hsm: Generate fully signed transactions and return them to caller

So far we have been generating the tx both in the HSM and in the
caller, and had to rely on them generating exactly the same
transaction. This makes it a lot simpler by fully signing and
serializing the TX on the HSM side and the caller just needs to unpack
and broadcast it.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

hsmd/hsm.c

@@ -601,8 +601,8 @@ static void sign_withdrawal_tx(struct daemon_conn *master, const u8 *msg)
 	u32 change_keyindex;
 	struct utxo *inutxos;
 	const struct utxo **utxos;
-	secp256k1_ecdsa_signature *sigs;
 	u8 *wscript;
+	u8 **scriptSigs;
 	struct bitcoin_tx *tx;
 	struct ext_key ext;
 	struct pubkey changekey;
@@ -630,13 +630,13 @@ static void sign_withdrawal_tx(struct daemon_conn *master, const u8 *msg)
 		tmpctx, utxos, scriptpubkey, satoshi_out,
 		&changekey, change_out, NULL);
 
-	/* Now generate signatures. */
+	scriptSigs = tal_arr(tmpctx, u8*, tal_count(utxos));
-	sigs = tal_arr(tmpctx, secp256k1_ecdsa_signature, tal_count(utxos));
 	for (size_t i = 0; i < tal_count(utxos); i++) {
 		struct pubkey inkey;
 		struct privkey inprivkey;
 		const struct utxo *in = utxos[i];
 		u8 *subscript;
+		secp256k1_ecdsa_signature sig;
 
 		bitcoin_keypair(&inprivkey, &inkey, in->keyindex);
 		if (utxos[i]->is_p2sh)
@@ -645,12 +645,23 @@ static void sign_withdrawal_tx(struct daemon_conn *master, const u8 *msg)
 			subscript = NULL;
 		wscript = p2wpkh_scriptcode(tmpctx, &inkey);
 
-		sign_tx_input(tx, i, subscript, wscript,
+		sign_tx_input(tx, i, subscript, wscript, &inprivkey, &inkey,
-			      &inprivkey, &inkey, &sigs[i]);
+			      &sig);
+
+		tx->input[i].witness = bitcoin_witness_p2wpkh(tx, &sig, &inkey);
+
+		if (utxos[i]->is_p2sh)
+			scriptSigs[i] = bitcoin_scriptsig_p2sh_p2wpkh(tx, &inkey);
+		else
+			scriptSigs[i] = NULL;
 	}
 
+	/* Now complete the transaction by attaching the scriptSigs where necessary */
+	for (size_t i=0; i<tal_count(utxos); i++)
+		tx->input[i].script = scriptSigs[i];
+
 	daemon_conn_send(master,
-			 take(towire_hsm_sign_withdrawal_reply(tmpctx, sigs)));
+			 take(towire_hsm_sign_withdrawal_reply(tmpctx, tx)));
 	tal_free(tmpctx);
 }
 

hsmd/hsm_client_wire_csv

@@ -57,8 +57,7 @@ hsm_sign_withdrawal,,num_inputs,u16
 hsm_sign_withdrawal,,inputs,num_inputs*struct utxo
 
 hsm_sign_withdrawal_reply,107
-hsm_sign_withdrawal_reply,,num_sigs,u16
+hsm_sign_withdrawal_reply,,tx,struct bitcoin_tx
-hsm_sign_withdrawal_reply,,sig,num_sigs*secp256k1_ecdsa_signature
 
 # Sign an invoice
 hsm_sign_invoice,8

wallet/walletrpc.c

@@ -194,9 +194,6 @@ static void json_withdraw(struct command *cmd,
 	u32 feerate_per_kw = get_feerate(cmd->ld->topology, FEERATE_NORMAL);
 	u64 fee_estimate;
 	struct utxo *utxos;
-	struct ext_key ext;
-	struct pubkey changekey;
-	secp256k1_ecdsa_signature *sigs;
 	struct bitcoin_tx *tx;
 	bool withdraw_all = false;
 
@@ -293,43 +290,12 @@ static void json_withdraw(struct command *cmd,
 
 	msg = hsm_sync_read(cmd, cmd->ld);
 
-	if (!fromwire_hsm_sign_withdrawal_reply(withdraw, msg, NULL, &sigs))
+	tx = tal(withdraw, struct bitcoin_tx);
+
+	if (!fromwire_hsm_sign_withdrawal_reply(msg, NULL, tx))
 		fatal("HSM gave bad sign_withdrawal_reply %s",
 		      tal_hex(withdraw, msg));
 
-	if (withdraw->changesatoshi) {
-		if (bip32_key_from_parent(cmd->ld->wallet->bip32_base,
-					  withdraw->change_key_index,
-					  BIP32_FLAG_KEY_PUBLIC, &ext)
-		    != WALLY_OK) {
-			command_fail(cmd, "Changekey generation failure");
-			return;
-		}
-
-		pubkey_from_der(ext.pub_key, sizeof(ext.pub_key), &changekey);
-	}
-	tx = withdraw_tx(withdraw, withdraw->utxos, withdraw->destination,
-			 withdraw->amount, &changekey, withdraw->changesatoshi,
-			 cmd->ld->wallet->bip32_base);
-
-	if (tal_count(sigs) != tal_count(tx->input))
-		fatal("HSM gave %zu sigs, needed %zu",
-		      tal_count(sigs), tal_count(tx->input));
-
-	/* Create input parts from signatures. */
-	for (size_t i = 0; i < tal_count(tx->input); i++) {
-		struct pubkey key;
-
-		if (!bip32_pubkey(cmd->ld->wallet->bip32_base,
-				  &key, withdraw->utxos[i]->keyindex))
-			fatal("Cannot generate BIP32 key for UTXO %u",
-			      withdraw->utxos[i]->keyindex);
-
-		/* P2SH inputs have same witness. */
-		tx->input[i].witness
-			= bitcoin_witness_p2wpkh(tx, &sigs[i], &key);
-	}
-
 	/* Now broadcast the transaction */
 	withdraw->hextx = tal_hex(withdraw, linearize_tx(cmd, tx));
 	bitcoind_sendrawtx(cmd->ld->topology->bitcoind, withdraw->hextx,