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 // a is 7bit
  11 // b is
  12 // c is a counter
  13 struct lfsr {
  14         uint8_t  a;
  15         uint8_t  b;
  16         uint32_t c;
  17 } lfsr;
  18
  19 // Normal init is set following variables with a random value IV
  20 // a == iv
  21 // b == iv << 1 | 1
  22 // * someone mentioned iv must be ODD.
  23 // Hack:
  24 // Now we have a special case with iv == 0
  25 // it sets b to 0 aswell to make sure we get a all zero keystream out
  26 // which is used in the initialisation phase sending the IV
  27 //
  28 void legic_prng_init(uint8_t iv) {
  29         lfsr.a = iv;
  30         lfsr.b = 0;  // hack to get a always 0 keystream
  31         lfsr.c = 0;
  32         if(iv)
  33                 lfsr.b = (iv << 1) | 1;
  34 }
  35
  36 void legic_prng_forward(int count) {
  37         if (count == 0) return;
  38
  39         lfsr.c += count;
  40         while(count--) {
  41                 // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
  42                 lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
  43                 lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
  44         }
  45 }
  46
  47 uint32_t legic_prng_count() {
  48         return lfsr.c;
  49 }
  50
  51 uint8_t legic_prng_get_bit() {
  52         uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
  53         return lfsr.b >> idx & 1;
  54 }
  55
  56 uint32_t legic_prng_get_bits(uint8_t len){
  57         uint32_t a = 0;
  58         for(uint8_t i = 0; i < len; ++i) {
  59                 a |= legic_prng_get_bit() << i;
  60                 legic_prng_forward(1);
  61         }
  62         return a;
  63 }