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annotate libtommath/bn_s_mp_mul_high_digs_fast.c @ 1902:4a6725ac957c

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Revert "Don't include sk keys at all in KEX list" This reverts git commit f972813ecdc7bb981d25b5a63638bd158f1c8e72. The sk algorithms need to remain in the sigalgs list so that they are included in the server-sig-algs ext-info message sent by the server. RFC8308 for server-sig-algs requires that all algorithms are listed (though OpenSSH client 8.4p1 tested doesn't require that)
author Matt Johnston <matt@ucc.asn.au>
date Thu, 24 Mar 2022 13:42:08 +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_MUL_HIGH_DIGS_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 /* this is a modified version of fast_s_mul_digs that only produces
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7 * output digits *above* digs. See the comments for fast_s_mul_digs
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8 * to see how it works.
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9 *
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10 * This is used in the Barrett reduction since for one of the multiplications
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11 * only the higher digits were needed. This essentially halves the work.
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12 *
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13 * Based on Algorithm 14.12 on pp.595 of HAC.
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14 */
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15 mp_err s_mp_mul_high_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs)
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16 {
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17 int olduse, pa, ix, iz;
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18 mp_err err;
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19 mp_digit W[MP_WARRAY];
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20 mp_word _W;
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21
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22 /* grow the destination as required */
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23 pa = a->used + b->used;
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24 if (c->alloc < pa) {
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25 if ((err = mp_grow(c, pa)) != MP_OKAY) {
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26 return err;
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27 }
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28 }
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29
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30 /* number of output digits to produce */
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31 pa = a->used + b->used;
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32 _W = 0;
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33 for (ix = digs; ix < pa; ix++) {
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34 int tx, ty, iy;
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35 mp_digit *tmpx, *tmpy;
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36
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37 /* get offsets into the two bignums */
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38 ty = MP_MIN(b->used-1, ix);
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39 tx = ix - ty;
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40
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41 /* setup temp aliases */
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42 tmpx = a->dp + tx;
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43 tmpy = b->dp + ty;
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44
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45 /* this is the number of times the loop will iterrate, essentially its
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46 while (tx++ < a->used && ty-- >= 0) { ... }
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47 */
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48 iy = MP_MIN(a->used-tx, ty+1);
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49
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50 /* execute loop */
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51 for (iz = 0; iz < iy; iz++) {
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52 _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
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53 }
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54
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55 /* store term */
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56 W[ix] = (mp_digit)_W & MP_MASK;
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57
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58 /* make next carry */
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59 _W = _W >> (mp_word)MP_DIGIT_BIT;
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60 }
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61
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62 /* setup dest */
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63 olduse = c->used;
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64 c->used = pa;
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65
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66 {
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67 mp_digit *tmpc;
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68
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69 tmpc = c->dp + digs;
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70 for (ix = digs; ix < pa; ix++) {
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71 /* now extract the previous digit [below the carry] */
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72 *tmpc++ = W[ix];
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73 }
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74
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75 /* clear unused digits [that existed in the old copy of c] */
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76 MP_ZERO_DIGITS(tmpc, olduse - ix);
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77 }
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78 mp_clamp(c);
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79 return MP_OKAY;
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80 }
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81 #endif