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42 lines
805 B
C
42 lines
805 B
C
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#include "legic_prng.h"
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/* legic's obfuscation function */
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struct lfsr {
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uint8_t a;
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uint8_t b;
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} lfsr;
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void legic_prng_init(uint8_t init) {
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lfsr.a = init;
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if(init == 0) /* hack to get a always 0 keystream */
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lfsr.b = 0;
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else
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lfsr.b = (init << 1) | 1;
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}
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void legic_prng_forward(int count) {
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uint8_t tmp;
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while(count--) {
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tmp = lfsr.a & 1;
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tmp ^= (lfsr.a & 0x40) >> 6;
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lfsr.a >>= 1;
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lfsr.a |= tmp << 6;
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tmp = lfsr.b & 1;
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tmp ^= (lfsr.b & 4) >> 2;
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tmp = ~tmp;
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tmp ^= (lfsr.b & 8) >> 3;
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tmp = ~tmp;
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tmp ^= (lfsr.b & 0x80) >> 7;
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lfsr.b >>= 1;
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lfsr.b |= tmp << 7;
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}
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}
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uint8_t legic_prng_get_bit() {
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uint8_t idx = 7-((lfsr.a & 4) | ((lfsr.a & 8) >> 2) | ((lfsr.a & 0x10) >> 4));
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return ((lfsr.b >> idx) & 1);
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}
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