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CHG: adjusted timing according to @sentinel 's traces
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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 // 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 }
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