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https://github.com/RfidResearchGroup/proxmark3.git
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62 lines
1.9 KiB
C
62 lines
1.9 KiB
C
//-----------------------------------------------------------------------------
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// This code is licensed to you under the terms of the GNU GPL, version 2 or,
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// at your option, any later version. See the LICENSE.txt file for the text of
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// the license.
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//-----------------------------------------------------------------------------
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// LEFIC's obfuscation function
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//-----------------------------------------------------------------------------
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#include "legic_prng.h"
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// the prng is a muxed value from two lsfr a, b
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// a is 7bit lsfr
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// b is 8bit lsfr
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// c keeps track on which step the prng is.
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// legic_prng_get_bit() = gets a bit muxed from a and b.
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static struct lfsr {
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uint8_t a;
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uint8_t b;
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uint32_t c;
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} lfsr;
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// Normal init is set following variables with a random value IV
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// a == iv
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// b == iv << 1 | 1
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// * someone mentioned iv must be ODD.
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// Hack:
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// Now we have a special case with iv == 0
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// it sets b to 0 as well to make sure we get a all zero keystream out
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// which is used in the initialisation phase sending the IV
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//
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void legic_prng_init(uint8_t iv) {
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lfsr.a = iv;
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lfsr.b = 0; // hack to get a always 0 keystream
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lfsr.c = 0;
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if (iv)
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lfsr.b = (iv << 1) | 1;
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}
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void legic_prng_forward(int count) {
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if (count == 0) return;
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lfsr.c += count;
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while (count--) {
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// According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
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lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
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lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
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}
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}
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uint8_t legic_prng_get_bit(void) {
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uint8_t idx = 7 - ((lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1));
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return lfsr.b >> idx & 1;
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}
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uint32_t legic_prng_get_bits(uint8_t len) {
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uint32_t a = 0;
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for (uint8_t i = 0; i < len; ++i) {
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a |= legic_prng_get_bit() << i;
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legic_prng_forward(1);
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}
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return a;
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}
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