Send null messages to mask verified syncs

First construct a null message of random size and contents and send it to the
destination. Then include that same padding in the verification sync.

Note that the sync message is additionally randomly padded like all other sync
messages.

This lets Verified sync messages appear the same as normal sync message traffic.

// FREEBIE

js/libtextsecure.js

@@ -39290,10 +39290,25 @@ MessageSender.prototype = {
         var myNumber = textsecure.storage.user.getNumber();
         var myDevice = textsecure.storage.user.getDeviceId();
         if (myDevice != 1) {
+            // First send a null message to mask the sync message.
+            var nullMessage = new textsecure.protobuf.NullMessage();
+
+            // Generate a random int from 1 and 512
+            var buffer = libsignal.crypto.getRandomBytes(1);
+            var paddingLength = (new Uint8Array(buffer)[0] & 0x1ff) + 1;
+
+            // Generate a random padding buffer of the chosen size
+            nullMessage.padding = libsignal.crypto.getRandomBytes(paddingLength);
+
+            var contentMessage = new textsecure.protobuf.Content();
+            contentMessage.nullMessage = nullMessage;
+
+            return this.sendIndividualProto(destination, contentMessage, Date.now()).then(function() {
                 var verified = new textsecure.protobuf.Verified();
                 verified.state = state;
                 verified.destination = destination;
                 verified.identityKey = identityKey;
+                verified.nullMessage = nullMessage.padding;
 
                 var syncMessage = this.createSyncMessage();
                 syncMessage.verified = verified;
@@ -39302,6 +39317,7 @@ MessageSender.prototype = {
                 contentMessage.syncMessage = syncMessage;
 
                 return this.sendIndividualProto(myNumber, contentMessage, Date.now());
+            }.bind(this));
         }
     },
 

libtextsecure/sendmessage.js

@@ -322,10 +322,25 @@ MessageSender.prototype = {
         var myNumber = textsecure.storage.user.getNumber();
         var myDevice = textsecure.storage.user.getDeviceId();
         if (myDevice != 1) {
+            // First send a null message to mask the sync message.
+            var nullMessage = new textsecure.protobuf.NullMessage();
+
+            // Generate a random int from 1 and 512
+            var buffer = libsignal.crypto.getRandomBytes(1);
+            var paddingLength = (new Uint8Array(buffer)[0] & 0x1ff) + 1;
+
+            // Generate a random padding buffer of the chosen size
+            nullMessage.padding = libsignal.crypto.getRandomBytes(paddingLength);
+
+            var contentMessage = new textsecure.protobuf.Content();
+            contentMessage.nullMessage = nullMessage;
+
+            return this.sendIndividualProto(destination, contentMessage, Date.now()).then(function() {
                 var verified = new textsecure.protobuf.Verified();
                 verified.state = state;
                 verified.destination = destination;
                 verified.identityKey = identityKey;
+                verified.nullMessage = nullMessage.padding;
 
                 var syncMessage = this.createSyncMessage();
                 syncMessage.verified = verified;
@@ -334,6 +349,7 @@ MessageSender.prototype = {
                 contentMessage.syncMessage = syncMessage;
 
                 return this.sendIndividualProto(myNumber, contentMessage, Date.now());
+            }.bind(this));
         }
     },