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