lightning/hsmd/hsmd.c

/*~ Welcome to the hsm daemon: keeper of our secrets!
 *
 * This is a separate daemon which keeps a root secret from which all others
 * are generated.  It starts with one client: lightningd, which can ask for
 * new sockets for other clients.  Each client has a simple capability map
 * which indicates what it's allowed to ask for.  We're entirely driven
 * by request, response.
 */
#include <bitcoin/address.h>
#include <bitcoin/privkey.h>
#include <bitcoin/pubkey.h>
#include <bitcoin/script.h>
#include <bitcoin/tx.h>
#include <ccan/array_size/array_size.h>
#include <ccan/cast/cast.h>
#include <ccan/container_of/container_of.h>
#include <ccan/crypto/hkdf_sha256/hkdf_sha256.h>
#include <ccan/endian/endian.h>
#include <ccan/fdpass/fdpass.h>
#include <ccan/intmap/intmap.h>
#include <ccan/io/fdpass/fdpass.h>
#include <ccan/io/io.h>
#include <ccan/noerr/noerr.h>
#include <ccan/ptrint/ptrint.h>
#include <ccan/read_write_all/read_write_all.h>
#include <ccan/take/take.h>
#include <ccan/tal/str/str.h>
#include <common/daemon_conn.h>
#include <common/derive_basepoints.h>
#include <common/funding_tx.h>
#include <common/hash_u5.h>
#include <common/key_derive.h>
#include <common/status.h>
#include <common/subdaemon.h>
#include <common/type_to_string.h>
#include <common/utils.h>
#include <common/version.h>
#include <common/withdraw_tx.h>
#include <errno.h>
#include <fcntl.h>
#include <hsmd/capabilities.h>
/*~ All gen_ files are autogenerated; in this case by tools/generate-wire.py */
#include <hsmd/gen_hsm_wire.h>
#include <inttypes.h>
#include <secp256k1_ecdh.h>
#include <sodium/randombytes.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <wally_bip32.h>
#include <wire/gen_peer_wire.h>
#include <wire/wire_io.h>

/*~ Each subdaemon is started with stdin connected to lightningd (for status
 * messages), and stderr untouched (for emergency printing).  File descriptors
 * 3 and beyond are set up on other sockets: for hsmd, fd 3 is the request
 * stream from lightningd. */
#define REQ_FD 3

/*~ Nobody will ever find it here!  hsm_secret is our root secret, the bip32
 * tree is derived from that, and cached here. */
static struct {
	struct secret hsm_secret;
	struct ext_key bip32;
} secretstuff;

/*~ We keep track of clients, but there's not much to keep. */
struct client {
	/* The ccan/io async io connection for this client: it closes, we die. */
	struct io_conn *conn;

	/*~ io_read_wire needs a pointer to store incoming messages until
	 * it has the complete thing; this is it. */
	u8 *msg_in;

	/* ~Useful for logging, but also used to derive the per-channel seed. */
	struct pubkey id;

	/* ~This is a unique value handed to us from lightningd, used for
	 * per-channel seed generation (a single id may have multiple channels
	 * over time).
	 *
	 * It's actually zero for the initial lightningd client connection and
	 * the ones for gossipd and connectd, which don't have channels
	 * associated. */
	u64 dbid;

	/* What is this client allowed to ask for? */
	u64 capabilities;
};

/*~ We keep a map of nonzero dbid -> clients, mainly for leak detection.
 * This is ccan/uintmap, which maps u64 to some (non-NULL) pointer.
 * I really dislike these kinds of declaration-via-magic macro things, as
 * tags can't find them without special hacks, but the payoff here is that
 * the map is typesafe: the compiler won't let you put anything in but a
 * struct client pointer. */
static UINTMAP(struct client *) clients;
/*~ Plus the three zero-dbid clients: master, gossipd and connnectd. */
static struct client *dbid_zero_clients[3];
static size_t num_dbid_zero_clients;

/*~ We need this deep inside bad_req_fmt, so we make it a global. */
static struct daemon_conn *status_conn;

/* This is used for various assertions and error cases. */
static bool is_lightningd(const struct client *client)
{
	return client == dbid_zero_clients[0];
}

/*~ FIXME: This is used by debug.c.  Doesn't apply to us, but lets us link. */
extern void dev_disconnect_init(int fd);
void dev_disconnect_init(int fd UNUSED) { }

/* Pre-declare this, due to mutual recursion */
static struct io_plan *handle_client(struct io_conn *conn, struct client *c);

/*~ ccan/compiler.h defines PRINTF_FMT as the gcc compiler hint so it will
 * check that fmt and other trailing arguments really are the correct type.
 *
 * This is a convenient helper to tell lightningd we've received a bad request
 * and closes the client connection.  This should never happen, of course, but
 * we definitely want to log if it does.
 */
static PRINTF_FMT(4,5)
	struct io_plan *bad_req_fmt(struct io_conn *conn,
				    struct client *c,
				    const u8 *msg_in,
				    const char *fmt, ...)
{
	va_list ap;
	char *str;

	va_start(ap, fmt);
	str = tal_fmt(tmpctx, fmt, ap);
	va_end(ap);

	/*~ If the client was actually lightningd, it's Game Over; we actually
	 * fail in this case, and it will too. */
	if (is_lightningd(c)) {
		status_broken("%s", str);
		master_badmsg(fromwire_peektype(msg_in), msg_in);
	}

	/*~ Note the use of NULL as the ctx arg to towire_hsmstatus_: only
	 * use NULL as the allocation when we're about to immediately free it
	 * or hand it off with take(), as here.  That makes it clear we don't
	 * expect it to linger, and in fact our memleak detection will
	 * complain if it does (unlike using the deliberately-transient
	 * tmpctx). */
	daemon_conn_send(status_conn,
			 take(towire_hsmstatus_client_bad_request(NULL,
								  &c->id,
								  str,
								  msg_in)));

	/*~ The way ccan/io works is that you return the "plan" for what to do
	 * next (eg. io_read).  io_close() is special: it means to close the
	 * connection. */
	return io_close(conn);
}

/* Convenience wrapper for when we simply can't parse. */
static struct io_plan *bad_req(struct io_conn *conn,
			       struct client *c,
			       const u8 *msg_in)
{
	return bad_req_fmt(conn, c, msg_in, "could not parse request");
}

/*~ This plan simply says: read the next packet into 'c->msg_in' (parent 'c'),
 * and then call handle_client with argument 'c' */
static struct io_plan *client_read_next(struct io_conn *conn, struct client *c)
{
	return io_read_wire(conn, c, &c->msg_in, handle_client, c);
}

/*~ This is the destructor on our client: we may call it manually, but
 * generally it's called because the io_conn associated with the client is
 * closed by the other end. */
static void destroy_client(struct client *c)
{
	if (!uintmap_del(&clients, c->dbid))
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "Failed to remove client dbid %"PRIu64, c->dbid);
}

static struct client *new_client(const tal_t *ctx,
				 const struct pubkey *id,
				 u64 dbid,
				 const u64 capabilities,
				 int fd)
{
	struct client *c = tal(ctx, struct client);

	/*~ All-zero pubkey is used for the initial master connection */
	if (id) {
		c->id = *id;
	} else {
		memset(&c->id, 0, sizeof(c->id));
	}
	c->dbid = dbid;

	c->capabilities = capabilities;
	/*~ This is the core of ccan/io: the connection creation calls a
	 * callback which returns the initial plan to execute: in our case,
	 * read a message.*/
	c->conn = io_new_conn(ctx, fd, client_read_next, c);

	/*~ tal_steal() moves a pointer to a new parent.  At this point, the
	 * hierarchy is:
	 *
	 *   ctx -> c
	 *   ctx -> c->conn
	 *
	 * We want to the c->conn to own 'c', so that if the io_conn closes,
	 * the client is freed:
	 *
	 *   ctx -> c->conn -> c.
	 */
	tal_steal(c->conn, c);

	/* We put the special zero-db HSM connections into an array, the rest
	 * go into the map. */
	if (dbid == 0) {
		assert(num_dbid_zero_clients < ARRAY_SIZE(dbid_zero_clients));
		dbid_zero_clients[num_dbid_zero_clients++] = c;
	} else {
		struct client *old_client = uintmap_get(&clients, dbid);

