bitcoin: implement sig_valid.

Update libsecp256k1 has a normalize function, which allows us to test
if the signature was in low-S form.

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

bitcoin/signature.c

@@ -252,8 +252,11 @@ size_t signature_to_der(secp256k1_context *secpctx,
 }
 
 /* Signature must have low S value. */
-bool sig_valid(const struct signature *sig)
+bool sig_valid(secp256k1_context *secpctx, const struct signature *sig)
 {
-	/* FIXME!  Need libsecp support. */
-	return true;
+	secp256k1_ecdsa_signature tmp;
+
+	if (secp256k1_ecdsa_signature_normalize(secpctx, &tmp, &sig->sig) == 0)
+		return true;
+	return false;
 }

bitcoin/signature.h

@@ -52,7 +52,7 @@ bool check_tx_sig(secp256k1_context *secpctx,
 		  const struct bitcoin_signature *sig);
 
 /* Signature must have low S value. */
-bool sig_valid(const struct signature *s);
+bool sig_valid(secp256k1_context *secpctx, const struct signature *sig);
 
 /* Give DER encoding of signature: returns length used (<= 72). */
 size_t signature_to_der(secp256k1_context *secpctx,

daemon/cryptopkt.c

@@ -356,7 +356,8 @@ static struct io_plan *check_proof(struct io_conn *conn, struct peer *peer)
 	if (!auth)
 		return io_close(conn);
 
-	if (!proto_to_signature(auth->session_sig, &sig)) {
+	if (!proto_to_signature(peer->dstate->secpctx, auth->session_sig,
+				&sig)) {
 		log_unusual(peer->log, "Invalid auth signature");
 		return io_close(conn);
 	}
@@ -428,7 +429,7 @@ static Pkt *authenticate_pkt(const tal_t *ctx,
 	Authenticate *auth = tal(ctx, Authenticate);
 	authenticate__init(auth);
 	auth->node_id = pubkey_to_proto(auth, secpctx, node_id);
-	auth->session_sig = signature_to_proto(auth, sig);
+	auth->session_sig = signature_to_proto(auth, secpctx, sig);
 	return pkt_wrap(ctx, auth, PKT__PKT_AUTH);
 }
 

daemon/packets.c

@@ -146,7 +146,8 @@ void queue_pkt_open_commit_sig(struct peer *peer)
 	peer->remote.commit->sig->stype = SIGHASH_ALL;
 	peer_sign_theircommit(peer, peer->remote.commit->tx,
 			      &peer->remote.commit->sig->sig);
-	s->sig = signature_to_proto(s, &peer->remote.commit->sig->sig);
+	s->sig = signature_to_proto(s, peer->dstate->secpctx,
+				    &peer->remote.commit->sig->sig);
 
 	queue_pkt(peer, PKT__PKT_OPEN_COMMIT_SIG, s);
 }
@@ -304,7 +305,7 @@ void queue_pkt_commit(struct peer *peer)
 
 	/* Now send message */
 	update_commit__init(u);
-	u->sig = signature_to_proto(u, &ci->sig->sig);
+	u->sig = signature_to_proto(u, peer->dstate->secpctx, &ci->sig->sig);
 
 	queue_pkt(peer, PKT__PKT_UPDATE_COMMIT, u);
 }
@@ -456,7 +457,7 @@ void queue_pkt_close_signature(struct peer *peer)
 	close_tx = peer_create_close_tx(peer, peer->closing.our_fee);
 
 	peer_sign_mutual_close(peer, close_tx, &our_close_sig);
-	c->sig = signature_to_proto(c, &our_close_sig);
+	c->sig = signature_to_proto(c, peer->dstate->secpctx, &our_close_sig);
 	c->close_fee = peer->closing.our_fee;
 	log_info(peer->log, "queue_pkt_close_signature: offered close fee %"
 		 PRIu64, c->close_fee);
@@ -530,7 +531,7 @@ static Pkt *check_and_save_commit_sig(struct peer *peer,
 
 	assert(!ci->sig);
 	sig->stype = SIGHASH_ALL;
-	if (!proto_to_signature(pb, &sig->sig))
+	if (!proto_to_signature(peer->dstate->secpctx, pb, &sig->sig))
 		return pkt_err(peer, "Malformed signature");
 
 	log_debug(peer->log, "Checking sig for %u/%u msatoshis, %zu/%zu htlcs",

daemon/peer.c

@@ -268,7 +268,7 @@ static bool closing_pkt_in(struct peer *peer, const Pkt *pkt)
 	 * transaction with the given `close_fee`, and MUST fail the
 	 * connection if it is not. */
 	theirsig.stype = SIGHASH_ALL;
-	if (!proto_to_signature(c->sig, &theirsig.sig))
+	if (!proto_to_signature(peer->dstate->secpctx, c->sig, &theirsig.sig))
 		return peer_comms_err(peer,
 				      pkt_err(peer, "Invalid signature format"));
 

protobuf_convert.c

@@ -4,12 +4,14 @@
 #include "protobuf_convert.h"
 #include <ccan/crypto/sha256/sha256.h>
 
-Signature *signature_to_proto(const tal_t *ctx, const struct signature *sig)
+Signature *signature_to_proto(const tal_t *ctx,
+			      secp256k1_context *secpctx,
+			      const struct signature *sig)
 {
 	Signature *pb = tal(ctx, Signature);
 	signature__init(pb);
 
-	assert(sig_valid(sig));
+	assert(sig_valid(secpctx, sig));
 
 	/* FIXME: Need a portable way to encode signatures in libsecp! */
 	
@@ -26,7 +28,9 @@ Signature *signature_to_proto(const tal_t *ctx, const struct signature *sig)
 	return pb;
 }
 
-bool proto_to_signature(const Signature *pb, struct signature *sig)
+bool proto_to_signature(secp256k1_context *secpctx,
+			const Signature *pb,
+			struct signature *sig)
 {
 	/* Kill me again. */
 	/* FIXME: Need a portable way to encode signatures in libsecp! */
@@ -40,7 +44,7 @@ bool proto_to_signature(const Signature *pb, struct signature *sig)
 	memcpy(sig->sig.data + 48, &pb->s3, 8);
 	memcpy(sig->sig.data + 56, &pb->s4, 8);
 
-	return sig_valid(sig);
+	return sig_valid(secpctx, sig);
 }
 
 BitcoinPubkey *pubkey_to_proto(const tal_t *ctx,

protobuf_convert.h

@@ -8,8 +8,12 @@
 
 /* Convert to-from protobuf to internal representation. */
 struct signature;
-Signature *signature_to_proto(const tal_t *ctx, const struct signature *sig);
-bool proto_to_signature(const Signature *pb, struct signature *sig);
+Signature *signature_to_proto(const tal_t *ctx,
+			      secp256k1_context *secpctx,
+			      const struct signature *sig);
+bool proto_to_signature(secp256k1_context *secpctx,
+			const Signature *pb,
+			struct signature *sig);
 
 /* Convert to-from protobuf to internal representation. */
 struct pubkey;