Mercurial > dropbear

annotate libtommath/bn_mp_log_u32.c @ 1788:1fc0012b9c38

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Fix handling of replies to global requests (#112) The current code assumes that all global requests want / need a reply. This isn't always true and the request itself indicates if it wants a reply or not. It causes a specific problem with [email protected] messages. These are sent by OpenSSH after authentication to inform the client of potential other host keys for the host. This can be used to add a new type of host key or to rotate host keys. The initial information message from the server is sent as a global request, but with want_reply set to false. This means that the server doesn't expect an answer to this message. Instead the client needs to send a prove request as a reply if it wants to receive proof of ownership for the host keys. The bug doesn't cause any current problems with due to how OpenSSH treats receiving the failure message. It instead treats it as a keepalive message and further ignores it. Arguably this is a protocol violation though of Dropbear and it is only accidental that it doesn't cause a problem with OpenSSH. The bug was found when adding host keys support to libssh, which is more strict protocol wise and treats the unexpected failure message an error, also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145 for more information. The fix here is to honor the want_reply flag in the global request and to only send a reply if the other side expects a reply.
author Dirkjan Bussink <d.bussink@gmail.com>
date Thu, 10 Dec 2020 16:13:13 +0100
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
1692
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_LOG_U32_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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5
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6 /* Compute log_{base}(a) */
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7 static mp_word s_pow(mp_word base, mp_word exponent)
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8 {
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9 mp_word result = 1uLL;
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10 while (exponent != 0u) {
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11 if ((exponent & 1u) == 1u) {
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12 result *= base;
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13 }
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14 exponent >>= 1;
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15 base *= base;
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16 }
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17
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18 return result;
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19 }
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20
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21 static mp_digit s_digit_ilogb(mp_digit base, mp_digit n)
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22 {
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23 mp_word bracket_low = 1uLL, bracket_mid, bracket_high, N;
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24 mp_digit ret, high = 1uL, low = 0uL, mid;
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25
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26 if (n < base) {
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27 return 0uL;
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28 }
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29 if (n == base) {
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30 return 1uL;
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31 }
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32
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33 bracket_high = (mp_word) base ;
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34 N = (mp_word) n;
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35
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36 while (bracket_high < N) {
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37 low = high;
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38 bracket_low = bracket_high;
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39 high <<= 1;
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40 bracket_high *= bracket_high;
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41 }
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42
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43 while (((mp_digit)(high - low)) > 1uL) {
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44 mid = (low + high) >> 1;
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45 bracket_mid = bracket_low * s_pow(base, (mp_word)(mid - low));
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46
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47 if (N < bracket_mid) {
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48 high = mid ;
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49 bracket_high = bracket_mid ;
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50 }
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51 if (N > bracket_mid) {
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52 low = mid ;
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53 bracket_low = bracket_mid ;
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54 }
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55 if (N == bracket_mid) {
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56 return (mp_digit) mid;
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57 }
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58 }
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59
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60 if (bracket_high == N) {
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61 ret = high;
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62 } else {
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63 ret = low;
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64 }
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65
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66 return ret;
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67 }
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68
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69 /* TODO: output could be "int" because the output of mp_radix_size is int, too,
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70 as is the output of mp_bitcount.
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71 With the same problem: max size is INT_MAX * MP_DIGIT not INT_MAX only!
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72 */
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73 mp_err mp_log_u32(const mp_int *a, uint32_t base, uint32_t *c)
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74 {
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75 mp_err err;
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76 mp_ord cmp;
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77 uint32_t high, low, mid;
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78 mp_int bracket_low, bracket_high, bracket_mid, t, bi_base;
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79
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80 err = MP_OKAY;
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81
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82 if (a->sign == MP_NEG) {
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83 return MP_VAL;
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84 }
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85
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86 if (MP_IS_ZERO(a)) {
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87 return MP_VAL;
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88 }
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89
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90 if (base < 2u) {
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91 return MP_VAL;
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92 }
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93
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94 /* A small shortcut for bases that are powers of two. */
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95 if ((base & (base - 1u)) == 0u) {
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96 int y, bit_count;
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97 for (y=0; (y < 7) && ((base & 1u) == 0u); y++) {
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98 base >>= 1;
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99 }
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100 bit_count = mp_count_bits(a) - 1;
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101 *c = (uint32_t)(bit_count/y);
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102 return MP_OKAY;
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103 }
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104
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105 if (a->used == 1) {
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106 *c = (uint32_t)s_digit_ilogb(base, a->dp[0]);
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107 return err;
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108 }
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109
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110 cmp = mp_cmp_d(a, base);
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111 if ((cmp == MP_LT) || (cmp == MP_EQ)) {
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112 *c = cmp == MP_EQ;
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113 return err;
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114 }
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115
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116 if ((err =
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117 mp_init_multi(&bracket_low, &bracket_high,
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118 &bracket_mid, &t, &bi_base, NULL)) != MP_OKAY) {
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119 return err;
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120 }
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121
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122 low = 0u;
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123 mp_set(&bracket_low, 1uL);
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124 high = 1u;
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125
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126 mp_set(&bracket_high, base);
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127
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128 /*
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129 A kind of Giant-step/baby-step algorithm.
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130 Idea shamelessly stolen from https://programmingpraxis.com/2010/05/07/integer-logarithms/2/
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131 The effect is asymptotic, hence needs benchmarks to test if the Giant-step should be skipped
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132 for small n.
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133 */
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134 while (mp_cmp(&bracket_high, a) == MP_LT) {
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135 low = high;
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136 if ((err = mp_copy(&bracket_high, &bracket_low)) != MP_OKAY) {
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137 goto LBL_ERR;
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138 }
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139 high <<= 1;
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140 if ((err = mp_sqr(&bracket_high, &bracket_high)) != MP_OKAY) {
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141 goto LBL_ERR;
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142 }
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143 }
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144 mp_set(&bi_base, base);
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145
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146 while ((high - low) > 1u) {
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147 mid = (high + low) >> 1;
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148
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149 if ((err = mp_expt_u32(&bi_base, (uint32_t)(mid - low), &t)) != MP_OKAY) {
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150 goto LBL_ERR;
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151 }
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152 if ((err = mp_mul(&bracket_low, &t, &bracket_mid)) != MP_OKAY) {
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153 goto LBL_ERR;
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154 }
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155 cmp = mp_cmp(a, &bracket_mid);
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156 if (cmp == MP_LT) {
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157 high = mid;
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158 mp_exch(&bracket_mid, &bracket_high);
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159 }
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160 if (cmp == MP_GT) {
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161 low = mid;
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162 mp_exch(&bracket_mid, &bracket_low);
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163 }
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Steffen Jaeckel <s@jaeckel.eu>
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164 if (cmp == MP_EQ) {
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165 *c = mid;
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166 goto LBL_END;
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167 }
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Steffen Jaeckel <s@jaeckel.eu>
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168 }
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169
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170 *c = (mp_cmp(&bracket_high, a) == MP_EQ) ? high : low;
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171
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172 LBL_END:
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Steffen Jaeckel <s@jaeckel.eu>
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173 LBL_ERR:
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174 mp_clear_multi(&bracket_low, &bracket_high, &bracket_mid,
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175 &t, &bi_base, NULL);
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176 return err;
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177 }
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178
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179
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180 #endif