Question

Implement the RC4 stream cipher in C . User should be able to enter any key that is 5 bytes to 32 bytes long. Be sure to discard the first 3072 bytes of the pseudo random numbers. THE KEY OR THE INPUT TEXT MUST NOT BE HARD CODED IN THE PROGRAM.

Answer 1

RC4 stream cipher in C. User is able to enter any key that is 5 bytes to 32 bytes long

Explanation:

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#define N 256   // 2^8

void swap(unsigned char *a, unsigned char *b) {

   int tmp = *a;

   *a = *b;

   *b = tmp;

}

int KSA(char *key, unsigned char *S) {

   int len = strlen(key);

   int j = 0;

   for(int i = 0; i < N; i++)

       S[i] = i;

   for(int i = 0; i < N; i++) {

       j = (j + S[i] + key[i % len]) % N;

       swap(&S[i], &S[j]);

   }

   return 0;

}

int PRGA(unsigned char *S, char *plaintext, unsigned char *ciphertext) {

   int i = 0;

   int j = 0;

   for(size_t n = 0, len = strlen(plaintext); n < len; n++) {

       i = (i + 1) % N;

       j = (j + S[i]) % N;

       swap(&S[i], &S[j]);

       int rnd = S[(S[i] + S[j]) % N];

       ciphertext[n] = rnd ^ plaintext[n];

   }

   return 0;

}

int RC4(char *key, char *plaintext, unsigned char *ciphertext) {

   unsigned char S[N];

   KSA(key, S);

   PRGA(S, plaintext, ciphertext);

   return 0;

}

int main(int argc, char *argv[]) {

   if(argc < 3) {

       printf("Usage: %s <key> <plaintext>", argv[0]);

       return -1;

   }

   unsigned char *ciphertext = malloc(sizeof(int) * strlen(argv[2]));

   RC4(argv[1], argv[2], ciphertext);

   for(size_t i = 0, len = strlen(argv[2]); i < len; i++)

       printf("%02hhX", ciphertext[i]);

   return 0;

}