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[proxmark3-svn] / common / legic_prng.c
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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
8e220a91 9#include "legic_prng.h"
8e220a91 10
11struct lfsr {
12 uint8_t a;
13 uint8_t b;
14} lfsr;
15
16void legic_prng_init(uint8_t init) {
17 lfsr.a = init;
18 if(init == 0) /* hack to get a always 0 keystream */
19 lfsr.b = 0;
20 else
21 lfsr.b = (init << 1) | 1;
22}
23
24void legic_prng_forward(int count) {
25 uint8_t tmp;
26 while(count--) {
27 tmp = lfsr.a & 1;
28 tmp ^= (lfsr.a & 0x40) >> 6;
29
30 lfsr.a >>= 1;
31 lfsr.a |= tmp << 6;
32
33 tmp = lfsr.b & 1;
34 tmp ^= (lfsr.b & 4) >> 2;
35 tmp = ~tmp;
36 tmp ^= (lfsr.b & 8) >> 3;
37 tmp = ~tmp;
38 tmp ^= (lfsr.b & 0x80) >> 7;
39
40 lfsr.b >>= 1;
41 lfsr.b |= tmp << 7;
42 }
43}
44
45uint8_t legic_prng_get_bit() {
46 uint8_t idx = 7-((lfsr.a & 4) | ((lfsr.a & 8) >> 2) | ((lfsr.a & 0x10) >> 4));
47 return ((lfsr.b >> idx) & 1);
48}
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