psbt: Add sighash types to PSBT when not DEFAULT or ALL

When an atypical sighash type is specified by the user, add it to the
PSBT so that further signing can enforce sighash type matching.

Author: achow101

src/psbt.cpp

@@ -424,6 +424,13 @@ PSBTError SignPSBTInput(const SigningProvider& provider, PartiallySignedTransact
     if (input.sighash_type && input.sighash_type != sighash) {
         return PSBTError::SIGHASH_MISMATCH;
     }
+    // Set the PSBT sighash field when sighash is not DEFAULT or ALL
+    // DEFAULT is allowed for non-taproot inputs since DEFAULT may be passed for them (e.g. the psbt being signed also has taproot inputs)
+    // Note that signing already aliases DEFAULT to ALL for non-taproot inputs.
+    if (utxo.scriptPubKey.IsPayToTaproot() ? sighash != SIGHASH_DEFAULT :
+                                            (sighash != SIGHASH_DEFAULT && sighash != SIGHASH_ALL)) {
+        input.sighash_type = sighash;
+    }
 
     // Check all existing signatures use the sighash type
     if (sighash == SIGHASH_DEFAULT) {
@@ -522,7 +529,8 @@ bool FinalizePSBT(PartiallySignedTransaction& psbtx)
     bool complete = true;
     const PrecomputedTransactionData txdata = PrecomputePSBTData(psbtx);
     for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
-        complete &= (SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, &txdata, std::nullopt, nullptr, true) == PSBTError::OK);
+        PSBTInput& input = psbtx.inputs.at(i);
+        complete &= (SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, &txdata, input.sighash_type, nullptr, true) == PSBTError::OK);
     }
 
     return complete;

test/functional/rpc_psbt.py

@@ -301,6 +301,53 @@ def test_sighash_mismatch(self):
 
         wallet.unloadwallet()
 
+    def test_sighash_adding(self):
+        self.log.info("Test adding of sighash type field")
+        self.nodes[0].createwallet("sighash_adding")
+        wallet = self.nodes[0].get_wallet_rpc("sighash_adding")
+        def_wallet = self.nodes[0].get_wallet_rpc(self.default_wallet_name)
+
+        outputs = [{wallet.getnewaddress(address_type="bech32"): 1}]
+        outputs.append({wallet.getnewaddress(address_type="bech32m"): 1})
+        descs = wallet.listdescriptors(True)["descriptors"]
+        def_wallet.send(outputs)
+        self.generate(self.nodes[0], 6)
+        utxos = wallet.listunspent()
+
+        # Make a PSBT
+        psbt = wallet.walletcreatefundedpsbt(utxos, [{def_wallet.getnewaddress(): 0.5}])["psbt"]
+
+        # Process the PSBT with the wallet
+        wallet_psbt = wallet.walletprocesspsbt(psbt=psbt, sighashtype="ALL|ANYONECANPAY", finalize=False)["psbt"]
+
+        # Separately process the PSBT with descriptors
+        desc_psbt = self.nodes[0].descriptorprocesspsbt(psbt=psbt, descriptors=descs, sighashtype="ALL|ANYONECANPAY", finalize=False)["psbt"]
+
+        for psbt in [wallet_psbt, desc_psbt]:
+            # Check that the PSBT has a sighash field on all inputs
+            dec_psbt = self.nodes[0].decodepsbt(psbt)
+            for input in dec_psbt["inputs"]:
+                assert_equal(input["sighash"], "ALL|ANYONECANPAY")
+
+            # Make sure we can still finalize the transaction
+            fin_res = self.nodes[0].finalizepsbt(psbt)
+            assert_equal(fin_res["complete"], True)
+            fin_hex = fin_res["hex"]
+            assert_equal(self.nodes[0].testmempoolaccept([fin_hex])[0]["allowed"], True)
+
+            # Change the sighash field to a different value and make sure we can no longer finalize
+            mod_psbt = PSBT.from_base64(psbt)
+            mod_psbt.i[0].map[PSBT_IN_SIGHASH_TYPE] = (SIGHASH_ALL).to_bytes(4, byteorder="little")
+            mod_psbt.i[1].map[PSBT_IN_SIGHASH_TYPE] = (SIGHASH_ALL).to_bytes(4, byteorder="little")
+            psbt = mod_psbt.to_base64()
+            fin_res = self.nodes[0].finalizepsbt(psbt)
+            assert_equal(fin_res["complete"], False)
+
+        self.nodes[0].sendrawtransaction(fin_hex)
+        self.generate(self.nodes[0], 1)
+
+        wallet.unloadwallet()
+
     def assert_change_type(self, psbtx, expected_type):
         """Assert that the given PSBT has a change output with the given type."""
 
@@ -1139,6 +1186,7 @@ def test_psbt_input_keys(psbt_input, keys):
         assert_raises_rpc_error(-8, "'all' is not a valid sighash parameter.", self.nodes[2].descriptorprocesspsbt, psbt, [descriptor], sighashtype="all")
 
         self.test_sighash_mismatch()
+        self.test_sighash_adding()
 
 if __name__ == '__main__':
     PSBTTest(__file__).main()