rc4.c revision 8d520ff1dc2da35cdca849e982051b86468016d8 
1/*
2 * RC4 stream cipher
3 * Copyright (c) 2002-2005, Jouni Malinen <j@w1.fi>
4 *
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License version 2 as
7 * published by the Free Software Foundation.
8 *
9 * Alternatively, this software may be distributed under the terms of BSD
10 * license.
11 *
12 * See README and COPYING for more details.
13 */
14
15#include "includes.h"
16
17#include "common.h"
18#include "crypto.h"
19
20#define S_SWAP(a,b) do { u8 t = S[a]; S[a] = S[b]; S[b] = t; } while(0)
21
22int rc4_skip(const u8 *key, size_t keylen, size_t skip,
23	     u8 *data, size_t data_len)
24{
25	u32 i, j, k;
26	u8 S[256], *pos;
27	size_t kpos;
28
29	/* Setup RC4 state */
30	for (i = 0; i < 256; i++)
31		S[i] = i;
32	j = 0;
33	kpos = 0;
34	for (i = 0; i < 256; i++) {
35		j = (j + S[i] + key[kpos]) & 0xff;
36		kpos++;
37		if (kpos >= keylen)
38			kpos = 0;
39		S_SWAP(i, j);
40	}
41
42	/* Skip the start of the stream */
43	i = j = 0;
44	for (k = 0; k < skip; k++) {
45		i = (i + 1) & 0xff;
46		j = (j + S[i]) & 0xff;
47		S_SWAP(i, j);
48	}
49
50	/* Apply RC4 to data */
51	pos = data;
52	for (k = 0; k < data_len; k++) {
53		i = (i + 1) & 0xff;
54		j = (j + S[i]) & 0xff;
55		S_SWAP(i, j);
56		*pos++ ^= S[(S[i] + S[j]) & 0xff];
57	}
58
59	return 0;
60}
61