From 6eaa8da9dcc7543194435f706c72272783331df7 Mon Sep 17 00:00:00 2001
From: marshmellow42 <marshmellowrf@gmail.com>
Date: Thu, 4 Jun 2015 12:53:19 -0400
Subject: [PATCH] Add CCITT Kermit CRC check for FDX-B demod

---
 client/cmddata.c | 16 ++++++++++---
 common/crc16.c   | 60 +++++++++++++++++++++++++++++-------------------
 common/crc16.h   |  1 +
 3 files changed, 51 insertions(+), 26 deletions(-)

diff --git a/client/cmddata.c b/client/cmddata.c
index dad0a711..00caefd9 100644
--- a/client/cmddata.c
+++ b/client/cmddata.c
@@ -23,6 +23,7 @@
 #include "lfdemod.h"
 #include "usb_cmd.h"
 #include "crc.h"
+#include "crc16.h"
 
 uint8_t DemodBuffer[MAX_DEMOD_BUF_LEN];
 uint8_t g_debugMode;
@@ -1501,9 +1502,10 @@ int CmdFDXBdemodBI(const char *Cmd){
 	if (g_debugMode) {
 		char *bin = sprint_bin_break(BitStream,size,16);
 		PrintAndLog("DEBUG BinStream:\n%s",bin);
+		PrintAndLog("0x%s", sprint_hex(BitStream,16));
 	}
 	PrintAndLog("\nFDX-B / ISO 11784/5 Animal Tag ID Found:");
-	if (g_debugMode) PrintAndLog("startmarker %d;   Size %d", preambleIndex, size);
+	if (g_debugMode) PrintAndLog("Start marker %d;   Size %d", preambleIndex, size);
 
 	//return 1;
 	//got a good demod
@@ -1515,14 +1517,22 @@ int CmdFDXBdemodBI(const char *Cmd){
 	uint32_t crc16 = bytebits_to_byteLSBF(BitStream+64,16);
 	uint32_t extended = bytebits_to_byteLSBF(BitStream+80,24);
 
+	uint64_t rawid = ((uint64_t)bytebits_to_byteLSBF(BitStream,32)<<32) | bytebits_to_byteLSBF(BitStream+32,32);
+	uint8_t raw[8];
+	num_to_bytes(rawid, 8, raw);
+	uint8_t *ID = SwapEndian64(raw, 8, 4);
+
+	if (g_debugMode) PrintAndLog("Raw ID Hex: %s", sprint_hex(ID,8));
+
+	uint16_t calcCrc = crc16_ccitt_kermit(ID, 8);
 	PrintAndLog("Animal ID:     %u-%012llu", countryCode, NationalCode);
 	PrintAndLog("National Code: %012llu", NationalCode);
 	PrintAndLog("CountryCode:   %u", countryCode);
 	PrintAndLog("Extended Data: %s", dataBlockBit ? "True" : "False");
 	PrintAndLog("reserved Code: %u", reservedCode);
 	PrintAndLog("Animal Tag:    %s", animalBit ? "True" : "False");
-	PrintAndLog("CRC:           0x%02X", crc16);
-	PrintAndLog("Extended:      0x%X", extended);
+	PrintAndLog("CRC:           0x%04X - [%04X] - %s", crc16, calcCrc, (calcCrc == crc16) ? "Passed" : "Failed");
+	PrintAndLog("Extended:      0x%X\n", extended);
 	
 	return 1;
 }
diff --git a/common/crc16.c b/common/crc16.c
index 973cd103..d48df3b2 100644
--- a/common/crc16.c
+++ b/common/crc16.c
@@ -11,35 +11,49 @@
 
 unsigned short update_crc16( unsigned short crc, unsigned char c )
 {
-  unsigned short i, v, tcrc = 0;
+	unsigned short i, v, tcrc = 0;
 
-  v = (crc ^ c) & 0xff;
-  for (i = 0; i < 8; i++) {
-      tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
-      v >>= 1;
-  }
+	v = (crc ^ c) & 0xff;
+	for (i = 0; i < 8; i++) {
+		tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
+		v >>= 1;
+	}
 
-  return ((crc >> 8) ^ tcrc)&0xffff;
+	return ((crc >> 8) ^ tcrc)&0xffff;
 }
 
 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
-    
-	if (length == 0)
-        return (~remainder);
-			
-    for (int byte = 0; byte < length; ++byte) {
-        remainder ^= (message[byte] << 8);
-        for (uint8_t bit = 8; bit > 0; --bit) {
-            if (remainder & 0x8000) {
-                remainder = (remainder << 1) ^ polynomial;
-            } else {
-                remainder = (remainder << 1);
-            }
-        }
-    }
-    return remainder;
+
+	if (length == 0) return (~remainder);
+
+	for (int byte = 0; byte < length; ++byte) {
+		remainder ^= (message[byte] << 8);
+		for (uint8_t bit = 8; bit > 0; --bit) {
+			if (remainder & 0x8000) {
+				remainder = (remainder << 1) ^ polynomial;
+			} else {
+				remainder = (remainder << 1);
+			}
+		}
+	}
+	return remainder;
 }
 
 uint16_t crc16_ccitt(uint8_t const *message, int length) {
-    return crc16(message, length, 0xffff, 0x1021);
+	return crc16(message, length, 0xffff, 0x1021);
+}
+
+uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
+	if (length == 0) return 0;
+	uint32_t crc = 0, q = 0;
+	uint8_t c;
+
+	for (int i = 0; i < length; i++){
+		c = message[i];
+		q = (crc ^ c ) & 0xF;
+		crc = (crc >> 4) ^ (q * 0x1081);
+		q = (crc ^ (c >> 4)) & 0xF;
+		crc = (crc >> 4) ^ (q * 0x1081);
+	}
+	return crc;
 }
diff --git a/common/crc16.h b/common/crc16.h
index d16d83b5..3656ce27 100644
--- a/common/crc16.h
+++ b/common/crc16.h
@@ -12,4 +12,5 @@
 unsigned short update_crc16(unsigned short crc, unsigned char c);
 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial);
 uint16_t crc16_ccitt(uint8_t const *message, int length);
+uint16_t crc16_ccitt_kermit(uint8_t const *message, int length);
 #endif
-- 
2.39.2