rpc: strip derivation paths from psbt during combine

Previously setting bip32derivs to false with walletprocesspsbt does not
include bip32_derivs for inputs, but does include bip32_derivs for
outputs.

User may want to strip all bip32 derivation paths for privacy reasons
however. It may make sense to do this after signing your inputs and
outputs during a manual coinjoin for example.

Therefore add functionality to `combinepsbt` permitting stripping of
all bip32_derivation paths found in all provided psbts.

As this RPC can be called with a single psbt, it can be used to strip
derivation paths from any psbt.

src/psbt.cpp

@@ -66,6 +66,19 @@ bool PartiallySignedTransaction::AddOutput(const CTxOut& txout, const PSBTOutput
     return true;
 }
 
+void PartiallySignedTransaction::StripDerivationPaths()
+{
+    // Loop over the inputs and strip m_tap_bip32_paths
+    for (unsigned int i = 0; i < inputs.size(); ++i) {
+        inputs[i].hd_keypaths.clear();
+    }
+
+    // Loop over the outputs and strip m_tap_bip32_paths
+    for (unsigned int i = 0; i < outputs.size(); ++i) {
+        outputs[i].hd_keypaths.clear();
+    }
+}
+
 bool PartiallySignedTransaction::GetInputUTXO(CTxOut& utxo, int input_index) const
 {
     const PSBTInput& input = inputs[input_index];

src/psbt.h

@@ -968,6 +968,7 @@ struct PartiallySignedTransaction
     bool AddInput(const CTxIn& txin, PSBTInput& psbtin);
     bool AddOutput(const CTxOut& txout, const PSBTOutput& psbtout);
     PartiallySignedTransaction() = default;
+    void StripDerivationPaths();
     explicit PartiallySignedTransaction(const CMutableTransaction& tx);
     /**
      * Finds the UTXO for a given input index

src/rpc/client.cpp

@@ -178,6 +178,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "createpsbt", 2, "locktime" },
     { "createpsbt", 3, "replaceable" },
     { "combinepsbt", 0, "txs"},
+    { "combinepsbt", 1, "stripderivs"},
     { "joinpsbts", 0, "txs"},
     { "finalizepsbt", 1, "extract"},
     { "converttopsbt", 1, "permitsigdata"},

src/rpc/rawtransaction.cpp

@@ -1464,6 +1464,7 @@ static RPCHelpMan combinepsbt()
                             {"psbt", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A base64 string of a PSBT"},
                         },
                         },
+                    {"stripderivs", RPCArg::Type::BOOL, RPCArg::Default{false}, "Strip BIP 32 derivation paths for out inputs and outputs if they are present"},
                 },
                 RPCResult{
                     RPCResult::Type::STR, "", "The base64-encoded partially signed transaction"
@@ -1473,6 +1474,8 @@ static RPCHelpMan combinepsbt()
                 },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
+    bool strip_derivation_paths = request.params[1].isNull() ? false : request.params[1].get_bool();
+
     // Unserialize the transactions
     std::vector<PartiallySignedTransaction> psbtxs;
     UniValue txs = request.params[0].get_array();
@@ -1485,6 +1488,9 @@ static RPCHelpMan combinepsbt()
         if (!DecodeBase64PSBT(psbtx, txs[i].get_str(), error)) {
             throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
         }
+        if (strip_derivation_paths) {
+            psbtx.StripDerivationPaths();
+        }
         psbtxs.push_back(psbtx);
     }