libavformat/asfcrypt.c
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 /*
  * ASF decryption
  * Copyright (c) 2007 Reimar Doeffinger
  * This is a rewrite of code contained in freeme/freeme2
  *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
  * FFmpeg is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
  *
  * You should have received a copy of the GNU Lesser General Public
  * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
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 #include "libavutil/bswap.h"
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 #include "libavutil/common.h"
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 #include "libavutil/des.h"
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 #include "libavutil/intreadwrite.h"
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 #include "libavutil/rc4.h"
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 #include "asfcrypt.h"
 
 /**
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  * @brief find multiplicative inverse modulo 2 ^ 32
  * @param v number to invert, must be odd!
  * @return number so that result * v = 1 (mod 2^32)
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  */
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 static uint32_t inverse(uint32_t v)
 {
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     // v ^ 3 gives the inverse (mod 16), could also be implemented
     // as table etc. (only lowest 4 bits matter!)
     uint32_t inverse = v * v * v;
     // uses a fixpoint-iteration that doubles the number
     // of correct lowest bits each time
     inverse *= 2 - v * inverse;
     inverse *= 2 - v * inverse;
     inverse *= 2 - v * inverse;
     return inverse;
 }
 
 /**
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  * @brief read keys from keybuf into keys
  * @param keybuf buffer containing the keys
  * @param keys output key array containing the keys for encryption in
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  *             native endianness
  */
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 static void multiswap_init(const uint8_t keybuf[48], uint32_t keys[12])
 {
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     int i;
     for (i = 0; i < 12; i++)
         keys[i] = AV_RL32(keybuf + (i << 2)) | 1;
 }
 
 /**
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  * @brief invert the keys so that encryption become decryption keys and
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  *        the other way round.
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  * @param keys key array of ints to invert
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  */
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 static void multiswap_invert_keys(uint32_t keys[12])
 {
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     int i;
     for (i = 0; i < 5; i++)
         keys[i] = inverse(keys[i]);
     for (i = 6; i < 11; i++)
         keys[i] = inverse(keys[i]);
 }
 
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 static uint32_t multiswap_step(const uint32_t keys[12], uint32_t v)
 {
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     int i;
     v *= keys[0];
     for (i = 1; i < 5; i++) {
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         v  = (v >> 16) | (v << 16);
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         v *= keys[i];
     }
     v += keys[5];
     return v;
 }
 
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 static uint32_t multiswap_inv_step(const uint32_t keys[12], uint32_t v)
 {
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     int i;
     v -= keys[5];
     for (i = 4; i > 0; i--) {
         v *= keys[i];
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         v  = (v >> 16) | (v << 16);
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     }
     v *= keys[0];
     return v;
 }
 
 /**
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  * @brief "MultiSwap" encryption
  * @param keys 32 bit numbers in machine endianness,
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  *             0-4 and 6-10 must be inverted from decryption
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  * @param key another key, this one must be the same for the decryption
  * @param data data to encrypt
  * @return encrypted data
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  */
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 static uint64_t multiswap_enc(const uint32_t keys[12],
                               uint64_t key, uint64_t data)
 {
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     uint32_t a = data;
     uint32_t b = data >> 32;
     uint32_t c;
     uint32_t tmp;
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     a  += key;
     tmp = multiswap_step(keys, a);
     b  += tmp;
     c   = (key >> 32) + tmp;
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     tmp = multiswap_step(keys + 6, b);
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     c  += tmp;
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     return ((uint64_t)c << 32) | tmp;
 }
 
 /**
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  * @brief "MultiSwap" decryption
  * @param keys 32 bit numbers in machine endianness,
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  *             0-4 and 6-10 must be inverted from encryption
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  * @param key another key, this one must be the same as for the encryption
  * @param data data to decrypt
  * @return decrypted data
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  */
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 static uint64_t multiswap_dec(const uint32_t keys[12],
                               uint64_t key, uint64_t data)
 {
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     uint32_t a;
     uint32_t b;
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     uint32_t c   = data >> 32;
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     uint32_t tmp = data;
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     c  -= tmp;
     b   = multiswap_inv_step(keys + 6, tmp);
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     tmp = c - (key >> 32);
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     b  -= tmp;
     a   = multiswap_inv_step(keys, tmp);
     a  -= key;
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     return ((uint64_t)b << 32) | a;
 }
 
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 void ff_asfcrypt_dec(const uint8_t key[20], uint8_t *data, int len)
 {
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     struct AVDES des;
     struct AVRC4 rc4;
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     int num_qwords      = len >> 3;
     uint8_t *qwords     = data;
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     uint64_t rc4buff[8] = { 0 };
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     uint64_t packetkey;
     uint32_t ms_keys[12];
     uint64_t ms_state;
     int i;
     if (len < 16) {
         for (i = 0; i < len; i++)
             data[i] ^= key[i];
         return;
     }
 
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     av_rc4_init(&rc4, key, 12 * 8, 1);
     av_rc4_crypt(&rc4, (uint8_t *)rc4buff, NULL, sizeof(rc4buff), NULL, 1);
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     multiswap_init((uint8_t *)rc4buff, ms_keys);
 
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     packetkey  = AV_RN64(&qwords[num_qwords * 8 - 8]);
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     packetkey ^= rc4buff[7];
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     av_des_init(&des, key + 12, 64, 1);
     av_des_crypt(&des, (uint8_t *)&packetkey, (uint8_t *)&packetkey, 1, NULL, 1);
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     packetkey ^= rc4buff[6];
 
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     av_rc4_init(&rc4, (uint8_t *)&packetkey, 64, 1);
     av_rc4_crypt(&rc4, data, data, len, NULL, 1);
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     ms_state = 0;
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     for (i = 0; i < num_qwords - 1; i++, qwords += 8)
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         ms_state = multiswap_enc(ms_keys, ms_state, AV_RL64(qwords));
     multiswap_invert_keys(ms_keys);
     packetkey = (packetkey << 32) | (packetkey >> 32);
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     packetkey = av_le2ne64(packetkey);
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     packetkey = multiswap_dec(ms_keys, ms_state, packetkey);
     AV_WL64(qwords, packetkey);
 }