From 803760ca80aacab972d9b2790c838cb1b6347ad1 Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Mon, 29 Jun 2015 00:45:33 +0100
Subject: [PATCH] Treat an all-zeros message as bad.

---
 mode_s.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mode_s.c b/mode_s.c
index 3c50d87..6510c25 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -333,6 +333,7 @@ int scoreModesMessage(unsigned char *msg, int validbits)
     int msgtype, msgbits, crc, iid;
     uint32_t addr;
     struct errorinfo *ei;
+    static unsigned char all_zeros[14] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
 
     if (validbits < 56)
         return -2;
@@ -343,6 +344,9 @@ int scoreModesMessage(unsigned char *msg, int validbits)
     if (validbits < msgbits)
         return -2;
 
+    if (!memcmp(all_zeros, msg, msgbits/8))
+        return -2;
+
     crc = modesChecksum(msg, msgbits);
 
     switch (msgtype) {