elements code sample

elements.cpp

static bool CreateTransactionInternal(
        CWallet& wallet,
        const std::vector<CRecipient>& vecSend,
        CTransactionRef& tx,
        CAmount& nFeeRet,
        int& nChangePosInOut,
        bilingual_str& error,
        const CCoinControl& coin_control,
        FeeCalculation& fee_calc_out,
        bool sign,
        BlindDetails* blind_details,
        const IssuanceDetails* issuance_details) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
{
    if (blind_details || issuance_details) {
        assert(g_con_elementsmode);
    }

    if (blind_details) {
        // Clear out previous blinding/data info as needed
        resetBlindDetails(blind_details);
    }

    AssertLockHeld(wallet.cs_wallet);

    CMutableTransaction txNew; // The resulting transaction that we make
    txNew.nLockTime = GetLocktimeForNewTransaction(wallet.chain(), wallet.GetLastBlockHash(), wallet.GetLastBlockHeight());

    CoinSelectionParams coin_selection_params; // Parameters for coin selection, init with dummy
    coin_selection_params.m_avoid_partial_spends = coin_control.m_avoid_partial_spends;

    CScript dummy_script = CScript() << 0x00;
    CAmountMap map_recipients_sum;
    // Always assume that we are at least sending policyAsset.
    map_recipients_sum[::policyAsset] = 0;
    std::vector<std::unique_ptr<ReserveDestination>> reservedest;
    // Set the long term feerate estimate to the wallet's consolidate feerate
    coin_selection_params.m_long_term_feerate = wallet.m_consolidate_feerate;
    const OutputType change_type = wallet.TransactionChangeType(coin_control.m_change_type ? *coin_control.m_change_type : wallet.m_default_change_type, vecSend);
    reservedest.emplace_back(new ReserveDestination(&wallet, change_type)); // policy asset

    std::set<CAsset> assets_seen;
    unsigned int outputs_to_subtract_fee_from = 0; // The number of outputs which we are subtracting the fee from
    for (const auto& recipient : vecSend)
    {
        // Pad change keys to cover total possible number of assets
        // One already exists(for policyAsset), so one for each destination
        if (assets_seen.insert(recipient.asset).second) {
            reservedest.emplace_back(new ReserveDestination(&wallet, change_type));
        }

        // Skip over issuance outputs, no need to select those coins
        if (recipient.asset == CAsset(uint256S("1")) || recipient.asset == CAsset(uint256S("2"))) {
            continue;
        }

        map_recipients_sum[recipient.asset] += recipient.nAmount;

        if (recipient.fSubtractFeeFromAmount) {
            outputs_to_subtract_fee_from++;
            coin_selection_params.m_subtract_fee_outputs = true;
        }
    }

    // Create change script that will be used if we need change
    // ELEMENTS: A map that keeps track of the change script for each asset and also
    // the index of the reservedest used for that script (-1 if none).
    std::map<CAsset, std::pair<int, CScript>> mapScriptChange;
    // For manually set change, we need to use the blinding pubkey associated
    // with the manually-set address rather than generating one from the wallet
    std::map<CAsset, std::optional<CPubKey>> mapBlindingKeyChange;

    // coin control: send change to custom address
    if (coin_control.destChange.size() > 0) {
        for (const auto& dest : coin_control.destChange) {
            // No need to test we cover all assets.  We produce error for that later.
            mapScriptChange[dest.first] = std::pair<int, CScript>(-1, GetScriptForDestination(dest.second));
            if (IsBlindDestination(dest.second)) {
                mapBlindingKeyChange[dest.first] = GetDestinationBlindingKey(dest.second);
            } else {
                mapBlindingKeyChange[dest.first] = std::nullopt;
            }
        }
    } else { // no coin control: send change to newly generated address
        // Note: We use a new key here to keep it from being obvious which side is the change.
        //  The drawback is that by not reusing a previous key, the change may be lost if a
        //  backup is restored, if the backup doesn't have the new private key for the change.
        //  If we reused the old key, it would be possible to add code to look for and
        //  rediscover unknown transactions that were written with keys of ours to recover
        //  post-backup change.

