Update README.md

[proxmark3-svn] / common / legic_prng.c

diff --git a/common/legic_prng.c b/common/legic_prng.c
index 322429ad716266924d72507ce02c248ac654dd38..df711aa10f2b5c58e74a2dae619e25958dff6a32 100644 (file)
--- a/common/legic_prng.c
+++ b/common/legic_prng.c
@@ -6,36 +6,60 @@
 // LEFIC's obfuscation function
 //-----------------------------------------------------------------------------
 
-#include "../include/legic_prng.h"
-
+#include "legic_prng.h"
+// the prng is a muxed value from two lsfr a, b
+// a is 7bit lsfr
+// b is 8bit lsfr
+// c keeps track on which step the prng is.
+// legic_prng_get_bit() = gets a bit muxed from a and b. 
 struct lfsr {
-  uint8_t  a;
-  uint8_t  b;
-  uint32_t c;
+       uint8_t  a;
+       uint8_t  b;
+       uint32_t c;
 } lfsr;
 
-void legic_prng_init(uint8_t init) {
-  lfsr.c = 0;
-  lfsr.a = init;
-  if(init == 0) /* hack to get a always 0 keystream */
-    lfsr.b = 0;
-  else
-    lfsr.b = (init << 1) | 1;
+// Normal init is set following variables with a random value IV
+// a == iv
+// b == iv << 1 | 1
+// * someone mentioned iv must be ODD.
+// Hack:
+// Now we have a special case with iv == 0
+// it sets b to 0 aswell to make sure we get a all zero keystream out
+// which is used in the initialisation phase sending the IV
+// 
+void legic_prng_init(uint8_t iv) {
+       lfsr.a = iv;
+       lfsr.b = 0;  // hack to get a always 0 keystream 
+       lfsr.c = 0;
+       if(iv)
+               lfsr.b = (iv << 1) | 1;
 }
 
 void legic_prng_forward(int count) {
-  lfsr.c += count;
-  while(count--) {
-    lfsr.a = lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6;
-    lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
-  }
+       if (count == 0) return;
+       
+       lfsr.c += count;
+       while(count--) {
+               // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
+               lfsr.a = (lfsr.a >> 1 | (lfsr.a ^ lfsr.a >> 6) << 6) & 0x7F;
+               lfsr.b = lfsr.b >> 1 | (lfsr.b ^ lfsr.b >> 2 ^ lfsr.b >> 3 ^ lfsr.b >> 7) << 7;
+       }
 }
 
-int legic_prng_count() {
-  return lfsr.c;
+uint32_t legic_prng_count() {
+       return lfsr.c;
 }
 
 uint8_t legic_prng_get_bit() {
-  uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
-  return lfsr.b >> idx & 1;
+       uint8_t idx = 7 - ( (lfsr.a & 4) | (lfsr.a >> 2 & 2) | (lfsr.a >> 4 & 1) );
+       return lfsr.b >> idx & 1;
 }
+
+uint32_t legic_prng_get_bits(uint8_t len){
+       uint32_t a = 0;
+       for(uint8_t i = 0; i < len; ++i) {
+               a |= legic_prng_get_bit() << i;
+               legic_prng_forward(1);
+       }
+       return a;
+}
\ No newline at end of file