Merge #16512: rpc: Shuffle inputs and outputs after joining psbts

c0b5d9710322a614a50ab5da081558cf6a38ad2a Test that joinpsbts randomly shuffles the inputs (Andrew Chow)
6f405a1d3b38395e35571b68aae55cae50e0762a Shuffle inputs and outputs after joining psbts (Andrew Chow)

Pull request description:

  `joinpsbts` currently just adds the inputs and outputs in the order of that the PSBTs were provided. This makes it extremely easy to identify which outputs belong to which inputs. This PR changes that so that all of the inputs and outputs are shuffled in the joined transaction.

ACKs for top commit:
  instagibbs:
    utACK https://github.com/bitcoin/bitcoin/pull/16512/commits/c0b5d9710322a614a50ab5da081558cf6a38ad2a
  jonatack:
    ACK c0b5d9710322a614a50ab5da081558cf6a38ad2a modulo suggestions for later.

Tree-SHA512: 14a0b7aae07d92e6d2c76a3a3b228b481e1964cb7d34f97515bdda18e2ea05a9f97c5a22affc143b86ae8b95c3cb239849fb54219d65512bc2112264dca915c8

3 files changed, 38 insertions, 1 deletions

diff --git a/doc/release-notes-16512.md b/doc/release-notes-16512.md
new file mode 100644
index 0000000000..9aa9cf36f9
--- /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 fb8ea8c227..4003299662 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>
@@ -1615,8 +1616,30 @@ 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());
 }
 
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index 5a04e0c8d8..cb8320a6c2 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -382,6 +382,16 @@ class PSBTTest(BitcoinTestFramework):
         joined_decoded = self.nodes[0].decodepsbt(joined)
         assert len(joined_decoded['inputs']) == 4 and len(joined_decoded['outputs']) == 2 and "final_scriptwitness" not in joined_decoded['inputs'][3] and "final_scriptSig" not in joined_decoded['inputs'][3]
 
+        # Check that joining shuffles the inputs and outputs
+        # 10 attempts should be enough to get a shuffled join
+        shuffled = False
+        for i in range(0, 10):
+            shuffled_joined = self.nodes[0].joinpsbts([psbt, psbt2])
+            shuffled |= joined != shuffled_joined
+            if shuffled:
+                break
+        assert shuffled
+
         # Newly created PSBT needs UTXOs and updating
         addr = self.nodes[1].getnewaddress("", "p2sh-segwit")
         txid = self.nodes[0].sendtoaddress(addr, 7)