        // One change script per output asset.
        size_t index = 0;
        for (const auto& value : map_recipients_sum) {
            // Reserve a new key pair from key pool. If it fails, provide a dummy
            // destination in case we don't need change.
            CTxDestination dest;
            bilingual_str dest_err;
            if (index >= reservedest.size() || !reservedest[index]->GetReservedDestination(dest, true, dest_err)) {
                if (dest_err.empty()) {
                    dest_err = _("Please call keypoolrefill first");
                }
                error = _("Transaction needs a change address, but we can't generate it.") + Untranslated(" ") + dest_err;
                // ELEMENTS: We need to put a dummy destination here. Core uses an empty script
                //  but we can't because empty scripts indicate fees (which trigger assertion
                //  failures in `BlindTransaction`). We also set the index to -1, indicating
                //  that this destination is not actually used, and therefore should not be
                //  returned by the `ReturnDestination` loop below.
                mapScriptChange[value.first] = std::pair<int, CScript>(-1, dummy_script);
            } else {
                mapScriptChange[value.first] = std::pair<int, CScript>(index, GetScriptForDestination(dest));
                ++index;
            }
        }

        // Also make sure we have change scripts for the pre-selected inputs.
        std::vector<COutPoint> vPresetInputs;
        coin_control.ListSelected(vPresetInputs);
        for (const COutPoint& presetInput : vPresetInputs) {
            CAsset asset;
            std::map<uint256, CWalletTx>::const_iterator it = wallet.mapWallet.find(presetInput.hash);
            CTxOut txout;
            if (it != wallet.mapWallet.end()) {
                 asset = it->second.GetOutputAsset(wallet, presetInput.n);
            } else if (coin_control.GetExternalOutput(presetInput, txout)) {
                asset = txout.nAsset.GetAsset();
            } else {
                // Ignore this here, will fail more gracefully later.
                continue;
            }

            if (mapScriptChange.find(asset) != mapScriptChange.end()) {
                // This asset already has a change script.
                continue;
            }

            CTxDestination dest;
            bilingual_str dest_err;
            if (index >= reservedest.size() || !reservedest[index]->GetReservedDestination(dest, true, dest_err)) {
                if (dest_err.empty()) {
                    dest_err = _("Keypool ran out, please call keypoolrefill first");
                }
                error = _("Transaction needs a change address, but we can't generate it.") + Untranslated(" ") + dest_err;
                return false;
            }

            CScript scriptChange = GetScriptForDestination(dest);
            // A valid destination implies a change script (and
            // vice-versa). An empty change script will abort later, if the
            // change keypool ran out, but change is required.
            CHECK_NONFATAL(IsValidDestination(dest) != (scriptChange == dummy_script));
            mapScriptChange[asset] = std::pair<int, CScript>(index, scriptChange);
            ++index;
        }
    }
    assert(mapScriptChange.size() > 0);
    CTxOut change_prototype_txout(mapScriptChange.begin()->first, 0, mapScriptChange.begin()->second.second);
    // TODO CA: Set this for each change output
    coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
    if (g_con_elementsmode) {
        if (blind_details) {
            change_prototype_txout.nAsset.vchCommitment.resize(33);
            change_prototype_txout.nValue.vchCommitment.resize(33);
            change_prototype_txout.nNonce.vchCommitment.resize(33);
            coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
            coin_selection_params.change_output_size += (MAX_RANGEPROOF_SIZE + DEFAULT_SURJECTIONPROOF_SIZE + WITNESS_SCALE_FACTOR - 1)/WITNESS_SCALE_FACTOR;
        } else {
            change_prototype_txout.nAsset.vchCommitment.resize(33);
            change_prototype_txout.nValue.vchCommitment.resize(9);
            change_prototype_txout.nNonce.vchCommitment.resize(1);
            coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
        }
    }

    // Get size of spending the change output
    int change_spend_size = CalculateMaximumSignedInputSize(change_prototype_txout, &wallet);
    // If the wallet doesn't know how to sign change output, assume p2sh-p2wpkh
    // as lower-bound to allow BnB to do it's thing
    if (change_spend_size == -1) {
        coin_selection_params.change_spend_size = DUMMY_NESTED_P2WPKH_INPUT_SIZE;
    } else {
        coin_selection_params.change_spend_size = (size_t)change_spend_size;
    }

    // Set discard feerate
    coin_selection_params.m_discard_feerate = GetDiscardRate(wallet);

    // Get the fee rate to use effective values in coin selection
    FeeCalculation feeCalc;
    coin_selection_params.m_effective_feerate = GetMinimumFeeRate(wallet, coin_control, &feeCalc);
    // Do not, ever, assume that it's fine to change the fee rate if the user has explicitly
    // provided one
    if (coin_control.m_feerate && coin_selection_params.m_effective_feerate > *coin_control.m_feerate) {
        error = strprintf(_("Fee rate (%s) is lower than the minimum fee rate setting (%s)"), coin_control.m_feerate->ToString(FeeEstimateMode::SAT_VB), coin_selection_params.m_effective_feerate.ToString(FeeEstimateMode::SAT_VB));
        return false;
    }
    if (feeCalc.reason == FeeReason::FALLBACK && !wallet.m_allow_fallback_fee) {
        // eventually allow a fallback fee
        error = _("Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.");
        return false;
    }

