sphinx: Store shared secrets on the origin node

We could recompute them once we receive a reply and need to decrypt
it, but why go through the trouble when we can just store them?

lightningd/dev_newhtlc.c

@@ -62,6 +62,7 @@ static void json_dev_newhtlc(struct command *cmd,
 	u8 *onion;
 	struct htlc_end *hend;
 	struct pubkey *path = tal_arrz(cmd, struct pubkey, 1);
+	struct sha256 *shared_secrets;
 
 	if (!json_get_params(buffer, params,
 			     "peerid", &peeridtok,
@@ -126,7 +127,7 @@ static void json_dev_newhtlc(struct command *cmd,
 	path[0] = *peer->id;
 	randombytes_buf(&sessionkey, sizeof(sessionkey));
 	packet = create_onionpacket(cmd, path, hopsdata, sessionkey, rhash.u.u8,
-				    sizeof(rhash));
+				    sizeof(rhash), &shared_secrets);
 	onion = serialize_onionpacket(cmd, packet);
 
 	log_debug(peer->log, "JSON command to add new HTLC");
@@ -137,6 +138,7 @@ static void json_dev_newhtlc(struct command *cmd,
 	hend->msatoshis = msatoshi;
 	hend->other_end = NULL;
 	hend->pay_command = (void *)cmd;
+	hend->path_secrets = tal_steal(hend, shared_secrets);
 
 	/* FIXME: If subdaemon dies? */
 	msg = towire_channel_offer_htlc(cmd, msatoshi, expiry, &rhash, onion);

lightningd/htlc_end.h

@@ -28,7 +28,14 @@ struct htlc_end {
 	u32 outgoing_cltv_value;
 	u32 cltv_expiry;
 	struct sha256 payment_hash;
+
+	/* If we are forwarding, remember the shared secret for an
+	 * eventual reply */
 	struct sha256 shared_secret;
+
+	/* If we are the origin, remember all shared secrets, so we
+	 * can unwrap an eventual reply */
+	struct sha256 *path_secrets;
 };
 
 static inline const struct htlc_end *keyof_htlc_end(const struct htlc_end *e)

lightningd/pay.c

@@ -163,6 +163,7 @@ static void json_sendpay(struct command *cmd,
 	u64 amount, lastamount;
 	struct onionpacket *packet;
 	u8 *msg;
+	struct sha256 *path_secrets;
 
 	if (!json_get_params(buffer, params,
 			     "route", &routetok,
@@ -321,8 +322,8 @@ static void json_sendpay(struct command *cmd,
 	randombytes_buf(&sessionkey, sizeof(sessionkey));
 
 	/* Onion will carry us from first peer onwards. */
-	packet = create_onionpacket(cmd, ids, hop_data, sessionkey,
-				    rhash.u.u8, sizeof(struct sha256));
+	packet = create_onionpacket(cmd, ids, hop_data, sessionkey, rhash.u.u8,
+				    sizeof(struct sha256), &path_secrets);
 	onion = serialize_onionpacket(cmd, packet);
 
 	if (pc)
@@ -344,6 +345,7 @@ static void json_sendpay(struct command *cmd,
 	pc->out->msatoshis = amount;
 	pc->out->other_end = NULL;
 	pc->out->pay_command = pc;
+	pc->out->path_secrets = tal_steal(pc->out, path_secrets);
 
 	log_info(ld->log, "Sending %"PRIu64" over %zu hops to deliver %"PRIu64,
 		 amount, n_hops, lastamount);

lightningd/sphinx.c

@@ -362,7 +362,8 @@ struct onionpacket *create_onionpacket(
 	struct hop_data hops_data[],
 	const u8 *sessionkey,
 	const u8 *assocdata,
-	const size_t assocdatalen
+	const size_t assocdatalen,
+	struct sha256 **path_secrets
 	)
 {
 	struct onionpacket *packet = talz(ctx, struct onionpacket);
@@ -403,6 +404,11 @@ struct onionpacket *create_onionpacket(
 	}
 	memcpy(packet->mac, nexthmac, sizeof(nexthmac));
 	memcpy(&packet->ephemeralkey, &params[0].ephemeralkey, sizeof(secp256k1_pubkey));
+
+	*path_secrets = tal_arr(ctx, struct sha256, num_hops);
+	for (i=0; i<num_hops; i++) {
+		create_shared_secret((*path_secrets)[i].u.u8, &path[i].pubkey, sessionkey);
+	}
 	return packet;
 }
 
