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annotate libtommath/bn_s_mp_sqr_fast.c @ 1820:e9854650d45b

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Clarify help text for dropbear -e environment option
author Matt Johnston <matt@ucc.asn.au>
date Thu, 19 Aug 2021 23:17:34 +0800
parents 1051e4eea25a
children
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1692
1051e4eea25a Update LibTomMath to 1.2.0 (#84)
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_SQR_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 /* the jist of squaring...
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7 * you do like mult except the offset of the tmpx [one that
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8 * starts closer to zero] can't equal the offset of tmpy.
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9 * So basically you set up iy like before then you min it with
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10 * (ty-tx) so that it never happens. You double all those
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11 * you add in the inner loop
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12
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13 After that loop you do the squares and add them in.
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14 */
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15
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16 mp_err s_mp_sqr_fast(const mp_int *a, mp_int *b)
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17 {
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18 int olduse, pa, ix, iz;
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19 mp_digit W[MP_WARRAY], *tmpx;
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20 mp_word W1;
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21 mp_err err;
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22
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23 /* grow the destination as required */
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24 pa = a->used + a->used;
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25 if (b->alloc < pa) {
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26 if ((err = mp_grow(b, pa)) != MP_OKAY) {
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27 return err;
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28 }
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29 }
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30
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31 /* number of output digits to produce */
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32 W1 = 0;
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33 for (ix = 0; ix < pa; ix++) {
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34 int tx, ty, iy;
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35 mp_word _W;
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36 mp_digit *tmpy;
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37
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38 /* clear counter */
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39 _W = 0;
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40
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41 /* get offsets into the two bignums */
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42 ty = MP_MIN(a->used-1, ix);
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43 tx = ix - ty;
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44
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45 /* setup temp aliases */
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46 tmpx = a->dp + tx;
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47 tmpy = a->dp + ty;
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48
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49 /* this is the number of times the loop will iterrate, essentially
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50 while (tx++ < a->used && ty-- >= 0) { ... }
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51 */
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52 iy = MP_MIN(a->used-tx, ty+1);
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53
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54 /* now for squaring tx can never equal ty
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55 * we halve the distance since they approach at a rate of 2x
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56 * and we have to round because odd cases need to be executed
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57 */
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58 iy = MP_MIN(iy, ((ty-tx)+1)>>1);
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59
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60 /* execute loop */
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61 for (iz = 0; iz < iy; iz++) {
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62 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
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63 }
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64
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65 /* double the inner product and add carry */
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66 _W = _W + _W + W1;
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67
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68 /* even columns have the square term in them */
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69 if (((unsigned)ix & 1u) == 0u) {
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70 _W += (mp_word)a->dp[ix>>1] * (mp_word)a->dp[ix>>1];
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71 }
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72
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73 /* store it */
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74 W[ix] = (mp_digit)_W & MP_MASK;
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75
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76 /* make next carry */
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77 W1 = _W >> (mp_word)MP_DIGIT_BIT;
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78 }
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79
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80 /* setup dest */
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81 olduse = b->used;
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82 b->used = a->used+a->used;
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83
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84 {
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85 mp_digit *tmpb;
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86 tmpb = b->dp;
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87 for (ix = 0; ix < pa; ix++) {
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88 *tmpb++ = W[ix] & MP_MASK;
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89 }
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90
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91 /* clear unused digits [that existed in the old copy of c] */
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92 MP_ZERO_DIGITS(tmpb, olduse - ix);
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93 }
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94 mp_clamp(b);
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95 return MP_OKAY;
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96 }
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97 #endif