    // Calculate the cost of change
    // Cost of change is the cost of creating the change output + cost of spending the change output in the future.
    // For creating the change output now, we use the effective feerate.
    // For spending the change output in the future, we use the discard feerate for now.
    // So cost of change = (change output size * effective feerate) + (size of spending change output * discard feerate)
    coin_selection_params.m_change_fee = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.change_output_size);
    coin_selection_params.m_cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.m_change_fee;

    // vouts to the payees
    if (!coin_selection_params.m_subtract_fee_outputs) {
        coin_selection_params.tx_noinputs_size = 11; // Static vsize overhead + outputs vsize. 4 nVersion, 4 nLocktime, 1 input count, 1 output count, 1 witness overhead (dummy, flag, stack size)
        if (g_con_elementsmode) {
            coin_selection_params.tx_noinputs_size += 46; // fee output: 9 bytes value, 1 byte scriptPubKey, 33 bytes asset, 1 byte nonce, 1 byte each for null rangeproof/surjectionproof
        }
    }
    // ELEMENTS: If we have blinded inputs but no blinded outputs (which, since the wallet
    //  makes an effort to not produce change, is a common case) then we need to add a
    //  dummy output.
    bool may_need_blinded_dummy = !!blind_details;
    for (const auto& recipient : vecSend)
    {
        CTxOut txout(recipient.asset, recipient.nAmount, recipient.scriptPubKey);
        txout.nNonce.vchCommitment = std::vector<unsigned char>(recipient.confidentiality_key.begin(), recipient.confidentiality_key.end());

        // Include the fee cost for outputs.
        if (!coin_selection_params.m_subtract_fee_outputs) {
            coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout, PROTOCOL_VERSION);
        }

        if (recipient.asset == policyAsset && IsDust(txout, wallet.chain().relayDustFee()))
        {
            error = _("Transaction amount too small");
            return false;
        }
        txNew.vout.push_back(txout);

        // ELEMENTS
        if (blind_details) {
            blind_details->o_pubkeys.push_back(recipient.confidentiality_key);
            if (blind_details->o_pubkeys.back().IsFullyValid()) {
                may_need_blinded_dummy = false;
                blind_details->num_to_blind++;
                blind_details->only_recipient_blind_index = txNew.vout.size()-1;
                if (!coin_selection_params.m_subtract_fee_outputs) {
                    coin_selection_params.tx_noinputs_size += (MAX_RANGEPROOF_SIZE + DEFAULT_SURJECTIONPROOF_SIZE + WITNESS_SCALE_FACTOR - 1)/WITNESS_SCALE_FACTOR;
                }
            }
        }
    }
    if (may_need_blinded_dummy && !coin_selection_params.m_subtract_fee_outputs) {
        // dummy output: 33 bytes value, 2 byte scriptPubKey, 33 bytes asset, 1 byte nonce, 66 bytes dummy rangeproof, 1 byte null surjectionproof
        // FIXME actually, we currently just hand off to BlindTransaction which will put
        //  a full rangeproof and surjectionproof. We should fix this when we overhaul
        //  the blinding logic.
        coin_selection_params.tx_noinputs_size += 70 + 66 +(MAX_RANGEPROOF_SIZE + DEFAULT_SURJECTIONPROOF_SIZE + WITNESS_SCALE_FACTOR - 1)/WITNESS_SCALE_FACTOR;
    }
    // If we are going to issue an asset, add the issuance data to the noinputs_size so that
    // we allocate enough coins for them.
    if (issuance_details) {
        size_t issue_count = 0;
        for (unsigned int i = 0; i < txNew.vout.size(); i++) {
            if (txNew.vout[i].nAsset.IsExplicit() && txNew.vout[i].nAsset.GetAsset() == CAsset(uint256S("1"))) {
                issue_count++;
            } else if (txNew.vout[i].nAsset.IsExplicit() && txNew.vout[i].nAsset.GetAsset() == CAsset(uint256S("2"))) {
                issue_count++;
            }
        }
        if (issue_count > 0) {
            // Allocate space for blinding nonce, entropy, and whichever of nAmount/nInflationKeys is null
            coin_selection_params.tx_noinputs_size += 2 * 32 + 2 * (2 - issue_count);
        }
        // Allocate non-null nAmount/nInflationKeys and rangeproofs
        if (issuance_details->blind_issuance) {
            coin_selection_params.tx_noinputs_size += issue_count * (33 * WITNESS_SCALE_FACTOR + MAX_RANGEPROOF_SIZE + WITNESS_SCALE_FACTOR - 1) / WITNESS_SCALE_FACTOR;
        } else {
            coin_selection_params.tx_noinputs_size += issue_count * 9;
        }
    }

