[Sync] Add intelligent re-encryption support.

chrome/browser/sync/util/cryptographer.cc

 // Copyright (c) 2011 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 <algorithm>
 
 #include "base/base64.h"
 #include "chrome/browser/sync/util/cryptographer.h"
 #include "chrome/browser/password_manager/encryptor.h"
 
@@ skipping 37 matching lines @@
 
 bool Cryptographer::CanDecrypt(const sync_pb::EncryptedData& data) const {
   return nigoris_.end() != nigoris_.find(data.key_name());
 }
 
 bool Cryptographer::CanDecryptUsingDefaultKey(
     const sync_pb::EncryptedData& data) const {
   return default_nigori_ && (data.key_name() == default_nigori_->first);
 }
 
-bool Cryptographer::Encrypt(const ::google::protobuf::MessageLite& message,
-                            sync_pb::EncryptedData* encrypted) const {
-  if (!encrypted || !default_nigori_) {
+bool Cryptographer::Encrypt(
+    const ::google::protobuf::MessageLite& message,
+    sync_pb::EncryptedData* encrypted) const {
+  DCHECK(encrypted);
+  if (!default_nigori_) {
     LOG(ERROR) << "Cryptographer not ready, failed to encrypt.";
     return false;
   }
 
   std::string serialized;
   if (!message.SerializeToString(&serialized)) {
     LOG(ERROR) << "Message is invalid/missing a required field.";
     return false;
   }
 
+  if (CanDecryptUsingDefaultKey(*encrypted)) {
+    const std::string& original_serialized = DecryptToString(*encrypted);
+    if (original_serialized == serialized) {
+      DVLOG(2) << "Re-encryption unnecessary, encrypted data already matches.";
+      return true;
+    }
+  }
+
   encrypted->set_key_name(default_nigori_->first);
   if (!default_nigori_->second->Encrypt(serialized,
                                         encrypted->mutable_blob())) {
     LOG(ERROR) << "Failed to encrypt data.";
     return false;
   }
   return true;
 }
 
 bool Cryptographer::Decrypt(const sync_pb::EncryptedData& encrypted,
@@ skipping 330 matching lines @@
         continue;
       }
       nigoris_[key.name()] = make_linked_ptr(new_nigori.release());
     }
   }
   DCHECK(nigoris_.end() != nigoris_.find(default_key_name));
   default_nigori_ = &*nigoris_.find(default_key_name);
 }
 
 }  // namespace browser_sync