libavformat/asfcrypt.c

   1 /*
   2  * ASF decryption
   3  * Copyright (c) 2007 Reimar Doeffinger
   4  * This is a rewrite of code contained in freeme/freeme2
   5  *
   6  * This file is part of FFmpeg.
   7  *
   8  * FFmpeg is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * FFmpeg is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with FFmpeg; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #include "libavutil/common.h"
  24 #include "libavutil/intreadwrite.h"
  25 #include "libavutil/bswap.h"
  26 #include "libavutil/des.h"
  27 #include "libavutil/rc4.h"
  28 #include "asfcrypt.h"
  29
  30 /**
  31  * \brief find multiplicative inverse modulo 2 ^ 32
  32  * \param v number to invert, must be odd!
  33  * \return number so that result * v = 1 (mod 2^32)
  34  */
  35 static uint32_t inverse(uint32_t v) {
  36     // v ^ 3 gives the inverse (mod 16), could also be implemented
  37     // as table etc. (only lowest 4 bits matter!)
  38     uint32_t inverse = v * v * v;
  39     // uses a fixpoint-iteration that doubles the number
  40     // of correct lowest bits each time
  41     inverse *= 2 - v * inverse;
  42     inverse *= 2 - v * inverse;
  43     inverse *= 2 - v * inverse;
  44     return inverse;
  45 }
  46
  47 /**
  48  * \brief read keys from keybuf into keys
  49  * \param keybuf buffer containing the keys
  50  * \param keys output key array containing the keys for encryption in
  51  *             native endianness
  52  */
  53 static void multiswap_init(const uint8_t keybuf[48], uint32_t keys[12]) {
  54     int i;
  55     for (i = 0; i < 12; i++)
  56         keys[i] = AV_RL32(keybuf + (i << 2)) | 1;
  57 }
  58
  59 /**
  60  * \brief invert the keys so that encryption become decryption keys and
  61  *        the other way round.
  62  * \param keys key array of ints to invert
  63  */
  64 static void multiswap_invert_keys(uint32_t keys[12]) {
  65     int i;
  66     for (i = 0; i < 5; i++)
  67         keys[i] = inverse(keys[i]);
  68     for (i = 6; i < 11; i++)
  69         keys[i] = inverse(keys[i]);
  70 }
  71
  72 static uint32_t multiswap_step(const uint32_t keys[12], uint32_t v) {
  73     int i;
  74     v *= keys[0];
  75     for (i = 1; i < 5; i++) {
  76         v = (v >> 16) | (v << 16);
  77         v *= keys[i];
  78     }
  79     v += keys[5];
  80     return v;
  81 }
  82
  83 static uint32_t multiswap_inv_step(const uint32_t keys[12], uint32_t v) {
  84     int i;
  85     v -= keys[5];
  86     for (i = 4; i > 0; i--) {
  87         v *= keys[i];
  88         v = (v >> 16) | (v << 16);
  89     }
  90     v *= keys[0];
  91     return v;
  92 }
  93
  94 /**
  95  * \brief "MultiSwap" encryption
  96  * \param keys 32 bit numbers in machine endianness,
  97  *             0-4 and 6-10 must be inverted from decryption
  98  * \param key another key, this one must be the same for the decryption
  99  * \param data data to encrypt
 100  * \return encrypted data
 101  */
 102 static uint64_t multiswap_enc(const uint32_t keys[12], uint64_t key, uint64_t data) {
 103     uint32_t a = data;
 104     uint32_t b = data >> 32;
 105     uint32_t c;
 106     uint32_t tmp;
 107     a += key;
 108     tmp = multiswap_step(keys    , a);
 109     b += tmp;
 110     c = (key >> 32) + tmp;
 111     tmp = multiswap_step(keys + 6, b);
 112     c += tmp;
 113     return ((uint64_t)c << 32) | tmp;
 114 }
 115
 116 /**
 117  * \brief "MultiSwap" decryption
 118  * \param keys 32 bit numbers in machine endianness,
 119  *             0-4 and 6-10 must be inverted from encryption
 120  * \param key another key, this one must be the same as for the encryption
 121  * \param data data to decrypt
 122  * \return decrypted data
 123  */
 124 static uint64_t multiswap_dec(const uint32_t keys[12], uint64_t key, uint64_t data) {
 125     uint32_t a;
 126     uint32_t b;
 127     uint32_t c = data >> 32;
 128     uint32_t tmp = data;
 129     c -= tmp;
 130     b = multiswap_inv_step(keys + 6, tmp);
 131     tmp = c - (key >> 32);
 132     b -= tmp;
 133     a = multiswap_inv_step(keys    , tmp);
 134     a -= key;
 135     return ((uint64_t)b << 32) | a;
 136 }
 137
 138 void ff_asfcrypt_dec(const uint8_t key[20], uint8_t *data, int len) {
 139     int num_qwords = len >> 3;
 140     uint64_t *qwords = (uint64_t *)data;
 141     uint64_t rc4buff[8];
 142     uint64_t packetkey;
 143     uint32_t ms_keys[12];
 144     uint64_t ms_state;
 145     int i;
 146     if (len < 16) {
 147         for (i = 0; i < len; i++)
 148             data[i] ^= key[i];
 149         return;
 150     }
 151
 152     memset(rc4buff, 0, sizeof(rc4buff));
 153     ff_rc4_enc(key, 12, (uint8_t *)rc4buff, sizeof(rc4buff));
 154     multiswap_init((uint8_t *)rc4buff, ms_keys);
 155
 156     packetkey = qwords[num_qwords - 1];
 157     packetkey ^= rc4buff[7];
 158     packetkey = be2me_64(packetkey);
 159     packetkey = ff_des_encdec(packetkey, AV_RB64(key + 12), 1);
 160     packetkey = be2me_64(packetkey);
 161     packetkey ^= rc4buff[6];
 162
 163     ff_rc4_enc((uint8_t *)&packetkey, 8, data, len);
 164
 165     ms_state = 0;
 166     for (i = 0; i < num_qwords - 1; i++, qwords++)
 167         ms_state = multiswap_enc(ms_keys, ms_state, AV_RL64(qwords));
 168     multiswap_invert_keys(ms_keys);
 169     packetkey = (packetkey << 32) | (packetkey >> 32);
 170     packetkey = le2me_64(packetkey);
 171     packetkey = multiswap_dec(ms_keys, ms_state, packetkey);
 172     AV_WL64(qwords, packetkey);
 173 }