Make default media device ID salts random by default

chrome/browser/media/media_device_id_salt.cc

 // Copyright 2013 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 "chrome/browser/media/media_device_id_salt.h"
 
 #include "base/base64.h"
 #include "base/rand_util.h"
 #include "chrome/browser/profiles/profile_io_data.h"
 #include "chrome/common/pref_names.h"
 #include "components/prefs/pref_service.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/resource_context.h"
 
 using content::BrowserThread;
 
-namespace {
-
-std::string CreateSalt() {
-  std::string salt;
-  base::Base64Encode(base::RandBytesAsString(16), &salt);
-  DCHECK(!salt.empty());
-  return salt;
-}
-
-}  // namespace
-
-MediaDeviceIDSalt::MediaDeviceIDSalt(PrefService* pref_service,
-                                     bool incognito) {
+MediaDeviceIDSalt::MediaDeviceIDSalt(PrefService* pref_service) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  if (incognito) {
-    incognito_salt_ = CreateSalt();
-    return;
+  media_device_id_salt_.Init(prefs::kMediaDeviceIdSalt, pref_service);
+  if (media_device_id_salt_.GetValue().empty()) {

[jam, 2016/06/14 16:20:33]

i'm confused, the cl description says "This results in different hashed device
IDs on each session, which helps prevent user fingerprinting." which I
understood to mean that there is a different salt each time chrome starts. am i
understanding correctly? If so, then how come the salt is saved in prefs?

[Guido Urdaneta, 2016/06/14 17:30:57]

Clarified via IM and in the updated CL description.

+    media_device_id_salt_.SetValue(
+        content::ResourceContext::CreateRandomMediaDeviceIDSalt());
   }
 
-  media_device_id_salt_.Init(prefs::kMediaDeviceIdSalt, pref_service);
-  if (media_device_id_salt_.GetValue().empty())
-    media_device_id_salt_.SetValue(CreateSalt());
-
   media_device_id_salt_.MoveToThread(
       BrowserThread::GetMessageLoopProxyForThread(BrowserThread::IO));
 }
 
 MediaDeviceIDSalt::~MediaDeviceIDSalt() {
 }
 
 void MediaDeviceIDSalt::ShutdownOnUIThread() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
-  if (incognito_salt_.empty())
-    media_device_id_salt_.Destroy();
+  media_device_id_salt_.Destroy();
 }
 
 std::string MediaDeviceIDSalt::GetSalt() const {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  if (incognito_salt_.size())
-    return incognito_salt_;
   return media_device_id_salt_.GetValue();
 }
 
 void MediaDeviceIDSalt::RegisterProfilePrefs(
     user_prefs::PrefRegistrySyncable* registry) {
   registry->RegisterStringPref(prefs::kMediaDeviceIdSalt, std::string());
 }
 
 void MediaDeviceIDSalt::Reset(PrefService* pref_service) {
-  pref_service->SetString(prefs::kMediaDeviceIdSalt,
-                          CreateSalt());
+  pref_service->SetString(
+      prefs::kMediaDeviceIdSalt,
+      content::ResourceContext::CreateRandomMediaDeviceIDSalt());
 }