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, 13 insertions, 1 deletions

diff --git a/src/device/device_default.cpp b/src/device/device_default.cpp
index e7b452d40..d70ece229 100644
--- a/src/device/device_default.cpp
+++ b/src/device/device_default.cpp
@@ -263,6 +263,11 @@ namespace hw {
             return true;
         }
 
+        bool device_default::derive_view_tag(const crypto::key_derivation &derivation, const std::size_t output_index, crypto::view_tag &view_tag) {
+            crypto::derive_view_tag(derivation, output_index, view_tag);
+            return true;
+        }
+
         bool device_default::conceal_derivation(crypto::key_derivation &derivation, const crypto::public_key &tx_pub_key, const std::vector<crypto::public_key> &additional_tx_pub_keys, const crypto::key_derivation &main_derivation, const std::vector<crypto::key_derivation> &additional_derivations){
             return true;
         }
@@ -291,7 +296,8 @@ namespace hw {
                                                             const cryptonote::tx_destination_entry &dst_entr, const boost::optional<cryptonote::account_public_address> &change_addr, const size_t output_index,
                                                             const bool &need_additional_txkeys, const std::vector<crypto::secret_key> &additional_tx_keys,
                                                             std::vector<crypto::public_key> &additional_tx_public_keys,
-                                                            std::vector<rct::key> &amount_keys,  crypto::public_key &out_eph_public_key) {
+                                                            std::vector<rct::key> &amount_keys,  crypto::public_key &out_eph_public_key,
+                                                            const bool use_view_tags, crypto::view_tag &view_tag) {
 
             crypto::key_derivation derivation;
 
@@ -331,6 +337,12 @@ namespace hw {
                 derivation_to_scalar(derivation, output_index, scalar1);
                 amount_keys.push_back(rct::sk2rct(scalar1));
             }
+
+            if (use_view_tags)
+            {
+                derive_view_tag(derivation, output_index, view_tag);
+            }
+
             r = derive_public_key(derivation, output_index, dst_entr.addr.m_spend_public_key, out_eph_public_key);
             CHECK_AND_ASSERT_MES(r, false, "at creation outs: failed to derive_public_key(" << derivation << ", " << output_index << ", "<< dst_entr.addr.m_spend_public_key << ")");