Add view tags to outputs to reduce wallet scanning time

Implements view tags as proposed by @UkoeHB in MRL issue
https://github.com/monero-project/research-lab/issues/73

At tx construction, the sender adds a 1-byte view tag to each
output. The view tag is derived from the sender-receiver
shared secret. When scanning for outputs, the receiver can
check the view tag for a match, in order to reduce scanning
time. When the view tag does not match, the wallet avoids the
more expensive EC operations when deriving the output public
key using the shared secret.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/blockchain_db/lmdb/db_lmdb.cpp b/src/blockchain_db/lmdb/db_lmdb.cpp
index 3522c4a1b..e2ac9df0b 100644
--- a/src/blockchain_db/lmdb/db_lmdb.cpp
+++ b/src/blockchain_db/lmdb/db_lmdb.cpp
@@ -1045,8 +1045,9 @@ uint64_t BlockchainLMDB::add_output(const crypto::hash& tx_hash,
   CURSOR(output_txs)
   CURSOR(output_amounts)
 
-  if (tx_output.target.type() != typeid(txout_to_key))
-    throw0(DB_ERROR("Wrong output type: expected txout_to_key"));
+  crypto::public_key output_public_key;
+  if (!get_output_public_key(tx_output, output_public_key))
+    throw0(DB_ERROR("Could not get an output public key from a tx output."));
   if (tx_output.amount == 0 && !commitment)
     throw0(DB_ERROR("RCT output without commitment"));
 
@@ -1074,7 +1075,7 @@ uint64_t BlockchainLMDB::add_output(const crypto::hash& tx_hash,
   else
     ok.amount_index = 0;
   ok.output_id = m_num_outputs;
-  ok.data.pubkey = boost::get < txout_to_key > (tx_output.target).key;
+  ok.data.pubkey = output_public_key;
   ok.data.unlock_time = unlock_time;
   ok.data.height = m_height;
   if (tx_output.amount == 0)