Mercurial > dropbear

annotate libtommath/bn_s_mp_mul_digs.c @ 1844:7a3cd7b598d8

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author Matt Johnston <matt@ucc.asn.au>
date Mon, 18 Oct 2021 23:31:23 +0800
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_MUL_DIGS_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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6 /* multiplies |a| * |b| and only computes upto digs digits of result
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7 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
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8 * many digits of output are created.
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9 */
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10 mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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11 {
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12 mp_int t;
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13 mp_err err;
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14 int pa, pb, ix, iy;
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15 mp_digit u;
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16 mp_word r;
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17 mp_digit tmpx, *tmpt, *tmpy;
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18
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19 /* can we use the fast multiplier? */
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20 if ((digs < MP_WARRAY) &&
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21 (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
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22 return s_mp_mul_digs_fast(a, b, c, digs);
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23 }
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24
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25 if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
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26 return err;
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27 }
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28 t.used = digs;
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29
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30 /* compute the digits of the product directly */
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31 pa = a->used;
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32 for (ix = 0; ix < pa; ix++) {
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33 /* set the carry to zero */
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34 u = 0;
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35
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36 /* limit ourselves to making digs digits of output */
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37 pb = MP_MIN(b->used, digs - ix);
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38
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39 /* setup some aliases */
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40 /* copy of the digit from a used within the nested loop */
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41 tmpx = a->dp[ix];
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42
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43 /* an alias for the destination shifted ix places */
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44 tmpt = t.dp + ix;
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46 /* an alias for the digits of b */
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47 tmpy = b->dp;
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48
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49 /* compute the columns of the output and propagate the carry */
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50 for (iy = 0; iy < pb; iy++) {
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51 /* compute the column as a mp_word */
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52 r = (mp_word)*tmpt +
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53 ((mp_word)tmpx * (mp_word)*tmpy++) +
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54 (mp_word)u;
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55
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56 /* the new column is the lower part of the result */
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57 *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
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58
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59 /* get the carry word from the result */
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60 u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
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61 }
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62 /* set carry if it is placed below digs */
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63 if ((ix + iy) < digs) {
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64 *tmpt = u;
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65 }
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66 }
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67
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68 mp_clamp(&t);
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69 mp_exch(&t, c);
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71 mp_clear(&t);
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72 return MP_OKAY;
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73 }
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74 #endif