.String()
+				sk = &skString
+			}
+		}
 		results = append(results, Result{
 			Context: SearchContextResponse{
 				Start: startToken.String(),
@@ -248,7 +257,7 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 				ProfileInfo: profileInfos,
 			},
 			Rank:   eventScore[event.EventID()].Score,
-			Result: synctypes.ToClientEvent(event, synctypes.FormatAll, sender),
+			Result: synctypes.ToClientEvent(event, synctypes.FormatAll, sender, sk),
 		})
 		roomGroup := groups[event.RoomID()]
 		roomGroup.Results = append(roomGroup.Results, event.EventID())
diff --git a/syncapi/storage/shared/storage_consumer.go b/syncapi/storage/shared/storage_consumer.go
index 5bd3b1f01..799e3d166 100644
--- a/syncapi/storage/shared/storage_consumer.go
+++ b/syncapi/storage/shared/storage_consumer.go
@@ -507,8 +507,20 @@ func (d *Database) CleanSendToDeviceUpdates(
 
 // getMembershipFromEvent returns the value of content.membership iff the event is a state event
 // with type 'm.room.member' and state_key of userID. Otherwise, an empty string is returned.
-func getMembershipFromEvent(ev gomatrixserverlib.PDU, userID string) (string, string) {
-	if ev.Type() != "m.room.member" || !ev.StateKeyEquals(userID) {
+func getMembershipFromEvent(ctx context.Context, ev gomatrixserverlib.PDU, userID string, rsAPI api.SyncRoomserverAPI) (string, string) {
+	if ev.StateKey() == nil || *ev.StateKey() == "" {
+		return "", ""
+	}
+	fullUser, err := spec.NewUserID(userID, true)
+	if err != nil {
+		return "", ""
+	}
+	senderID, err := rsAPI.QuerySenderIDForUser(ctx, ev.RoomID(), *fullUser)
+	if err != nil {
+		return "", ""
+	}
+
+	if ev.Type() != "m.room.member" || !ev.StateKeyEquals(string(senderID)) {
 		return "", ""
 	}
 	membership, err := ev.Membership()
diff --git a/syncapi/storage/shared/storage_sync.go b/syncapi/storage/shared/storage_sync.go
index df9613850..8e79b71df 100644
--- a/syncapi/storage/shared/storage_sync.go
+++ b/syncapi/storage/shared/storage_sync.go
@@ -430,7 +430,7 @@ func (d *DatabaseTransaction) GetStateDeltas(
 		for _, ev := range stateStreamEvents {
 			// Look for our membership in the state events and skip over any
 			// membership events that are not related to us.
-			membership, prevMembership := getMembershipFromEvent(ev.PDU, userID)
+			membership, prevMembership := getMembershipFromEvent(ctx, ev.PDU, userID, rsAPI)
 			if membership == "" {
 				continue
 			}
@@ -556,7 +556,7 @@ func (d *DatabaseTransaction) GetStateDeltasForFullStateSync(
 
 	for roomID, stateStreamEvents := range state {
 		for _, ev := range stateStreamEvents {
-			if membership, _ := getMembershipFromEvent(ev.PDU, userID); membership != "" {
+			if membership, _ := getMembershipFromEvent(ctx, ev.PDU, userID, rsAPI); membership != "" {
 				if membership != spec.Join { // We've already added full state for all joined rooms above.
 					deltas[roomID] = types.StateDelta{
 						Membership:    membership,
diff --git a/syncapi/streams/stream_invite.go b/syncapi/streams/stream_invite.go
index a8b0a7b66..3a5badd92 100644
--- a/syncapi/streams/stream_invite.go
+++ b/syncapi/streams/stream_invite.go
@@ -70,11 +70,20 @@ func (p *InviteStreamProvider) IncrementalSync(
 			user = *sender
 		}
 
+		sk := inviteEvent.StateKey()
+		if sk != nil && *sk != "" {
+			skUserID, err := p.rsAPI.QueryUserIDForSender(ctx, inviteEvent.RoomID(), spec.SenderID(*inviteEvent.StateKey()))
+			if err == nil && skUserID != nil {
+				skString := skUserID.String()
+				sk = &skString
+			}
+		}
+
 		// skip ignored user events
 		if _, ok := req.IgnoredUsers.List[user.String()]; ok {
 			continue
 		}
-		ir := types.NewInviteResponse(inviteEvent, user)
+		ir := types.NewInviteResponse(inviteEvent, user, sk)
 		req.Response.Rooms.Invite[roomID] = ir
 	}
 
