rc4.c revision c5ec7f57ead87efa365800228aa0b09a12d9e6c4 
1/*
2 * RC4 stream cipher
3 * Copyright (c) 2002-2005, Jouni Malinen <j@w1.fi>
4 *
5 * This software may be distributed under the terms of the BSD license.
6 * See README for more details.
7 */
8
9#include "includes.h"
10
11#include "common.h"
12#include "crypto.h"
13
14#define S_SWAP(a,b) do { u8 t = S[a]; S[a] = S[b]; S[b] = t; } while(0)
15
16int rc4_skip(const u8 *key, size_t keylen, size_t skip,
17	     u8 *data, size_t data_len)
18{
19	u32 i, j, k;
20	u8 S[256], *pos;
21	size_t kpos;
22
23	/* Setup RC4 state */
24	for (i = 0; i < 256; i++)
25		S[i] = i;
26	j = 0;
27	kpos = 0;
28	for (i = 0; i < 256; i++) {
29		j = (j + S[i] + key[kpos]) & 0xff;
30		kpos++;
31		if (kpos >= keylen)
32			kpos = 0;
33		S_SWAP(i, j);
34	}
35
36	/* Skip the start of the stream */
37	i = j = 0;
38	for (k = 0; k < skip; k++) {
39		i = (i + 1) & 0xff;
40		j = (j + S[i]) & 0xff;
41		S_SWAP(i, j);
42	}
43
44	/* Apply RC4 to data */
45	pos = data;
46	for (k = 0; k < data_len; k++) {
47		i = (i + 1) & 0xff;
48		j = (j + S[i]) & 0xff;
49		S_SWAP(i, j);
50		*pos++ ^= S[(S[i] + S[j]) & 0xff];
51	}
52
53	return 0;
54}
55