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mp_mul.c
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mp_mul.c
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#include "tommath_private.h"
#ifdef MP_MUL_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* high level multiplication (handles sign) */
mp_err mp_mul(const mp_int *a, const mp_int *b, mp_int *c)
{
mp_err err;
int min = MP_MIN(a->used, b->used),
max = MP_MAX(a->used, b->used),
digs = a->used + b->used + 1;
bool neg = (a->sign != b->sign);
if ((a == b) &&
MP_HAS(S_MP_SQR_TOOM) && /* use Toom-Cook? */
(a->used >= MP_SQR_TOOM_CUTOFF)) {
err = s_mp_sqr_toom(a, c);
} else if ((a == b) &&
MP_HAS(S_MP_SQR_KARATSUBA) && /* Karatsuba? */
(a->used >= MP_SQR_KARATSUBA_CUTOFF)) {
err = s_mp_sqr_karatsuba(a, c);
} else if ((a == b) &&
MP_HAS(S_MP_SQR_COMBA) && /* can we use the fast comba multiplier? */
(((a->used * 2) + 1) < MP_WARRAY) &&
(a->used <= MP_MAX_COMBA)) {
err = s_mp_sqr_comba(a, c);
} else if ((a == b) &&
MP_HAS(S_MP_SQR)) {
err = s_mp_sqr(a, c);
} else if (MP_HAS(S_MP_MUL_BALANCE) &&
/* Check sizes. The smaller one needs to be larger than the Karatsuba cut-off.
* The bigger one needs to be at least about one MP_MUL_KARATSUBA_CUTOFF bigger
* to make some sense, but it depends on architecture, OS, position of the
* stars... so YMMV.
* Using it to cut the input into slices small enough for s_mp_mul_comba
* was actually slower on the author's machine, but YMMV.
*/
(min >= MP_MUL_KARATSUBA_CUTOFF) &&
((max / 2) >= MP_MUL_KARATSUBA_CUTOFF) &&
/* Not much effect was observed below a ratio of 1:2, but again: YMMV. */
(max >= (2 * min))) {
err = s_mp_mul_balance(a,b,c);
} else if (MP_HAS(S_MP_MUL_TOOM) &&
(min >= MP_MUL_TOOM_CUTOFF)) {
err = s_mp_mul_toom(a, b, c);
} else if (MP_HAS(S_MP_MUL_KARATSUBA) &&
(min >= MP_MUL_KARATSUBA_CUTOFF)) {
err = s_mp_mul_karatsuba(a, b, c);
} else if (MP_HAS(S_MP_MUL_COMBA) &&
/* can we use the fast multiplier?
*
* The fast multiplier can be used if the output will
* have less than MP_WARRAY digits and the number of
* digits won't affect carry propagation
*/
(digs < MP_WARRAY) &&
(min <= MP_MAX_COMBA)) {
err = s_mp_mul_comba(a, b, c, digs);
} else if (MP_HAS(S_MP_MUL)) {
err = s_mp_mul(a, b, c, digs);
} else {
err = MP_VAL;
}
c->sign = ((c->used > 0) && neg) ? MP_NEG : MP_ZPOS;
return err;
}
#endif