/*

 * RC4 stream cipher

 * Copyright (c) 2002-2005, Jouni Malinen <j@w1.fi>

 *

 * This software may be distributed under the terms of the BSD license.

 * See README for more details.

 */

#include "includes.h"

#include "common.h"

#include "crypto.h"

#define S_SWAP(a,b) do { u8 t = S[a]; S[a] = S[b]; S[b] = t; } while(0)

int rc4_skip(const u8 *key, size_t keylen, size_t skip,

       u8 *data, size_t data_len)

{

  u32 i, j, k;

  u8 S[256], *pos;

  size_t kpos;

  /* Setup RC4 state */

  for (i = 0; i < 256; i++)

    S[i] = i;

  j = 0;

  kpos = 0;

  for (i = 0; i < 256; i++) {

    j = (j + S[i] + key[kpos]) & 0xff;

    kpos++;

    if (kpos >= keylen)

      kpos = 0;

    S_SWAP(i, j);

  }

  /* Skip the start of the stream */

  i = j = 0;

  for (k = 0; k < skip; k++) {

    i = (i + 1) & 0xff;

    j = (j + S[i]) & 0xff;

    S_SWAP(i, j);

  }

  /* Apply RC4 to data */

  pos = data;

  for (k = 0; k < data_len; k++) {

    i = (i + 1) & 0xff;

    j = (j + S[i]) & 0xff;

    S_SWAP(i, j);

    *pos++ ^= S[(S[i] + S[j]) & 0xff];

  }

  return 0;

}