    // Include the fees for things that aren't inputs, excluding the change output
    const CAmount not_input_fees = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.tx_noinputs_size);
    CAmountMap map_selection_target = map_recipients_sum;
    map_selection_target[policyAsset] += not_input_fees;

    // Get available coins
    std::vector<COutput> vAvailableCoins;
    AvailableCoins(wallet, vAvailableCoins, &coin_control, 1, MAX_MONEY, MAX_MONEY, 0);

    // Choose coins to use
    std::optional<SelectionResult> result = SelectCoins(wallet, vAvailableCoins, /* nTargetValue */ map_selection_target, coin_control, coin_selection_params);
    if (!result) {
        error = _("Insufficient funds");
        return false;
    }

    // If all of our inputs are explicit, we don't need a blinded dummy
    if (may_need_blinded_dummy) {
        may_need_blinded_dummy = false;
        for (const auto& coin : result->GetInputSet()) {
            if (!coin.txout.nValue.IsExplicit()) {
                may_need_blinded_dummy = true;
                break;
            }
        }
    }

    // Always make a change output
    // We will reduce the fee from this change output later, and remove the output if it is too small.
    // ELEMENTS: wrap this all in a loop, set nChangePosInOut specifically for policy asset
    CAmountMap map_change_and_fee = result->GetSelectedValue() - map_recipients_sum;
    // Zero out any non-policy assets which have zero change value
    for (auto it = map_change_and_fee.begin(); it != map_change_and_fee.end(); ) {
        if (it->first != policyAsset && it->second == 0) {
            it = map_change_and_fee.erase(it);
        } else {
            ++it;
        }
    }

    // Uniformly randomly place change outputs for all assets, except that the policy-asset
    // change may have a fixed position.
    std::vector<std::optional<CAsset>> change_pos{txNew.vout.size() + map_change_and_fee.size()};
    if (nChangePosInOut == -1) {
       // randomly set policyasset change position
    } else if ((unsigned int)nChangePosInOut >= change_pos.size()) {
        error = _("Transaction change output index out of range");
        return false;
    } else {
        change_pos[nChangePosInOut] = policyAsset;
    }

    for (const auto& asset_change_and_fee : map_change_and_fee) {
        // No need to randomly set the policyAsset change if has been set manually
        if (nChangePosInOut >= 0 && asset_change_and_fee.first == policyAsset) {
            continue;
        }

        int index;
        do {
            index = GetRandInt(change_pos.size());
        } while (change_pos[index]);

        change_pos[index] = asset_change_and_fee.first;
        if (asset_change_and_fee.first == policyAsset) {
            nChangePosInOut = index;
        }
    }

    // Create all the change outputs in their respective places, inserting them
    // in increasing order so that none of them affect each others' indices
    for (unsigned int i = 0; i < change_pos.size(); i++) {
        if (!change_pos[i]) {
            continue;
        }

        const CAsset& asset = *change_pos[i];
        const CAmount& change_and_fee = map_change_and_fee.at(asset);

        assert(change_and_fee >= 0);

        const std::map<CAsset, std::pair<int, CScript>>::const_iterator itScript = mapScriptChange.find(asset);
        if (itScript == mapScriptChange.end()) {
            error = Untranslated(strprintf("No change destination provided for asset %s", asset.GetHex()));
            return false;
        }
        CTxOut newTxOut(asset, change_and_fee, itScript->second.second);

        if (blind_details) {
            std::optional<CPubKey> blind_pub = std::nullopt;
            // We cannot blind zero-valued outputs, and anyway they will be dropped
            // later in this function during the dust check
            if (change_and_fee > 0) {
                const auto itBlindingKey = mapBlindingKeyChange.find(asset);
                if (itBlindingKey != mapBlindingKeyChange.end()) {
                    // If the change output was specified, use the blinding key that
                    // came with the specified address (if any)
                    blind_pub = itBlindingKey->second;
                } else {
                    // Otherwise, we generated it from our own wallet, so get the
                    // blinding key from our own wallet.
                    blind_pub = wallet.GetBlindingPubKey(itScript->second.second);
                }
            } else {
                assert(asset == policyAsset);
            }

            if (blind_pub) {
                blind_details->o_pubkeys.insert(blind_details->o_pubkeys.begin() + i, *blind_pub);
                assert(blind_pub->IsFullyValid());

                blind_details->num_to_blind++;
                blind_details->change_to_blind++;
                blind_details->only_change_pos = i;
                // Place the blinding pubkey here in case of fundraw calls
                newTxOut.nNonce.vchCommitment = std::vector<unsigned char>(blind_pub->begin(), blind_pub->end());
            } else {
                blind_details->o_pubkeys.insert(blind_details->o_pubkeys.begin() + i, CPubKey());
            }
        }
        // Insert change output
        txNew.vout.insert(txNew.vout.begin() + i, newTxOut);
    }

