[Sync] Add intelligent re-encryption support.

chrome/browser/sync/util/cryptographer.cc

Index: chrome/browser/sync/util/cryptographer.cc
diff --git a/chrome/browser/sync/util/cryptographer.cc b/chrome/browser/sync/util/cryptographer.cc
index 59e96103ba71c56891b7dc3a0baf41e69f35a40a..9db70c85cb7fd48444418a038bc156f8d17416e4 100644
--- a/chrome/browser/sync/util/cryptographer.cc
+++ b/chrome/browser/sync/util/cryptographer.cc
@@ -57,9 +57,36 @@ bool Cryptographer::CanDecryptUsingDefaultKey(
   return default_nigori_ && (data.key_name() == default_nigori_->first);
 }
 
+bool Cryptographer::EncryptIfDifferent(
+    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)) {
+    std::string original_serialized = DecryptToString(*encrypted);
+    if (original_serialized == serialized) {
+      DVLOG(2) << "Re-encryption unnecessary, encrypted data already matches.";
+      return true;
+    }
+  }
+
+  return EncryptImpl(serialized, default_nigori_, encrypted);
+}
+
 bool Cryptographer::Encrypt(const ::google::protobuf::MessageLite& message,
                             sync_pb::EncryptedData* encrypted) const {
-  if (!encrypted || !default_nigori_) {
+  DCHECK(encrypted);
+  if (!default_nigori_) {
     LOG(ERROR) << "Cryptographer not ready, failed to encrypt.";
     return false;
   }
@@ -70,9 +97,15 @@ bool Cryptographer::Encrypt(const ::google::protobuf::MessageLite& message,
     return false;
   }
 
+  return EncryptImpl(serialized, default_nigori_, encrypted);
+}
+
+bool Cryptographer::EncryptImpl(const std::string& serialized,
+                                const NigoriMap::value_type* nigori,
+                                sync_pb::EncryptedData* encrypted) const {
   encrypted->set_key_name(default_nigori_->first);
-  if (!default_nigori_->second->Encrypt(serialized,
-                                        encrypted->mutable_blob())) {
+  if (!nigori->second->Encrypt(serialized,
+                               encrypted->mutable_blob())) {
     LOG(ERROR) << "Failed to encrypt data.";
     return false;
   }
@@ -119,7 +152,7 @@ bool Cryptographer::GetKeys(sync_pb::EncryptedData* encrypted) const {
   }
 
   // Encrypt the bag with the default Nigori.
-  return Encrypt(bag, encrypted);
+  return EncryptIfDifferent(bag, encrypted);
 }
 
 bool Cryptographer::AddKey(const KeyParams& params) {