Configure encryption on m.room.encryption events

This is the first step in having a cross-room "enable encryption" button.

If encryption is enabled, add an event handler which will set up encryption
when we receive an m.room.encryption event (either at initial /sync, or in
subsequent syncs.)

lib/client.js

@@ -142,6 +142,7 @@ function MatrixClient(opts) {
         this.sessionStore.storeEndToEndDevicesForUser(
             opts.userId, myDevices
         );
+        setupCryptoEventHandler(this);
     }
     this.scheduler = opts.scheduler;
     if (this.scheduler) {
@@ -632,6 +633,44 @@ MatrixClient.prototype.isEventSenderVerified = function(event) {
     return false;
 };
 
+/**
+ * Register a listener for m.room.encryption events which will enable encryption
+ * for a room.
+ *
+ * @param {MatrixClient} client
+ */
+function setupCryptoEventHandler(client) {
+    client.on("event", function(event) {
+        if (event.getType() != "m.room.encryption") {
+            return;
+        }
+        onCryptoEvent(client, event);
+    });
+}
+
+function onCryptoEvent(client, event) {
+    var roomId = event.getRoomId();
+
+    // if we already have encryption in this room, we should ignore this event
+    // (for now at least. maybe we should alert the user somehow?)
+    var content = event.getContent();
+    var existingConfig = client.sessionStore.getEndToEndRoom(roomId);
+    if (existingConfig) {
+        if (JSON.stringify(existingConfig) != JSON.stringify(content)) {
+            console.error("Ignoring m.room.encryption event which requests " +
+                          "a change of config in " + roomId);
+            return;
+        }
+    }
+
+    try {
+        client.setRoomEncryption(roomId, content).done();
+    } catch (e) {
+        console.error("Error configuring encryption in room " + roomId +
+                      ": " + e);
+    }
+}
+
 /**
  * Enable end-to-end encryption for a room.
  * @param {string} roomId The room ID to enable encryption in.
@@ -640,12 +679,16 @@ MatrixClient.prototype.isEventSenderVerified = function(event) {
  */
 MatrixClient.prototype.setRoomEncryption = function(roomId, config) {
     if (!this._olmDevice) {
-        return q.reject(new Error("End-to-End encryption disabled"));
+        throw new Error("End-to-End encryption disabled");
     }
 
     var self = this;
 
     if (config.algorithm === OLM_ALGORITHM) {
+        // remove spurious keys
+        config = {
+            algorithm: OLM_ALGORITHM,
+        };
         this.sessionStore.storeEndToEndRoom(roomId, config);
 
         var room = this.getRoom(roomId);