hsmd: Add PSBT keypath utility functions

common/Makefile

@@ -69,6 +69,7 @@ COMMON_SRC_NOGEN :=				\
 	common/ping.c				\
 	common/private_channel_announcement.c	\
 	common/psbt_internal.c			\
+	common/psbt_keypath.c			\
 	common/psbt_open.c			\
 	common/pseudorand.c			\
 	common/random_select.c			\

common/psbt_keypath.c

@@ -0,0 +1,33 @@
+#include "config.h"
+#include <bitcoin/tx.h>
+#include <common/psbt_keypath.h>
+#include <common/utils.h>
+#include <wally_bip32.h>
+#include <wally_psbt.h>
+
+void psbt_set_keypath(u32 index, const struct ext_key *ext, struct wally_map *map_in) {
+	u8 fingerprint[BIP32_KEY_FINGERPRINT_LEN];
+	if (bip32_key_get_fingerprint(
+		    (struct ext_key *) ext, fingerprint, sizeof(fingerprint)) != WALLY_OK)
+		abort();
+
+	u32 path[1];
+	path[0] = index;
+
+	if (wally_map_add_keypath_item(map_in,
+				       ext->pub_key, sizeof(ext->pub_key),
+				       fingerprint, sizeof(fingerprint),
+				       path, 1) != WALLY_OK)
+		abort();
+}
+
+void psbt_add_keypath_to_last_output(struct bitcoin_tx *tx,
+				     u32 key_index,
+				     const struct ext_key *ext) {
+	size_t outndx = tx->psbt->num_outputs - 1;
+	struct wally_map *map_in = &tx->psbt->outputs[outndx].keypaths;
+
+	tal_wally_start();
+	psbt_set_keypath(key_index, ext, map_in);
+	tal_wally_end(tx->psbt);
+}

common/psbt_keypath.h

@@ -0,0 +1,32 @@
+#ifndef LIGHTNING_COMMON_PSBT_KEYPATH_H
+#define LIGHTNING_COMMON_PSBT_KEYPATH_H
+
+#include "config.h"
+#include <ccan/short_types/short_types.h>
+
+struct bitcoin_tx;
+struct ext_key;
+struct wally_map;
+
+/* psbt_set_keypath - Set the keypath of a PSBT output.
+ *
+ * @index - child index of the wallet key
+ * @ext - extended public key of the immediate parent of the wallet key
+ * @map_in - wally keypaths map
+ */
+void psbt_set_keypath(u32 index,
+		      const struct ext_key *ext,
+		      struct wally_map *map_in);
+
+/* psbt_add_keypath_to_last_output - augment the last output with the
+ * given wallet keypath
+ *
+ * @tx - transaction to modify
+ * @index - child index of the wallet key
+ * @ext - extended public key of the immediate parent of the wallet key
+ */
+void psbt_add_keypath_to_last_output(struct bitcoin_tx *tx,
+				     u32 index,
+				     const struct ext_key *ext);
+
+#endif /* LIGHTNING_COMMON_PSBT_KEYPATH_H */