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

   1 #include "legic_prng.h"
   2 /* legic's obfuscation function */
   3
   4 struct lfsr {
   5   uint8_t a;
   6   uint8_t b;
   7 } lfsr;
   8
   9 void legic_prng_init(uint8_t init) {
  10   lfsr.a = init;
  11   if(init == 0) /* hack to get a always 0 keystream */
  12     lfsr.b = 0;
  13   else
  14     lfsr.b = (init << 1) | 1;
  15 }
  16
  17 void legic_prng_forward(int count) {
  18   uint8_t tmp;
  19   while(count--) {
  20     tmp  =  lfsr.a & 1;
  21     tmp ^= (lfsr.a & 0x40) >> 6;
  22
  23     lfsr.a >>= 1;
  24     lfsr.a |= tmp << 6;
  25
  26     tmp  =  lfsr.b & 1;
  27     tmp ^= (lfsr.b & 4) >> 2;
  28     tmp  = ~tmp;
  29     tmp ^= (lfsr.b & 8) >> 3;
  30     tmp  = ~tmp;
  31     tmp ^= (lfsr.b & 0x80) >> 7;
  32
  33     lfsr.b >>= 1;
  34     lfsr.b |= tmp << 7;
  35   }
  36 }
  37
  38 uint8_t legic_prng_get_bit() {
  39   uint8_t idx = 7-((lfsr.a & 4) | ((lfsr.a & 8) >> 2) | ((lfsr.a & 0x10) >> 4));
  40   return ((lfsr.b >> idx) & 1);
  41 }