Send a 'm.new_device' when we get a message for an unknown group session

lib/crypto/OlmDevice.js

@@ -539,7 +539,9 @@ OlmDevice.prototype._saveInboundGroupSession = function(
  * @param {string} senderKey
  * @param {string} sessionId
  * @param {function} func
- * @return {object} result of func
+ * @return {object} Object with two keys "result": result of func, "exists"
+ *  whether the session exists. if the session doesn't exist then the function
+ *  isn't called and the "result" is undefined.
  * @private
  */
 OlmDevice.prototype._getInboundGroupSession = function(
@@ -550,7 +552,7 @@ OlmDevice.prototype._getInboundGroupSession = function(
     );
 
     if (r === null) {
-        throw new Error("Unknown inbound group session id");
+        return {sessionExists: false};
     }
 
     r = JSON.parse(r);
@@ -567,7 +569,7 @@ OlmDevice.prototype._getInboundGroupSession = function(
     var session = new Olm.InboundGroupSession();
     try {
         session.unpickle(this._pickleKey, r.session);
-        return func(session);
+        return {sessionExists: true, result: func(session)};
     } finally {
         session.free();
     }
@@ -603,7 +605,7 @@ OlmDevice.prototype.addInboundGroupSession = function(
  * @param {string} sessionId session identifier
  * @param {string} body      base64-encoded body of the encrypted message
  *
- * @return {string} plaintext
+ * @return {object} {result: "plaintext"|undefined, sessionExists: Boolean}
  */
 OlmDevice.prototype.decryptGroupMessage = function(
     roomId, senderKey, sessionId, body

lib/crypto/algorithms/megolm.js

@@ -272,6 +272,9 @@ MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
             sender_key: self._olmDevice.deviceCurve25519Key,
             ciphertext: ciphertext,
             session_id: session_id,
+             // Include our device ID so that recipients can send us a
+             // m.new_device message if they don't have our session key.
+            device_id: self._deviceId,
         };
 
         return encryptedContent;
@@ -356,7 +359,8 @@ utils.inherits(MegolmDecryption, base.DecryptionAlgorithm);
  *
  * @param {object} event raw event
  *
- * @return {object} decrypted payload (with properties 'type', 'content')
+ * @return {object} object with 'result' key with decrypted payload (with
+ *  properties 'type', 'content') and a 'sessionKey' key.
  *
  * @throws {module:crypto/algorithms/base.DecryptionError} if there is a
  *   problem decrypting the event
@@ -377,7 +381,11 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         var res = this._olmDevice.decryptGroupMessage(
             event.room_id, content.sender_key, content.session_id, content.ciphertext
         );
-        return JSON.parse(res);
+        if (res.sessionExists) {
+            return {result: JSON.parse(res.result), sessionExists: true};
+        } else {
+            return {sessionExists: false};
+        }
     } catch (e) {
         throw new base.DecryptionError(e);
     }

lib/crypto/algorithms/olm.js

@@ -142,7 +142,9 @@ utils.inherits(OlmDecryption, base.DecryptionAlgorithm);
  *
  * @param {object} event raw event
  *
- * @return {object} decrypted payload (with properties 'type', 'content')
+ * @return {object} result object with result property with the decrypted
+ *  payload (with properties 'type', 'content'), and a "sessionExists" key
+ *  always set to true.
  *
  * @throws {module:crypto/algorithms/base.DecryptionError} if there is a
  *   problem decrypting the event
@@ -198,7 +200,7 @@ OlmDecryption.prototype.decryptEvent = function(event) {
     // TODO: Check the sender user id matches the sender key.
     // TODO: check the room_id and fingerprint
     if (payloadString !== null) {
-        return JSON.parse(payloadString);
+        return {result: JSON.parse(payloadString), sessionExists: true};
     } else {
         throw new base.DecryptionError("Bad Encrypted Message");
     }

lib/crypto/index.js

@@ -85,6 +85,8 @@ function Crypto(baseApis, eventEmitter, sessionStore, userId, deviceId) {
     );
 
     _registerEventHandlers(this, eventEmitter);
+
+    this._lastNewDeviceMessageTsByUserDeviceRoom = {};
 }
 
 function _registerEventHandlers(crypto, eventEmitter) {
@@ -820,7 +822,63 @@ Crypto.prototype.decryptEvent = function(event) {
     var alg = new AlgClass({
         olmDevice: this._olmDevice,
     });
-    return alg.decryptEvent(event);
+    var r = alg.decryptEvent(event);
+    if (r.sessionExists) {
+        return r.result;
+    } else {
+        // We've got a message for a session we don't have.
+        // Maybe the sender forgot to tell us about the session.
+        // Remind the sender that we exists so that they might
+        // tell us about the sender.
+        if (event.getRoomId !== undefined && event.getSender !== undefined) {
+            this._sendPingToDevice(
+                event.getSender(), event.content.device, event.getRoomId
+            );
+        }
+
+        throw new algorithms.DecryptionError("Unknown inbound session id");
+    }
+};
+
+/**
+ * Send a "m.new_device" message to remind it that we exist and are a member
+ * of a room.
+ *
+ * This is rate limited to send a message at most once an hour per desination.
+ *
+ * @param {string} userId   The ID of the user to ping.
+ * @param {string} deviceId The ID of the device to ping.
+ * @param {string} roomId   The ID of the room we want to remind them about.
+ */
+Crypto.prototype._sendPingToDevice = function(userId, deviceId, roomId) {
+    if (deviceId === undefined) {
+        deviceId = "*";
+    }
+
+    var lastMessageTsMap = this._lastNewDeviceMessageTsByUserDeviceRoom;
+    var lastTsByDevice = lastMessageTsMap[userId] || {};
+    var lastTsByRoom = lastTsByDevice[deviceId] || {};
+    var lastTs = lastTsByRoom[roomId];
+    var timeNowMs = Date.now();
+    var oneHourMs = 1000 * 60 * 60;
+
+    if (lastTs === undefined || lastTs + oneHourMs < timeNowMs) {
+        var content = {
+            userId: {
+                deviceId: {
+                    device_id: this._deviceId,
+                    rooms: [roomId],
+                }
+            }
+        };
+
+        lastTsByRoom[roomId] = timeNowMs;
+
+        this._baseApis.sendToDevice(
+            "m.new_device", // OH HAI!
+            content
+        ).done(function() {});
+    }
 };
 
 /**