hsmtool: add a 'testnet' option to 'dumponchaindescriptors'

Actually, it's more complex to translate the xpub descriptor to
testnet because of the descriptor checksum.

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tools/hsmtool.c

@@ -36,7 +36,7 @@ static void show_usage(const char *progname)
 	printf("	- guesstoremote <P2WPKH address> <node id> <tries> "
 	       "<path/to/hsm_secret> [hsm_secret password]\n");
 	printf("	- dumponchaindescriptors <path/to/hsm_secret> [password] "
-		"[private]\n");
+		"[private] [testnet]\n");
 	exit(0);
 }
 
@@ -373,11 +373,12 @@ static int guess_to_remote(const char *address, struct node_id *node_id,
 }
 
 static int dumponchaindescriptors(const char *hsm_secret_path, const char *passwd,
-				  const bool private)
+				  const bool private, const bool testnet)
 {
 	struct secret hsm_secret;
 	u8 bip32_seed[BIP32_ENTROPY_LEN_256];
 	u32 salt = 0;
+	u32 version = testnet ? BIP32_VER_TEST_PRIVATE : BIP32_VER_MAIN_PRIVATE;
 	struct ext_key master_extkey;
 	char *enc_xpub, *descriptor, checksum[DESCRIPTOR_CHECKSUM_LENGTH + 1];
 
@@ -397,9 +398,7 @@ static int dumponchaindescriptors(const char *hsm_secret_path, const char *passw
 		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);
+				     version, 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");
@@ -492,7 +491,8 @@ int main(int argc, char *argv[])
 
 		return dumponchaindescriptors(argv[2],
 					      argc > 3 && !streq(argv[3], "") ? argv[3] : NULL,
-					      argc > 4 && streq(argv[4], "private"));
+					      argc > 4 && streq(argv[4], "private"),
+					      argc > 5 && streq(argv[5], "testnet"));
 	}
 
 	show_usage(argv[0]);