Update README.md

[proxmark3-svn] / common / legic_prng.c

diff --git a/common/legic_prng.c b/common/legic_prng.c
index 93229251124563174032c946d693059b3a735ce9..df711aa10f2b5c58e74a2dae619e25958dff6a32 100644 (file)
--- a/common/legic_prng.c
+++ b/common/legic_prng.c
@@ -7,22 +7,37 @@
 //-----------------------------------------------------------------------------
 
 #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;
 } lfsr;
 
-void legic_prng_init(uint8_t init) {
-       lfsr.a = init;
+// 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(init)
-               lfsr.b = (init << 1) | 1;
+       if(iv)
+               lfsr.b = (iv << 1) | 1;
 }
 
 void legic_prng_forward(int count) {
+       if (count == 0) return;
+       
        lfsr.c += count;
        while(count--) {
                // According: http://www.proxmark.org/forum/viewtopic.php?pid=5437#p5437
@@ -38,4 +53,13 @@ uint32_t legic_prng_count() {
 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;
+}
+
+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