put legic back in the system and add rumpletux's fast legic / prng code from forum

[proxmark3-svn] / common / legic_prng.c

diff --git a/common/legic_prng.c b/common/legic_prng.c
new file mode 100644 (file)
index 0000000..ee8e0dc
--- /dev/null
+++ b/common/legic_prng.c
@@ -0,0 +1,41 @@
+#include "legic_prng.h"
+/* legic's obfuscation function */
+
+struct lfsr {
+  uint8_t a;
+  uint8_t b;
+} 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;
+}
+
+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;
+  }
+}
+
+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);
+}