[CT437]: Add WK06 materials

year4/semester2/CT437/materials/code/lfsr.c

@@ -0,0 +1,55 @@
+//
+// Example for a 16-bit NFLSR
+//
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+#define LFSR_SIZE  16
+#define DEFAULT_START_STATE 0xACE1  // Default initial state for NLFSR
+
+uint16_t lfsr;
+uint8_t used[65535];
+
+uint8_t non_linear_feedback(uint16_t state) {
+  // Example non-linear function: XOR of specific bits and AND of others
+  uint8_t bit1 = (state >> 0) & 1;
+  uint8_t bit2 = (state >> 2) & 1;
+  uint8_t bit3 = (state >> 3) & 1;
+  uint8_t bit4 = (state >> 5) & 1;
+  uint8_t bit5 = (state >> 9) & 1;
+  return (bit1 ^ bit2 ^ bit3 ^ bit4 ^ bit5);
+}
+
+uint8_t clock_and_shift_lfsr() {
+  uint8_t ret = lfsr & 1;
+  uint8_t new_bit = non_linear_feedback(lfsr);
+  lfsr = (lfsr >> 1) | (new_bit << (LFSR_SIZE - 1));
+
+  return ret;
+}
+
+int main() {
+  uint16_t input;
+  uint16_t cycle = 0;
+  memset(used, sizeof(used), 0);
+
+  printf("Enter IV for LFSR: "); scanf("%hd", &input);
+  input = input & 0xFFFF;
+  lfsr = input;
+
+  while (1) {
+    cycle++;
+    printf("%d", clock_and_shift_lfsr());
+    if (used[lfsr] > 0)
+      break;
+    else
+      used[lfsr] = 1;
+    // printf("%d\n", nlfsr);
+  }
+
+  printf("\nCycle length: %d bits.\n", cycle);
+  return 0;
+}
+