git.zerfleddert.de Git - proxmark3-svn/blob - common/legic_prng.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 // LEFIC's obfuscation function
   7 //-----------------------------------------------------------------------------
   8
   9 #include "legic_prng.h"
  10
  11 struct lfsr {
  12         uint8_t  a;
  13         uint8_t  b;
  14         uint32_t c;
  15 } lfsr;
  16
  17 void legic_prng_init(uint8_t init) {
  18         lfsr.a = init;
  19         lfsr.b = 0;  // hack to get a always 0 keystream
  20         lfsr.c = 0;
  21         if(init)
  22                 lfsr.b = (init << 1) | 1;
  23 }
  24
  25 void legic_prng_forward(int count) {
  26         lfsr.c += count;
  27         while(count--) {
  28                 // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
  29                 lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
  30                 lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
  31         }
  32 }
  33
  34 uint32_t legic_prng_count() {
  35         return lfsr.c;
  36 }
  37
  38 uint8_t legic_prng_get_bit() {
  39         uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
  40         return lfsr.b >> idx & 1;
  41 }