X-Git-Url: https://git.zerfleddert.de/cgi-bin/gitweb.cgi/proxmark3-svn/blobdiff_plain/bd20f8f47847787e1f3e933043933272908c5beb..f6c56cd204e1548b3cf4404b44f44fb3e13b45d9:/common/legic_prng.c?ds=sidebyside

diff --git a/common/legic_prng.c b/common/legic_prng.c
index 1d28fdb7..df711aa1 100644
--- a/common/legic_prng.c
+++ b/common/legic_prng.c
@@ -7,42 +7,59 @@
 //-----------------------------------------------------------------------------
 
 #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;
+	uint8_t  a;
+	uint8_t  b;
+	uint32_t c;
 } lfsr;
 
-void legic_prng_init(uint8_t init) {
-  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) {
-  uint8_t tmp;
-  while(count--) {
-    tmp  =  lfsr.a & 1;
-    tmp ^= (lfsr.a & 0x40) >> 6;
-    
-    lfsr.a >>= 1;
-    lfsr.a |= tmp << 6;
-    
-    tmp  =  lfsr.b & 1;
-    tmp ^= (lfsr.b & 4) >> 2;
-    tmp  = ~tmp;
-    tmp ^= (lfsr.b & 8) >> 3;
-    tmp  = ~tmp;
-    tmp ^= (lfsr.b & 0x80) >> 7;
-    
-    lfsr.b >>= 1;
-    lfsr.b |= tmp << 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;
+	}
+}
+
+uint32_t legic_prng_count() {
+	return lfsr.c;
 }
 
 uint8_t legic_prng_get_bit() {
-  uint8_t idx = 7-((lfsr.a & 4) | ((lfsr.a & 8) >> 2) | ((lfsr.a & 0x10) >> 4));
-  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