[CT437]: Add WK06 materials

year4/semester2/CT437/exam

@@ -0,0 +1,3 @@
+- always question about rsa and diffie helman
+- don't expecct any diffie helman calculations on the exam, that would be ridiciulous
+

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

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+//
+// 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;
+}
+

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

@@ -0,0 +1,55 @@
+//
+// Example for a 16-bit NFLSR
+//
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+#define NLFSR_SIZE  16
+#define DEFAULT_START_STATE 0xACE1  // Default initial state for NLFSR
+
+uint16_t nlfsr;
+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_nlfsr() {
+  uint8_t ret = nlfsr & 1;
+  uint8_t new_bit = non_linear_feedback(nlfsr);
+  nlfsr = (nlfsr >> 1) | (new_bit << (NLFSR_SIZE - 1));
+
+  return ret;
+}
+
+int main() {
+  uint16_t input;
+  uint16_t cycle = 0;
+  memset(used, sizeof(used), 0);
+
+  printf("Enter IV for NLFSR: "); scanf("%hd", &input);
+  input = input & 0xFFFF;
+  nlfsr = input;
+
+  while (1) {
+    cycle++;
+    printf("%d", clock_and_shift_nlfsr());
+    if (used[nlfsr] > 0)
+      break;
+    else
+      used[nlfsr] = 1;
+    // printf("%d\n", nlfsr);
+  }
+
+  printf("\nCycle length: %d bits.\n", cycle);
+  return 0;
+}
+

year4/semester2/CT437/materials/code/rc4.c

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+//
+// Example implementation of RC4
+//
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+#define A_LEN   256
+#define SWAP(a, b) { uint8_t temp = a; a = b; b = temp; }
+
+void rc4_init(uint8_t *key, uint8_t *S, int key_length) {
+    int i, j = 0;
+    for (i = 0; i < 256; i++) {
+        S[i] = i;
+    }
+    for (i = 0; i < 256; i++) {
+        j = (j + S[i] + key[i % key_length]) % 256;
+        SWAP(S[i], S[j]);
+    }
+}
+
+void rc4_generate_keystream(uint8_t *S, uint8_t *keystream, int length) {
+    int i = 0, j = 0, k;
+    for (k = 0; k < length; k++) {
+        i = (i + 1) % 256;
+        j = (j + S[i]) % 256;
+        SWAP(S[i], S[j]);
+        keystream[k] = S[(S[i] + S[j]) % 256];
+    }
+}
+
+void rc4_encrypt_decrypt(uint8_t *data, uint8_t *keystream, int length) {
+    for (int i = 0; i < length; i++) {
+        data[i] ^= keystream[i];
+    }
+}
+
+int main() {
+    uint8_t key[A_LEN];
+    uint8_t S[A_LEN];
+    uint8_t data[A_LEN];
+    int data_length;
+    uint8_t keystream[A_LEN];
+
+    printf("Enter key: "); scanf("%s", key);
+    printf("Enter plaintext: "); scanf("%s", data);
+    data_length = strlen((char *)data);
+
+    rc4_init(key, S, strlen((char *)key));
+    rc4_generate_keystream(S, keystream, data_length);
+    rc4_encrypt_decrypt(data, keystream, data_length);
+
+    printf("Encrypted: ");
+    for (int i = 0; i < data_length; i++) {
+        printf("%02X ", data[i]);
+    }
+    printf("\n");
+
+    // Decrypting the data
+    rc4_encrypt_decrypt(data, keystream, data_length);
+
+    printf("Decrypted: %s\n", data);
+
+    return 0;
+}
+