		/* Close conn and free any old client of this dbid. */
		if (old_client)
			io_close(old_client->conn);

		if (!uintmap_add(&clients, dbid, c))
			status_failed(STATUS_FAIL_INTERNAL_ERROR,
				      "Failed inserting dbid %"PRIu64, dbid);
		tal_add_destructor(c, destroy_client);
	}

	return c;
}

/* This is the common pattern for the tail of each handler in this file. */
static struct io_plan *req_reply(struct io_conn *conn,
				 struct client *c,
				 const u8 *msg_out TAKES)
{
	/*~ Write this out, then read the next one.  This works perfectly for
	 * a simple request/response system like this.
	 *
	 * Internally, the ccan/io subsystem gathers all the file descriptors,
	 * figures out which want to write and read, asks the OS which ones
	 * are available, and for those file descriptors, tries to do the
	 * reads/writes we've asked it.	 It handles retry in the case where a
	 * read or write is done partially.
	 *
	 * Since the OS does buffering internally (on my system, over 100k
	 * worth) writes will normally succeed immediately.  However, if the
	 * client is slow or malicious, and doesn't read from the socket as
	 * fast as we're writing, eventually the socket buffer will fill up;
	 * we don't care, because ccan/io will wait until there's room to
	 * write this reply before it will read again.  The client just hurts
	 * themselves, and there's no Denial of Service on us.
	 *
	 * If we were to queue outgoing messages ourselves, we *would* have to
	 * consider such scenarios; this is why our daemons generally avoid
	 * buffering from untrusted parties. */
	return io_write_wire(conn, msg_out, client_read_next, c);
}

/*~ This returns the secret and/or public key for this node. */
static void node_key(struct privkey *node_privkey, struct pubkey *node_id)
{
	u32 salt = 0;
	struct privkey unused_s;
	struct pubkey unused_k;

	/* If caller specifies NULL, they don't want the results. */
	if (node_privkey == NULL)
		node_privkey = &unused_s;
	else if (node_id == NULL)
		node_id = &unused_k;

	/*~ So, there is apparently a 1 in 2^127 chance that a random value is
	 * not a valid private key, so this never actually loops. */
	do {
		/*~ ccan/crypto/hkdf_sha256 implements RFC5869 "Hardened Key
		 * Derivation Functions".  That means that if a derived key
		 * leaks somehow, the other keys are not compromised. */
		hkdf_sha256(node_privkey, sizeof(*node_privkey),
			    &salt, sizeof(salt),
			    &secretstuff.hsm_secret,
			    sizeof(secretstuff.hsm_secret),
			    "nodeid", 6);
		salt++;
	} while (!secp256k1_ec_pubkey_create(secp256k1_ctx, &node_id->pubkey,
					     node_privkey->secret.data));
}

/*~ This secret is the basis for all per-channel secrets: the per-channel seeds
 * will be generated by mixing in the dbid and the peer node_id. */
static void hsm_channel_secret_base(struct secret *channel_seed_base)
{
	hkdf_sha256(channel_seed_base, sizeof(struct secret), NULL, 0,
		    &secretstuff.hsm_secret, sizeof(secretstuff.hsm_secret),
		    /*~ Initially, we didn't support multiple channels per
		     * peer at all: a channel had to be completely forgotten
		     * before another could exist.  That was slightly relaxed,
		     * but the phrase "peer seed" is wired into the seed
		     * generation here, so we need to keep it that way for
		     * existing clients, rather than using "channel seed". */
		    "peer seed", strlen("peer seed"));
}

/*~ This gets the seed for this particular channel. */
static void get_channel_seed(const struct pubkey *peer_id, u64 dbid,
			     struct secret *channel_seed)
{
	struct secret channel_base;
	u8 input[PUBKEY_DER_LEN + sizeof(dbid)];
	/*~ Again, "per-peer" should be "per-channel", but Hysterical Raisins */
	const char *info = "per-peer seed";

	/*~ We use the DER encoding of the pubkey, because it's platform
	 * independent.  Since the dbid is unique, however, it's completely
	 * unnecessary, but again, existing users can't be broken. */
	/* FIXME: lnd has a nicer BIP32 method for deriving secrets which we
	 * should migrate to. */
	hsm_channel_secret_base(&channel_base);
	pubkey_to_der(input, peer_id);
	/*~ For all that talk about platform-independence, note that this
	 * field is endian-dependent!  But let's face it, little-endian won.
	 * In related news, we don't support EBCDIC or middle-endian. */
	memcpy(input + PUBKEY_DER_LEN, &dbid, sizeof(dbid));

	hkdf_sha256(channel_seed, sizeof(*channel_seed),
		    input, sizeof(input),
		    &channel_base, sizeof(channel_base),
		    info, strlen(info));
}

/*~ Called at startup to derive the bip32 field. */
static void populate_secretstuff(void)
{
	u8 bip32_seed[BIP32_ENTROPY_LEN_256];
	u32 salt = 0;
	struct ext_key master_extkey, child_extkey;

	/* Fill in the BIP32 tree for bitcoin addresses. */
	do {
		hkdf_sha256(bip32_seed, sizeof(bip32_seed),
			    &salt, sizeof(salt),
			    &secretstuff.hsm_secret,
			    sizeof(secretstuff.hsm_secret),
			    "bip32 seed", strlen("bip32 seed"));
		salt++;
	} while (bip32_key_from_seed(bip32_seed, sizeof(bip32_seed),
				     BIP32_VER_TEST_PRIVATE,
				     0, &master_extkey) != WALLY_OK);

	/* BIP 32:
	 *
	 * The default wallet layout
	 *
	 * An HDW is organized as several 'accounts'. Accounts are numbered,
	 * the default account ("") being number 0. Clients are not required
	 * to support more than one account - if not, they only use the
	 * default account.
	 *
	 * Each account is composed of two keypair chains: an internal and an
	 * external one. The external keychain is used to generate new public
	 * addresses, while the internal keychain is used for all other
	 * operations (change addresses, generation addresses, ..., anything
	 * that doesn't need to be communicated). Clients that do not support
	 * separate keychains for these should use the external one for
	 * everything.
	 *
	 *  - m/iH/0/k corresponds to the k'th keypair of the external chain of account number i of the HDW derived from master m.
	 */
	/* Hence child 0, then child 0 again to get extkey to derive from. */
	if (bip32_key_from_parent(&master_extkey, 0, BIP32_FLAG_KEY_PRIVATE,
				  &child_extkey) != WALLY_OK)
		/*~ status_failed() is a helper which exits and sends lightningd
		 * a message about what happened.  For hsmd, that's fatal to
		 * lightningd. */
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "Can't derive child bip32 key");

	if (bip32_key_from_parent(&child_extkey, 0, BIP32_FLAG_KEY_PRIVATE,
				  &secretstuff.bip32) != WALLY_OK)
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "Can't derive private bip32 key");
}

/*~ Get the keys for this given BIP32 index: if privkey is NULL, we
 * don't fill it in. */
static void bitcoin_key(struct privkey *privkey, struct pubkey *pubkey,
			u32 index)
{
	struct ext_key ext;
	struct privkey unused_priv;

	if (privkey == NULL)
		privkey = &unused_priv;

	if (index >= BIP32_INITIAL_HARDENED_CHILD)
		status_failed(STATUS_FAIL_MASTER_IO,
			      "Index %u too great", index);

	/*~ This uses libwally, which doesn't dovetail directly with
	 * libsecp256k1 even though it, too, uses it internally. */
	if (bip32_key_from_parent(&secretstuff.bip32, index,
				  BIP32_FLAG_KEY_PRIVATE, &ext) != WALLY_OK)
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "BIP32 of %u failed", index);

	/* libwally says: The private key with prefix byte 0; remove it
	 * for libsecp256k1. */
	memcpy(privkey->secret.data, ext.priv_key+1, 32);
	if (!secp256k1_ec_pubkey_create(secp256k1_ctx, &pubkey->pubkey,
					privkey->secret.data))
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "BIP32 pubkey %u create failed", index);
}

/*~ We store our root secret in a "hsm_secret" file (like all of c-lightning,
 * we run in the user's .lightningd directory). */
static void maybe_create_new_hsm(void)
{
	/*~ Note that this is opened for write-only, even though the permissions
	 * are set to read-only.  That's perfectly valid! */
	int fd = open("hsm_secret", O_CREAT|O_EXCL|O_WRONLY, 0400);
	if (fd < 0) {
		/* If this is not the first time we've run, it will exist. */
		if (errno == EEXIST)
			return;
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "creating: %s", strerror(errno));
	}

