tool to find correct byte for 8-bit XOR CRC

tools/xorcheck.py

@@ -0,0 +1,55 @@
+#!/usr/bin/python
+
+#  xorcheck.py - find xor values for 8-bit CRC
+# 
+#  Adam Laurie <adam@algroup.co.uk>
+#  http://rfidiot.org/
+# 
+#  This code is copyright (c) Adam Laurie, 2009, All rights reserved.
+#  For non-commercial use only, the following terms apply - for all other
+#  uses, please contact the author:
+#
+#    This code is free software; you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation; either version 2 of the License, or
+#    (at your option) any later version.
+#
+#    This code is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+
+import sys
+import os
+
+if(len(sys.argv) < 3):
+	print
+	print sys.argv[0] + ' - generate final byte for XOR CRC'
+	print 'Usage: ' + sys.argv[0] + ' <ID Byte1> <ID Byte2> ... <CRC>'
+	print
+	print '\tSpecifying the bytes of a UID with a known CRC will generate the last byte value'
+	print '\tneeded to generate that CRC with a rolling XOR. All bytes should be specified in HEX.'
+	print
+	print 'Example:'
+	print
+	print '\txorcheck.py 04 00 80 64 ba'
+	print
+	print 'Should produce the output:'
+	print
+	print '\tTarget matched with Byte value: 5A'
+	print
+	os._exit(True)
+
+target= int(sys.argv[len(sys.argv) - 1],16)
+
+for candidate in range(256):
+	crc= 0x00
+	for i in range(len(sys.argv) - 2):
+		crc ^= int(sys.argv[i + 1],16)
+	crc ^= candidate
+	if (crc == target):
+		print
+		print 'Target matched with Byte value: %02X' % candidate
+		print
+