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annotate libtomcrypt/src/modes/xts/xts_decrypt.c @ 1690:2a06ef4ba80d

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author Matt Johnston <matt@ucc.asn.au>
date Tue, 26 May 2020 23:16:11 +0800
parents 6dba84798cd5
children
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1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
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2 *
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3 * LibTomCrypt is a library that provides various cryptographic
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4 * algorithms in a highly modular and flexible manner.
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5 *
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6 * The library is free for all purposes without any express
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7 * guarantee it works.
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8 */
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9 #include "tomcrypt.h"
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10
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11 /**
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12 Source donated by Elliptic Semiconductor Inc (www.ellipticsemi.com) to the LibTom Projects
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13 */
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14
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15 #ifdef LTC_XTS_MODE
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16
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17 static int _tweak_uncrypt(const unsigned char *C, unsigned char *P, unsigned char *T, symmetric_xts *xts)
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18 {
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19 unsigned long x;
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20 int err;
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21
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22 /* tweak encrypt block i */
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23 #ifdef LTC_FAST
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24 for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
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25 *(LTC_FAST_TYPE_PTR_CAST(&P[x])) = *(LTC_FAST_TYPE_PTR_CAST(&C[x])) ^ *(LTC_FAST_TYPE_PTR_CAST(&T[x]));
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26 }
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27 #else
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28 for (x = 0; x < 16; x++) {
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29 P[x] = C[x] ^ T[x];
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30 }
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31 #endif
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32
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33 err = cipher_descriptor[xts->cipher].ecb_decrypt(P, P, &xts->key1);
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34
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35 #ifdef LTC_FAST
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36 for (x = 0; x < 16; x += sizeof(LTC_FAST_TYPE)) {
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37 *(LTC_FAST_TYPE_PTR_CAST(&P[x])) ^= *(LTC_FAST_TYPE_PTR_CAST(&T[x]));
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38 }
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39 #else
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40 for (x = 0; x < 16; x++) {
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41 P[x] = P[x] ^ T[x];
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42 }
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43 #endif
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44
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45 /* LFSR the tweak */
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46 xts_mult_x(T);
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47
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48 return err;
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49 }
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50
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51 /** XTS Decryption
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52 @param ct [in] Ciphertext
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53 @param ptlen Length of plaintext (and ciphertext)
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54 @param pt [out] Plaintext
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55 @param tweak [in] The 128--bit encryption tweak (e.g. sector number)
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56 @param xts The XTS structure
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57 Returns CRYPT_OK upon success
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58 */
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59 int xts_decrypt(const unsigned char *ct, unsigned long ptlen, unsigned char *pt, unsigned char *tweak,
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60 symmetric_xts *xts)
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61 {
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62 unsigned char PP[16], CC[16], T[16];
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63 unsigned long i, m, mo, lim;
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64 int err;
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65
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66 /* check inputs */
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67 LTC_ARGCHK(pt != NULL);
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68 LTC_ARGCHK(ct != NULL);
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69 LTC_ARGCHK(tweak != NULL);
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70 LTC_ARGCHK(xts != NULL);
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71
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72 /* check if valid */
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73 if ((err = cipher_is_valid(xts->cipher)) != CRYPT_OK) {
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74 return err;
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75 }
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76
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77 /* get number of blocks */
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78 m = ptlen >> 4;
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79 mo = ptlen & 15;
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80
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81 /* must have at least one full block */
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82 if (m == 0) {
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83 return CRYPT_INVALID_ARG;
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84 }
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85
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86 if (mo == 0) {
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87 lim = m;
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88 } else {
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89 lim = m - 1;
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90 }
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91
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92 if (cipher_descriptor[xts->cipher].accel_xts_decrypt && lim > 0) {
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93
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94 /* use accelerated decryption for whole blocks */
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95 if ((err = cipher_descriptor[xts->cipher].accel_xts_decrypt(ct, pt, lim, tweak, &xts->key1, &xts->key2)) !=
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96 CRYPT_OK) {
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97 return err;
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98 }
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99 ct += lim * 16;
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100 pt += lim * 16;
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101
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102 /* tweak is encrypted on output */
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103 XMEMCPY(T, tweak, sizeof(T));
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104 } else {
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105 /* encrypt the tweak */
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106 if ((err = cipher_descriptor[xts->cipher].ecb_encrypt(tweak, T, &xts->key2)) != CRYPT_OK) {
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107 return err;
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108 }
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109
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110 for (i = 0; i < lim; i++) {
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111 if ((err = _tweak_uncrypt(ct, pt, T, xts)) != CRYPT_OK) {
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112 return err;
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113 }
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114 ct += 16;
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115 pt += 16;
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116 }
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117 }
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118
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119 /* if ptlen not divide 16 then */
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120 if (mo > 0) {
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121 XMEMCPY(CC, T, 16);
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122 xts_mult_x(CC);
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123
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124 /* PP = tweak decrypt block m-1 */
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125 if ((err = _tweak_uncrypt(ct, PP, CC, xts)) != CRYPT_OK) {
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126 return err;
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127 }
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128
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129 /* Pm = first ptlen % 16 bytes of PP */
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130 for (i = 0; i < mo; i++) {
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131 CC[i] = ct[16 + i];
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132 pt[16 + i] = PP[i];
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133 }
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134 for (; i < 16; i++) {
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135 CC[i] = PP[i];
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136 }
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137
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138 /* Pm-1 = Tweak uncrypt CC */
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139 if ((err = _tweak_uncrypt(CC, pt, T, xts)) != CRYPT_OK) {
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140 return err;
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141 }
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142 }
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143
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144 /* Decrypt the tweak back */
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145 if ((err = cipher_descriptor[xts->cipher].ecb_decrypt(T, tweak, &xts->key2)) != CRYPT_OK) {
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146 return err;
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147 }
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148
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149 return CRYPT_OK;
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150 }
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151
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152 #endif
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153
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154 /* ref: $Format:%D$ */
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155 /* git commit: $Format:%H$ */
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156 /* commit time: $Format:%ai$ */