    // Add fee output.
    if (g_con_elementsmode) {
        CTxOut fee(::policyAsset, 0, CScript());
        assert(fee.IsFee());
        txNew.vout.push_back(fee);
        if (blind_details) {
            blind_details->o_pubkeys.push_back(CPubKey());
        }
    }
    assert(nChangePosInOut != -1);
    auto change_position = txNew.vout.begin() + nChangePosInOut;
    // end ELEMENTS

    // Set token input if reissuing
    int reissuance_index = -1;
    uint256 token_blinding;

    // Elements: Shuffle here to preserve random ordering for surjection proofs
    // selected_coins = std::vector<CInputCoin>(setCoins.begin(), setCoins.end());
    // Shuffle(selected_coins.begin(), selected_coins.end(), FastRandomContext());
    // Shuffle selected coins and fill in final vin
    std::vector<CInputCoin> selected_coins = result->GetShuffledInputVector();

    // Note how the sequence number is set to non-maxint so that
    // the nLockTime set above actually works.
    //
    // BIP125 defines opt-in RBF as any nSequence < maxint-1, so
    // we use the highest possible value in that range (maxint-2)
    // to avoid conflicting with other possible uses of nSequence,
    // and in the spirit of "smallest possible change from prior
    // behavior."
    const uint32_t nSequence{coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf) ? MAX_BIP125_RBF_SEQUENCE : CTxIn::MAX_SEQUENCE_NONFINAL};
    for (const auto& coin : selected_coins) {
        txNew.vin.push_back(CTxIn(coin.outpoint, CScript(), nSequence));

        if (issuance_details && coin.asset == issuance_details->reissuance_token) {
            reissuance_index = txNew.vin.size() - 1;
            token_blinding = coin.bf_asset;
        }
    }

    // ELEMENTS add issuance details and blinding details
    std::vector<CKey> issuance_asset_keys;
    std::vector<CKey> issuance_token_keys;
    if (issuance_details) {
        // Fill in issuances now that inputs are set
        assert(txNew.vin.size() > 0);
        int asset_index = -1;
        int token_index = -1;
        for (unsigned int i = 0; i < txNew.vout.size(); i++) {
            if (txNew.vout[i].nAsset.IsExplicit() && txNew.vout[i].nAsset.GetAsset() == CAsset(uint256S("1"))) {
                asset_index = i;
            } else if (txNew.vout[i].nAsset.IsExplicit() && txNew.vout[i].nAsset.GetAsset() == CAsset(uint256S("2"))) {
                token_index = i;
            }
        }
        // Initial issuance request
        if (issuance_details->reissuance_asset.IsNull() && issuance_details->reissuance_token.IsNull() && (asset_index != -1 || token_index != -1)) {
            uint256 entropy;
            CAsset asset;
            CAsset token;
            // Initial issuance always uses vin[0]
            GenerateAssetEntropy(entropy, txNew.vin[0].prevout, issuance_details->contract_hash);
            CalculateAsset(asset, entropy);
            CalculateReissuanceToken(token, entropy, issuance_details->blind_issuance);
            CScript blindingScript(CScript() << OP_RETURN << std::vector<unsigned char>(txNew.vin[0].prevout.hash.begin(), txNew.vin[0].prevout.hash.end()) << txNew.vin[0].prevout.n);
            txNew.vin[0].assetIssuance.assetEntropy = issuance_details->contract_hash;
            // We're making asset outputs, fill out asset type and issuance input
            if (asset_index != -1) {
                txNew.vin[0].assetIssuance.nAmount = txNew.vout[asset_index].nValue;

                txNew.vout[asset_index].nAsset = asset;
                if (issuance_details->blind_issuance && blind_details) {
                    issuance_asset_keys.push_back(wallet.GetBlindingKey(&blindingScript));
                    blind_details->num_to_blind++;
                }
            }
            // We're making reissuance token outputs
            if (token_index != -1) {
                txNew.vin[0].assetIssuance.nInflationKeys = txNew.vout[token_index].nValue;
                txNew.vout[token_index].nAsset = token;
                if (issuance_details->blind_issuance && blind_details) {
                    issuance_token_keys.push_back(wallet.GetBlindingKey(&blindingScript));
                    blind_details->num_to_blind++;

                    // If we're blinding a token issuance and no assets, we must make
                    // the asset issuance a blinded commitment to 0
                    if (asset_index == -1) {
                        txNew.vin[0].assetIssuance.nAmount = 0;
                        issuance_asset_keys.push_back(wallet.GetBlindingKey(&blindingScript));
                        blind_details->num_to_blind++;
                    }
                }
            }
        // Asset being reissued with explicitly named asset/token
        } else if (asset_index != -1) {
            assert(reissuance_index != -1);
            // Fill in output with issuance
            txNew.vout[asset_index].nAsset = issuance_details->reissuance_asset;

