[Sync] Do not use base::RandBytesAsString to generate random string

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/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/events/commit_request_event.h"
 #include "components/sync/engine_impl/events/commit_response_event.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;
+
+const char kLegalCharacters[] =
+    "abcdefghijklmnopqrstuvwxyz"
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    "0123456789"
+    " .-,/\\:;";
+
+const int first_char_index = 0;
+const int last_char_index = sizeof(kLegalCharacters) - 2;
+
+std::string RandomString(int length) {

[maxbogue, 2017/01/20 01:24:56]

I was poking around trying to understand RandInt and sizeof and found this:

https://cs.chromium.org/chromium/src/components/history/core/browser/delete_d...

What do you think of copying that approach instead of hard-coding
kLegalCharacters? I think it's cleaner, though you should keep the
.reserve(length) call yours has.

[Gang Wu, 2017/01/20 07:46:56]

Done.

+  std::string random;
+  random.reserve(length);
+  for (int i = 0; i < length; ++i) {
+    random.push_back(
+        kLegalCharacters[base::RandInt(first_char_index, last_char_index)]);
+  }
+
+  return random;
 }
 
+}  // namespace
+
 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 20 matching lines @@
 
   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));
+    commit_message->set_padding(RandomString(kPaddingSize));
   }
 
   // Set extensions activity if bookmark commits are present.
   ExtensionsActivity::Records extensions_activity_buffer;
   if (extensions_activity != nullptr) {
     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);
     }
@@ skipping 110 matching lines @@
 
 void Commit::CleanUp() {
   for (ContributionMap::const_iterator it = contributions_.begin();
        it != contributions_.end(); ++it) {
     it->second->CleanUp();
   }
   cleaned_up_ = true;
 }
 
 }  // namespace syncer