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 */
s_mp_sub(const mp_int * a,const mp_int * b,mp_int * c)7 mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
8 {
9 int olduse, min, max;
10 mp_err err;
11
12 /* find sizes */
13 min = b->used;
14 max = a->used;
15
16 /* init result */
17 if (c->alloc < max) {
18 if ((err = mp_grow(c, max)) != MP_OKAY) {
19 return err;
20 }
21 }
22 olduse = c->used;
23 c->used = max;
24
25 {
26 mp_digit u, *tmpa, *tmpb, *tmpc;
27 int i;
28
29 /* alias for digit pointers */
30 tmpa = a->dp;
31 tmpb = b->dp;
32 tmpc = c->dp;
33
34 /* set carry to zero */
35 u = 0;
36 for (i = 0; i < min; i++) {
37 /* T[i] = A[i] - B[i] - U */
38 *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 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
46
47 /* Clear carry from T[i] */
48 *tmpc++ &= MP_MASK;
49 }
50
51 /* now copy higher words if any, e.g. if A has more digits than B */
52 for (; i < max; i++) {
53 /* T[i] = A[i] - U */
54 *tmpc = *tmpa++ - u;
55
56 /* U = carry bit of T[i] */
57 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
58
59 /* Clear carry from T[i] */
60 *tmpc++ &= MP_MASK;
61 }
62
63 /* clear digits above used (since we may not have grown result above) */
64 MP_ZERO_DIGITS(tmpc, olduse - c->used);
65 }
66
67 mp_clamp(c);
68 return MP_OKAY;
69 }
70
71 #endif
72