hsmtool: only write 32 bytes to hsm_secret on generatehsm

Fixes #4302
Changelog-fixed: hsmtool: the `generatehsm` command now generates an appropriately-sized hsm_secret
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tests/test_wallet.py

@@ -1097,7 +1097,6 @@ def test_hsmtool_dump_descriptors(node_factory, bitcoind):
     assert len(bitcoind.rpc.listunspent(1, 1, [addr])) == 1
 
 
-@pytest.mark.xfail(strict=True)
 @unittest.skipIf(VALGRIND, "It does not play well with prompt and key derivation.")
 def test_hsmtool_generatehsm(node_factory):
     l1 = node_factory.get_node()

tools/hsmtool.c

@@ -532,7 +532,9 @@ static int generate_hsm(const char *hsm_secret_path)
 	if (fd < 0) {
 		errx(ERROR_USAGE, "Unable to create hsm_secret file");
 	}
-	if (!write_all(fd, bip32_seed, bip32_seed_len))
+	/* Write only the first 32 bytes, length of the (plaintext) seed in the
+	 * hsm_secret. */
+	if (!write_all(fd, bip32_seed, 32))
 		errx(ERROR_USAGE, "Error writing secret to hsm_secret file");
 
 	if (fsync(fd) != 0)