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rc4.c
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1 /*
2  * RC4 encryption/decryption/pseudo-random number generator
3  * Copyright (c) 2007 Reimar Doeffinger
4  *
5  * loosely based on LibTomCrypt by Tom St Denis
6  *
7  * This file is part of FFmpeg.
8  *
9  * FFmpeg is free software; you can redistribute it and/or
10  * modify it under the terms of the GNU Lesser General Public
11  * License as published by the Free Software Foundation; either
12  * version 2.1 of the License, or (at your option) any later version.
13  *
14  * FFmpeg is distributed in the hope that it will be useful,
15  * but WITHOUT ANY WARRANTY; without even the implied warranty of
16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
17  * Lesser General Public License for more details.
18  *
19  * You should have received a copy of the GNU Lesser General Public
20  * License along with FFmpeg; if not, write to the Free Software
21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
22  */
23 #include "avutil.h"
24 #include "common.h"
25 #include "rc4.h"
26 
27 typedef struct AVRC4 AVRC4;
28 
29 int av_rc4_init(AVRC4 *r, const uint8_t *key, int key_bits, int decrypt) {
30  int i, j;
31  uint8_t y;
32  uint8_t *state = r->state;
33  int keylen = key_bits >> 3;
34  if (key_bits & 7)
35  return -1;
36  for (i = 0; i < 256; i++)
37  state[i] = i;
38  y = 0;
39  // j is i % keylen
40  for (j = 0, i = 0; i < 256; i++, j++) {
41  if (j == keylen) j = 0;
42  y += state[i] + key[j];
43  FFSWAP(uint8_t, state[i], state[y]);
44  }
45  r->x = 1;
46  r->y = state[1];
47  return 0;
48 }
49 
50 void av_rc4_crypt(AVRC4 *r, uint8_t *dst, const uint8_t *src, int count, uint8_t *iv, int decrypt) {
51  uint8_t x = r->x, y = r->y;
52  uint8_t *state = r->state;
53  while (count-- > 0) {
54  uint8_t sum = state[x] + state[y];
55  FFSWAP(uint8_t, state[x], state[y]);
56  *dst++ = src ? *src++ ^ state[sum] : state[sum];
57  x++;
58  y += state[x];
59  }
60  r->x = x; r->y = y;
61 }