From 6f405a1d3b38395e35571b68aae55cae50e0762a Mon Sep 17 00:00:00 2001
From: Andrew Chow <achow101-github@achow101.com>
Date: Wed, 31 Jul 2019 18:02:24 -0400
Subject: [PATCH] Shuffle inputs and outputs after joining psbts

---
 doc/release-notes-16512.md |  4 ++++
 src/rpc/rawtransaction.cpp | 25 ++++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 doc/release-notes-16512.md

diff --git a/doc/release-notes-16512.md b/doc/release-notes-16512.md
new file mode 100644
index 00000000000..9aa9cf36f92
--- /dev/null
+++ b/doc/release-notes-16512.md
@@ -0,0 +1,4 @@
+RPC changes
+-----------
+The RPC `joinpsbts` will shuffle the order of the inputs and outputs of the resulting joined psbt.
+Previously inputs and outputs were added in the order that the PSBTs were provided which makes correlating inputs to outputs extremely easy.
diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
index 0ab504de06c..a2630431241 100644
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -17,6 +17,7 @@
 #include <policy/rbf.h>
 #include <primitives/transaction.h>
 #include <psbt.h>
+#include <random.h>
 #include <rpc/rawtransaction_util.h>
 #include <rpc/server.h>
 #include <rpc/util.h>
@@ -1604,8 +1605,30 @@ UniValue joinpsbts(const JSONRPCRequest& request)
         merged_psbt.unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
     }
 
+    // Generate list of shuffled indices for shuffling inputs and outputs of the merged PSBT
+    std::vector<int> input_indices(merged_psbt.inputs.size());
+    std::iota(input_indices.begin(), input_indices.end(), 0);
+    std::vector<int> output_indices(merged_psbt.outputs.size());
+    std::iota(output_indices.begin(), output_indices.end(), 0);
+
+    // Shuffle input and output indicies lists
+    Shuffle(input_indices.begin(), input_indices.end(), FastRandomContext());
+    Shuffle(output_indices.begin(), output_indices.end(), FastRandomContext());
+
+    PartiallySignedTransaction shuffled_psbt;
+    shuffled_psbt.tx = CMutableTransaction();
+    shuffled_psbt.tx->nVersion = merged_psbt.tx->nVersion;
+    shuffled_psbt.tx->nLockTime = merged_psbt.tx->nLockTime;
+    for (int i : input_indices) {
+        shuffled_psbt.AddInput(merged_psbt.tx->vin[i], merged_psbt.inputs[i]);
+    }
+    for (int i : output_indices) {
+        shuffled_psbt.AddOutput(merged_psbt.tx->vout[i], merged_psbt.outputs[i]);
+    }
+    shuffled_psbt.unknown.insert(merged_psbt.unknown.begin(), merged_psbt.unknown.end());
+
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
-    ssTx << merged_psbt;
+    ssTx << shuffled_psbt;
     return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
 }