[Sync] Enable Compression from Client to Server by experiment

components/sync/engine_impl/commit.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/engine_impl/commit.h"
 
 #include <utility>
 
+#include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/rand_util.h"
 #include "base/trace_event/trace_event.h"
 #include "components/sync/base/data_type_histogram.h"
 #include "components/sync/engine/events/commit_request_event.h"
 #include "components/sync/engine/events/commit_response_event.h"
+#include "components/sync/engine/net/http_bridge.h"
 #include "components/sync/engine_impl/commit_processor.h"
 #include "components/sync/engine_impl/commit_util.h"
 #include "components/sync/engine_impl/cycle/sync_cycle.h"
 #include "components/sync/engine_impl/syncer.h"
 #include "components/sync/engine_impl/syncer_proto_util.h"
 
 namespace syncer {
 
+namespace {
+// The number of random ASCII bytes we'll add to CommitMessage. We choose 256
+// because it is not too large (to hurt performance and compression ratio), but
+// it is not too small to easily be canceled out using statistical analysis.
+const size_t kPaddingSize = 256;
+}
+
 Commit::Commit(ContributionMap contributions,
                const sync_pb::ClientToServerMessage& message,
                ExtensionsActivity::Records extensions_activity_buffer)
     : contributions_(std::move(contributions)),
       message_(message),
       extensions_activity_buffer_(extensions_activity_buffer),
       cleaned_up_(false) {}
 
 Commit::~Commit() {
   DCHECK(cleaned_up_);
@@ skipping 18 matching lines @@
   if (contributions.empty())
     return nullptr;
 
   sync_pb::ClientToServerMessage message;
   message.set_message_contents(sync_pb::ClientToServerMessage::COMMIT);
   message.set_share(account_name);
 
   sync_pb::CommitMessage* commit_message = message.mutable_commit();
   commit_message->set_cache_guid(cache_guid);
 
+  // Set padding to mitigate CRIME attack.
+  if (base::FeatureList::IsEnabled(syncer::kSyncClientToServerCompression)) {
+    commit_message->set_padding(base::RandBytesAsString(kPaddingSize));
+  }
+
   // Set extensions activity if bookmark commits are present.
   ExtensionsActivity::Records extensions_activity_buffer;
   ContributionMap::const_iterator it = contributions.find(BOOKMARKS);
   if (it != contributions.end() && it->second->GetNumEntries() != 0) {
     commit_util::AddExtensionsActivityToMessage(
         extensions_activity, &extensions_activity_buffer, commit_message);
   }
 
   // Set the client config params.
   commit_util::AddClientConfigParamsToMessage(
@@ skipping 105 matching lines @@
 
 void Commit::CleanUp() {
   for (ContributionMap::const_iterator it = contributions_.begin();
        it != contributions_.end(); ++it) {
     it->second->CleanUp();
   }
   cleaned_up_ = true;
 }
 
 }  // namespace syncer