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annotate libtommath/bn_s_mp_montgomery_reduce_fast.c @ 1861:2b3a8026a6ce

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Add re-exec for server This allows ASLR to re-randomize the address space for every connection, preventing some vulnerabilities from being exploitable by repeated probing. Overhead (memory and time) is yet to be confirmed. At present this is only enabled on Linux. Other BSD platforms with fexecve() would probably also work though have not been tested.
author Matt Johnston <matt@ucc.asn.au>
date Sun, 30 Jan 2022 10:14:56 +0800
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_S_MP_MONTGOMERY_REDUCE_FAST_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 /* computes xR**-1 == x (mod N) via Montgomery Reduction
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7 *
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8 * This is an optimized implementation of montgomery_reduce
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9 * which uses the comba method to quickly calculate the columns of the
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10 * reduction.
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11 *
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12 * Based on Algorithm 14.32 on pp.601 of HAC.
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13 */
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14 mp_err s_mp_montgomery_reduce_fast(mp_int *x, const mp_int *n, mp_digit rho)
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15 {
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16 int ix, olduse;
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17 mp_err err;
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18 mp_word W[MP_WARRAY];
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19
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20 if (x->used > MP_WARRAY) {
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21 return MP_VAL;
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22 }
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23
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24 /* get old used count */
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25 olduse = x->used;
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26
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27 /* grow a as required */
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28 if (x->alloc < (n->used + 1)) {
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29 if ((err = mp_grow(x, n->used + 1)) != MP_OKAY) {
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30 return err;
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31 }
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32 }
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33
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34 /* first we have to get the digits of the input into
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35 * an array of double precision words W[...]
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36 */
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37 {
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38 mp_word *_W;
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39 mp_digit *tmpx;
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40
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41 /* alias for the W[] array */
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42 _W = W;
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43
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44 /* alias for the digits of x*/
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45 tmpx = x->dp;
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46
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47 /* copy the digits of a into W[0..a->used-1] */
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48 for (ix = 0; ix < x->used; ix++) {
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49 *_W++ = *tmpx++;
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50 }
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51
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52 /* zero the high words of W[a->used..m->used*2] */
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53 if (ix < ((n->used * 2) + 1)) {
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54 MP_ZERO_BUFFER(_W, sizeof(mp_word) * (size_t)(((n->used * 2) + 1) - ix));
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55 }
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56 }
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57
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58 /* now we proceed to zero successive digits
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59 * from the least significant upwards
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60 */
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61 for (ix = 0; ix < n->used; ix++) {
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62 /* mu = ai * m' mod b
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63 *
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64 * We avoid a double precision multiplication (which isn't required)
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65 * by casting the value down to a mp_digit. Note this requires
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66 * that W[ix-1] have the carry cleared (see after the inner loop)
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67 */
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68 mp_digit mu;
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69 mu = ((W[ix] & MP_MASK) * rho) & MP_MASK;
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70
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71 /* a = a + mu * m * b**i
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72 *
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73 * This is computed in place and on the fly. The multiplication
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74 * by b**i is handled by offseting which columns the results
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75 * are added to.
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76 *
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77 * Note the comba method normally doesn't handle carries in the
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78 * inner loop In this case we fix the carry from the previous
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79 * column since the Montgomery reduction requires digits of the
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80 * result (so far) [see above] to work. This is
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81 * handled by fixing up one carry after the inner loop. The
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82 * carry fixups are done in order so after these loops the
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83 * first m->used words of W[] have the carries fixed
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84 */
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85 {
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86 int iy;
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87 mp_digit *tmpn;
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88 mp_word *_W;
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89
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90 /* alias for the digits of the modulus */
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91 tmpn = n->dp;
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92
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93 /* Alias for the columns set by an offset of ix */
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94 _W = W + ix;
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95
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96 /* inner loop */
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97 for (iy = 0; iy < n->used; iy++) {
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98 *_W++ += (mp_word)mu * (mp_word)*tmpn++;
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99 }
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100 }
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101
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102 /* now fix carry for next digit, W[ix+1] */
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103 W[ix + 1] += W[ix] >> (mp_word)MP_DIGIT_BIT;
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104 }
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105
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106 /* now we have to propagate the carries and
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107 * shift the words downward [all those least
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108 * significant digits we zeroed].
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109 */
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110 {
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111 mp_digit *tmpx;
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112 mp_word *_W, *_W1;
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113
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114 /* nox fix rest of carries */
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115
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116 /* alias for current word */
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117 _W1 = W + ix;
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118
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119 /* alias for next word, where the carry goes */
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120 _W = W + ++ix;
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121
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122 for (; ix < ((n->used * 2) + 1); ix++) {
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123 *_W++ += *_W1++ >> (mp_word)MP_DIGIT_BIT;
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124 }
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125
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126 /* copy out, A = A/b**n
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127 *
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128 * The result is A/b**n but instead of converting from an
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129 * array of mp_word to mp_digit than calling mp_rshd
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130 * we just copy them in the right order
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131 */
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132
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133 /* alias for destination word */
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134 tmpx = x->dp;
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135
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136 /* alias for shifted double precision result */
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137 _W = W + n->used;
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138
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139 for (ix = 0; ix < (n->used + 1); ix++) {
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140 *tmpx++ = *_W++ & (mp_word)MP_MASK;
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141 }
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142
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143 /* zero oldused digits, if the input a was larger than
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144 * m->used+1 we'll have to clear the digits
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145 */
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146 MP_ZERO_DIGITS(tmpx, olduse - ix);
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147 }
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148
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149 /* set the max used and clamp */
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150 x->used = n->used + 1;
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151 mp_clamp(x);
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152
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153 /* if A >= m then A = A - m */
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154 if (mp_cmp_mag(x, n) != MP_LT) {
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155 return s_mp_sub(x, n, x);
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156 }
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157 return MP_OKAY;
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158 }
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159 #endif