Implementing Rc4 algorithm

Question

I need to implement a Rc4 algorithm with a seed: 1 2 3 6 and the plain text cryptology. I am following this guideline we were provided in class, but it's not initializing S correctly.

my output is

and needs to be

My code was previously printing negative values , not sure why but I managed to fix that error. Thought everything was good to go but it's not. Sorry for the pictures, I figured it was easier to explain what I was following for my code structure. I am mod 4 the seed since it contains 4 characters, could that possibly be my error?

#include <iostream>
#include <string>
#include <string.h>


using std::endl;
using std::string;

void swap(unsigned int *x, unsigned int *y);



int main()
{
string plaintext = "cryptology";

char cipherText[256] = { ' ' };
unsigned int S[256] = { 0 };
unsigned int t[256] = { 0 };
unsigned int seed[4] = { 1, 2, 3, 6 };      // seed used for test case 1 
unsigned int temp = 0;
int runningTotal = 0;

unsigned int key = 0;



// inilializing s and t
for (int i = 0; i < 256; i++)
{

    S[i] = i;
    t[i] = seed[i % 4];
}

for (int i = 0; i < 256; i++)
{
    runningTotal += S[i] + t[i];
    runningTotal %= 256;
    swap(&S[runningTotal], &S[i]);

     std::cout  << S[i] <<" ";
}
runningTotal = 0;

for (int i = 0; i < plaintext.size(); i++)
{
    runningTotal %= 256;
    swap(&S[i], &S[runningTotal]);

    temp = (unsigned int)S[i] + (unsigned int)S[runningTotal];
    temp %= 256;

    key = S[temp];
    std::cout << endl;
    cipherText[i] = plaintext[i] ^ key;



}
std::cout << " this is cipher text " << endl;
std::cout << cipherText << endl;






system("pause");
return 0;
}
void swap(unsigned int *x, unsigned int *y)
{
unsigned int temp = 0;

temp = *x;
*x = *y;
*y = temp;






}

r3mainer · Accepted Answer · 2016-01-27 23:51:06Z

Actually I think you're generating S[] correctly. I can only assume you're supposed to do something different with the key. (Perhaps's its an ASCII string instead of four byte values? Check your assignment notes.)

There is a problem later on, however. In the stream generation loop, you're supposed to do the increment and swap operations before you fetch a byte from S[].

for (int k = 0; k < plaintext.size(); k++)
{
   i = (i+1) % 256;                             // increment S[] index
   runningTotal = (runningTotal + S[i]) % 256;  // swap bytes
   swap(&S[i], &S[runningTotal]);

   temp = (S[i] + S[runningTotal]) % 256;       // fetch byte from S and
   cipherText[k] = plaintext[k] ^ S[temp];      // XOR with plaintext
}

NOTE: Although unrelated to your question, your code could be made a lot tidier by using unsigned char values instead of ints. That would eliminate the % 256 instructions that are littered all over the place. (But be careful during initialization, because i<256 will always be true if i is an unsigned char.)

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