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 // the prng is a muxed value from two lsfr a, b
  11 // a is 7bit lsfr
  12 // b is 8bit lsfr
  13 // c keeps track on which step the prng is.
  14 // legic_prng_get_bit() = gets a bit muxed from a and b.
  15 static struct lfsr {
  16     uint8_t  a;
  17     uint8_t  b;
  18     uint32_t c;
  19 } lfsr;
  20
  21 // Normal init is set following variables with a random value IV
  22 // a == iv
  23 // b == iv << 1 | 1
  24 // * someone mentioned iv must be ODD.
  25 // Hack:
  26 // Now we have a special case with iv == 0
  27 // it sets b to 0 as well to make sure we get a all zero keystream out
  28 // which is used in the initialisation phase sending the IV
  29 //
  30 void legic_prng_init(uint8_t iv) {
  31     lfsr.a = iv;
  32     lfsr.b = 0;  // hack to get a always 0 keystream
  33     lfsr.c = 0;
  34     if (iv)
  35         lfsr.b = (iv << 1) | 1;
  36 }
  37
  38 void legic_prng_forward(int count) {
  39     if (count == 0) return;
  40
  41     lfsr.c += count;
  42     while (count--) {
  43         // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
  44         lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
  45         lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
  46     }
  47 }
  48
  49 uint8_t legic_prng_get_bit(void) {
  50     uint8_t idx = 7 - ((lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1));
  51     return lfsr.b >> idx & 1;
  52 }
  53
  54 uint32_t legic_prng_get_bits(uint8_t len) {
  55     uint32_t a = 0;
  56     for (uint8_t i = 0; i < len; ++i) {
  57         a |= legic_prng_get_bit() << i;
  58         legic_prng_forward(1);
  59     }
  60     return a;
  61 }