	/*~ This is libsodium's cryptographic randomness routine: we assume
	 * it's doing a good job. */
	randombytes_buf(&secretstuff.hsm_secret, sizeof(secretstuff.hsm_secret));
	/*~ ccan/read_write_all has a more convenient return than write() where
	 * we'd have to check the return value == the length we gave: write()
	 * can return short on normal files if we run out of disk space. */
	if (!write_all(fd, &secretstuff.hsm_secret, sizeof(secretstuff.hsm_secret))) {
		/* ccan/noerr contains useful routines like this, which don't
		 * clobber errno, so we can use it in our error report. */
		unlink_noerr("hsm_secret");
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "writing: %s", strerror(errno));
	}
	/*~ fsync (mostly!) ensures that the file has reached the disk. */
	if (fsync(fd) != 0) {
		unlink_noerr("hsm_secret");
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "fsync: %s", strerror(errno));
	}
	/*~ This should never fail if fsync succeeded.  But paranoia good, and
	 * bugs exist. */
	if (close(fd) != 0) {
		unlink_noerr("hsm_secret");
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "closing: %s", strerror(errno));
	}
	/*~ We actually need to sync the *directory itself* to make sure the
	 * file exists!  You're only allowed to open directories read-only in
	 * modern Unix though. */
	fd = open(".", O_RDONLY);
	if (fd < 0) {
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "opening: %s", strerror(errno));
	}
	if (fsync(fd) != 0) {
		unlink_noerr("hsm_secret");
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "fsyncdir: %s", strerror(errno));
	}
	close(fd);
	/*~ status_unusual() is good for things which are interesting and
	 * definitely won't spam the logs.  Only status_broken() is higher;
	 * status_info() is lower, then status_debug() and finally
	 * status_io(). */
	status_unusual("HSM: created new hsm_secret file");
}

/*~ We always load the HSM file, even if we just created it above.  This
 * both unifies the code paths, and provides a nice sanity check that the
 * file contents are as they will be for future invocations. */
static void load_hsm(void)
{
	int fd = open("hsm_secret", O_RDONLY);
	if (fd < 0)
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "opening: %s", strerror(errno));
	if (!read_all(fd, &secretstuff.hsm_secret, sizeof(secretstuff.hsm_secret)))
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "reading: %s", strerror(errno));
	close(fd);

	populate_secretstuff();
}

/*~ This is the response to lightningd's HSM_INIT request, which is the first
 * thing it sends. */
static struct io_plan *init_hsm(struct io_conn *conn,
				struct client *c,
				const u8 *msg_in)
{
	struct pubkey node_id;

	/* This must be lightningd. */
	assert(is_lightningd(c));

	/*~ The fromwire_* routines are autogenerated, based on the message
	 * definitions in hsm_client_wire.csv.  The format of those files is
	 * an extension of the simple comma-separated format output by the
	 * BOLT tools/extract-formats.py tool. */
	if (!fromwire_hsm_init(msg_in))
		return bad_req(conn, c, msg_in);

	maybe_create_new_hsm();
	load_hsm();

	/*~ We tell lightning our node id and (public) bip32 seed. */
	node_key(NULL, &node_id);

	/*~ Note: marshalling a bip32 tree only marshals the public side,
	 * not the secrets!  So we're not actually handing them out here!
	 */
	return req_reply(conn, c,
			 take(towire_hsm_init_reply(NULL, &node_id,
						    &secretstuff.bip32)));
}

/*~ The client has asked us to extract the shared secret from an EC Diffie
 * Hellman token.  This doesn't leak any information, but requires the private
 * key, so the hsmd performs it.  It's used to set up an encryption key for the
 * connection handshaking (BOLT #8) and for the onion wrapping (BOLT #4). */
static struct io_plan *handle_ecdh(struct io_conn *conn,
				   struct client *c,
				   const u8 *msg_in)
{
	struct privkey privkey;
	struct pubkey point;
	struct secret ss;

	if (!fromwire_hsm_ecdh_req(msg_in, &point))
		return bad_req(conn, c, msg_in);

	/*~ We simply use the secp256k1_ecdh function, which really shouldn't
	 * fail (iff the point is invalid). */
	node_key(&privkey, NULL);
	if (secp256k1_ecdh(secp256k1_ctx, ss.data, &point.pubkey,
			   privkey.secret.data) != 1) {
		return bad_req_fmt(conn, c, msg_in, "secp256k1_ecdh fail");
	}

	/*~ In the normal case, we return the shared secret, and then read
	 * the next msg. */
	return req_reply(conn, c, take(towire_hsm_ecdh_resp(NULL, &ss)));
}

/*~ The specific routine to sign the channel_announcement message.  This is
 * defined in BOLT #7, and requires *two* signatures: one from this node's key
 * (to prove it's from us), and one from the bitcoin key used to create the
 * funding transaction (to prove we own the output). */
static struct io_plan *handle_cannouncement_sig(struct io_conn *conn,
						struct client *c,
						const u8 *msg_in)
{
	/*~ Our autogeneration code doesn't define field offsets, so we just
	 * copy this from the spec itself.
	 *
	 * Note that 'check-source' will actually find and check this quote
	 * against the spec (if available); whitespace is ignored and
	 * ... means some content is skipped, but it works remarkably well to
	 * track spec changes. */

	/* BOLT #7:
	 *
	 * - MUST compute the double-SHA256 hash `h` of the message, beginning
	 *   at offset 256, up to the end of the message.
	 *     - Note: the hash skips the 4 signatures but hashes the rest of the
	 *       message, including any future fields appended to the end.
	 */
	/* First type bytes are the msg type */
	size_t offset = 2 + 256;
	struct privkey node_pkey;
	secp256k1_ecdsa_signature node_sig, bitcoin_sig;
	struct sha256_double hash;
	u8 *reply;
	u8 *ca;
	struct pubkey funding_pubkey;
	struct privkey funding_privkey;
	struct secret channel_seed;

	/*~ You'll find FIXMEs like this scattered through the code.
	 * Sometimes they suggest simple improvements which someone like
	 * yourself should go ahead an implement.  Sometimes they're deceptive
	 * quagmires which will cause you nothing but grief.  You decide! */

	/* FIXME: We should cache these. */
	get_channel_seed(&c->id, c->dbid, &channel_seed);
	derive_funding_key(&channel_seed, &funding_pubkey, &funding_privkey);

	/*~ fromwire_ routines which need to do allocation take a tal context
	 * as their first field; tmpctx is good here since we won't need it
	 * after this function. */
	if (!fromwire_hsm_cannouncement_sig_req(tmpctx, msg_in, &ca))
		return bad_req(conn, c, msg_in);

	if (tal_count(ca) < offset)
		return bad_req_fmt(conn, c, msg_in,
				   "bad cannounce length %zu",
				   tal_count(ca));

	/*~ Christian uses TODO(cdecker), but I'm sure he won't mind if you fix
	 * this for him! */
	/* TODO(cdecker) Check that this is actually a valid
	 * channel_announcement */
	node_key(&node_pkey, NULL);
	sha256_double(&hash, ca + offset, tal_count(ca) - offset);

	sign_hash(&node_pkey, &hash, &node_sig);
	sign_hash(&funding_privkey, &hash, &bitcoin_sig);

	reply = towire_hsm_cannouncement_sig_reply(NULL, &node_sig,
						   &bitcoin_sig);
	return req_reply(conn, c, take(reply));
}

/*~ The specific routine to sign the channel_update message. */
static struct io_plan *handle_channel_update_sig(struct io_conn *conn,
						 struct client *c,
						 const u8 *msg_in)
{
	/* BOLT #7:
	 *
	 * - MUST set `signature` to the signature of the double-SHA256 of the
	 *   entire remaining packet after `signature`, using its own
	 *   `node_id`.
	 */
	/* 2 bytes msg type + 64 bytes signature */
	size_t offset = 66;
	struct privkey node_pkey;
	struct sha256_double hash;
	secp256k1_ecdsa_signature sig;
	struct short_channel_id scid;
	u32 timestamp, fee_base_msat, fee_proportional_mill;
	u64 htlc_minimum_msat;
	u64 htlc_maximum_msat;
	u8 message_flags, channel_flags;
	u16 cltv_expiry_delta;
	struct bitcoin_blkid chain_hash;
	u8 *cu;

	if (!fromwire_hsm_cupdate_sig_req(tmpctx, msg_in, &cu))
		return bad_req(conn, c, msg_in);

	if (!fromwire_channel_update_option_channel_htlc_max(cu, &sig,
			&chain_hash, &scid, &timestamp, &message_flags,
			&channel_flags, &cltv_expiry_delta,
			&htlc_minimum_msat, &fee_base_msat,
			&fee_proportional_mill, &htlc_maximum_msat)) {
		return bad_req_fmt(conn, c, msg_in, "Bad inner channel_update");
	}
	if (tal_count(cu) < offset)
		return bad_req_fmt(conn, c, msg_in,
				   "inner channel_update too short");

	node_key(&node_pkey, NULL);
	sha256_double(&hash, cu + offset, tal_count(cu) - offset);

	sign_hash(&node_pkey, &hash, &sig);

	cu = towire_channel_update_option_channel_htlc_max(tmpctx, &sig, &chain_hash,
				   &scid, timestamp, message_flags, channel_flags,
				   cltv_expiry_delta, htlc_minimum_msat,
				   fee_base_msat, fee_proportional_mill,
				   htlc_maximum_msat);
	return req_reply(conn, c, take(towire_hsm_cupdate_sig_reply(NULL, cu)));
}

