## ffmpeg / libavformat / asfcrypt.c @ 883fe4d5

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1 | 72be7db4 | Reimar DÃ¶ffinger | ```
/*
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2 | ```
* ASF decryption
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3 | ```
* Copyright (c) 2007 Reimar Doeffinger
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4 | ```
* This is a rewrite of code contained in freeme/freeme2
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5 | ```
*
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6 | ```
* This file is part of FFmpeg.
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7 | ```
*
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8 | ```
* FFmpeg is free software; you can redistribute it and/or
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9 | ```
* modify it under the terms of the GNU Lesser General Public
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10 | ```
* License as published by the Free Software Foundation; either
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11 | ```
* version 2.1 of the License, or (at your option) any later version.
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12 | ```
*
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13 | ```
* FFmpeg is distributed in the hope that it will be useful,
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14 | ```
* but WITHOUT ANY WARRANTY; without even the implied warranty of
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15 | ```
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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16 | ```
* Lesser General Public License for more details.
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17 | ```
*
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18 | ```
* You should have received a copy of the GNU Lesser General Public
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19 | ```
* License along with FFmpeg; if not, write to the Free Software
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20 | ```
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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21 | ```
*/
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22 | 245976da | Diego Biurrun | |

23 | #include "libavutil/common.h" |
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24 | #include "libavutil/intreadwrite.h" |
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25 | #include "libavutil/bswap.h" |
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26 | #include "libavutil/des.h" |
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27 | #include "libavutil/rc4.h" |
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28 | 72be7db4 | Reimar DÃ¶ffinger | #include "asfcrypt.h" |

29 | |||

30 | ```
/**
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31 | ```
* \brief find multiplicative inverse modulo 2 ^ 32
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32 | ```
* \param v number to invert, must be odd!
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33 | ```
* \return number so that result * v = 1 (mod 2^32)
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34 | ```
*/
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35 | ```
static uint32_t inverse(uint32_t v) {
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36 | ```
// v ^ 3 gives the inverse (mod 16), could also be implemented
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37 | ```
// as table etc. (only lowest 4 bits matter!)
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38 | uint32_t inverse = v * v * v; |
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39 | ```
// uses a fixpoint-iteration that doubles the number
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40 | ```
// of correct lowest bits each time
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41 | ```
inverse *= 2 - v * inverse;
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42 | ```
inverse *= 2 - v * inverse;
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43 | ```
inverse *= 2 - v * inverse;
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44 | ```
return inverse;
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45 | } |
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46 | |||

47 | ```
/**
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48 | ```
* \brief read keys from keybuf into keys
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49 | ```
* \param keybuf buffer containing the keys
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50 | ```
* \param keys output key array containing the keys for encryption in
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51 | ```
* native endianness
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52 | ```
*/
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53 | static void multiswap_init(const uint8_t keybuf[48], uint32_t keys[12]) { |
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54 | ```
int i;
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55 | for (i = 0; i < 12; i++) |
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56 | keys[i] = AV_RL32(keybuf + (i << 2)) | 1; |
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57 | } |
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58 | |||

59 | ```
/**
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60 | ```
* \brief invert the keys so that encryption become decryption keys and
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61 | ```
* the other way round.
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62 | ```
* \param keys key array of ints to invert
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63 | ```
*/
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64 | static void multiswap_invert_keys(uint32_t keys[12]) { |
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65 | ```
int i;
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66 | for (i = 0; i < 5; i++) |
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67 | keys[i] = inverse(keys[i]); |
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68 | for (i = 6; i < 11; i++) |
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69 | keys[i] = inverse(keys[i]); |
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70 | } |
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71 | |||

72 | static uint32_t multiswap_step(const uint32_t keys[12], uint32_t v) { |
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73 | ```
int i;
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74 | ```
v *= keys[0];
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75 | for (i = 1; i < 5; i++) { |
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76 | v = (v >> 16) | (v << 16); |
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77 | v *= keys[i]; |
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78 | } |
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79 | ```
v += keys[5];
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80 | ```
return v;
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81 | } |
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82 | |||

83 | static uint32_t multiswap_inv_step(const uint32_t keys[12], uint32_t v) { |
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84 | ```
int i;
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85 | ```
v -= keys[5];
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86 | for (i = 4; i > 0; i--) { |
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87 | v *= keys[i]; |
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88 | v = (v >> 16) | (v << 16); |
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89 | } |
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90 | ```
v *= keys[0];
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91 | ```
return v;
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92 | } |
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93 | |||