diff --git a/syncapi/streams/stream_pdu.go b/syncapi/streams/stream_pdu.go
index d214980bd..f728d4aea 100644
--- a/syncapi/streams/stream_pdu.go
+++ b/syncapi/streams/stream_pdu.go
@@ -605,13 +605,17 @@ func (p *PDUStreamProvider) lazyLoadMembers(
 			// If this is a gapped incremental sync, we still want this membership
 			isGappedIncremental := limited && incremental
 			// We want this users membership event, keep it in the list
-			stateKey := *event.StateKey()
-			if _, ok := timelineUsers[stateKey]; ok || isGappedIncremental || stateKey == device.UserID {
+			userID := ""
+			stateKeyUserID, err := p.rsAPI.QueryUserIDForSender(ctx, roomID, spec.SenderID(*event.StateKey()))
+			if err == nil && stateKeyUserID != nil {
+				userID = stateKeyUserID.String()
+			}
+			if _, ok := timelineUsers[userID]; ok || isGappedIncremental || userID == device.UserID {
 				newStateEvents = append(newStateEvents, event)
 				if !stateFilter.IncludeRedundantMembers {
-					p.lazyLoadCache.StoreLazyLoadedUser(device, roomID, stateKey, event.EventID())
+					p.lazyLoadCache.StoreLazyLoadedUser(device, roomID, userID, event.EventID())
 				}
-				delete(timelineUsers, stateKey)
+				delete(timelineUsers, userID)
 			}
 		} else {
 			newStateEvents = append(newStateEvents, event)
diff --git a/syncapi/syncapi.go b/syncapi/syncapi.go
index ecbe05dd8..64a4af757 100644
--- a/syncapi/syncapi.go
+++ b/syncapi/syncapi.go
@@ -60,7 +60,7 @@ func AddPublicRoutes(
 	}
 
 	eduCache := caching.NewTypingCache()
-	notifier := notifier.NewNotifier()
+	notifier := notifier.NewNotifier(rsAPI)
 	streams := streams.NewSyncStreamProviders(syncDB, userAPI, rsAPI, eduCache, caches, notifier)
 	notifier.SetCurrentPosition(streams.Latest(context.Background()))
 	if err = notifier.Load(context.Background(), syncDB); err != nil {
diff --git a/syncapi/synctypes/clientevent.go b/syncapi/synctypes/clientevent.go
index 66fb1d01f..358a0c971 100644
--- a/syncapi/synctypes/clientevent.go
+++ b/syncapi/synctypes/clientevent.go
@@ -55,18 +55,27 @@ func ToClientEvents(serverEvs []gomatrixserverlib.PDU, format ClientEventFormat,
 		if err == nil && userID != nil {
 			sender = *userID
 		}
-		evs = append(evs, ToClientEvent(se, format, sender))
+
+		sk := se.StateKey()
+		if sk != nil && *sk != "" {
+			skUserID, err := userIDForSender(se.RoomID(), spec.SenderID(*sk))
+			if err == nil && skUserID != nil {
+				skString := skUserID.String()
+				sk = &skString
+			}
+		}
+		evs = append(evs, ToClientEvent(se, format, sender, sk))
 	}
 	return evs
 }
 
 // ToClientEvent converts a single server event to a client event.
-func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender spec.UserID) ClientEvent {
+func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender spec.UserID, stateKey *string) ClientEvent {
 	ce := ClientEvent{
 		Content:        spec.RawJSON(se.Content()),
 		Sender:         sender.String(),
 		Type:           se.Type(),
-		StateKey:       se.StateKey(),
+		StateKey:       stateKey,
 		Unsigned:       spec.RawJSON(se.Unsigned()),
 		OriginServerTS: se.OriginServerTS(),
 		EventID:        se.EventID(),
@@ -77,3 +86,23 @@ func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender sp
 	}
 	return ce
 }
+
+// ToClientEvent converts a single server event to a client event.
+// It provides default logic for event.SenderID & event.StateKey -> userID conversions.
+func ToClientEventDefault(userIDQuery spec.UserIDForSender, event gomatrixserverlib.PDU) ClientEvent {
+	sender := spec.UserID{}
+	userID, err := userIDQuery(event.RoomID(), event.SenderID())
+	if err == nil && userID != nil {
+		sender = *userID
+	}
+
+	sk := event.StateKey()
+	if sk != nil && *sk != "" {
+		skUserID, err := userIDQuery(event.RoomID(), spec.SenderID(*event.StateKey()))
+		if err == nil && skUserID != nil {
+			skString := skUserID.String()
+			sk = &skString
+		}
+	}
+	return ToClientEvent(event, FormatAll, sender, sk)
+}
diff --git a/syncapi/synctypes/clientevent_test.go b/syncapi/synctypes/clientevent_test.go
index 341795081..63c65b2af 100644
--- a/syncapi/synctypes/clientevent_test.go
+++ b/syncapi/synctypes/clientevent_test.go
@@ -48,7 +48,8 @@ func TestToClientEvent(t *testing.T) { // nolint: gocyclo
 	if err != nil {
 		t.Fatalf("failed to create userID: %s", err)
 	}
-	ce := ToClientEvent(ev, FormatAll, *userID)
+	sk := ""
+	ce := ToClientEvent(ev, FormatAll, *userID, &sk)
 	if ce.EventID != ev.EventID() {
 		t.Errorf("ClientEvent.EventID: wanted %s, got %s", ev.EventID(), ce.EventID)
 	}
@@ -107,7 +108,8 @@ func TestToClientFormatSync(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create userID: %s", err)
 	}
-	ce := ToClientEvent(ev, FormatSync, *userID)
+	sk := ""
+	ce := ToClientEvent(ev, FormatSync, *userID, &sk)
 	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 a3dc7f54b..cb3c362d5 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, userID spec.UserID) *InviteResponse {
+func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID, stateKey *string) *InviteResponse {
 	res := InviteResponse{}
 	res.InviteState.Events = []json.RawMessage{}
 
