FLARM decoder

pyModeS/decoder/flarm/core.c

@@ -0,0 +1,124 @@
+#include "core.h"
+
+/*
+ *
+ * https://pastebin.com/YK2f8bfm
+ *
+ * NEW ENCRYPTION
+ *
+ * Swiss glider anti-colission system moved to a new encryption scheme: XXTEA
+ * The algorithm encrypts all the packet after the header: total 20 bytes or 5 long int words of data
+ *
+ * XXTEA description and code are found here: http://en.wikipedia.org/wiki/XXTEA
+ * The system uses 6 iterations of the main loop.
+ *
+ * The system version 6 sends two type of packets: position and ... some unknown data
+ * The difference is made by bit 0 of byte 3 of the packet: for position data this bit is zero.
+ *
+ * For position data the key used depends on the time and transmitting device address.
+ * The key is as well obscured by a weird algorithm.
+ * The code to generate the key is:
+ *
+ * */
+
+void make_key(int *key, long time, long address)
+{
+  const long key1[4] = {0xe43276df, 0xdca83759, 0x9802b8ac, 0x4675a56b};
+  const long key1b[4] = {0xfc78ea65, 0x804b90ea, 0xb76542cd, 0x329dfa32};
+  const long *table = ((((time >> 23) & 255) & 0x01) != 0) ? key1b : key1;
+
+  for (int i = 0; i < 4; i++)
+  {
+    key[i] = obscure(table[i] ^ ((time >> 6) ^ address), 0x045D9F3B) ^ 0x87B562F4;
+  }
+}
+
+long obscure(long key, unsigned long seed)
+{
+  unsigned int m1 = seed * (key ^ (key >> 16));
+  unsigned int m2 = seed * (m1 ^ (m1 >> 16));
+  return m2 ^ (m2 >> 16);
+}
+
+/*
+ * NEW PACKET FORMAT:
+ *
+ * Byte     Bits
+ *  0     AAAA AAAA    device address
+ *  1     AAAA AAAA
+ *  2     AAAA AAAA
+ *  3     00aa 0000    aa = 10 or 01
+ *
+ *  4     vvvv vvvv    vertical speed
+ *  5     xxxx xxvv
+ *  6     gggg gggg    GPS status
+ *  7     tttt gggg    plane type
+ *
+ *  8     LLLL LLLL    Latitude
+ *  9     LLLL LLLL
+ * 10     aaaa aLLL
+ * 11     aaaa aaaa    Altitude
+ *
+ * 12     NNNN NNNN    Longitude
+ * 13     NNNN NNNN
+ * 14     xxxx NNNN
+ * 15     FFxx xxxx    multiplying factor
+ *
+ * 16     SSSS SSSS    as in version 4
+ * 17     ssss ssss
+ * 18     KKKK KKKK
+ * 19     kkkk kkkk
+ *
+ * 20     EEEE EEEE
+ * 21     eeee eeee
+ * 22     PPPP PPPP
+ * 24     pppp pppp
+ * */
+
+/*
+ * https://en.wikipedia.org/wiki/XXTEA
+ */
+
+void btea(uint32_t *v, int n, uint32_t const key[4])
+{
+  uint32_t y, z, sum;
+  unsigned p, rounds, e;
+  if (n > 1)
+  { /* Coding Part */
+    /* Unused, should remove? */
+    rounds = 6 + 52 / n;
+    sum = 0;
+    z = v[n - 1];
+    do
+    {
+      sum += DELTA;
+      e = (sum >> 2) & 3;
+      for (p = 0; p < (unsigned)n - 1; p++)
+      {
+        y = v[p + 1];
+        z = v[p] += MX;
+      }
+      y = v[0];
+      z = v[n - 1] += MX;
+    } while (--rounds);
+  }
+  else if (n < -1)
+  { /* Decoding Part */
+    n = -n;
+    rounds = 6; // + 52 / n;
+    sum = rounds * DELTA;
+    y = v[0];
+    do
+    {
+      e = (sum >> 2) & 3;
+      for (p = n - 1; p > 0; p--)
+      {
+        z = v[p - 1];
+        y = v[p] -= MX;
+      }
+      z = v[n - 1];
+      y = v[0] -= MX;
+      sum -= DELTA;
+    } while (--rounds);
+  }
+}