libavutil/rc4.c

   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 }