            // Fill in issuance
            // Blinding revealing underlying asset
            txNew.vin[reissuance_index].assetIssuance.assetBlindingNonce = token_blinding;
            txNew.vin[reissuance_index].assetIssuance.assetEntropy = issuance_details->entropy;
            txNew.vin[reissuance_index].assetIssuance.nAmount = txNew.vout[asset_index].nValue;

            // If blinded token derivation, blind the issuance
            CAsset temp_token;
            CalculateReissuanceToken(temp_token, issuance_details->entropy, true);
            if (temp_token == issuance_details->reissuance_token && blind_details) {
            CScript blindingScript(CScript() << OP_RETURN << std::vector<unsigned char>(txNew.vin[reissuance_index].prevout.hash.begin(), txNew.vin[reissuance_index].prevout.hash.end()) << txNew.vin[reissuance_index].prevout.n);
                issuance_asset_keys.resize(reissuance_index);
                issuance_asset_keys.push_back(wallet.GetBlindingKey(&blindingScript));
                blind_details->num_to_blind++;
            }
        }
    }

    // Do "initial blinding" for fee estimation purposes
    TxSize tx_sizes;
    CMutableTransaction tx_blinded = txNew;
    if (blind_details) {
        if (!fillBlindDetails(blind_details, &wallet, tx_blinded, selected_coins, error)) {
            return false;
        }
        txNew = tx_blinded; // sigh, `fillBlindDetails` may have modified txNew

        int ret = BlindTransaction(blind_details->i_amount_blinds, blind_details->i_asset_blinds, blind_details->i_assets, blind_details->i_amounts, blind_details->o_amount_blinds, blind_details->o_asset_blinds, blind_details->o_pubkeys, issuance_asset_keys, issuance_token_keys, tx_blinded);
        assert(ret != -1);
        if (ret != blind_details->num_to_blind) {
            error = _("Unable to blind the transaction properly. This should not happen.");
            return false;
        }

        tx_sizes = CalculateMaximumSignedTxSize(CTransaction(tx_blinded), &wallet, &coin_control);
    } else {
        tx_sizes = CalculateMaximumSignedTxSize(CTransaction(txNew), &wallet, &coin_control);
    }
    // end ELEMENTS

    // Calculate the transaction fee
    int nBytes = tx_sizes.vsize;
    if (nBytes == -1) {
        error = _("Missing solving data for estimating transaction size");
        return false;
    }
    nFeeRet = coin_selection_params.m_effective_feerate.GetFee(nBytes);

    // Subtract fee from the change output if not subtracting it from recipient outputs
    CAmount fee_needed = nFeeRet;
    if (!coin_selection_params.m_subtract_fee_outputs) {
        change_position->nValue = change_position->nValue.GetAmount() - fee_needed;
    }

    // We want to drop the change to fees if:
    // 1. The change output would be dust
    // 2. The change is within the (almost) exact match window, i.e. it is less than or equal to the cost of the change output (cost_of_change)
    CAmount change_amount = change_position->nValue.GetAmount();
    if (IsDust(*change_position, coin_selection_params.m_discard_feerate) || change_amount <= coin_selection_params.m_cost_of_change)
    {
        bool was_blinded = blind_details && blind_details->o_pubkeys[nChangePosInOut].IsValid();

        // If the change was blinded, and was the only blinded output, we cannot drop it
        // without causing the transaction to fail to balance. So keep it, and merely
        // zero it out.
        if (was_blinded && blind_details->num_to_blind == 1) {
            assert (may_need_blinded_dummy);
            change_position->scriptPubKey = CScript() << OP_RETURN;
            change_position->nValue = 0;
        } else {
            txNew.vout.erase(change_position);

            change_pos[nChangePosInOut] = std::nullopt;
            tx_blinded.vout.erase(tx_blinded.vout.begin() + nChangePosInOut);
            if (tx_blinded.witness.vtxoutwit.size() > (unsigned) nChangePosInOut) {
                tx_blinded.witness.vtxoutwit.erase(tx_blinded.witness.vtxoutwit.begin() + nChangePosInOut);
            }
            if (blind_details) {

                blind_details->o_amounts.erase(blind_details->o_amounts.begin() + nChangePosInOut);
                blind_details->o_assets.erase(blind_details->o_assets.begin() + nChangePosInOut);
                blind_details->o_pubkeys.erase(blind_details->o_pubkeys.begin() + nChangePosInOut);
                // If change_amount == 0, we did not increment num_to_blind initially
                // and therefore do not need to decrement it here.
                if (was_blinded) {
                    blind_details->num_to_blind--;
                    blind_details->change_to_blind--;