/*~ This gets the basepoints for a channel; it's not privite information really
 * (we tell the peer this to establish a channel, as it sets up the keys used
 * for each transaction).
 *
 * Note that this is asked by lightningd, so it tells us what channels it wants.
 */
static struct io_plan *handle_get_channel_basepoints(struct io_conn *conn,
						     struct client *c,
						     const u8 *msg_in)
{
	struct pubkey peer_id;
	u64 dbid;
	struct secret seed;
	struct basepoints basepoints;
	struct pubkey funding_pubkey;

	if (!fromwire_hsm_get_channel_basepoints(msg_in, &peer_id, &dbid))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&peer_id, dbid, &seed);
	derive_basepoints(&seed, &funding_pubkey, &basepoints, NULL, NULL);

	return req_reply(conn, c,
			 take(towire_hsm_get_channel_basepoints_reply(NULL,
							      &basepoints,
							      &funding_pubkey)));
}

/*~ This is another lightningd-only interface; signing a commit transaction.
 * This is dangerous, since if we sign a revoked commitment tx we'll lose
 * funds, thus it's only available to lightningd.
 *
 *
 * Oh look, another FIXME! */
/* FIXME: Ensure HSM never does this twice for same dbid! */
static struct io_plan *handle_sign_commitment_tx(struct io_conn *conn,
						 struct client *c,
						 const u8 *msg_in)
{
	struct pubkey peer_id, remote_funding_pubkey, local_funding_pubkey;
	u64 dbid, funding_amount;
	struct secret channel_seed;
	struct bitcoin_tx *tx;
	secp256k1_ecdsa_signature sig;
	struct secrets secrets;
	const u8 *funding_wscript;

	if (!fromwire_hsm_sign_commitment_tx(tmpctx, msg_in,
					     &peer_id, &dbid,
					     &tx,
					     &remote_funding_pubkey,
					     &funding_amount))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&peer_id, dbid, &channel_seed);
	derive_basepoints(&channel_seed,
			  &local_funding_pubkey, NULL, &secrets, NULL);

	/*~ Bitcoin signatures cover the (part of) the script they're
	 * executing; the rules are a bit complex in general, but for
	 * Segregated Witness it's simply the current script. */
	funding_wscript = bitcoin_redeem_2of2(tmpctx,
					      &local_funding_pubkey,
					      &remote_funding_pubkey);
	/*~ Segregated Witness also added the input amount to the signing
	 * algorithm; it's only part of the input implicitly (it's part of the
	 * output it's spending), so in our 'bitcoin_tx' structure it's a
	 * pointer, as we don't always know it (and zero is a valid amount, so
	 * NULL is better to mean 'unknown' and has the nice property that
	 * you'll crash if you assume it's there and you're wrong. */
	tx->input[0].amount = tal_dup(tx->input, u64, &funding_amount);
	sign_tx_input(tx, 0, NULL, funding_wscript,
		      &secrets.funding_privkey,
		      &local_funding_pubkey,
		      &sig);

	return req_reply(conn, c,
			 take(towire_hsm_sign_commitment_tx_reply(NULL, &sig)));
}

/*~ This is used by channeld to create signatures for the remote peer's
 * commitment transaction.  It's functionally identical to signing our own,
 * but we expect to do this repeatedly as commitment transactions are
 * updated.
 *
 * The HSM almost certainly *should* do more checks before signing!
 */
/* FIXME: make sure it meets some criteria? */
static struct io_plan *handle_sign_remote_commitment_tx(struct io_conn *conn,
							struct client *c,
							const u8 *msg_in)
{
	struct pubkey remote_funding_pubkey, local_funding_pubkey;
	u64 funding_amount;
	struct secret channel_seed;
	struct bitcoin_tx *tx;
	secp256k1_ecdsa_signature sig;
	struct secrets secrets;
	const u8 *funding_wscript;

	if (!fromwire_hsm_sign_remote_commitment_tx(tmpctx, msg_in,
						    &tx,
						    &remote_funding_pubkey,
						    &funding_amount))
		bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);
	derive_basepoints(&channel_seed,
			  &local_funding_pubkey, NULL, &secrets, NULL);

	funding_wscript = bitcoin_redeem_2of2(tmpctx,
					      &local_funding_pubkey,
					      &remote_funding_pubkey);
	/* Need input amount for signing */
	tx->input[0].amount = tal_dup(tx->input, u64, &funding_amount);
	sign_tx_input(tx, 0, NULL, funding_wscript,
		      &secrets.funding_privkey,
		      &local_funding_pubkey,
		      &sig);

	return req_reply(conn, c, take(towire_hsm_sign_tx_reply(NULL, &sig)));
}

/*~ This is used by channeld to create signatures for the remote peer's
 * HTLC transactions. */
static struct io_plan *handle_sign_remote_htlc_tx(struct io_conn *conn,
						  struct client *c,
						  const u8 *msg_in)
{
	struct secret channel_seed;
	struct bitcoin_tx *tx;
	secp256k1_ecdsa_signature sig;
	struct secrets secrets;
	struct basepoints basepoints;
	struct pubkey remote_per_commit_point;
	u64 amount;
	u8 *wscript;
	struct privkey htlc_privkey;
	struct pubkey htlc_pubkey;

	if (!fromwire_hsm_sign_remote_htlc_tx(tmpctx, msg_in,
					      &tx, &wscript, &amount,
					      &remote_per_commit_point))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);
	derive_basepoints(&channel_seed, NULL, &basepoints, &secrets, NULL);

	if (!derive_simple_privkey(&secrets.htlc_basepoint_secret,
				   &basepoints.htlc,
				   &remote_per_commit_point,
				   &htlc_privkey))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving htlc privkey");

	if (!derive_simple_key(&basepoints.htlc,
			       &remote_per_commit_point,
			       &htlc_pubkey))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving htlc pubkey");

	/* Need input amount for signing */
	tx->input[0].amount = tal_dup(tx->input, u64, &amount);
	sign_tx_input(tx, 0, NULL, wscript, &htlc_privkey, &htlc_pubkey, &sig);

	return req_reply(conn, c, take(towire_hsm_sign_tx_reply(NULL, &sig)));
}

/*~ This covers several cases where onchaind is creating a transaction which
 * sends funds to our internal wallet. */
/* FIXME: Derive output address for this client, and check it here! */
static struct io_plan *handle_sign_to_us_tx(struct io_conn *conn,
					    struct client *c,
					    const u8 *msg_in,
					    struct bitcoin_tx *tx,
					    const struct privkey *privkey,
					    const u8 *wscript,
					    u64 input_amount)
{
	secp256k1_ecdsa_signature sig;
	struct pubkey pubkey;

	if (!pubkey_from_privkey(privkey, &pubkey))
		return bad_req_fmt(conn, c, msg_in, "bad pubkey_from_privkey");

	if (tal_count(tx->input) != 1)
		return bad_req_fmt(conn, c, msg_in, "bad txinput count");

	tx->input[0].amount = tal_dup(tx->input, u64, &input_amount);
	sign_tx_input(tx, 0, NULL, wscript, privkey, &pubkey, &sig);

	return req_reply(conn, c, take(towire_hsm_sign_tx_reply(NULL, &sig)));
}

/*~ When we send a commitment transaction onchain (unilateral close), there's
 * a delay before we can spend it.  onchaind does an explicit transaction to
 * transfer it to the wallet so that doesn't need to remember how to spend
 * this complex transaction. */
static struct io_plan *handle_sign_delayed_payment_to_us(struct io_conn *conn,
							 struct client *c,
							 const u8 *msg_in)
{
	u64 commit_num, input_amount;
	struct secret channel_seed, basepoint_secret;
	struct pubkey basepoint;
	struct bitcoin_tx *tx;
	struct sha256 shaseed;
	struct pubkey per_commitment_point;
	struct privkey privkey;
	u8 *wscript;

	/*~ We don't derive the wscript ourselves, but perhaps we should? */
	if (!fromwire_hsm_sign_delayed_payment_to_us(tmpctx, msg_in,
						     &commit_num,
						     &tx, &wscript,
						     &input_amount))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);

