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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 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 aswell 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 uint32_t legic_prng_count() {
50 return lfsr.c;
51 }
52
53 uint8_t legic_prng_get_bit() {
54 uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
55 return lfsr.b >> idx & 1;
56 }
57
58 uint32_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;
65 }
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