Mercurial > dropbear

annotate libtommath/bn_s_mp_sub.c @ 1646:6d1bbe7d5fa5 DROPBEAR_2019.77

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2019.77
author Matt Johnston <matt@ucc.asn.au>
date Sat, 23 Mar 2019 21:46:29 +0800
parents 8bba51a55704
children f52919ffd3b1
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
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1 #include <tommath_private.h>
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2 #ifdef BN_S_MP_SUB_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://libtom.org
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16 */
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17
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18 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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19 int
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20 s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
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21 {
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22 int olduse, res, min, max;
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23
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24 /* find sizes */
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25 min = b->used;
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26 max = a->used;
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27
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28 /* init result */
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29 if (c->alloc < max) {
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30 if ((res = mp_grow (c, max)) != MP_OKAY) {
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31 return res;
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32 }
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33 }
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34 olduse = c->used;
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35 c->used = max;
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36
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37 {
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38 mp_digit u, *tmpa, *tmpb, *tmpc;
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39 int i;
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40
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41 /* alias for digit pointers */
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42 tmpa = a->dp;
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43 tmpb = b->dp;
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44 tmpc = c->dp;
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45
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46 /* set carry to zero */
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47 u = 0;
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48 for (i = 0; i < min; i++) {
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49 /* T[i] = A[i] - B[i] - U */
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50 *tmpc = (*tmpa++ - *tmpb++) - u;
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51
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52 /* U = carry bit of T[i]
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53 * Note this saves performing an AND operation since
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54 * if a carry does occur it will propagate all the way to the
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55 * MSB. As a result a single shift is enough to get the carry
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56 */
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57 u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
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58
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59 /* Clear carry from T[i] */
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60 *tmpc++ &= MP_MASK;
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61 }
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62
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63 /* now copy higher words if any, e.g. if A has more digits than B */
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64 for (; i < max; i++) {
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65 /* T[i] = A[i] - U */
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66 *tmpc = *tmpa++ - u;
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67
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68 /* U = carry bit of T[i] */
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69 u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
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70
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71 /* Clear carry from T[i] */
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72 *tmpc++ &= MP_MASK;
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73 }
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74
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75 /* clear digits above used (since we may not have grown result above) */
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76 for (i = c->used; i < olduse; i++) {
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77 *tmpc++ = 0;
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78 }
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79 }
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80
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81 mp_clamp (c);
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82 return MP_OKAY;
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83 }
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84
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85 #endif
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86
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87 /* ref: $Format:%D$ */
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88 /* git commit: $Format:%H$ */
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89 /* commit time: $Format:%ai$ */