tools: add a new 'dumponchaindescriptors' command to hsmtool

This adds a command which outputs the two output descriptors
corresponding to our onchain wallet.

This can be useful for an external service to monitor / send fund to our
wallet.

Further, an "xpriv" version of such descriptors could be used to import
onchain funds on a new wallet.

Changelog-Added: lightning-hsmtool: a new command was added to hsmtool for dumping descriptors of the onchain wallet
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tools/Makefile

@@ -17,7 +17,7 @@ tools/headerversions: FORCE tools/headerversions.o $(CCAN_OBJS)
 
 tools/check-bolt: tools/check-bolt.o $(CCAN_OBJS) $(TOOLS_COMMON_OBJS)
 
-tools/hsmtool: tools/hsmtool.o $(CCAN_OBJS) $(TOOLS_COMMON_OBJS) $(BITCOIN_OBJS) common/amount.o common/bech32.o common/bigsize.o common/derive_basepoints.o common/node_id.o common/type_to_string.o wire/fromwire.o wire/towire.o
+tools/hsmtool: tools/hsmtool.o $(CCAN_OBJS) $(TOOLS_COMMON_OBJS) $(BITCOIN_OBJS) common/amount.o common/bech32.o common/bigsize.o common/derive_basepoints.o common/descriptor_checksum.o common/node_id.o common/type_to_string.o wire/fromwire.o wire/towire.o
 
 tools/lightning-hsmtool: tools/hsmtool
 	cp $< $@

tools/hsmtool.c

@@ -6,8 +6,10 @@
 #include <ccan/read_write_all/read_write_all.h>
 #include <ccan/str/str.h>
 #include <ccan/tal/path/path.h>
+#include <ccan/tal/str/str.h>
 #include <common/bech32.h>
 #include <common/derive_basepoints.h>
+#include <common/descriptor_checksum.h>
 #include <common/node_id.h>
 #include <common/type_to_string.h>
 #include <common/utils.h>
@@ -39,6 +41,7 @@ static void show_usage(const char *progname)
 	printf("	- guesstoremote <P2WPKH address> <node id> <tries> "
 	       "<path/to/hsm_secret> [hsm_secret password]\n");
 	printf("	- generatehsm <path/to/new//hsm_secret>\n");
+	printf("	- dumponchaindescriptors <path/to/hsm_secret> <password>\n");
 	exit(0);
 }
 
@@ -513,6 +516,58 @@ static int generate_hsm(const char *hsm_secret_path)
 	return 0;
 }
 
+static int dumponchaindescriptors(const char *hsm_secret_path, const char *passwd)
+{
+	struct secret hsm_secret;
+	u8 bip32_seed[BIP32_ENTROPY_LEN_256];
+	u32 salt = 0;
+	struct ext_key master_extkey;
+	char *enc_xpub, *descriptor;
+	struct descriptor_checksum checksum;
+
+	if (passwd)
+		get_encrypted_hsm_secret(&hsm_secret, hsm_secret_path, passwd);
+	else
+		get_hsm_secret(&hsm_secret, hsm_secret_path);
+
+	/* We use m/0/0/k as the derivation tree for onchain funds. */
+
+	/* The root seed is derived from hsm_secret using hkdf.. */
+	do {
+		hkdf_sha256(bip32_seed, sizeof(bip32_seed),
+			    &salt, sizeof(salt),
+			    &hsm_secret, sizeof(hsm_secret),
+			    "bip32 seed", strlen("bip32 seed"));
+		salt++;
+		/* ..Which is used to derive m/ */
+	} while (bip32_key_from_seed(bip32_seed, sizeof(bip32_seed),
+				     /* An xpub can easily be converted to a tpub */
+				     BIP32_VER_MAIN_PRIVATE,
+				     0, &master_extkey) != WALLY_OK);
+
+	if (bip32_key_to_base58(&master_extkey, BIP32_FLAG_KEY_PUBLIC, &enc_xpub) != WALLY_OK)
+		errx(ERROR_LIBWALLY, "Can't encode xpub");
+
+	/* Now we format the descriptor strings (we only ever create P2WPKH and
+	 * P2SH-P2WPKH outputs). */
+
+	descriptor = tal_fmt(NULL, "wpkh(%s/0/0/*)", enc_xpub);
+	if (!descriptor_checksum(descriptor, strlen(descriptor), &checksum))
+		errx(ERROR_LIBWALLY, "Can't derive descriptor checksum for wpkh");
+	printf("%s#%s\n", descriptor, checksum.csum);
+	tal_free(descriptor);
+
+	descriptor = tal_fmt(NULL, "sh(wpkh(%s/0/0/*))", enc_xpub);
+	if (!descriptor_checksum(descriptor, strlen(descriptor), &checksum))
+		errx(ERROR_LIBWALLY, "Can't derive descriptor checksum for sh(wpkh)");
+	printf("%s#%s\n", descriptor, checksum.csum);
+	tal_free(descriptor);
+
+	wally_free_string(enc_xpub);
+
+	return 0;
+}
+
 int main(int argc, char *argv[])
 {
 	const char *method;
@@ -573,5 +628,12 @@ int main(int argc, char *argv[])
 		return generate_hsm(hsm_secret_path);
 	}
 
+	if (streq(method, "dumponchaindescriptors")) {
+		if (argc < 3)
+			show_usage(argv[0]);
+
+		return dumponchaindescriptors(argv[2], argc > 3 ? argv[3] : NULL);
+	}
+
 	show_usage(argv[0]);
 }