small improvements in auth (#694)

[proxmark3-svn] / common / legic_prng.c

diff --git a/common/legic_prng.c b/common/legic_prng.c
index ee8e0dc7ab0451f28bff23b4a671c51bf616cc29..4f3b1ffee1167ab51a067502451cb4e634c88cef 100644 (file)
--- a/common/legic_prng.c
+++ b/common/legic_prng.c
@@ -1,12 +1,21 @@
+//-----------------------------------------------------------------------------
+// This code is licensed to you under the terms of the GNU GPL, version 2 or,
+// at your option, any later version. See the LICENSE.txt file for the text of
+// the license.
+//-----------------------------------------------------------------------------
+// LEFIC's obfuscation function
+//-----------------------------------------------------------------------------
+
 #include "legic_prng.h"
-/* legic's obfuscation function */
 
 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.c = 0;
   lfsr.a = init;
   if(init == 0) /* hack to get a always 0 keystream */
     lfsr.b = 0;
@@ -15,27 +24,18 @@ void legic_prng_init(uint8_t init) {
 }
 
 void legic_prng_forward(int count) {
-  uint8_t tmp;
+  lfsr.c += count;
   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;
+    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;
   }
 }
 
+int 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;
 }