From fa0d9b6d2a452c867210210d180d546fde053886 Mon Sep 17 00:00:00 2001
From: Dusty Daemon <dustinpaystaxes@gmail.com>
Date: Mon, 11 Nov 2024 11:00:31 +1030
Subject: [PATCH] psbt: Add ability to extract signature
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

A routine for getting a signature back out of an input’s list of pending signatures via pubkey search.

This is needed for certain kinds of restarts as we lose our peer’s signature from memory but a copy is kept in the PSBT.
---
 bitcoin/psbt.c                  | 30 ++++++++++++++++++++++++++++++
 bitcoin/psbt.h                  |  8 ++++++++
 bitcoin/test/run-psbt-from-tx.c |  3 +++
 3 files changed, 41 insertions(+)

diff --git a/bitcoin/psbt.c b/bitcoin/psbt.c
index 5a1a24946..7afb5a4fe 100644
--- a/bitcoin/psbt.c
+++ b/bitcoin/psbt.c
@@ -311,6 +311,36 @@ bool psbt_input_have_signature(const struct wally_psbt *psbt,
 	return ok;
 }
 
+bool psbt_input_get_signature(const tal_t *ctx,
+			      const struct wally_psbt *psbt,
+			      size_t in,
+			      const struct pubkey *pubkey,
+			      struct bitcoin_signature **sig)
+{
+	u8 pk_der[PUBKEY_CMPR_LEN];
+	size_t index_plus_one;
+	struct wally_map_item *item;
+	bool ok;
+
+	assert(in < psbt->num_inputs);
+
+	pubkey_to_der(pk_der, pubkey);
+	*sig = NULL;
+
+	ok = wally_psbt_input_find_signature(&psbt->inputs[in], pk_der,
+					     sizeof(pk_der),
+					     &index_plus_one) == WALLY_OK;
+	if (ok) {
+		item = &psbt->inputs[in].signatures.items[index_plus_one - 1];
+		*sig = tal(ctx, struct bitcoin_signature);
+		if (!signature_from_der(item->value, item->value_len, *sig)) {
+			*sig = tal_free(*sig);
+			return false;
+		}
+	}
+	return ok;
+}
+
 void psbt_input_set_wit_utxo(struct wally_psbt *psbt, size_t in,
 			     const u8 *scriptPubkey, struct amount_sat amt)
 {
diff --git a/bitcoin/psbt.h b/bitcoin/psbt.h
index 31abd2d42..8d9eda2a0 100644
--- a/bitcoin/psbt.h
+++ b/bitcoin/psbt.h
@@ -186,6 +186,14 @@ WARN_UNUSED_RESULT bool psbt_input_have_signature(const struct wally_psbt *psbt,
 						  const struct pubkey *pubkey,
 						  bool *signature_found);
 
+/* Returns false on error. On success *sig is set to the signature otherwise
+ * *sig is set to NULL. */
+WARN_UNUSED_RESULT bool psbt_input_get_signature(const tal_t *ctx,
+						 const struct wally_psbt *psbt,
+						 size_t in,
+						 const struct pubkey *pubkey,
+						 struct bitcoin_signature **sig);
+
 void psbt_input_set_witscript(struct wally_psbt *psbt, size_t in, const u8 *wscript);
 
 /* psbt_input_set_unknown - Set the given Key-Value in the psbt's input keymap
diff --git a/bitcoin/test/run-psbt-from-tx.c b/bitcoin/test/run-psbt-from-tx.c
index 3c71675e5..85e2ca677 100644
--- a/bitcoin/test/run-psbt-from-tx.c
+++ b/bitcoin/test/run-psbt-from-tx.c
@@ -68,6 +68,9 @@ void sha256_double(struct sha256_double *shadouble UNNEEDED, const void *p UNNEE
 /* Generated stub for signature_to_der */
 size_t signature_to_der(u8 der[73] UNNEEDED, const struct bitcoin_signature *sig UNNEEDED)
 { fprintf(stderr, "signature_to_der called!\n"); abort(); }
+/* Generated stub for signature_to_der */
+bool signature_from_der(const u8 *der UNNEEDED, size_t len UNNEEDED, struct bitcoin_signature *sig UNNEEDED)
+{ fprintf(stderr, "signature_from_der called!\n"); abort(); }
 /* Generated stub for towire_sha256_double */
 void towire_sha256_double(u8 **pptr UNNEEDED, const struct sha256_double *sha256d UNNEEDED)
 { fprintf(stderr, "towire_sha256_double called!\n"); abort(); }