diff --git a/src/matrix/Sync3.ts b/src/matrix/Sync3.ts
index 7062f72b..85c74db6 100644
--- a/src/matrix/Sync3.ts
+++ b/src/matrix/Sync3.ts
@@ -80,11 +80,22 @@ type RoomSubscriptionsRequest = {
     [roomId: string]: RoomSubscriptionRequest
 };
 
+type ExtensionsRequest = {
+    to_device?: ToDeviceExtensionRequest;
+};
+
+type ToDeviceExtensionRequest = {
+    enabled: boolean;
+    since?: string;
+    limit?: number;
+};
+
 interface Sync3RequestBody {
     session_id: string;
     lists: Sync3List[];
     room_subscriptions?: RoomSubscriptionsRequest;
     unsubscribe_rooms?: string[];
+    extensions?: ExtensionsRequest;
 };
 
 // Response types are below: See api.md for what these fields mean.
@@ -95,6 +106,7 @@ interface Sync3Response {
     ops: Op[];
     counts: number[];
     pos: number;
+    extensions: ExtensionsResponse;
 };
 
 interface Op {
@@ -119,6 +131,15 @@ interface RoomResponse {
     highlight_count: number;
 };
 
+type ExtensionsResponse = {
+    to_device?: ToDeviceExtensionResponse;
+};
+
+type ToDeviceExtensionResponse = {
+    next_batch: string;
+    events: any[];
+};
+
 // Some internal data structure types
 
 type IndexToRoomId = {
@@ -322,6 +343,7 @@ export class Sync3 {
 
         // track the number of times we've failed to work out how long to wait between requests
         let backoffCounter = 0;
+        let toDeviceSince; // TODO: load/save this?
 
         while (this.status.get() !== SyncStatus.Stopped) {
             let isFirstSync = this.status.get() === SyncStatus.InitialSync;
@@ -338,6 +360,13 @@ export class Sync3 {
             let requestBody: Sync3RequestBody = {
                 session_id: sessionId,
                 lists: [list],
+                extensions: {
+                    to_device: {
+                        enabled: true,
+                        limit: 100,
+                        since: toDeviceSince,
+                    },
+                },
             };
             if (this.nextRoomSubscriptions.length > 0) {
                 // we may have been interruped before to update the room subscriptions, if this array
@@ -352,6 +381,9 @@ export class Sync3 {
                 let resp: Sync3Response;
                 this.currentRequest = await this.hsApi.sync3(requestBody, pos);
                 resp = await this.currentRequest.response();
+                if (resp.extensions?.to_device?.next_batch) {
+                    toDeviceSince = resp.extensions?.to_device?.next_batch;
+                }
                 backoffCounter = 0;
                 // regardless of whether we process the sync response without error, update the room subs
                 // to reflect the new reality.
@@ -409,8 +441,23 @@ export class Sync3 {
         await this.logger.run("sync3", async log => {
             const syncTxn = await this.openSyncTxn();
             try {
-                // session.prepareSync // E2EE decrypts room keys
-                // this.session.writeSync()  // write account data, device lists, etc.
+                // E2EE decrypts room keys
+                const v2DeviceResponse = {
+                    to_device: {
+                        events: resp.extensions?.to_device?.events,
+                    },
+                };
+                const lock = await this.session.obtainSyncLock(v2DeviceResponse);
+                const sessionPreparation = await this.session.prepareSync(v2DeviceResponse, lock, syncTxn, log);
+                // TODO:
+                // response.device_one_time_keys_count
+                // response.device_lists
+                // response.account_data
+                const changes = await this.session.writeSync(
+                    v2DeviceResponse, null, sessionPreparation, syncTxn, log
+                );
+                await this.session.afterSync(changes, log);
+                // TODO add any rooms with new keys but no sync response to the list of rooms to be synced
                 await Promise.all(updates.map(async (roomResponse) => {
                     await log.wrap("room", async () => {
                         // get or create a room