	/*~ ccan/crypto/shachain how we efficiently derive 2^48 ordered
	 * preimages from a single seed; the twist is that as the preimages
	 * are revealed, you can generate the previous ones yourself, needing
	 * to only keep log(N) of them at any time. */
	if (!derive_shaseed(&channel_seed, &shaseed))
		return bad_req_fmt(conn, c, msg_in, "bad derive_shaseed");

	/*~ BOLT #3 describes exactly how this is used to generate the Nth
	 * per-commitment point. */
	if (!per_commit_point(&shaseed, &per_commitment_point, commit_num))
		return bad_req_fmt(conn, c, msg_in,
				   "bad per_commitment_point %"PRIu64,
				   commit_num);

	/*~ ... which is combined with the basepoint to generate then N'th key.
	 */
	if (!derive_delayed_payment_basepoint(&channel_seed,
					      &basepoint,
					      &basepoint_secret))
		return bad_req_fmt(conn, c, msg_in, "failed deriving basepoint");

	if (!derive_simple_privkey(&basepoint_secret,
				   &basepoint,
				   &per_commitment_point,
				   &privkey))
		return bad_req_fmt(conn, c, msg_in, "failed deriving privkey");

	return handle_sign_to_us_tx(conn, c, msg_in,
				    tx, &privkey, wscript, input_amount);
}

/*~ This is used when the a commitment transaction is onchain, and has an HTLC
 * output paying to us (because we have the preimage); this signs that
 * transaction, which lightningd will broadcast to collect the funds. */
static struct io_plan *handle_sign_remote_htlc_to_us(struct io_conn *conn,
						     struct client *c,
						     const u8 *msg_in)
{
	u64 input_amount;
	struct secret channel_seed, htlc_basepoint_secret;
	struct pubkey htlc_basepoint;
	struct bitcoin_tx *tx;
	struct pubkey remote_per_commitment_point;
	struct privkey privkey;
	u8 *wscript;

	if (!fromwire_hsm_sign_remote_htlc_to_us(tmpctx, msg_in,
						 &remote_per_commitment_point,
						 &tx, &wscript,
						 &input_amount))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);

	if (!derive_htlc_basepoint(&channel_seed, &htlc_basepoint,
				   &htlc_basepoint_secret))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed derive_htlc_basepoint");

	if (!derive_simple_privkey(&htlc_basepoint_secret,
				   &htlc_basepoint,
				   &remote_per_commitment_point,
				   &privkey))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving htlc privkey");

	return handle_sign_to_us_tx(conn, c, msg_in,
				    tx, &privkey, wscript, input_amount);
}

/*~ This is used when the remote peer's commitment transaction is revoked;
 * we can use the revocation secret to spend the outputs.  For simplicity,
 * we do them one at a time, though. */
static struct io_plan *handle_sign_penalty_to_us(struct io_conn *conn,
						 struct client *c,
						 const u8 *msg_in)
{
	u64 input_amount;
	struct secret channel_seed, revocation_secret, revocation_basepoint_secret;
	struct pubkey revocation_basepoint;
	struct bitcoin_tx *tx;
	struct pubkey point;
	struct privkey privkey;
	u8 *wscript;

	if (!fromwire_hsm_sign_penalty_to_us(tmpctx, msg_in,
					     &revocation_secret,
					     &tx, &wscript,
					     &input_amount))
		return bad_req(conn, c, msg_in);

	if (!pubkey_from_secret(&revocation_secret, &point))
		return bad_req_fmt(conn, c, msg_in, "Failed deriving pubkey");

	get_channel_seed(&c->id, c->dbid, &channel_seed);
	if (!derive_revocation_basepoint(&channel_seed,
					 &revocation_basepoint,
					 &revocation_basepoint_secret))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving revocation basepoint");

	if (!derive_revocation_privkey(&revocation_basepoint_secret,
				       &revocation_secret,
				       &revocation_basepoint,
				       &point,
				       &privkey))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving revocation privkey");

	return handle_sign_to_us_tx(conn, c, msg_in,
				    tx, &privkey, wscript, input_amount);
}

/*~ This is used when the a commitment transaction is onchain, and has an HTLC
 * output paying to them, which has timed out; this signs that transaction,
 * which lightningd will broadcast to collect the funds. */
static struct io_plan *handle_sign_local_htlc_tx(struct io_conn *conn,
						 struct client *c,
						 const u8 *msg_in)
{
	u64 commit_num, input_amount;
	struct secret channel_seed, htlc_basepoint_secret;
	struct sha256 shaseed;
	struct pubkey per_commitment_point, htlc_basepoint;
	struct bitcoin_tx *tx;
	u8 *wscript;
	secp256k1_ecdsa_signature sig;
	struct privkey htlc_privkey;
	struct pubkey htlc_pubkey;

	if (!fromwire_hsm_sign_local_htlc_tx(tmpctx, msg_in,
					     &commit_num, &tx, &wscript,
					     &input_amount))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);

	if (!derive_shaseed(&channel_seed, &shaseed))
		return bad_req_fmt(conn, c, msg_in, "bad derive_shaseed");

	if (!per_commit_point(&shaseed, &per_commitment_point, commit_num))
		return bad_req_fmt(conn, c, msg_in,
				   "bad per_commitment_point %"PRIu64,
				   commit_num);

	if (!derive_htlc_basepoint(&channel_seed,
				   &htlc_basepoint,
				   &htlc_basepoint_secret))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving htlc basepoint");

	if (!derive_simple_privkey(&htlc_basepoint_secret,
				   &htlc_basepoint,
				   &per_commitment_point,
				   &htlc_privkey))
		return bad_req_fmt(conn, c, msg_in,
				   "Failed deriving htlc privkey");

	if (!pubkey_from_privkey(&htlc_privkey, &htlc_pubkey))
		return bad_req_fmt(conn, c, msg_in, "bad pubkey_from_privkey");

	if (tal_count(tx->input) != 1)
		return bad_req_fmt(conn, c, msg_in, "bad txinput count");

	/* FIXME: Check that output script is correct! */
	tx->input[0].amount = tal_dup(tx->input, u64, &input_amount);
	sign_tx_input(tx, 0, NULL, wscript, &htlc_privkey, &htlc_pubkey, &sig);

	return req_reply(conn, c, take(towire_hsm_sign_tx_reply(NULL, &sig)));
}

/*~ This get the Nth a per-commitment point, and for N > 2, returns the
 * grandparent per-commitment secret.  This pattern is because after
 * negotiating commitment N-1, we send them the next per-commitment point,
 * and reveal the previous per-commitment secret as a promise not to spend
 * the previous commitment transaction. */
static struct io_plan *handle_get_per_commitment_point(struct io_conn *conn,
						       struct client *c,
						       const u8 *msg_in)
{
	struct secret channel_seed;
	struct sha256 shaseed;
	struct pubkey per_commitment_point;
	u64 n;
	struct secret *old_secret;

	if (!fromwire_hsm_get_per_commitment_point(msg_in, &n))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);
	if (!derive_shaseed(&channel_seed, &shaseed))
		return bad_req_fmt(conn, c, msg_in, "bad derive_shaseed");

	if (!per_commit_point(&shaseed, &per_commitment_point, n))
		return bad_req_fmt(conn, c, msg_in,
				   "bad per_commit_point %"PRIu64, n);

	if (n >= 2) {
		old_secret = tal(tmpctx, struct secret);
		if (!per_commit_secret(&shaseed, old_secret, n - 2)) {
			return bad_req_fmt(conn, c, msg_in,
					   "Cannot derive secret %"PRIu64,
					   n - 2);
		}
	} else
		old_secret = NULL;

	/*~ hsm_client_wire.csv marks the secret field here optional, so it only
	 * gets included if the parameter is non-NULL.  We violate 80 columns
	 * pretty badly here, but it's a recommendation not a religion. */
	return req_reply(conn, c,
			 take(towire_hsm_get_per_commitment_point_reply(NULL,
									&per_commitment_point,
									old_secret)));
}

/*~ This is used when the remote peer claims to have knowledge of future
 * commitment states (option_data_loss_protect in the spec) which means we've
 * been restored from backup or something, and may have already revealed
 * secrets.  We carefully check that this is true, here. */
static struct io_plan *handle_check_future_secret(struct io_conn *conn,
						  struct client *c,
						  const u8 *msg_in)
{
	struct secret channel_seed;
	struct sha256 shaseed;
	u64 n;
	struct secret secret, suggested;

	if (!fromwire_hsm_check_future_secret(msg_in, &n, &suggested))
		return bad_req(conn, c, msg_in);

	get_channel_seed(&c->id, c->dbid, &channel_seed);
	if (!derive_shaseed(&channel_seed, &shaseed))
		return bad_req_fmt(conn, c, msg_in, "bad derive_shaseed");

	if (!per_commit_secret(&shaseed, &secret, n))
		return bad_req_fmt(conn, c, msg_in,
				   "bad commit secret #%"PRIu64, n);

