New "Halfway RingCT" outputs for coinbase transactions

When RingCT is enabled, outputs from coinbase transactions
are created as a single output, and stored as RingCT output,
with a fake mask. Their amount is not hidden on the blockchain
itself, but they are then able to be used as fake inputs in
a RingCT ring. Since the output amounts are hidden, their
"dustiness" is not an obstacle anymore to mixing, and this
makes the coinbase transactions a lot smaller, as well as
helping the TXO set to grow more slowly.

Also add a new "Null" type of rct signature, which decreases
the size required when no signatures are to be stored, as
in a coinbase tx.

src/cryptonote_core/blockchain.cpp

@@ -959,7 +959,7 @@ bool Blockchain::validate_miner_transaction(const block& b, size_t cumulative_bl
     money_in_use += o.amount;
   partial_block_reward = false;
 
-  if (version >= 3) {
+  if (version == 3) {
     for (auto &o: b.miner_tx.vout) {
       if (!is_valid_decomposed_amount(o.amount)) {
         LOG_PRINT_L1("miner tx output " << print_money(o.amount) << " is not a valid decomposed amount");
@@ -1128,7 +1128,9 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
    block size, so first miner transaction generated with fake amount of money, and with phase we know think we know expected block size
    */
   //make blocks coin-base tx looks close to real coinbase tx to get truthful blob size
-  bool r = construct_miner_tx(height, median_size, already_generated_coins, txs_size, fee, miner_address, b.miner_tx, ex_nonce, 11, m_hardfork->get_current_version());
+  uint8_t hf_version = m_hardfork->get_current_version();
+  size_t max_outs = hf_version >= 4 ? 1 : 11;
+  bool r = construct_miner_tx(height, median_size, already_generated_coins, txs_size, fee, miner_address, b.miner_tx, ex_nonce, max_outs, hf_version);
   CHECK_AND_ASSERT_MES(r, false, "Failed to construc miner tx, first chance");
   size_t cumulative_size = txs_size + get_object_blobsize(b.miner_tx);
 #if defined(DEBUG_CREATE_BLOCK_TEMPLATE)
@@ -1137,7 +1139,7 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
 #endif
   for (size_t try_count = 0; try_count != 10; ++try_count)
   {
-    r = construct_miner_tx(height, median_size, already_generated_coins, cumulative_size, fee, miner_address, b.miner_tx, ex_nonce, 11, m_hardfork->get_current_version());
+    r = construct_miner_tx(height, median_size, already_generated_coins, cumulative_size, fee, miner_address, b.miner_tx, ex_nonce, max_outs, hf_version);
 
     CHECK_AND_ASSERT_MES(r, false, "Failed to construc miner tx, second chance");
     size_t coinbase_blob_size = get_object_blobsize(b.miner_tx);
@@ -2354,17 +2356,6 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
       }
     }
 
-    // for v3, we force txes with all mixable inputs to be rct
-    if (m_hardfork->get_current_version() >= 4)
-    {
-      if (n_unmixable == 0 && tx.version == 1)
-      {
-        LOG_PRINT_L1("Tx " << get_transaction_hash(tx) << " is not rct and does not have unmixable inputs");
-        tvc.m_not_rct = true;
-        return false;
-      }
-    }
-
     if (mixin < 2)
     {
       if (n_unmixable == 0)
@@ -2543,6 +2534,11 @@ bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc,
     const rct::rctSig &rv = tx.rct_signatures;
     switch (rv.type)
     {
+    case rct::RCTTypeNull: {
+      // we only accept no signatures for coinbase txes
+      LOG_PRINT_L1("Null rct signature on non-coinbase tx");
+      return false;
+    }
     case rct::RCTTypeSimple: {
       // check all this, either recontructed (so should really pass), or not
       {

src/cryptonote_core/cryptonote_basic.h

@@ -233,6 +233,8 @@ namespace cryptonote
         FIELD(rct_signatures)
         switch (rct_signatures.type)
         {
+        case rct::RCTTypeNull:
+          break;
         case rct::RCTTypeSimple:
           if (rct_signatures.mixRing.size() && rct_signatures.mixRing.size() != vin.size())
             return false;
@@ -276,6 +278,7 @@ namespace cryptonote
     vout.clear();
     extra.clear();
     signatures.clear();
+    rct_signatures.type = rct::RCTTypeNull;
   }
 
