[CT437]: Add WK06 materials

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

@@ -0,0 +1,67 @@
+//
+// 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;
+}
+