Line data    Source code

       1             : #include "tommath_private.h"
       2             : #ifdef BN_S_MP_SUB_C
       3             : /* LibTomMath, multiple-precision integer library -- Tom St Denis */
       4             : /* SPDX-License-Identifier: Unlicense */
       5             : 
       6             : /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
       7      766362 : mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
       8             : {
       9       51227 :    int    olduse, min, max;
      10       51227 :    mp_err err;
      11             : 
      12             :    /* find sizes */
      13      766362 :    min = b->used;
      14      766362 :    max = a->used;
      15             : 
      16             :    /* init result */
      17      766362 :    if (c->alloc < max) {
      18          58 :       if ((err = mp_grow(c, max)) != MP_OKAY) {
      19           0 :          return err;
      20             :       }
      21             :    }
      22      766362 :    olduse = c->used;
      23      766362 :    c->used = max;
      24             : 
      25             :    {
      26       51227 :       mp_digit u, *tmpa, *tmpb, *tmpc;
      27       51227 :       int i;
      28             : 
      29             :       /* alias for digit pointers */
      30      766362 :       tmpa = a->dp;
      31      766362 :       tmpb = b->dp;
      32      766362 :       tmpc = c->dp;
      33             : 
      34             :       /* set carry to zero */
      35      766362 :       u = 0;
      36    42038436 :       for (i = 0; i < min; i++) {
      37             :          /* T[i] = A[i] - B[i] - U */
      38    41272074 :          *tmpc = (*tmpa++ - *tmpb++) - u;
      39             : 
      40             :          /* U = carry bit of T[i]
      41             :           * Note this saves performing an AND operation since
      42             :           * if a carry does occur it will propagate all the way to the
      43             :           * MSB.  As a result a single shift is enough to get the carry
      44             :           */
      45    41272074 :          u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
      46             : 
      47             :          /* Clear carry from T[i] */
      48    41272074 :          *tmpc++ &= MP_MASK;
      49             :       }
      50             : 
      51             :       /* now copy higher words if any, e.g. if A has more digits than B  */
      52      784710 :       for (; i < max; i++) {
      53             :          /* T[i] = A[i] - U */
      54       18348 :          *tmpc = *tmpa++ - u;
      55             : 
      56             :          /* U = carry bit of T[i] */
      57       18348 :          u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
      58             : 
      59             :          /* Clear carry from T[i] */
      60       18348 :          *tmpc++ &= MP_MASK;
      61             :       }
      62             : 
      63             :       /* clear digits above used (since we may not have grown result above) */
      64      766362 :       MP_ZERO_DIGITS(tmpc, olduse - c->used);
      65             :    }
      66             : 
      67      766362 :    mp_clamp(c);
      68      766362 :    return MP_OKAY;
      69             : }
      70             : 
      71             : #endif