                    // FIXME: If we drop the change *and* this means we have only one
                    //  blinded output *and* we have no blinded inputs, then this puts
                    //  us in a situation where BlindTransaction will fail. This is
                    //  prevented in fillBlindDetails, which adds an OP_RETURN output
                    //  to handle this case. So do this ludicrous hack to accomplish
                    //  this. This whole lump of un-followable-logic needs to be replaced
                    //  by a complete rewriting of the wallet blinding logic.
                    if (blind_details->num_to_blind < 2) {
                        resetBlindDetails(blind_details, true /* don't wipe output data */);
                        if (!fillBlindDetails(blind_details, &wallet, txNew, selected_coins, error)) {
                            return false;
                        }
                    }
                }
            }
        }
        change_amount = 0;
        nChangePosInOut = -1;

        // Because we have dropped this change, the tx size and required fee will be different, so let's recalculate those
        tx_sizes = CalculateMaximumSignedTxSize(CTransaction(tx_blinded), &wallet, &coin_control);
        nBytes = tx_sizes.vsize;
        fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
    }

    // The only time that fee_needed should be less than the amount available for fees (in change_and_fee - change_amount) is when
    // we are subtracting the fee from the outputs. If this occurs at any other time, it is a bug.
    assert(coin_selection_params.m_subtract_fee_outputs || fee_needed <= map_change_and_fee.at(policyAsset) - change_amount);

    // Update nFeeRet in case fee_needed changed due to dropping the change output
    if (fee_needed <= map_change_and_fee.at(policyAsset) - change_amount) {
        nFeeRet = map_change_and_fee.at(policyAsset) - change_amount;
    }

    // Reduce output values for subtractFeeFromAmount
    if (coin_selection_params.m_subtract_fee_outputs) {
        CAmount to_reduce = fee_needed + change_amount - map_change_and_fee.at(policyAsset);
        int i = 0;
        bool fFirst = true;
        for (const auto& recipient : vecSend)
        {
            if (i == nChangePosInOut) {
                ++i;
            }
            CTxOut& txout = txNew.vout[i];

            if (recipient.fSubtractFeeFromAmount)
            {
                CAmount value = txout.nValue.GetAmount();
                if (recipient.asset != policyAsset) {
                    error = Untranslated(strprintf("Wallet does not support more than one type of fee at a time, therefore can not subtract fee from address amount, which is of a different asset id. fee asset: %s recipient asset: %s", policyAsset.GetHex(), recipient.asset.GetHex()));
                    return false;
                }

                value -= to_reduce / outputs_to_subtract_fee_from; // Subtract fee equally from each selected recipient

                if (fFirst) // first receiver pays the remainder not divisible by output count
                {
                    fFirst = false;
                    value -= to_reduce % outputs_to_subtract_fee_from;
                }

                // Error if this output is reduced to be below dust
                if (IsDust(txout, wallet.chain().relayDustFee())) {
                    if (value < 0) {
                        error = _("The transaction amount is too small to pay the fee");
                    } else {
                        error = _("The transaction amount is too small to send after the fee has been deducted");
                    }
                    return false;
                }

                txout.nValue = value;
            }
            ++i;
        }
        nFeeRet = fee_needed;
    }

    // ELEMENTS: Give up if change keypool ran out and change is required
    for (const auto& maybe_change_asset : change_pos) {
        if (maybe_change_asset) {
            auto used = mapScriptChange.extract(*maybe_change_asset);
            if (used.mapped().second == dummy_script) {
                return false;
            }
        }
    }

    // ELEMENTS update fee output
    if (g_con_elementsmode) {
        for (auto& txout : txNew.vout) {
            if (txout.IsFee()) {
                txout.nValue = nFeeRet;
                break;
            }
        }
    }

    // ELEMENTS do actual blinding
    if (blind_details) {
        // Print blinded transaction info before we possibly blow it away when !sign.
        std::string summary = "CreateTransaction created blinded transaction:\nIN: ";
        for (unsigned int i = 0; i < selected_coins.size(); ++i) {
            if (i > 0) {
                summary += "    ";
            }
            summary += strprintf("#%d: %s [%s] (%s [%s])\n", i,
                selected_coins[i].value,
                selected_coins[i].txout.nValue.IsExplicit() ? "explicit" : "blinded",
                selected_coins[i].asset.GetHex(),
                selected_coins[i].txout.nAsset.IsExplicit() ? "explicit" : "blinded"
            );
        }
        summary += "OUT: ";
        for (unsigned int i = 0; i < txNew.vout.size(); ++i) {
            if (i > 0) {
                summary += "     ";
            }
            const CTxOut& unblinded = txNew.vout[i];
            summary += strprintf("#%d: %s%s [%s] (%s [%s])\n", i,
                txNew.vout[i].IsFee() ? "[fee] " : "",
                unblinded.nValue.GetAmount(),
                blind_details->o_pubkeys[i].IsValid() ? "blinded" : "explicit",
                unblinded.nAsset.GetAsset().GetHex(),
                blind_details->o_pubkeys[i].IsValid() ? "blinded" : "explicit"
            );
        }
        wallet.WalletLogPrintf(summary+"\n");