@@ -552,7 +552,7 @@ func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID) *InviteRe
 
 	// 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, userID)
+	inviteEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatSync, userID, stateKey)
 	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 a79ce5417..c1b7f70bd 100644
--- a/syncapi/types/types_test.go
+++ b/syncapi/types/types_test.go
@@ -65,8 +65,14 @@ func TestNewInviteResponse(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	skUserID, err := spec.NewUserID("@neilalexander:dendrite.neilalexander.dev", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	skString := skUserID.String()
+	sk := &skString
 
-	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev}, *sender)
+	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev}, *sender, sk)
 	j, err := json.Marshal(res)
 	if err != nil {
 		t.Fatal(err)
diff --git a/userapi/consumers/roomserver.go b/userapi/consumers/roomserver.go
index df507eb26..b2dc477aa 100644
--- a/userapi/consumers/roomserver.go
+++ b/userapi/consumers/roomserver.go
@@ -306,7 +306,16 @@ func (s *OutputRoomEventConsumer) processMessage(ctx context.Context, event *rst
 		if queryErr == nil && userID != nil {
 			sender = *userID
 		}
-		cevent := synctypes.ToClientEvent(event, synctypes.FormatAll, sender)
+
+		sk := event.StateKey()
+		if sk != nil && *sk != "" {
+			skUserID, queryErr := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), spec.SenderID(*event.StateKey()))
+			if queryErr == nil && skUserID != nil {
+				skString := skUserID.String()
+				sk = &skString
+			}
+		}
+		cevent := synctypes.ToClientEvent(event, synctypes.FormatAll, sender, sk)
 		var member *localMembership
 		member, err = newLocalMembership(&cevent)
 		if err != nil {
@@ -539,12 +548,21 @@ func (s *OutputRoomEventConsumer) notifyLocal(ctx context.Context, event *rstype
 	if err == nil && userID != nil {
 		sender = *userID
 	}
+
+	sk := event.StateKey()
+	if sk != nil && *sk != "" {
+		skUserID, queryErr := s.rsAPI.QueryUserIDForSender(ctx, event.RoomID(), spec.SenderID(*event.StateKey()))
+		if queryErr == nil && skUserID != nil {
+			skString := skUserID.String()
+			sk = &skString
+		}
+	}
 	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, sender),
+		Event: synctypes.ToClientEvent(event, synctypes.FormatSync, sender, sk),
 		// 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
@@ -792,10 +810,20 @@ func (s *OutputRoomEventConsumer) notifyHTTP(ctx context.Context, event *rstypes
 				Type:     event.Type(),
 			},
 		}
-		if mem, err := event.Membership(); err == nil {
+		if mem, memberErr := event.Membership(); memberErr == nil {
 			req.Notification.Membership = mem
 		}
-		if event.StateKey() != nil && *event.StateKey() == fmt.Sprintf("@%s:%s", localpart, s.cfg.Matrix.ServerName) {
+		userID, err := spec.NewUserID(fmt.Sprintf("@%s:%s", localpart, s.cfg.Matrix.ServerName), true)
+		if err != nil {
+			logger.WithError(err).Errorf("Failed to convert local user to userID %s", localpart)
+			return nil, err
+		}
+		localSender, err := s.rsAPI.QuerySenderIDForUser(ctx, event.RoomID(), *userID)
+		if err != nil {
+			logger.WithError(err).Errorf("Failed to get local user senderID for room %s: %s", userID.String(), event.RoomID())
+			return nil, err
+		}
+		if event.StateKey() != nil && *event.StateKey() == string(localSender) {
 			req.Notification.UserIsTarget = true
 		}
 	}
diff --git a/userapi/util/notify_test.go b/userapi/util/notify_test.go
index 27dd373c2..3017069bc 100644
--- a/userapi/util/notify_test.go
+++ b/userapi/util/notify_test.go
@@ -104,8 +104,9 @@ func TestNotifyUserCountsAsync(t *testing.T) {
 		if err != nil {
 			t.Error(err)
 		}
+		sk := ""
 		if err := db.InsertNotification(ctx, aliceLocalpart, serverName, dummyEvent.EventID(), 0, nil, &api.Notification{
-			Event: synctypes.ToClientEvent(dummyEvent, synctypes.FormatAll, *sender),
+			Event: synctypes.ToClientEvent(dummyEvent, synctypes.FormatAll, *sender, &sk),
 		}); err != nil {
 			t.Error(err)
 		}