Hacking - The Art of Exploitation, 2nd Edition

Cryptology 443

Again, the correct key byte is determined. Of course, for the sake of

demonstration, values for X have been strategically picked. To give you a

true sense of the statistical nature of the attack against a full RC4 imple-

mentation, the following source code has been included:

fms.c

#include <stdio.h>

/ RC4 stream cipher /
int RC4(int IV, int key) {
int K[256];
int S[256];
int seed[16];
int i, j, k, t;

//Seed = IV + key;
for(k=0; k<3; k++)
seed[k] = IV[k];
for(k=0; k<13; k++)
seed[k+3] = key[k];

// -= Key Scheduling Algorithm (KSA) =-
//Initialize the arrays.
for(k=0; k<256; k++) {
S[k] = k;
K[k] = seed[k%16];
}

j=0;
for(i=0; i < 256; i++) {
j = (j + S[i] + K[i])%256;
t=S[i]; S[i]=S[j]; S[j]=t; // Swap(S[i], S[j]);
}

// First step of PRGA for first keystream byte
i = 0;
j = 0;

i = i + 1;
j = j + S[i];

t=S[i]; S[i]=S[j]; S[j]=t; // Swap(S[i], S[j]);

k = (S[i] + S[j])%256;

return S[k];
}

int main(int argc, char *argv[]) {
int K[256];
int S[256];

int IV[3];