DSC Security Contact added CRC-8 calc/check

* Added "little-endian" CRC-8 calculation * Moved reverse8() to util.c * Quieted WT450 callback false positive messages
bitfliq · Oct 8, 2015 · c64e7ff · c64e7ff
1 parent fdb1d08
commit c64e7ff
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Showing 5 changed files with 70 additions and 14 deletions.
diff --git a/include/util.h b/include/util.h
@@ -18,6 +18,11 @@
 #define max(a,b) ((a) > (b) ? (a) : (b))
 #define min(a,b) ((a) < (b) ? (a) : (b))
 
+/// Reverse the bits in an 8 bit byte
+/// @param x: input byte
+/// @return bit reversed byte
+uint8_t reverse8(uint8_t x);
+
 /// Generic Cyclic Redundancy Check CRC-8
 ///
 /// Example polynomial: 0x31 = x8 + x5 + x4 + 1	(x8 is implicit)
@@ -29,6 +34,17 @@
 /// @return CRC value
 uint8_t crc8(uint8_t const message[], unsigned nBytes, uint8_t polynomial);
 
+/// "Little-endian" Cyclic Redundancy Check CRC-8 LE
+///
+/// Based on code generated from PyCyc, (pycrc.org)
+///
+/// @param message[]: array of bytes to check
+/// @param nBytes: number of bytes in message
+/// @param polynomial: byte is from x^7 to x^0 (x^8 is implicitly one)
+/// @param init: starting crc value
+/// @return CRC value
+uint8_t crc8le(uint8_t const message[], unsigned nBytes, uint8_t polynomial, uint8_t init);
+
 
 // buffer to hold localized timestamp YYYY-MM-DD HH:MM:SS
 #define LOCAL_TIME_BUFLEN	32

diff --git a/src/devices/alecto.c b/src/devices/alecto.c
@@ -48,13 +48,6 @@
  *********************************************************************************************
  */
 
-uint8_t reverse8(uint8_t x) {
-    x = (x & 0xF0) >> 4 | (x & 0x0F) << 4;
-    x = (x & 0xCC) >> 2 | (x & 0x33) << 2;
-    x = (x & 0xAA) >> 1 | (x & 0x55) << 1;
-    return x;
-}
-
 uint8_t bcd_decode8(uint8_t x) {
     return ((x & 0xF0) >> 4) * 10 + (x & 0x0F);
 }

diff --git a/src/devices/dsc.c b/src/devices/dsc.c
@@ -56,11 +56,23 @@
  *  CRC (cr)    = 8 bits, CRC, type/polynom to be determined
  *
  * The ESN in practice is 24 bits, The type + remaining 5 nybbles, 
+ *
+ * The CRC is 8 bit, "little endian", Polynomial 0xf5, Inital value 0x3d
+ *
+ * CRC algorithm found with CRC reveng (reveng.sourceforge.net)
+ *
+ * CRC Model Parameters:
+ * width=8  poly=0xf5  init=0x3d  refin=true  refout=true  xorout=0x00  check=0xfd  name=(none)
+ *
  */
 
 #include "rtl_433.h"
 #include "util.h"
 
+#define DSC_CT_MSGLEN		5	
+#define DSC_CT_CRC_POLY		0xf5
+#define DSC_CT_CRC_INIT		0x3d
+
 
 static int DSC_callback(bitbuffer_t *bitbuffer) {
     bitrow_t *bb = bitbuffer->bb;
@@ -134,13 +146,15 @@ static int DSC_callback(bitbuffer_t *bitbuffer) {
 
 	local_time_str(0, time_str);
 
-	// TODO - figure out CRC, I haven't been able to find an
-        // 8 bit CRC and polynomial that seems to match
-
-	printf("%s DSC Contact ESN: %06X, Status: %02X, CRC: %02X\n",
-	       time_str, esn, status, crc);
+	if (crc8le(bytes, DSC_CT_MSGLEN, DSC_CT_CRC_POLY, DSC_CT_CRC_INIT) == 0) {
+	    printf("%s DSC Contact ESN: %06X, Status: %02X, CRC: %02X\n",
+		   time_str, esn, status, crc);
 
-	valid_cnt++; // Have a valid packet.
+	    valid_cnt++; // Have a valid packet.
+	} else {
+	    fprintf(stderr,"%s DSC Contact bad CRC: %06X, Status: %02X, CRC: %02X\n",
+		   time_str, esn, status, crc);
+	}
     }
 
     if (valid_cnt) {

diff --git a/src/devices/wt450.c b/src/devices/wt450.c
@@ -81,8 +81,10 @@ static int wt450_callback(bitbuffer_t *bitbuffer) {
 
    if ( bitbuffer->bits_per_row[0] != 36 )
    {
-      fprintf(stderr, "wt450_callback: wrong size of bit per row %d\n",
+      if (debug_output) 
+         fprintf(stderr, "wt450_callback: wrong size of bit per row %d\n",
               bitbuffer->bits_per_row[0] );
+
       return 0;
    }
 

diff --git a/src/util.c b/src/util.c
@@ -11,6 +11,14 @@
 #include "util.h"
 #include <stdio.h>
 
+uint8_t reverse8(uint8_t x) {
+    x = (x & 0xF0) >> 4 | (x & 0x0F) << 4;
+    x = (x & 0xCC) >> 2 | (x & 0x33) << 2;
+    x = (x & 0xAA) >> 1 | (x & 0x55) << 1;
+    return x;
+}
+
+
 uint8_t crc8(uint8_t const message[], unsigned nBytes, uint8_t polynomial) {
     uint8_t remainder = 0;	
     unsigned byte, bit;
@@ -30,6 +38,29 @@ uint8_t crc8(uint8_t const message[], unsigned nBytes, uint8_t polynomial) {
 }
 
 
+uint8_t crc8le(uint8_t const message[], unsigned nBytes, uint8_t polynomial, uint8_t init) {
+    uint8_t crc = init, i;	
+    unsigned byte;
+    uint8_t bit;
+
+
+    for (byte = 0; byte < nBytes; ++byte) {
+	for (i = 0x01; i & 0xff; i <<= 1) {
+	    bit = (crc & 0x80) == 0x80;	    
+	    if (message[byte] & i) {
+		bit = !bit;
+	    }
+	    crc <<= 1;
+	    if (bit) {
+		crc ^= polynomial;
+	    }
+	}
+	crc &= 0xff;
+    }
+
+    return reverse8(crc);
+}
+
 void local_time_str(time_t time_secs, char *buf) {
 	time_t etime;
 	struct tm *tm_info;