secrets: peer_sign_theircommit() helper.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

daemon/secrets.c

@@ -41,6 +41,18 @@ struct peer_secrets {
 	struct sha256 revocation_seed;
 };
 
+void peer_sign_theircommit(const struct peer *peer, struct signature *sig)
+{
+	/* Commit tx only has one input: that of the anchor. */
+	sign_tx_input(peer->dstate->secpctx,
+		      peer->them.commit, 0,
+		      peer->anchor.redeemscript,
+		      tal_count(peer->anchor.redeemscript),
+		      &peer->secrets->commit,
+		      &peer->us.commitkey,
+		      sig);
+}
+
 static void new_keypair(struct lightningd_state *dstate,
 			struct privkey *privkey, struct pubkey *pubkey)
 {

daemon/secrets.h

@@ -12,6 +12,8 @@ struct sha256;
 void privkey_sign(struct peer *peer, const void *src, size_t len,
 		  struct signature *sig);
 
+void peer_sign_theircommit(const struct peer *peer, struct signature *sig);
+
 void peer_secrets_init(struct peer *peer);
 
 void peer_get_revocation_hash(const struct peer *peer, u64 index,