Reset megolm session when people join/leave the room

lib/crypto-algorithms/base.js

@@ -90,6 +90,13 @@ EncryptionAlgorithm.prototype.initRoomEncryption = function(roomMembers) {
  * @return {module:client.Promise} Promise which resolves to the new event body
  */
 
+/**
+ * Called when the membership of a member of the room changes.
+ *
+ * @param {module:models/event.MatrixEvent} event  event causing the change
+ * @param {module:models/room-member} member  user whose membership changed
+ */
+EncryptionAlgorithm.prototype.onRoomMembership = function(event, member) {};
 
 /**
  * base type for decryption implementations
@@ -125,7 +132,7 @@ module.exports.DecryptionAlgorithm = DecryptionAlgorithm;
  *
  * @method module:crypto-algorithms/base.DecryptionAlgorithm#onRoomKeyEvent
  *
- * @param {module:modules/event~MatrixEvent} event key event
+ * @param {module:models/event.MatrixEvent} event key event
  */
 DecryptionAlgorithm.prototype.onRoomKeyEvent = function(params) {
     // ignore by default

lib/crypto-algorithms/megolm.js

@@ -40,6 +40,7 @@ function MegolmEncryption(params) {
     base.EncryptionAlgorithm.call(this, params);
     this._prepPromise = null;
     this._outboundSessionId = null;
+    this._discardNewSession = false;
 }
 utils.inherits(MegolmEncryption, base.EncryptionAlgorithm);
 
@@ -59,7 +60,7 @@ MegolmEncryption.prototype._ensureOutboundSession = function(room) {
 
     if (this._outboundSessionId) {
         // prep already done
-        return q();
+        return q(this._outboundSessionId);
     }
 
     var session_id = this._olmDevice.createOutboundGroupSession();
@@ -138,11 +139,19 @@ MegolmEncryption.prototype._ensureOutboundSession = function(room) {
             undefined, "PUT", path, undefined, encryptedContent
         );
     }).then(function() {
-        // don't set this until the keys are sent successfully; if we get an
-        // error, the user can restart by resending the message.
-        self._outboundSessionId = session_id;
+        if (self._discardNewSession) {
+            // we've had cause to reset the session_id since starting this process.
+            // we'll use the current session for any currently pending events, but
+            // don't save it as the current _outboundSessionId, so that new events
+            // will use a new session.
+            console.log("Session generation complete, but discarding");
+        } else {
+            self._outboundSessionId = session_id;
+        }
+        return session_id;
     }).finally(function() {
         self._prepPromise = null;
+        self._discardNewSession = false;
     });
 
     return this._prepPromise;
@@ -159,7 +168,7 @@ MegolmEncryption.prototype._ensureOutboundSession = function(room) {
  */
 MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     var self = this;
-    return this._ensureOutboundSession(room).then(function() {
+    return this._ensureOutboundSession(room).then(function(session_id) {
         var payloadJson = {
             room_id: self._roomId,
             type: eventType,
@@ -167,14 +176,14 @@ MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
         };
 
         var ciphertext = self._olmDevice.encryptGroupMessage(
-            self._outboundSessionId, JSON.stringify(payloadJson)
+            session_id, JSON.stringify(payloadJson)
         );
 
         var encryptedContent = {
             algorithm: olmlib.MEGOLM_ALGORITHM,
             sender_key: self._olmDevice.deviceCurve25519Key,
             body: ciphertext,
-            session_id: self._outboundSessionId,
+            session_id: session_id,
             signature: "FIXME",
         };
 
@@ -182,6 +191,36 @@ MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     });
 };
 
+/**
+ * @inheritdoc
+ *
+ * @param {module:models/event.MatrixEvent} event  event causing the change
+ * @param {module:models/room-member} member  user whose membership changed
+ */
+MegolmEncryption.prototype.onRoomMembership = function(event, member) {
+    // start a new outbound session whenever someone joins or leaves the room.
+    //
+    // technically we don't need to reset on all membership transitions (eg,
+    // leave->ban), but we might as well.
+
+    // when people join the room, we could get away with sharing the current
+    // state of the ratchet with them; however, it's somewhat easier for now
+    // just to reset the session and start a new one.
+
+    if (this._outboundSessionId) {
+        console.log("Discarding outbound megolm session due to change in " +
+                    "membership of " + member.userId);
+        this._outboundSessionId = null;
+    }
+
+    if (this._prepPromise) {
+        console.log("Discarding as-yet-incomplete megolm session due to " +
+                    "change in membership of " + member.userId);
+        this._discardNewSession = true;
+    }
+};
+
+
 /**
  * Megolm decryption implementation
  *
@@ -231,7 +270,7 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
 /**
  * @inheritdoc
  *
- * @param {module:modules/event~MatrixEvent} event key event
+ * @param {module:models/event.MatrixEvent} event key event
  */
 MegolmDecryption.prototype.onRoomKeyEvent = function(event) {
     console.log("Adding key from ", event);