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 "rc4.h"
19
20#define S_SWAP(a,b) do { u8 t = S[a]; S[a] = S[b]; S[b] = t; } while(0)
21
22/**
23 * rc4 - XOR RC4 stream to given data with skip-stream-start
24 * @key: RC4 key
25 * @keylen: RC4 key length
26 * @skip: number of bytes to skip from the beginning of the RC4 stream
27 * @data: data to be XOR'ed with RC4 stream
28 * @data_len: buf length
29 *
30 * Generate RC4 pseudo random stream for the given key, skip beginning of the
31 * stream, and XOR the end result with the data buffer to perform RC4
32 * encryption/decryption.
33 */
34void rc4_skip(const u8 *key, size_t keylen, size_t skip,
35	      u8 *data, size_t data_len)
36{
37	u32 i, j, k;
38	u8 S[256], *pos;
39	size_t kpos;
40
41	/* Setup RC4 state */
42	for (i = 0; i < 256; i++)
43		S[i] = i;
44	j = 0;
45	kpos = 0;
46	for (i = 0; i < 256; i++) {
47		j = (j + S[i] + key[kpos]) & 0xff;
48		kpos++;
49		if (kpos >= keylen)
50			kpos = 0;
51		S_SWAP(i, j);
52	}
53
54	/* Skip the start of the stream */
55	i = j = 0;
56	for (k = 0; k < skip; k++) {
57		i = (i + 1) & 0xff;
58		j = (j + S[i]) & 0xff;
59		S_SWAP(i, j);
60	}
61
62	/* Apply RC4 to data */
63	pos = data;
64	for (k = 0; k < data_len; k++) {
65		i = (i + 1) & 0xff;
66		j = (j + S[i]) & 0xff;
67		S_SWAP(i, j);
68		*pos++ ^= S[(S[i] + S[j]) & 0xff];
69	}
70}
71