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[proxmark3-svn] / 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
11 struct lfsr {
12 uint8_t a;
13 uint8_t b;
14 uint32_t c;
15 } lfsr;
16
17 void legic_prng_init(uint8_t init) {
18 lfsr.a = init;
19 lfsr.b = 0; // hack to get a always 0 keystream
20 lfsr.c = 0;
21 if(init)
22 lfsr.b = (init << 1) | 1;
23 }
24
25 void legic_prng_forward(int count) {
26 lfsr.c += count;
27 while(count--) {
28 // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
29 lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
30 lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
31 }
32 }
33
34 uint32_t legic_prng_count() {
35 return lfsr.c;
36 }
37
38 uint8_t legic_prng_get_bit() {
39 uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
40 return lfsr.b >> idx & 1;
41 }
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