hsmd: routine to sign HTLC tx merged with our own tx.

Since HTLC txs when using anchors are
SIGHASH_SINGLE|SIGHASH_ANYONECANPAY, we can attach other inputs to
give it a higher feerate.  But we need the HSMd to actually sign the
combo.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/hsm_version.h

@@ -14,6 +14,7 @@
  * v4 with check_pubkey: 48b3992745aa3c6ab6ce5cdaee9082cb7d70017f523d322015e9710bf49fd193
  * v4 with sign_any_penalty_to_us: ead7963185194a515d1f14d2c44401392575299d68ce9a13d8a12baff3cf4f35
  * v4 with sign_anchorspend: 8a30722e38b56e82af566b9629ff18da01fcebd1e80ec67f04d8b3a2fa66d81c
+ * v4 with sign_htlc_tx_mingle: b9247e75d41ee1b3fc2f7db0bac8f4e92d544ab2f017d430ae3a000589c384e5
  */
 #define HSM_MIN_VERSION 3
 #define HSM_MAX_VERSION 4

hsmd/hsmd.c

@@ -686,6 +686,7 @@ static struct io_plan *handle_client(struct io_conn *conn, struct client *c)
 	case WIRE_HSMD_SIGN_ANY_REMOTE_HTLC_TO_US:
 	case WIRE_HSMD_SIGN_ANY_LOCAL_HTLC_TX:
 	case WIRE_HSMD_SIGN_ANCHORSPEND:
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE:
 		/* Hand off to libhsmd for processing */
 		return req_reply(conn, c,
 				 take(hsmd_handle_client_message(
@@ -720,6 +721,7 @@ static struct io_plan *handle_client(struct io_conn *conn, struct client *c)
 	case WIRE_HSMD_PREAPPROVE_KEYSEND_REPLY:
 	case WIRE_HSMD_CHECK_PUBKEY_REPLY:
 	case WIRE_HSMD_SIGN_ANCHORSPEND_REPLY:
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE_REPLY:
 		return bad_req_fmt(conn, c, c->msg_in,
 				   "Received an incoming message of type %s, "
 				   "which is not a request",

hsmd/hsmd_wire.csv

@@ -380,3 +380,13 @@ msgdata,hsmd_sign_any_local_htlc_tx,option_anchor_outputs,bool,
 msgdata,hsmd_sign_any_local_htlc_tx,input,u32,
 msgdata,hsmd_sign_any_local_htlc_tx,peerid,node_id,
 msgdata,hsmd_sign_any_local_htlc_tx,channel_dbid,u64,
+
+msgtype,hsmd_sign_htlc_tx_mingle,149
+msgdata,hsmd_sign_htlc_tx_mingle,peerid,node_id,
+msgdata,hsmd_sign_htlc_tx_mingle,channel_dbid,u64,
+msgdata,hsmd_sign_htlc_tx_mingle,num_inputs,u16,
+msgdata,hsmd_sign_htlc_tx_mingle,inputs,utxo,num_inputs
+msgdata,hsmd_sign_htlc_tx_mingle,psbt,wally_psbt,
+
+msgtype,hsmd_sign_htlc_tx_mingle_reply,150
+msgdata,hsmd_sign_htlc_tx_mingle_reply,psbt,wally_psbt,

hsmd/libhsmd.c

@@ -129,6 +129,7 @@ bool hsmd_check_client_capabilities(struct hsmd_client *client,
 	case WIRE_HSMD_SIGN_ANY_REMOTE_HTLC_TO_US:
 	case WIRE_HSMD_SIGN_ANY_LOCAL_HTLC_TX:
 	case WIRE_HSMD_SIGN_ANCHORSPEND:
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE:
 		return (client->capabilities & HSM_CAP_MASTER) != 0;
 
 	/*~ These are messages sent by the HSM so we should never receive them. */
@@ -163,6 +164,7 @@ bool hsmd_check_client_capabilities(struct hsmd_client *client,
 	case WIRE_HSMD_DERIVE_SECRET_REPLY:
 	case WIRE_HSMD_CHECK_PUBKEY_REPLY:
 	case WIRE_HSMD_SIGN_ANCHORSPEND_REPLY:
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE_REPLY:
 		break;
 	}
 	return false;
@@ -1504,6 +1506,26 @@ static u8 *handle_sign_anchorspend(struct hsmd_client *c, const u8 *msg_in)
 	return towire_hsmd_sign_anchorspend_reply(NULL, psbt);
 }
 
+/*~ Called from lightningd */
+static u8 *handle_sign_htlc_tx_mingle(struct hsmd_client *c, const u8 *msg_in)
+{
+	struct node_id peer_id;
+	u64 dbid;
+	struct utxo **utxos;
+	struct wally_psbt *psbt;
+
+	/* FIXME: Check output goes to us. */
+	if (!fromwire_hsmd_sign_htlc_tx_mingle(tmpctx, msg_in,
+					       &peer_id, &dbid, &utxos, &psbt))
+		return hsmd_status_malformed_request(c, msg_in);
+
+	/* Sign all the UTXOs (htlc_inout input is already signed with
+	 * SIGHASH_SINGLE|SIGHASH_ANYONECANPAY) */
+	sign_our_inputs(utxos, psbt);
+
+	return towire_hsmd_sign_htlc_tx_mingle_reply(NULL, psbt);
+}
+
 /*~ This is another lightningd-only interface; signing a commit transaction.
  * This is dangerous, since if we sign a revoked commitment tx we'll lose
  * funds, thus it's only available to lightningd.
@@ -1909,6 +1931,8 @@ u8 *hsmd_handle_client_message(const tal_t *ctx, struct hsmd_client *client,
 		return handle_sign_any_penalty_to_us(client, msg);
 	case WIRE_HSMD_SIGN_ANCHORSPEND:
 		return handle_sign_anchorspend(client, msg);
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE:
+		return handle_sign_htlc_tx_mingle(client, msg);
 
 	case WIRE_HSMD_DEV_MEMLEAK:
 	case WIRE_HSMD_ECDH_RESP:
@@ -1940,6 +1964,7 @@ u8 *hsmd_handle_client_message(const tal_t *ctx, struct hsmd_client *client,
 	case WIRE_HSMD_PREAPPROVE_KEYSEND_REPLY:
 	case WIRE_HSMD_CHECK_PUBKEY_REPLY:
 	case WIRE_HSMD_SIGN_ANCHORSPEND_REPLY:
+	case WIRE_HSMD_SIGN_HTLC_TX_MINGLE_REPLY:
 		break;
 	}
 	return hsmd_status_bad_request(client, msg, "Unknown request");
@@ -1957,6 +1982,7 @@ u8 *hsmd_init(struct secret hsm_secret,
 		WIRE_HSMD_CHECK_PUBKEY,
 		WIRE_HSMD_SIGN_ANY_DELAYED_PAYMENT_TO_US,
 		WIRE_HSMD_SIGN_ANCHORSPEND,
+		WIRE_HSMD_SIGN_HTLC_TX_MINGLE,
 	};
 
 	/*~ Don't swap this. */