Merge bitcoin/bitcoin#24494: wallet: generate random change target for each tx for better privacy

9053f64fcbd26d87c26ae6b982d17756a6ea0896 [doc] release notes for random change target (glozow)
46f2fed6c5e0fa623bfeabf61ba4811d5cf8f47c [wallet] remove MIN_CHANGE (glozow)
a44236addd01cff4e4d751e0f379d399fbfc8eae [wallet] randomly generate change targets (glozow)
1e52e6bd0a8888efb4ed247d74ec7ca9dfc2e002 refactor coin selection for parameterizable change target (glozow)

Pull request description:

  Closes #24458 - the wallet always chooses 1 million sats as its change target, making it easier to fingerprint transactions created by the Core wallet. Instead of using a fixed value, choose one randomly each time (within a range). Using 50ksat (around $20) as the lower bound and `min(1 million sat, 2 * average payment value)` as the upper bound.
  RFC: If the payment is <25ksat, this doesn't work, so we're using the range (payment amount, 50ksat) instead.

ACKs for top commit:
  achow101:
    ACK 9053f64fcbd26d87c26ae6b982d17756a6ea0896
  Xekyo:
    reACK 9053f64fcbd26d87c26ae6b982d17756a6ea0896

Tree-SHA512: 45ce5d064697065549473347648e29935733f3deffc71a6ab995449431f60302d1f9911a0994dfdb960b48c48b5d8859f168b396ff2a62db67d535a7db041d35

1 files changed, 2 insertions, 4 deletions

diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
index 4ece886488..d3103492a4 100644
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -33,7 +33,6 @@
 #include <QTreeWidget>
 
 using wallet::CCoinControl;
-using wallet::MIN_CHANGE;
 
 QList<CAmount> CoinControlDialog::payAmounts;
 bool CoinControlDialog::fSubtractFeeFromAmount = false;
@@ -486,11 +485,10 @@ void CoinControlDialog::updateLabels(CCoinControl& m_coin_control, WalletModel *
             if (!CoinControlDialog::fSubtractFeeFromAmount)
                 nChange -= nPayFee;
 
-            // Never create dust outputs; if we would, just add the dust to the fee.
-            if (nChange > 0 && nChange < MIN_CHANGE)
-            {
+            if (nChange > 0) {
                 // Assumes a p2pkh script size
                 CTxOut txout(nChange, CScript() << std::vector<unsigned char>(24, 0));
+                // Never create dust outputs; if we would, just add the dust to the fee.
                 if (IsDust(txout, model->node().getDustRelayFee()))
                 {
                     nPayFee += nChange;