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bd20f8f4 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
7838f4be 9#include "legic_prng.h"
93b7aa8f 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.
8e220a91 15struct lfsr {
c71c5ee1 16 uint8_t a;
17 uint8_t b;
18 uint32_t c;
8e220a91 19} lfsr;
20
ad5bc8cc 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 aswell to make sure we get a all zero keystream out
28// which is used in the initialisation phase sending the IV
29//
30void legic_prng_init(uint8_t iv) {
31 lfsr.a = iv;
c71c5ee1 32 lfsr.b = 0; // hack to get a always 0 keystream
33 lfsr.c = 0;
ad5bc8cc 34 if(iv)
35 lfsr.b = (iv << 1) | 1;
8e220a91 36}
37
38void legic_prng_forward(int count) {
ad5bc8cc 39 if (count == 0) return;
40
c71c5ee1 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 }
8e220a91 47}
48
c71c5ee1 49uint32_t legic_prng_count() {
50 return lfsr.c;
3612a8a8 51}
52
8e220a91 53uint8_t legic_prng_get_bit() {
c71c5ee1 54 uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
55 return lfsr.b >> idx & 1;
ad5bc8cc 56}
57
58uint32_t legic_prng_get_bits(uint8_t len){
59 uint32_t a = 0;
60 for(uint8_t i = 0; i < len; ++i) {
61 a |= legic_prng_get_bit() << i;
62 legic_prng_forward(1);
63 }
64 return a;
c71c5ee1 65}
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