Shuffle inputs and outputs after joining psbts

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.

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());
 }