signpsbt: add signonly parameter to restrict/enforce what inputs to sign.

This is an extra safety check for dual funding, where we only want to sign
the inputs we provided!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `signpsbt` takes an optional `signonly` array to limit what inputs to sign.

contrib/pyln-client/pyln/client/lightning.py

@@ -1155,12 +1155,13 @@ class LightningRpc(UnixDomainSocketRpc):
         }
         return self.call("utxopsbt", payload)
 
-    def signpsbt(self, psbt):
+    def signpsbt(self, psbt, signonly=None):
         """
         Add internal wallet's signatures to PSBT
         """
         payload = {
             "psbt": psbt,
+            "signonly": signonly,
         }
         return self.call("signpsbt", payload)
 

tests/test_wallet.py

@@ -738,17 +738,42 @@ def test_sign_and_send_psbt(node_factory, bitcoind, chainparams):
     with pytest.raises(RpcError, match=r"No wallet inputs to sign"):
         l1.rpc.signpsbt(l2_funding['psbt'])
 
+    # With signonly it will fail if it can't sign it.
+    with pytest.raises(RpcError, match=r"is unknown"):
+        l1.rpc.signpsbt(l2_funding['psbt'], signonly=[0])
+
     # Add some of our own PSBT inputs to it
     l1_funding = l1.rpc.fundpsbt(satoshi=Millisatoshi(3 * amount * 1000),
                                  feerate=7500,
                                  startweight=42,
                                  reserve=True)
+    l1_num_inputs = len(bitcoind.rpc.decodepsbt(l1_funding['psbt'])['tx']['vin'])
+    l2_num_inputs = len(bitcoind.rpc.decodepsbt(l2_funding['psbt'])['tx']['vin'])
 
-    # Join and add an output
+    # Join and add an output (reorders!)
     joint_psbt = bitcoind.rpc.joinpsbts([l1_funding['psbt'], l2_funding['psbt'],
                                          output_pbst])
 
-    half_signed_psbt = l1.rpc.signpsbt(joint_psbt)['signed_psbt']
+    # Ask it to sign inputs it doesn't know, it will fail.
+    with pytest.raises(RpcError, match=r"is unknown"):
+        l1.rpc.signpsbt(joint_psbt,
+                        signonly=list(range(l1_num_inputs + 1)))
+
+    # Similarly, it can't sign inputs it doesn't know.
+    sign_success = []
+    for i in range(l1_num_inputs + l2_num_inputs):
+        try:
+            l1.rpc.signpsbt(joint_psbt, signonly=[i])
+        except RpcError:
+            continue
+        sign_success.append(i)
+    assert len(sign_success) == l1_num_inputs
+
+    # But it can sign all the valid ones at once.
+    half_signed_psbt = l1.rpc.signpsbt(joint_psbt, signonly=sign_success)['signed_psbt']
+    for s in sign_success:
+        assert bitcoind.rpc.decodepsbt(half_signed_psbt)['inputs'][s]['partial_signatures'] is not None
+
     totally_signed = l2.rpc.signpsbt(half_signed_psbt)['signed_psbt']
 
     broadcast_tx = l1.rpc.sendpsbt(totally_signed)

wallet/walletrpc.c

@@ -1221,8 +1221,17 @@ struct command_result *param_psbt(struct command *cmd,
 			    json_tok_full(buffer, tok));
 }
 
+static bool in_only_inputs(const u32 *only_inputs, u32 this)
+{
+	for (size_t i = 0; i < tal_count(only_inputs); i++)
+		if (only_inputs[i] == this)
+			return true;
+	return false;
+}
+
 static struct command_result *match_psbt_inputs_to_utxos(struct command *cmd,
 							 struct wally_psbt *psbt,
+							 const u32 *only_inputs,
 							 struct utxo ***utxos)
 {
 	*utxos = tal_arr(cmd, struct utxo *, 0);
@@ -1230,11 +1239,21 @@ static struct command_result *match_psbt_inputs_to_utxos(struct command *cmd,
 		struct utxo *utxo;
 		struct bitcoin_txid txid;
 
+		if (only_inputs && !in_only_inputs(only_inputs, i))
+			continue;
+
 		wally_tx_input_get_txid(&psbt->tx->inputs[i], &txid);
 		utxo = wallet_utxo_get(*utxos, cmd->ld->wallet,
 				       &txid, psbt->tx->inputs[i].index);
-		if (!utxo)
+		if (!utxo) {
+			if (only_inputs)
+				return command_fail(cmd, LIGHTNINGD,
+						    "Aborting PSBT signing. UTXO %s:%u is unknown (and specified by signonly)",
+						    type_to_string(tmpctx, struct bitcoin_txid,
+								   &txid),
+						    psbt->tx->inputs[i].index);
 			continue;
+		}
 
 		/* Oops we haven't reserved this utxo yet! */
 		if (!is_reserved(utxo, get_block_height(cmd->ld->topology)))
@@ -1249,6 +1268,32 @@ static struct command_result *match_psbt_inputs_to_utxos(struct command *cmd,
 	return NULL;
 }
 
+static struct command_result *param_input_numbers(struct command *cmd,
+						  const char *name,
+						  const char *buffer,
+						  const jsmntok_t *tok,
+						  u32 **input_nums)
+{
+	struct command_result *res;
+	const jsmntok_t *arr, *t;
+	size_t i;
+
+	res = param_array(cmd, name, buffer, tok, &arr);
+	if (res)
+		return res;
+
+	*input_nums = tal_arr(cmd, u32, arr->size);
+	json_for_each_arr(i, t, arr) {
+		u32 *num;
+		res = param_number(cmd, name, buffer, t, &num);
+		if (res)
+			return res;
+		(*input_nums)[i] = *num;
+		tal_free(num);
+	}
+	return NULL;
+}
+
 static struct command_result *json_signpsbt(struct command *cmd,
 					    const char *buffer,
 					    const jsmntok_t *obj UNNEEDED,
@@ -1258,17 +1303,27 @@ static struct command_result *json_signpsbt(struct command *cmd,
 	struct json_stream *response;
 	struct wally_psbt *psbt, *signed_psbt;
 	struct utxo **utxos;
+	u32 *input_nums;
 
 	if (!param(cmd, buffer, params,
 		   p_req("psbt", param_psbt, &psbt),
+		   p_opt("signonly", param_input_numbers, &input_nums),
 		   NULL))
 		return command_param_failed();
 
+	/* Sanity check! */
+	for (size_t i = 0; i < tal_count(input_nums); i++) {
+		if (input_nums[i] >= psbt->num_inputs)
+			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+					    "signonly[%zu]: %u out of range",
+					    i, input_nums[i]);
+	}
+
 	/* We have to find/locate the utxos that are ours on this PSBT,
 	 * so that the HSM knows how/what to sign for (it's possible some of
 	 * our utxos require more complicated data to sign for e.g.
 	 * closeinfo outputs */
-	res = match_psbt_inputs_to_utxos(cmd, psbt, &utxos);
+	res = match_psbt_inputs_to_utxos(cmd, psbt, input_nums, &utxos);
 	if (res)
 		return res;
 
@@ -1334,7 +1389,7 @@ static struct command_result *json_sendpsbt(struct command *cmd,
 	/* We have to find/locate the utxos that are ours on this PSBT,
 	 * so that we know who to mark as used.
 	 */
-	res = match_psbt_inputs_to_utxos(cmd, psbt, &utxos);
+	res = match_psbt_inputs_to_utxos(cmd, psbt, NULL, &utxos);
 	if (res)
 		return res;