channeld: Verify the signature sent by the counterparty

This commit addresses an issue to enhance the resilience of core
lightning when receiving node announcements.

According to BOLT 7 (The announcement_signatures Message),
if the node_signature OR the bitcoin_signature is NOT correct,
it is recommended to either send a warning and close the connection or send an error and fail the channel.

In this commit, we take a strict approach. If any error is detected, we
send an error and fail the open channel operation.
This is because the announcement_signatures operation is optional,
and we assume that it must be correct.

lnprototest at commit dea47c29b5541dbfe7fe53cc2598330e897fa4f4 report
the following error now.

```
2023-07-06T21:03:20.930Z DEBUG   hsmd: Shutting down

ERROR    root:helpers.py:170 Traceback (most recent call last):
  File "/home/vincent/Github/lightning/external/lnprototest/tests/helpers.py", line 167, in run_runner
    runner.run(test)
  File "/home/vincent/Github/lightning/external/lnprototest/lnprototest/runner.py", line 99, in run
    all_done = sequence.action(self)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/home/vincent/Github/lightning/external/lnprototest/lnprototest/structure.py", line 55, in action
    all_done &= e.action(runner)
                ^^^^^^^^^^^^^^^^
  File "/home/vincent/Github/lightning/external/lnprototest/lnprototest/event.py", line 365, in action
    raise EventError(self, "{}: message was {}".format(err, msg.to_str()))
lnprototest.errors.EventError: `Expected msgtype-warning, got msgtype-error: message was error channel_id=a37362839b13f61cfe82d35bd397b1264c389b245847cfb6111b38892546dc77 data=4661696c656420746f20766572696679206e6f64655f7369676e61747572652e` on event [{"event": "ExpectMsg", "file": "test_bolt2-01-close_channel.py", "pos": "157"},]
============================================================================================================================================================== short test summary info ===============================================================================================================================================================
FAILED tests/test_bolt2-01-close_channel.py::test_close_channel_shutdown_msg_normal_case_receiver_side - AssertionError: `Expected msgtype-shutdown, got msgtype-error: message was error channel_id=a37362839b13f61cfe82d35bd397b1264c389b245847cfb6111b38892546dc77 data=4661696c656420746f20766572696679206e6f64655f7369676e61747572652e` on event [{"event": "ExpectMsg", "file": "test_bolt2-01-close_channel.py", "pos": "75"},]
FAILED tests/test_bolt2-01-close_channel.py::test_close_channel_shutdown_msg_wrong_script_pubkey_receiver_side - AssertionError: `Expected msgtype-warning, got msgtype-error: message was error channel_id=a37362839b13f61cfe82d35bd397b1264c389b245847cfb6111b38892546dc77 data=4661696c656420746f20766572696679206e6f64655f7369676e61747572652e` on event [{"event": "ExpectMsg", "file": "test_bolt2-01-close_channel.py", "pos": "157"},]

```

Changelog-Fixes: channeld: Verify the signature sent in announcement_signatures by the counterparty
Reported-by: lnprototest
Signed-off-by: Vincenzo Palazzo <[email protected]>

channeld/channeld.c

@@ -647,9 +647,24 @@ static void handle_peer_channel_ready(struct peer *peer, const u8 *msg)
 	billboard_update(peer);
 }
 
+/* Checks that key is valid, and signed this hash
+ *
+ * FIXME: move this inside common/utils.h */
+static bool check_signed_hash_nodeid(const struct sha256_double *hash,
+				     const secp256k1_ecdsa_signature *signature,
+				     const struct node_id *id)
+{
+	struct pubkey key;
+
+	return pubkey_from_node_id(&key, id)
+		&& check_signed_hash(hash, signature, &key);
+}
+
 static void handle_peer_announcement_signatures(struct peer *peer, const u8 *msg)
 {
+	const u8 *cannounce;
 	struct channel_id chanid;
+	struct sha256_double hash;
 
 	if (!fromwire_announcement_signatures(msg,
 					      &chanid,
@@ -669,6 +684,44 @@ static void handle_peer_announcement_signatures(struct peer *peer, const u8 *msg
 				type_to_string(tmpctx, struct channel_id, &chanid));
 	}
 
+	/* BOLT 7:
+	 * - if the node_signature OR the bitcoin_signature is NOT correct:
+	 *  - MAY send a warning and close the connection, or send an error and fail the channel.
+	 *
+	 * In our case, we send an error and stop the open channel procedure. This approach is
+	 * considered overly strict since the peer can recover from it. However, this step is
+	 * optional. If the peer sends it, we assume that the signature must be correct.*/
+	 cannounce = create_channel_announcement(tmpctx, peer);
+
+	/* 2 byte msg type + 256 byte signatures */
+	int offset = 258;
+	sha256_double(&hash, cannounce + offset,
+		       tal_count(cannounce) - offset);
+
+	 if (!check_signed_hash_nodeid(&hash, &peer->announcement_node_sigs[REMOTE], &peer->node_ids[REMOTE])) {
+		peer_failed_err(peer->pps, &chanid,
+					 "Bad node_signature %s hash %s"
+					 " on announcement_signatures %s",
+					 type_to_string(tmpctx,
+							secp256k1_ecdsa_signature,
+							&peer->announcement_node_sigs[REMOTE]),
+					 type_to_string(tmpctx,
+							struct sha256_double,
+							&hash),
+					 tal_hex(tmpctx, cannounce));
+	}
+	if (!check_signed_hash(&hash, &peer->announcement_bitcoin_sigs[REMOTE], &peer->channel->funding_pubkey[REMOTE])) {
+		peer_failed_err(peer->pps, &chanid,
+					 "Bad bitcoin_signature %s hash %s"
+					 " on announcement_signatures %s",
+					 type_to_string(tmpctx,
+							secp256k1_ecdsa_signature,
+							&peer->announcement_bitcoin_sigs[REMOTE]),
+					 type_to_string(tmpctx,
+							struct sha256_double,
+							&hash),
+					 tal_hex(tmpctx, cannounce));
+	}
 	peer->have_sigs[REMOTE] = true;
 	billboard_update(peer);