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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