	/*~ Note the special secret_eq_consttime: we generate foo_eq for many
	 * types using ccan/structeq, but not 'struct secret' because any
	 * comparison risks leaking information about the secret if it is
	 * timing dependent. */
	return req_reply(conn, c,
			 take(towire_hsm_check_future_secret_reply(NULL,
				   secret_eq_consttime(&secret, &suggested))));
}

/* This is used by closingd to sign off on a mutual close tx. */
static struct io_plan *handle_sign_mutual_close_tx(struct io_conn *conn,
						   struct client *c,
						   const u8 *msg_in)
{
	struct secret channel_seed;
	struct bitcoin_tx *tx;
	struct pubkey remote_funding_pubkey, local_funding_pubkey;
	secp256k1_ecdsa_signature sig;
	struct secrets secrets;
	u64 funding_amount;
	const u8 *funding_wscript;

	if (!fromwire_hsm_sign_mutual_close_tx(tmpctx, msg_in,
					       &tx,
					       &remote_funding_pubkey,
					       &funding_amount))
		return bad_req(conn, c, msg_in);

	/* FIXME: We should know dust level, decent fee range and
	 * balances, and final_keyindex, and thus be able to check tx
	 * outputs! */
	get_channel_seed(&c->id, c->dbid, &channel_seed);
	derive_basepoints(&channel_seed,
			  &local_funding_pubkey, NULL, &secrets, NULL);

	funding_wscript = bitcoin_redeem_2of2(tmpctx,
					      &local_funding_pubkey,
					      &remote_funding_pubkey);
	/* Need input amount for signing */
	tx->input[0].amount = tal_dup(tx->input, u64, &funding_amount);
	sign_tx_input(tx, 0, NULL, funding_wscript,
		      &secrets.funding_privkey,
		      &local_funding_pubkey,
		      &sig);

	return req_reply(conn, c, take(towire_hsm_sign_tx_reply(NULL, &sig)));
}

/*~ Since we process requests then service them in strict order, and because
 * only lightningd can request a new client fd, we can get away with a global
 * here!  But because we are being tricky, I set it to an invalid value when
 * not in use, and sprinkle assertions around. */
static int pending_client_fd = -1;

/*~ This is the callback from below: having sent the reply, we now send the
 * fd for the client end of the new socketpair. */
static struct io_plan *send_pending_client_fd(struct io_conn *conn,
					      struct client *master)
{
	int fd = pending_client_fd;
	/* This must be the master. */
	assert(is_lightningd(master));
	assert(fd != -1);

	/* This sanity check shouldn't be necessary, but it's cheap. */
	pending_client_fd = -1;

	/*~There's arcane UNIX magic to send an open file descriptor over a
	 * UNIX domain socket.  There's no great way to autogenerate this
	 * though; especially for the receive side, so we always pass these
	 * manually immediately following the message.
	 *
	 * io_send_fd()'s third parameter is whether to close the local one
	 * after sending; that saves us YA callback.
	 */
	return io_send_fd(conn, fd, true, client_read_next, master);
}

/*~ This is used by by the master to create a new client connection (which
 * becomes the HSM_FD for the subdaemon after forking). */
static struct io_plan *pass_client_hsmfd(struct io_conn *conn,
					 struct client *c,
					 const u8 *msg_in)
{
	int fds[2];
	u64 dbid, capabilities;
	struct pubkey id;

	/* This must be lightningd itself. */
	assert(is_lightningd(c));

	if (!fromwire_hsm_client_hsmfd(msg_in, &id, &dbid, &capabilities))
		return bad_req(conn, c, msg_in);

	/* socketpair is a bi-directional pipe, which is what we want. */
	if (socketpair(AF_UNIX, SOCK_STREAM, 0, fds) != 0)
		status_failed(STATUS_FAIL_INTERNAL_ERROR, "creating fds: %s",
			      strerror(errno));

	status_trace("new_client: %"PRIu64, dbid);
	new_client(c, &id, dbid, capabilities, fds[0]);

	/*~ We stash this in a global, because we need to get both the fd and
	 * the client pointer to the callback.  The other way would be to
	 * create a boutique structure and hand that, but we don't need to. */
	pending_client_fd = fds[1];
	return io_write_wire(conn, take(towire_hsm_client_hsmfd_reply(NULL)),
			     send_pending_client_fd, c);
}

/*~ For almost every wallet tx we use the BIP32 seed, but not for onchain
 * unilateral closes from a peer: they (may) have an output to us using a
 * public key based on the channel basepoints.  It's a bit spammy to spend
 * those immediately just to make the wallet simpler, and we didn't appreciate
 * the problem when we designed the protocol for commitment transaction keys.
 *
 * So we store just enough about the channel it came from (which may be
 * long-gone) to regenerate the keys here.  That has the added advantage that
 * the secrets themselves stay within the HSM. */
static void hsm_unilateral_close_privkey(struct privkey *dst,
					 struct unilateral_close_info *info)
{
	struct secret channel_seed;
	struct basepoints basepoints;
	struct secrets secrets;

	get_channel_seed(&info->peer_id, info->channel_id, &channel_seed);
	derive_basepoints(&channel_seed, NULL, &basepoints, &secrets, NULL);

	if (!derive_simple_privkey(&secrets.payment_basepoint_secret,
				   &basepoints.payment, &info->commitment_point,
				   dst)) {
		status_failed(STATUS_FAIL_INTERNAL_ERROR,
			      "Deriving unilateral_close_privkey");
	}
}

/* This gets the bitcoin private key needed to spend from our wallet. */
static void hsm_key_for_utxo(struct privkey *privkey, struct pubkey *pubkey,
			     const struct utxo *utxo)
{
	if (utxo->close_info != NULL) {
		/* This is a their_unilateral_close/to-us output, so
		 * we need to derive the secret the long way */
		status_debug("Unilateral close output, deriving secrets");
		hsm_unilateral_close_privkey(privkey, utxo->close_info);
		pubkey_from_privkey(privkey, pubkey);
		status_debug("Derived public key %s from unilateral close",
			     type_to_string(tmpctx, struct pubkey, pubkey));
	} else {
		/* Simple case: just get derive via HD-derivation */
		bitcoin_key(privkey, pubkey, utxo->keyindex);
	}
}

/* This completes the tx by filling in the input scripts with signatures. */
static void sign_all_inputs(struct bitcoin_tx *tx, struct utxo **utxos)
{
	/* FIXME: sign_tx_input is dumb and needs all input->script to be
	 * NULL, so we gather these here and assign them at the end */
	u8 **scriptSigs = tal_arr(tmpctx, u8 *, tal_count(utxos));

	/*~ Deep in my mind there's a continuous battle: should arrays be
	 * named as singular or plural?  Is consistency the sign of a weak
	 * mind?
	 *
	 * ZmnSCPxj answers thusly: One must make peace with the fact, that
	 * the array itself is singular, yet its contents are plural. Do you
	 * name the array, or do you name its contents? Is the array itself
	 * the thing and the whole of the thing, or is it its contents that
	 * define what it is?
	 *
	 *... I'm not sure that helps! */
	assert(tal_count(tx->input) == tal_count(utxos));
	for (size_t i = 0; i < tal_count(utxos); i++) {
		struct pubkey inkey;
		struct privkey inprivkey;
		const struct utxo *in = utxos[i];
		u8 *subscript, *wscript;
		secp256k1_ecdsa_signature sig;

		/* Figure out keys to spend this. */
		hsm_key_for_utxo(&inprivkey, &inkey, in);

		/* It's either a p2wpkh or p2sh (we support that so people from
		 * the last bitcoin era can put funds into the wallet) */
		wscript = p2wpkh_scriptcode(tmpctx, &inkey);
		if (in->is_p2sh) {
			/* For P2SH-wrapped Segwit, the (implied) redeemScript
			 * is defined in BIP141 */
			subscript = bitcoin_redeem_p2sh_p2wpkh(tmpctx, &inkey);
			scriptSigs[i] = bitcoin_scriptsig_p2sh_p2wpkh(tx, &inkey);
		} else {
			/* Pure segwit uses an empty inputScript; NULL has
			 * tal_count() == 0, so it works great here. */
			subscript = NULL;
			scriptSigs[i] = NULL;
		}
		/* This is the core crypto magic. */
		sign_tx_input(tx, i, subscript, wscript, &inprivkey, &inkey,
			      &sig);

