[wallet] randomly generate change targets

If the wallet always chooses 1 million sats as its change target, it is
easier to fingerprint transactions created by the Core wallet.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index 228725646c..b2839f2afe 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -19,6 +19,8 @@
 #include <wallet/transaction.h>
 #include <wallet/wallet.h>
 
+#include <cmath>
+
 using interfaces::FoundBlock;
 
 namespace wallet {
@@ -395,9 +397,11 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
         results.push_back(*knapsack_result);
     }
 
-    // We include the minimum final change for SRD as we do want to avoid making really small change.
-    // KnapsackSolver does not need this because it includes MIN_CHANGE internally.
-    const CAmount srd_target = nTargetValue + coin_selection_params.m_change_fee + MIN_FINAL_CHANGE;
+    // Include change for SRD as we want to avoid making really small change if the selection just
+    // barely meets the target. Just use the lower bound change target instead of the randomly
+    // generated one, since SRD will result in a random change amount anyway; avoid making the
+    // target needlessly large.
+    const CAmount srd_target = nTargetValue + coin_selection_params.m_change_fee + CHANGE_LOWER;
     if (auto srd_result{SelectCoinsSRD(positive_groups, srd_target, coin_selection_params.rng_fast)}) {
         srd_result->ComputeAndSetWaste(coin_selection_params.m_cost_of_change);
         results.push_back(*srd_result);
@@ -681,6 +685,7 @@ static bool CreateTransactionInternal(
             coin_selection_params.m_subtract_fee_outputs = true;
         }
     }
+    coin_selection_params.m_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), rng_fast);
 
     // Create change script that will be used if we need change
     CScript scriptChange;