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 } lfsr;
  15
  16 void legic_prng_init(uint8_t init) {
  17   lfsr.a = init;
  18   if(init == 0) /* hack to get a always 0 keystream */
  19     lfsr.b = 0;
  20   else
  21     lfsr.b = (init << 1) | 1;
  22 }
  23
  24 void legic_prng_forward(int count) {
  25   while(count--) {
  26     lfsr.a = lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6;
  27     lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
  28   }
  29 }
  30
  31 uint8_t legic_prng_get_bit() {
  32   uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
  33   return lfsr.b >> idx & 1;
  34 }