		/* The witness is [sig] [key] */
		tx->input[i].witness = bitcoin_witness_p2wpkh(tx, &sig, &inkey);
	}

	/* Now complete the transaction by attaching the scriptSigs */
	for (size_t i = 0; i < tal_count(utxos); i++)
		tx->input[i].script = scriptSigs[i];
}

/*~ lightningd asks us to sign the transaction to fund a channel; it feeds us
 * the set of inputs and the local and remote pubkeys, and we sign it. */
static struct io_plan *handle_sign_funding_tx(struct io_conn *conn,
					      struct client *c,
					      const u8 *msg_in)
{
	u64 satoshi_out, change_out;
	u32 change_keyindex;
	struct pubkey local_pubkey, remote_pubkey;
	struct utxo **utxos;
	struct bitcoin_tx *tx;
	u16 outnum;
	struct pubkey *changekey;

	/* FIXME: Check fee is "reasonable" */
	if (!fromwire_hsm_sign_funding(tmpctx, msg_in,
				       &satoshi_out, &change_out,
				       &change_keyindex, &local_pubkey,
				       &remote_pubkey, &utxos))
		return bad_req(conn, c, msg_in);

	if (change_out) {
		changekey = tal(tmpctx, struct pubkey);
		bitcoin_key(NULL, changekey, change_keyindex);
	} else
		changekey = NULL;

	tx = funding_tx(tmpctx, &outnum,
			/*~ For simplicity, our generated code is not const
			 * correct.  The C rules around const and
			 * pointer-to-pointer are a bit weird, so we use
			 * ccan/cast which ensures the type is correct and
			 * we're not casting something random */
			cast_const2(const struct utxo **, utxos),
			satoshi_out, &local_pubkey, &remote_pubkey,
			change_out, changekey,
			NULL);

	sign_all_inputs(tx, utxos);
	return req_reply(conn, c, take(towire_hsm_sign_funding_reply(NULL, tx)));
}

/*~ lightningd asks us to sign a withdrawal; same as above but we in theory
 * we can do more to check the previous case is valid. */
static struct io_plan *handle_sign_withdrawal_tx(struct io_conn *conn,
						 struct client *c,
						 const u8 *msg_in)
{
	u64 satoshi_out, change_out;
	u32 change_keyindex;
	struct utxo **utxos;
	struct bitcoin_tx *tx;
	struct ext_key ext;
	struct pubkey changekey;
	u8 *scriptpubkey;

	if (!fromwire_hsm_sign_withdrawal(tmpctx, msg_in, &satoshi_out,
					  &change_out, &change_keyindex,
					  &scriptpubkey, &utxos))
		return bad_req(conn, c, msg_in);

	if (bip32_key_from_parent(&secretstuff.bip32, change_keyindex,
				  BIP32_FLAG_KEY_PUBLIC, &ext) != WALLY_OK)
		return bad_req_fmt(conn, c, msg_in,
				   "Failed to get key %u", change_keyindex);

	pubkey_from_der(ext.pub_key, sizeof(ext.pub_key), &changekey);
	tx = withdraw_tx(tmpctx, cast_const2(const struct utxo **, utxos),
			 scriptpubkey, satoshi_out,
			 &changekey, change_out, NULL);

	sign_all_inputs(tx, utxos);

	return req_reply(conn, c,
			 take(towire_hsm_sign_withdrawal_reply(NULL, tx)));
}

/*~ Lightning invoices, defined by BOLT 11, are signed.  This has been
 * surprisingly controversial; it means a node needs to be online to create
 * invoices.  However, it seems clear to me that in a world without
 * intermedaries you need proof that you have received an offer (the
 * signature), as well as proof that you've paid it (the preimage). */
static struct io_plan *handle_sign_invoice(struct io_conn *conn,
					   struct client *c,
					   const u8 *msg_in)
{
	/*~ We make up a 'u5' type to represent BOLT11's 5-bits-per-byte
	 * format: it's only for human consumption, as typedefs are almost
	 * entirely transparent to the C compiler. */
	u5 *u5bytes;
	u8 *hrpu8;
	char *hrp;
	struct sha256 sha;
        secp256k1_ecdsa_recoverable_signature rsig;
	struct hash_u5 hu5;
	struct privkey node_pkey;

	if (!fromwire_hsm_sign_invoice(tmpctx, msg_in, &u5bytes, &hrpu8))
		return bad_req(conn, c, msg_in);

	/* BOLT #11:
	 *
	 * A writer MUST set `signature` to a valid 512-bit secp256k1
	 * signature of the SHA2 256-bit hash of the human-readable part,
	 * represented as UTF-8 bytes, concatenated with the data part
	 * (excluding the signature) with zero bits appended to pad the data
	 * to the next byte boundary, with a trailing byte containing the
	 * recovery ID (0, 1, 2 or 3).
	 */

	/* FIXME: Check invoice! */

	/* tal_dup_arr() does what you'd expect: allocate an array by copying
	 * another; the cast is needed because the hrp is a 'char' array, not
	 * a 'u8' (unsigned char) as it's the "human readable" part.
	 *
	 * The final arg of tal_dup_arr() is how many extra bytes to allocate:
	 * it's so often zero that I've thought about dropping the argument, but
	 * in cases like this (adding a NUL terminator) it's perfect. */
	hrp = tal_dup_arr(tmpctx, char, (char *)hrpu8, tal_count(hrpu8), 1);
	hrp[tal_count(hrpu8)] = '\0';

	hash_u5_init(&hu5, hrp);
	hash_u5(&hu5, u5bytes, tal_count(u5bytes));
	hash_u5_done(&hu5, &sha);

	node_key(&node_pkey, NULL);
	/*~ By no small coincidence, this libsecp routine uses the exact
	 * recovery signature format mandated by BOLT 11. */
        if (!secp256k1_ecdsa_sign_recoverable(secp256k1_ctx, &rsig,
                                              (const u8 *)&sha,
                                              node_pkey.secret.data,
                                              NULL, NULL)) {
		return bad_req_fmt(conn, c, msg_in, "Failed to sign invoice");
	}

	return req_reply(conn, c,
			 take(towire_hsm_sign_invoice_reply(NULL, &rsig)));
}

/*~ It's optional for nodes to send node_announcement, but it lets us set our
 * favourite color and cool alias!  Plus other minor details like how to
 * connect to us. */
static struct io_plan *handle_sign_node_announcement(struct io_conn *conn,
						     struct client *c,
						     const u8 *msg_in)
{
	/* BOLT #7:
	 *
	 * The origin node:
	 *...
	 * - MUST set `signature` to the signature of the double-SHA256 of the
	 *   entire remaining packet after `signature` (using the key given by
	 *   `node_id`).
	 */
	/* 2 bytes msg type + 64 bytes signature */
	size_t offset = 66;
	struct sha256_double hash;
	struct privkey node_pkey;
	secp256k1_ecdsa_signature sig;
	u8 *reply;
	u8 *ann;

	if (!fromwire_hsm_node_announcement_sig_req(tmpctx, msg_in, &ann))
		return bad_req(conn, c, msg_in);

	if (tal_count(ann) < offset)
		return bad_req_fmt(conn, c, msg_in,
				   "Node announcement too short");

	/* FIXME(cdecker) Check the node announcement's content */
	node_key(&node_pkey, NULL);
	sha256_double(&hash, ann + offset, tal_count(ann) - offset);

	sign_hash(&node_pkey, &hash, &sig);

	reply = towire_hsm_node_announcement_sig_reply(NULL, &sig);
	return req_reply(conn, c, take(reply));
}