94 | ```
/**
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95 | ```
* \brief "MultiSwap" encryption
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96 | ```
* \param keys 32 bit numbers in machine endianness,
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97 | ```
* 0-4 and 6-10 must be inverted from decryption
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98 | ```
* \param key another key, this one must be the same for the decryption
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99 | ```
* \param data data to encrypt
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100 | ```
* \return encrypted data
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101 | ```
*/
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102 | static uint64_t multiswap_enc(const uint32_t keys[12], uint64_t key, uint64_t data) { |
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103 | uint32_t a = data; |
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104 | ```
uint32_t b = data >> 32;
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105 | uint32_t c; |
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106 | uint32_t tmp; |
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107 | a += key; |
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108 | tmp = multiswap_step(keys , a); |
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109 | b += tmp; |
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110 | ```
c = (key >> 32) + tmp;
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111 | ```
tmp = multiswap_step(keys + 6, b);
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112 | c += tmp; |
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113 | return ((uint64_t)c << 32) | tmp; |
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114 | } |
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115 | |||

116 | ```
/**
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117 | ```
* \brief "MultiSwap" decryption
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118 | ```
* \param keys 32 bit numbers in machine endianness,
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119 | ```
* 0-4 and 6-10 must be inverted from encryption
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120 | ```
* \param key another key, this one must be the same as for the encryption
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121 | ```
* \param data data to decrypt
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122 | ```
* \return decrypted data
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123 | ```
*/
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124 | static uint64_t multiswap_dec(const uint32_t keys[12], uint64_t key, uint64_t data) { |
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125 | uint32_t a; |
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126 | uint32_t b; |
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127 | ```
uint32_t c = data >> 32;
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128 | uint32_t tmp = data; |
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129 | c -= tmp; |
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130 | ```
b = multiswap_inv_step(keys + 6, tmp);
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131 | ```
tmp = c - (key >> 32);
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132 | b -= tmp; |
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133 | a = multiswap_inv_step(keys , tmp); |
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134 | a -= key; |
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135 | return ((uint64_t)b << 32) | a; |
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136 | } |
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137 | |||

138 | void ff_asfcrypt_dec(const uint8_t key[20], uint8_t *data, int len) { |
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139 | int num_qwords = len >> 3; |
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140 | uint64_t *qwords = (uint64_t *)data; |
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141 | ```
uint64_t rc4buff[8];
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142 | uint64_t packetkey; |
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143 | ```
uint32_t ms_keys[12];
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144 | uint64_t ms_state; |
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145 | ```
int i;
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146 | if (len < 16) { |
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147 | for (i = 0; i < len; i++) |
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148 | data[i] ^= key[i]; |
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149 | ```
return;
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150 | } |
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151 | |||

152 | memset(rc4buff, 0, sizeof(rc4buff)); |
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153 | ff_rc4_enc(key, 12, (uint8_t *)rc4buff, sizeof(rc4buff)); |
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154 | multiswap_init((uint8_t *)rc4buff, ms_keys); |
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155 | |||

156 | ```
packetkey = qwords[num_qwords - 1];
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157 | ```
packetkey ^= rc4buff[7];
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158 | packetkey = be2me_64(packetkey); |
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159 | packetkey = ff_des_encdec(packetkey, AV_RB64(key + 12), 1); |
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160 | packetkey = be2me_64(packetkey); |
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161 | ```
packetkey ^= rc4buff[6];
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162 | |||

163 | ```
ff_rc4_enc((uint8_t *)&packetkey, 8, data, len);
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164 | |||

165 | ```
ms_state = 0;
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166 | for (i = 0; i < num_qwords - 1; i++, qwords++) |
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167 | ms_state = multiswap_enc(ms_keys, ms_state, AV_RL64(qwords)); |
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168 | multiswap_invert_keys(ms_keys); |
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169 | packetkey = (packetkey << 32) | (packetkey >> 32); |
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170 | packetkey = le2me_64(packetkey); |
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171 | packetkey = multiswap_dec(ms_keys, ms_state, packetkey); |
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172 | AV_WL64(qwords, packetkey); |
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173 | } |