   inline

src/cryptonote_core/cryptonote_boost_serialization.h

@@ -249,6 +249,8 @@ namespace boost
   inline void serialize(Archive &a, rct::rctSigBase &x, const boost::serialization::version_type ver)
   {
     a & x.type;
+    if (x.type == rct::RCTTypeNull)
+      return;
     if (x.type != rct::RCTTypeFull && x.type != rct::RCTTypeSimple)
       throw boost::archive::archive_exception(boost::archive::archive_exception::other_exception, "Unsupported rct type");
     // a & x.message; message is not serialized, as it can be reconstructed from the tx data
@@ -264,6 +266,8 @@ namespace boost
   inline void serialize(Archive &a, rct::rctSig &x, const boost::serialization::version_type ver)
   {
     a & x.type;
+    if (x.type == rct::RCTTypeNull)
+      return;
     if (x.type != rct::RCTTypeFull && x.type != rct::RCTTypeSimple)
       throw boost::archive::archive_exception(boost::archive::archive_exception::other_exception, "Unsupported rct type");
     // a & x.message; message is not serialized, as it can be reconstructed from the tx data

src/cryptonote_core/cryptonote_format_utils.cpp

@@ -136,7 +136,10 @@ namespace cryptonote
     // from hard fork 2, we cut out the low significant digits. This makes the tx smaller, and
     // keeps the paid amount almost the same. The unpaid remainder gets pushed back to the
     // emission schedule
-    if (hard_fork_version >= 2) {
+    // from hard fork 4, we use a single "dusty" output. This makes the tx even smaller,
+    // and avoids the quantization. These outputs will be added as rct outputs with identity
+    // masks, to they can be used as rct inputs.
+    if (hard_fork_version >= 2 && hard_fork_version < 4) {
       block_reward = block_reward - block_reward % ::config::BASE_REWARD_CLAMP_THRESHOLD;
     }
 
@@ -146,12 +149,16 @@ namespace cryptonote
       [&out_amounts](uint64_t a_dust) { out_amounts.push_back(a_dust); });
 
     CHECK_AND_ASSERT_MES(1 <= max_outs, false, "max_out must be non-zero");
-    if (height == 0)
+    if (height == 0 || hard_fork_version >= 4)
     {
       // the genesis block was not decomposed, for unknown reasons
       while (max_outs < out_amounts.size())
       {
-        out_amounts[out_amounts.size() - 2] += out_amounts.back();
+        //out_amounts[out_amounts.size() - 2] += out_amounts.back();
+        //out_amounts.resize(out_amounts.size() - 1);
+        out_amounts[1] += out_amounts[0];
+        for (size_t n = 1; n < out_amounts.size(); ++n)
+          out_amounts[n - 1] = out_amounts[n];
         out_amounts.resize(out_amounts.size() - 1);
       }
     }
@@ -182,7 +189,11 @@ namespace cryptonote
 
     CHECK_AND_ASSERT_MES(summary_amounts == block_reward, false, "Failed to construct miner tx, summary_amounts = " << summary_amounts << " not equal block_reward = " << block_reward);
 
-    tx.version = 1;
+    if (hard_fork_version >= 4)
+      tx.version = 2;
+    else
+      tx.version = 1;
+
     //lock
     tx.unlock_time = height + CRYPTONOTE_MINED_MONEY_UNLOCK_WINDOW;
     tx.vin.push_back(in);
@@ -726,7 +737,7 @@ namespace cryptonote
       // zero out all amounts to mask rct outputs, real amounts are now encrypted
       for (size_t i = 0; i < tx.vin.size(); ++i)
       {
-        if (!(sources[i].mask == rct::identity()))
+        if (sources[i].rct)
           boost::get<txin_to_key>(tx.vin[i]).amount = 0;
       }
       for (size_t i = 0; i < tx.vout.size(); ++i)

src/cryptonote_core/cryptonote_format_utils.h

@@ -58,6 +58,7 @@ namespace cryptonote
     crypto::public_key real_out_tx_key; //incoming real tx public key
     size_t real_output_in_tx_index;     //index in transaction outputs vector
     uint64_t amount;                    //money
+    bool rct;                           //true if the output is rct
     rct::key mask;                      //ringct amount mask
 
     void push_output(uint64_t idx, const crypto::public_key &k, uint64_t amount) { outputs.push_back(std::make_pair(idx, rct::ctkey({rct::pk2rct(k), rct::zeroCommit(amount)}))); }