annotate libtommath/bn_mp_dr_reduce.c @ 1734:73646de50f13 DROPBEAR_2020.80

version 2020.80
author Matt Johnston <matt@ucc.asn.au>
date Fri, 26 Jun 2020 21:45:59 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_DR_REDUCE_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 /* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
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7 *
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8 * Based on algorithm from the paper
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9 *
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10 * "Generating Efficient Primes for Discrete Log Cryptosystems"
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11 * Chae Hoon Lim, Pil Joong Lee,
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12 * POSTECH Information Research Laboratories
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13 *
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14 * The modulus must be of a special format [see manual]
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15 *
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16 * Has been modified to use algorithm 7.10 from the LTM book instead
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17 *
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18 * Input x must be in the range 0 <= x <= (n-1)**2
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19 */
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20 mp_err mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
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21 {
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22 mp_err err;
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23 int i, m;
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24 mp_word r;
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25 mp_digit mu, *tmpx1, *tmpx2;
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27 /* m = digits in modulus */
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28 m = n->used;
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30 /* ensure that "x" has at least 2m digits */
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31 if (x->alloc < (m + m)) {
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32 if ((err = mp_grow(x, m + m)) != MP_OKAY) {
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33 return err;
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34 }
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35 }
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36
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37 /* top of loop, this is where the code resumes if
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38 * another reduction pass is required.
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39 */
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40 top:
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41 /* aliases for digits */
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42 /* alias for lower half of x */
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43 tmpx1 = x->dp;
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45 /* alias for upper half of x, or x/B**m */
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46 tmpx2 = x->dp + m;
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47
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48 /* set carry to zero */
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49 mu = 0;
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51 /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
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52 for (i = 0; i < m; i++) {
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53 r = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu;
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54 *tmpx1++ = (mp_digit)(r & MP_MASK);
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55 mu = (mp_digit)(r >> ((mp_word)MP_DIGIT_BIT));
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56 }
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57
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58 /* set final carry */
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59 *tmpx1++ = mu;
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60
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61 /* zero words above m */
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62 MP_ZERO_DIGITS(tmpx1, (x->used - m) - 1);
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63
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64 /* clamp, sub and return */
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65 mp_clamp(x);
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66
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67 /* if x >= n then subtract and reduce again
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68 * Each successive "recursion" makes the input smaller and smaller.
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69 */
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70 if (mp_cmp_mag(x, n) != MP_LT) {
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71 if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
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72 return err;
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73 }
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74 goto top;
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75 }
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76 return MP_OKAY;
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77 }
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78 #endif