Problem - Kevin Tafuro

The “official” RC4 key setup function isn’t generally secure without additional work,
but we need to have it around anyway:
#include
void RC4_set_key(RC4_CTX *c, size_t keybytes, unsigned char *key) {
int i, j;
unsigned char keyarr[256], swap;
c->i = c->j = 0;
for (i = j = 0; i < 256; i++, j = (j + 1) % keybytes) {
c->sbox[i] = i;
keyarr[i] = key[j];
}
for (i = j = 0; i < 256; i++) {
j += c->sbox[i] + keyarr[i];
j %= 256;
swap = c->sbox[i];
c->sbox[i] = c->sbox[j];
c->sbox[j] = swap;
}
}
The RC4 function has the following arguments:
c
Pointer to an RC4_CTX object.
n
Number of bytes to encrypt.
in
Buffer to encrypt.
out
Output buffer.
void RC4(RC4_CTX *c, size_t n, unsigned char *in, unsigned char *out) {
unsigned char swap;
while (n--) {
c->j += c->sbox[++c->i];
swap = c->sbox[c->i];
c->sbox[c->i] = c->sbox[c->j];
c->sbox[c->j] = swap;
swap = c->sbox[c->i] + c->sbox[c->j];
*out++ = *in++ ^ c->sbox[swap];
}
}
That’s it for an RC4 implementation. This function can be used incrementally or as
an “all-in-one” solution.
Now let’s look at how to key RC4 properly.
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Setting Up and Using RC4 | 235