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   uint8_t tmp;
  26   while(count--) {
  27     tmp  =  lfsr.a & 1;
  28     tmp ^= (lfsr.a & 0x40) >> 6;
  29
  30     lfsr.a >>= 1;
  31     lfsr.a |= tmp << 6;
  32
  33     tmp  =  lfsr.b & 1;
  34     tmp ^= (lfsr.b & 4) >> 2;
  35     tmp  = ~tmp;
  36     tmp ^= (lfsr.b & 8) >> 3;
  37     tmp  = ~tmp;
  38     tmp ^= (lfsr.b & 0x80) >> 7;
  39
  40     lfsr.b >>= 1;
  41     lfsr.b |= tmp << 7;
  42   }
  43 }
  44
  45 uint8_t legic_prng_get_bit() {
  46   uint8_t idx = 7-((lfsr.a & 4) | ((lfsr.a & 8) >> 2) | ((lfsr.a & 0x10) >> 4));
  47   return ((lfsr.b >> idx) & 1);
  48 }