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 "common.h"
  24 #include "rc4.h"
  25
  26 void ff_rc4_enc(const uint8_t *key, int keylen, uint8_t *data, int datalen) {
  27     int i, j;
  28     uint8_t x, y;
  29     uint8_t state[256];
  30     for (i = 0; i < 256; i++)
  31         state[i] = i;
  32     y = 0;
  33     // j is i % keylen
  34     for (j = 0, i = 0; i < 256; i++, j++) {
  35         if (j == keylen) j = 0;
  36         y += state[i] + key[j];
  37         FFSWAP(uint8_t, state[i], state[y]);
  38     }
  39     // state initialized, now do the real encryption
  40     x = 1; y = state[1];
  41     while (datalen-- > 0) {
  42         uint8_t sum = state[x] + state[y];
  43         FFSWAP(uint8_t, state[x], state[y]);
  44         *data++ ^= state[sum];
  45         x++;
  46         y += state[x];
  47     }
  48 }