        // Wipe output blinding factors and start over
        blind_details->o_amount_blinds.clear();
        blind_details->o_asset_blinds.clear();
        for (unsigned int i = 0; i < txNew.vout.size(); i++) {
            blind_details->o_amounts[i] = txNew.vout[i].nValue.GetAmount();
            assert(blind_details->o_assets[i] == txNew.vout[i].nAsset.GetAsset());
        }

        if (sign) {
            int ret = BlindTransaction(blind_details->i_amount_blinds, blind_details->i_asset_blinds, blind_details->i_assets, blind_details->i_amounts, blind_details->o_amount_blinds, blind_details->o_asset_blinds,  blind_details->o_pubkeys, issuance_asset_keys, issuance_token_keys, txNew);
            assert(ret != -1);
            if (ret != blind_details->num_to_blind) {
                wallet.WalletLogPrintf("ERROR: tried to blind %d outputs but only blinded %d\n", (int) blind_details->num_to_blind, (int) ret);
                error = _("Unable to blind the transaction properly. This should not happen.");
                return false;
            }
        }
    }

    // Release any change keys that we didn't use.
    for (const auto& it : mapScriptChange) {
        int index = it.second.first;
        if (index < 0) {
            continue;
        }

        reservedest[index]->ReturnDestination();
    }


    if (sign) {
        if (!wallet.SignTransaction(txNew)) {
            error = _("Signing transaction failed");
            return false;
        }
    }

    // Normalize the witness in case it is not serialized before mempool
    if (!txNew.HasWitness()) {
        txNew.witness.SetNull();
    }

    // Return the constructed transaction data.
    tx = MakeTransactionRef(std::move(txNew));

    // Limit size
    if ((sign && GetTransactionWeight(*tx) > MAX_STANDARD_TX_WEIGHT) ||
        (!sign && tx_sizes.weight > MAX_STANDARD_TX_WEIGHT))
    {
        error = _("Transaction too large");
        return false;
    }

    if (nFeeRet > wallet.m_default_max_tx_fee) {
        error = TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
        return false;
    }

    if (gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS)) {
        // Lastly, ensure this tx will pass the mempool's chain limits
        if (!wallet.chain().checkChainLimits(tx)) {
            error = _("Transaction has too long of a mempool chain");
            return false;
        }
    }

    // Before we return success, we assume any change key will be used to prevent
    // accidental re-use.
    for (auto& reservedest_ : reservedest) {
        reservedest_->KeepDestination();
    }
    fee_calc_out = feeCalc;

    wallet.WalletLogPrintf("Fee Calculation: Fee:%d Bytes:%u Tgt:%d (requested %d) Reason:\"%s\" Decay %.5f: Estimation: (%g - %g) %.2f%% %.1f/(%.1f %d mem %.1f out) Fail: (%g - %g) %.2f%% %.1f/(%.1f %d mem %.1f out)\n",
              nFeeRet, nBytes, feeCalc.returnedTarget, feeCalc.desiredTarget, StringForFeeReason(feeCalc.reason), feeCalc.est.decay,
              feeCalc.est.pass.start, feeCalc.est.pass.end,
              (feeCalc.est.pass.totalConfirmed + feeCalc.est.pass.inMempool + feeCalc.est.pass.leftMempool) > 0.0 ? 100 * feeCalc.est.pass.withinTarget / (feeCalc.est.pass.totalConfirmed + feeCalc.est.pass.inMempool + feeCalc.est.pass.leftMempool) : 0.0,
              feeCalc.est.pass.withinTarget, feeCalc.est.pass.totalConfirmed, feeCalc.est.pass.inMempool, feeCalc.est.pass.leftMempool,
              feeCalc.est.fail.start, feeCalc.est.fail.end,
              (feeCalc.est.fail.totalConfirmed + feeCalc.est.fail.inMempool + feeCalc.est.fail.leftMempool) > 0.0 ? 100 * feeCalc.est.fail.withinTarget / (feeCalc.est.fail.totalConfirmed + feeCalc.est.fail.inMempool + feeCalc.est.fail.leftMempool) : 0.0,
              feeCalc.est.fail.withinTarget, feeCalc.est.fail.totalConfirmed, feeCalc.est.fail.inMempool, feeCalc.est.fail.leftMempool);
    return true;
}