[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 a24e9cfd36d88d2bcf67cfa1f4141b7976e1291e..5d1e45009dac82da7320d3749196d16945cca5a6 100644
--- a/chrome/browser/sync/util/cryptographer.cc
+++ b/chrome/browser/sync/util/cryptographer.cc
@@ -55,9 +55,11 @@ bool Cryptographer::CanDecryptUsingDefaultKey(
   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;
   }
@@ -68,6 +70,14 @@ bool Cryptographer::Encrypt(const ::google::protobuf::MessageLite& message,
     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())) {