git.zerfleddert.de Git - proxmark3-svn/blob - common/crc16.c

   1 //-----------------------------------------------------------------------------
   2 // This code is licensed to you under the terms of the GNU GPL, version 2 or,
   3 // at your option, any later version. See the LICENSE.txt file for the text of
   4 // the license.
   5 //-----------------------------------------------------------------------------
   6 // CRC16
   7 //-----------------------------------------------------------------------------
   8
   9 #include "crc16.h"
  10 #define CRC16_POLY_CCITT 0x1021
  11 #define CRC16_POLY 0x8408
  12
  13 unsigned short update_crc16( unsigned short crc, unsigned char c )
  14 {
  15   unsigned short i, v, tcrc = 0;
  16
  17   v = (crc ^ c) & 0xff;
  18   for (i = 0; i < 8; i++) {
  19       tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ CRC16_POLY : tcrc >> 1;
  20       v >>= 1;
  21   }
  22
  23   return ((crc >> 8) ^ tcrc) & 0xffff;
  24 }
  25
  26 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
  27
  28         if (length == 0)
  29         return (~remainder);
  30
  31     for (int byte = 0; byte < length; ++byte) {
  32         remainder ^= (message[byte] << 8);
  33         for (uint8_t bit = 8; bit > 0; --bit) {
  34             if (remainder & 0x8000) {
  35                 remainder = (remainder << 1) ^ polynomial;
  36             } else {
  37                 remainder = (remainder << 1);
  38             }
  39         }
  40     }
  41     return remainder;
  42 }
  43
  44 uint16_t crc16_ccitt(uint8_t const *message, int length) {
  45     return crc16(message, length, 0xffff, CRC16_POLY_CCITT);
  46 }
  47
  48 uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
  49     return bit_reverse_uint16(crc16(message, length, 0x0000, CRC16_POLY_CCITT));
  50 }
  51 uint16_t bit_reverse_uint16 (uint16_t value) {
  52         const uint16_t mask0 = 0x5555;
  53         const uint16_t mask1 = 0x3333;
  54         const uint16_t mask2 = 0x0F0F;
  55         const uint16_t mask3 = 0x00FF;
  56
  57         value = (((~mask0) & value) >> 1) | ((mask0 & value) << 1);
  58         value = (((~mask1) & value) >> 2) | ((mask1 & value) << 2);
  59         value = (((~mask2) & value) >> 4) | ((mask2 & value) << 4);
  60         value = (((~mask3) & value) >> 8) | ((mask3 & value) << 8);
  61
  62         return value;
  63 }