@@ -462,7 +468,7 @@ struct route_step *process_onionpacket(
 	return step;
 }
 
-u8 *create_onionreply(tal_t *ctx, const u8 *shared_secret,
+u8 *create_onionreply(const tal_t *ctx, const u8 *shared_secret,
 		      const u8 *failure_msg)
 {
 	size_t msglen = tal_len(failure_msg);
@@ -487,7 +493,7 @@ u8 *create_onionreply(tal_t *ctx, const u8 *shared_secret,
 	return reply;
 }
 
-u8 *wrap_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *reply)
+u8 *wrap_onionreply(const tal_t *ctx, const u8 *shared_secret, const u8 *reply)
 {
 	u8 key[KEY_LEN];
 	size_t streamlen = tal_len(reply);
@@ -499,7 +505,7 @@ u8 *wrap_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *reply)
 	return result;
 }
 
-struct onionreply *unwrap_onionreply(tal_t *ctx, u8 **shared_secrets,
+struct onionreply *unwrap_onionreply(const tal_t *ctx, u8 **shared_secrets,
 				     const int numhops, const u8 *reply)
 {
 	tal_t *tmpctx = tal_tmpctx(ctx);

lightningd/sphinx.h

@@ -78,6 +78,7 @@ struct route_step {
  * @sessionkey: 32 byte random session key to derive secrets from
  * @assocdata: associated data to commit to in HMACs
  * @assocdatalen: length of the assocdata
+ * @path_secrets: (out) shared secrets generated for the entire path
  */
 struct onionpacket *create_onionpacket(
 	const tal_t * ctx,
@@ -85,7 +86,8 @@ struct onionpacket *create_onionpacket(
 	struct hop_data hops_data[],
 	const u8 * sessionkey,
 	const u8 *assocdata,
-	const size_t assocdatalen
+	const size_t assocdatalen,
+	struct sha256 **path_secrets
 	);
 
 /**
@@ -160,7 +162,7 @@ struct onionreply {
  *     HMAC
  * @failure_msg: message (must support tal_len)
  */
-u8 *create_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *failure_msg);
+u8 *create_onionreply(const tal_t *ctx, const u8 *shared_secret, const u8 *failure_msg);
 
 /**
  * wrap_onionreply - Add another encryption layer to the reply.
@@ -170,7 +172,7 @@ u8 *create_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *failure_msg
  *     encryption.
  * @reply: the reply to wrap
  */
-u8 *wrap_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *reply);
+u8 *wrap_onionreply(const tal_t *ctx, const u8 *shared_secret, const u8 *reply);
 
 /**
  * unwrap_onionreply - Remove layers, check integrity and parse reply
@@ -180,7 +182,7 @@ u8 *wrap_onionreply(tal_t *ctx, const u8 *shared_secret, const u8 *reply);
  * @numhops: path length and number of shared_secrets provided
  * @reply: the incoming reply
  */
-struct onionreply *unwrap_onionreply(tal_t *ctx, u8 **shared_secrets,
+struct onionreply *unwrap_onionreply(const tal_t *ctx, u8 **shared_secrets,
 				     const int numhops, const u8 *reply);
 
 #endif /* LIGHTNING_DAEMON_SPHINX_H */

test/test_sphinx.c

@@ -140,7 +140,8 @@ int main(int argc, char **argv)
 		u8 privkeys[argc - 1][32];
 		u8 sessionkey[32];
 		struct hop_data hops_data[num_hops];
-
+		struct sha256 *shared_secrets;
+		
 		memset(&sessionkey, 'A', sizeof(sessionkey));
 
 		int i;
@@ -163,7 +164,8 @@ int main(int argc, char **argv)
 							     hops_data,
 							     sessionkey,
 							     assocdata,
-							     sizeof(assocdata));
+							     sizeof(assocdata),
+							     &shared_secrets);
 
 		u8 *serialized = serialize_onionpacket(ctx, res);
 		if (!serialized)