common/legic_prng.c

   1 //-----------------------------------------------------------------------------
   2 // Copyright (C) Proxmark3 contributors. See AUTHORS.md for details.
   3 //
   4 // This program is free software: you can redistribute it and/or modify
   5 // it under the terms of the GNU General Public License as published by
   6 // the Free Software Foundation, either version 3 of the License, or
   7 // (at your option) any later version.
   8 //
   9 // This program is distributed in the hope that it will be useful,
  10 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 // GNU General Public License for more details.
  13 //
  14 // See LICENSE.txt for the text of the license.
  15 //-----------------------------------------------------------------------------
  16 // LEGIC's obfuscation function
  17 //-----------------------------------------------------------------------------
  18
  19 #include "legic_prng.h"
  20 // the prng is a muxed value from two lsfr a, b
  21 // a is 7bit lsfr
  22 // b is 8bit lsfr
  23 // c keeps track on which step the prng is.
  24 // legic_prng_get_bit() = gets a bit muxed from a and b.
  25 static struct lfsr {
  26     uint8_t  a;
  27     uint8_t  b;
  28     uint32_t c;
  29 } lfsr;
  30
  31 // Normal init is set following variables with a random value IV
  32 // a == iv
  33 // b == iv << 1 | 1
  34 // * someone mentioned iv must be ODD.
  35 // Hack:
  36 // Now we have a special case with iv == 0
  37 // it sets b to 0 as well to make sure we get a all zero keystream out
  38 // which is used in the initialisation phase sending the IV
  39 //
  40 void legic_prng_init(uint8_t iv) {
  41     lfsr.a = iv;
  42     lfsr.b = 0;  // hack to get a always 0 keystream
  43     lfsr.c = 0;
  44     if (iv)
  45         lfsr.b = (iv << 1) | 1;
  46 }
  47
  48 void legic_prng_forward(int count) {
  49     if (count == 0) return;
  50
  51     lfsr.c += count;
  52     while (count--) {
  53         // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
  54         lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
  55         lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
  56     }
  57 }
  58
  59 uint8_t legic_prng_get_bit(void) {
  60     uint8_t idx = 7 - ((lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1));
  61     return lfsr.b >> idx & 1;
  62 }
  63
  64 uint32_t legic_prng_get_bits(uint8_t len) {
  65     uint32_t a = 0;
  66     for (uint8_t i = 0; i < len; ++i) {
  67         a |= legic_prng_get_bit() << i;
  68         legic_prng_forward(1);
  69     }
  70     return a;
  71 }