/*~ This routine checks that a client is allowed to call the handler. */
static bool check_client_capabilities(struct client *client,
				      enum hsm_wire_type t)
{
	/*~ Here's a useful trick: enums in C are not real types, they're
	 * semantic sugar sprinkled over an int, bascally (in fact, older
	 * versions of gcc used to convert the values ints in the parser!).
	 *
	 * But GCC will do one thing for us: if we have a switch statement
	 * with a controlling expression which is an enum, it will warn us
	 * if a declared enum value is *not* handled in the switch, eg:
	 *     enumeration value ‘FOOBAR’ not handled in switch [-Werror=switch]
	 *
	 * This only works if there's no 'default' label, which is sometimes
	 * hard, as we *can* have non-enum values in our enum.  But the tradeoff
	 * is worth it so the compiler tells us everywhere we have to fix when
	 * we add a new enum identifier!
	 */
	switch (t) {
	case WIRE_HSM_ECDH_REQ:
		return (client->capabilities & HSM_CAP_ECDH) != 0;

	case WIRE_HSM_CANNOUNCEMENT_SIG_REQ:
	case WIRE_HSM_CUPDATE_SIG_REQ:
	case WIRE_HSM_NODE_ANNOUNCEMENT_SIG_REQ:
		return (client->capabilities & HSM_CAP_SIGN_GOSSIP) != 0;

	case WIRE_HSM_SIGN_DELAYED_PAYMENT_TO_US:
	case WIRE_HSM_SIGN_REMOTE_HTLC_TO_US:
	case WIRE_HSM_SIGN_PENALTY_TO_US:
	case WIRE_HSM_SIGN_LOCAL_HTLC_TX:
		return (client->capabilities & HSM_CAP_SIGN_ONCHAIN_TX) != 0;

	case WIRE_HSM_GET_PER_COMMITMENT_POINT:
	case WIRE_HSM_CHECK_FUTURE_SECRET:
		return (client->capabilities & HSM_CAP_COMMITMENT_POINT) != 0;

	case WIRE_HSM_SIGN_REMOTE_COMMITMENT_TX:
	case WIRE_HSM_SIGN_REMOTE_HTLC_TX:
		return (client->capabilities & HSM_CAP_SIGN_REMOTE_TX) != 0;

	case WIRE_HSM_SIGN_MUTUAL_CLOSE_TX:
		return (client->capabilities & HSM_CAP_SIGN_CLOSING_TX) != 0;

	case WIRE_HSM_INIT:
	case WIRE_HSM_CLIENT_HSMFD:
	case WIRE_HSM_SIGN_FUNDING:
	case WIRE_HSM_SIGN_WITHDRAWAL:
	case WIRE_HSM_SIGN_INVOICE:
	case WIRE_HSM_SIGN_COMMITMENT_TX:
	case WIRE_HSM_GET_CHANNEL_BASEPOINTS:
		return (client->capabilities & HSM_CAP_MASTER) != 0;

	/*~ These are messages sent by the HSM so we should never receive them.
	 * FIXME: Since we autogenerate these, we should really generate separate
	 * enums for replies to avoid this kind of clutter! */
	case WIRE_HSM_ECDH_RESP:
	case WIRE_HSM_CANNOUNCEMENT_SIG_REPLY:
	case WIRE_HSM_CUPDATE_SIG_REPLY:
	case WIRE_HSM_CLIENT_HSMFD_REPLY:
	case WIRE_HSM_SIGN_FUNDING_REPLY:
	case WIRE_HSM_NODE_ANNOUNCEMENT_SIG_REPLY:
	case WIRE_HSM_SIGN_WITHDRAWAL_REPLY:
	case WIRE_HSM_SIGN_INVOICE_REPLY:
	case WIRE_HSM_INIT_REPLY:
	case WIRE_HSMSTATUS_CLIENT_BAD_REQUEST:
	case WIRE_HSM_SIGN_COMMITMENT_TX_REPLY:
	case WIRE_HSM_SIGN_TX_REPLY:
	case WIRE_HSM_GET_PER_COMMITMENT_POINT_REPLY:
	case WIRE_HSM_CHECK_FUTURE_SECRET_REPLY:
	case WIRE_HSM_GET_CHANNEL_BASEPOINTS_REPLY:
		break;
	}
	return false;
}

/*~ This is the core of the HSM daemon: handling requests. */
static struct io_plan *handle_client(struct io_conn *conn, struct client *c)
{
	enum hsm_wire_type t = fromwire_peektype(c->msg_in);

	status_debug("Client: Received message %d from client", t);

	/* Before we do anything else, is this client allowed to do
	 * what he asks for? */
	if (!check_client_capabilities(c, t))
		return bad_req_fmt(conn, c, c->msg_in,
				   "does not have capability to run %d", t);

	/* Now actually go and do what the client asked for */
	switch (t) {
	case WIRE_HSM_INIT:
		return init_hsm(conn, c, c->msg_in);

	case WIRE_HSM_CLIENT_HSMFD:
		return pass_client_hsmfd(conn, c, c->msg_in);

	case WIRE_HSM_GET_CHANNEL_BASEPOINTS:
		return handle_get_channel_basepoints(conn, c, c->msg_in);

	case WIRE_HSM_ECDH_REQ:
		return handle_ecdh(conn, c, c->msg_in);

	case WIRE_HSM_CANNOUNCEMENT_SIG_REQ:
		return handle_cannouncement_sig(conn, c, c->msg_in);

	case WIRE_HSM_CUPDATE_SIG_REQ:
		return handle_channel_update_sig(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_FUNDING:
		return handle_sign_funding_tx(conn, c, c->msg_in);

	case WIRE_HSM_NODE_ANNOUNCEMENT_SIG_REQ:
		return handle_sign_node_announcement(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_INVOICE:
		return handle_sign_invoice(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_WITHDRAWAL:
		return handle_sign_withdrawal_tx(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_COMMITMENT_TX:
		return handle_sign_commitment_tx(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_DELAYED_PAYMENT_TO_US:
		return handle_sign_delayed_payment_to_us(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_REMOTE_HTLC_TO_US:
		return handle_sign_remote_htlc_to_us(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_PENALTY_TO_US:
		return handle_sign_penalty_to_us(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_LOCAL_HTLC_TX:
		return handle_sign_local_htlc_tx(conn, c, c->msg_in);

	case WIRE_HSM_GET_PER_COMMITMENT_POINT:
		return handle_get_per_commitment_point(conn, c, c->msg_in);

	case WIRE_HSM_CHECK_FUTURE_SECRET:
		return handle_check_future_secret(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_REMOTE_COMMITMENT_TX:
		return handle_sign_remote_commitment_tx(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_REMOTE_HTLC_TX:
		return handle_sign_remote_htlc_tx(conn, c, c->msg_in);

	case WIRE_HSM_SIGN_MUTUAL_CLOSE_TX:
		return handle_sign_mutual_close_tx(conn, c, c->msg_in);

	case WIRE_HSM_ECDH_RESP:
	case WIRE_HSM_CANNOUNCEMENT_SIG_REPLY:
	case WIRE_HSM_CUPDATE_SIG_REPLY:
	case WIRE_HSM_CLIENT_HSMFD_REPLY:
	case WIRE_HSM_SIGN_FUNDING_REPLY:
	case WIRE_HSM_NODE_ANNOUNCEMENT_SIG_REPLY:
	case WIRE_HSM_SIGN_WITHDRAWAL_REPLY:
	case WIRE_HSM_SIGN_INVOICE_REPLY:
	case WIRE_HSM_INIT_REPLY:
	case WIRE_HSMSTATUS_CLIENT_BAD_REQUEST:
	case WIRE_HSM_SIGN_COMMITMENT_TX_REPLY:
	case WIRE_HSM_SIGN_TX_REPLY:
	case WIRE_HSM_GET_PER_COMMITMENT_POINT_REPLY:
	case WIRE_HSM_CHECK_FUTURE_SECRET_REPLY:
	case WIRE_HSM_GET_CHANNEL_BASEPOINTS_REPLY:
		break;
	}

	return bad_req_fmt(conn, c, c->msg_in, "Unknown request");
}

static void master_gone(struct io_conn *unused UNUSED, struct client *c UNUSED)
{
	daemon_shutdown();
	/* Can't tell master, it's gone. */
	exit(2);
}

int main(int argc, char *argv[])
{
	struct client *master;

	setup_locale();

	/* This sets up tmpctx, various DEVELOPER options, backtraces, etc. */
	subdaemon_setup(argc, argv);

	/* A trivial daemon_conn just for writing. */
	status_conn = tal(NULL, struct daemon_conn);
	daemon_conn_init(status_conn, status_conn, STDIN_FILENO,
			 (void *)io_never, NULL);
	status_setup_async(status_conn);
	uintmap_init(&clients);

	master = new_client(NULL, NULL, 0, HSM_CAP_MASTER | HSM_CAP_SIGN_GOSSIP,
			    REQ_FD);

	/* First client == lightningd. */
	assert(is_lightningd(master));

	/* When conn closes, everything is freed. */
	io_set_finish(master->conn, master_gone, master);

	/*~ The two NULL args a list of timers, and the timer which expired:
	 * we don't have any timers. */
	io_loop(NULL, NULL);

	/*~ This should never be reached: io_loop only exits on io_break which
	 * we don't call, a timer expiry which we don't have, or all connections
	 * being closed, and closing the master calls master_gone. */
	abort();
}

/*~ Congratulations on making it through the first of the seven dwarves!
 * (And Christian wondered why I'm so fond of having separate daemons!).
 *
 * We continue our story in the next-more-complex daemon: connectd/connectd.c
 */