Merge #12699: [wallet] Shuffle transaction inputs before signing

2fb9c1e shuffle selected coins before transaction finalization (Gregory Sanders)

Pull request description:

  Currently inputs are ordered based on COutPoint ordering, which while doesn't leak additional internal wallet state, likely further fingerprints the wallet as a Core wallet to observers.

  Note: This slightly changed behavior of `fundrawtransaction` in that the newly-appended inputs will now be shuffled rather than in outpoint-order. This does not break API compatibility.

  Simple shuffling of the coins being returned will hopefully allow the wallet to blend in a bit more, in lieu of additional data to find what other wallets are doing, or another standard, ala @gmaxwell's suggested of ordering via scriptPubKey.

Tree-SHA512: 70689a6eccf9fa7fc6e3d884f2eba4b482446a1e6128beff7a98f446d0c60f7966c5a6c55e9b0b3d73a9b539ce54889a26c7efe78ab7f34af386d5e4f3fa6df2

Author: laanwj

src/wallet/wallet.cpp

@@ -2890,20 +2890,11 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CTransac
                     nChangePosInOut = -1;
                 }
 
-                // Fill vin
+                // Dummy fill vin for maximum size estimation
                 //
-                // 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.signalRbf ? MAX_BIP125_RBF_SEQUENCE : (CTxIn::SEQUENCE_FINAL - 1);
-                for (const auto& coin : setCoins)
-                    txNew.vin.push_back(CTxIn(coin.outpoint,CScript(),
-                                              nSequence));
+                for (const auto& coin : setCoins) {
+                    txNew.vin.push_back(CTxIn(coin.outpoint,CScript()));
+                }
 
                 nBytes = CalculateMaximumSignedTxSize(txNew, this);
                 if (nBytes < 0) {
@@ -2993,11 +2984,29 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CTransac
 
         if (nChangePosInOut == -1) reservekey.ReturnKey(); // Return any reserved key if we don't have change
 
+        // Shuffle selected coins and fill in final vin
+        txNew.vin.clear();
+        std::vector<CInputCoin> selected_coins(setCoins.begin(), setCoins.end());
+        std::shuffle(selected_coins.begin(), selected_coins.end(), FastRandomContext());
+
+        // 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.signalRbf ? MAX_BIP125_RBF_SEQUENCE : (CTxIn::SEQUENCE_FINAL - 1);
+        for (const auto& coin : selected_coins) {
+            txNew.vin.push_back(CTxIn(coin.outpoint, CScript(), nSequence));
+        }
+
         if (sign)
         {
             CTransaction txNewConst(txNew);
             int nIn = 0;
-            for (const auto& coin : setCoins)
+            for (const auto& coin : selected_coins)
             {
                 const CScript& scriptPubKey = coin.txout.scriptPubKey;
                 SignatureData sigdata;