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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 | |
11 | struct lfsr { | |
12 | uint8_t a; | |
13 | uint8_t b; | |
14 | } lfsr; | |
15 | ||
16 | void 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 | ||
24 | void 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 | ||
45 | uint8_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 | } |