Coverage Report

Created: 2023-03-20 06:28

/src/dropbear/libtommath/bn_s_mp_sub.c
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#include "tommath_private.h"
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#ifdef BN_S_MP_SUB_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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{
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   int    olduse, min, max;
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   mp_err err;
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   /* find sizes */
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   min = b->used;
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   max = a->used;
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   /* init result */
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   if (c->alloc < max) {
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      if ((err = mp_grow(c, max)) != MP_OKAY) {
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         return err;
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      }
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   }
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   olduse = c->used;
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   c->used = max;
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   {
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      mp_digit u, *tmpa, *tmpb, *tmpc;
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      int i;
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      /* alias for digit pointers */
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      tmpa = a->dp;
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      tmpb = b->dp;
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      tmpc = c->dp;
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      /* set carry to zero */
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      u = 0;
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      for (i = 0; i < min; i++) {
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         /* T[i] = A[i] - B[i] - U */
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         *tmpc = (*tmpa++ - *tmpb++) - u;
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         /* U = carry bit of T[i]
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          * Note this saves performing an AND operation since
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          * if a carry does occur it will propagate all the way to the
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          * MSB.  As a result a single shift is enough to get the carry
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          */
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         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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         /* Clear carry from T[i] */
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         *tmpc++ &= MP_MASK;
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      }
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      /* now copy higher words if any, e.g. if A has more digits than B  */
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      for (; i < max; i++) {
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         /* T[i] = A[i] - U */
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         *tmpc = *tmpa++ - u;
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         /* U = carry bit of T[i] */
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         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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         /* Clear carry from T[i] */
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         *tmpc++ &= MP_MASK;
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      }
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      /* clear digits above used (since we may not have grown result above) */
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      MP_ZERO_DIGITS(tmpc, olduse - c->used);
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   }
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   mp_clamp(c);
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   return MP_OKAY;
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}
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#endif