### view libtommath/bn_s_mp_karatsuba_sqr.c @ 1734:73646de50f13DROPBEAR_2020.80

version 2020.80
author Matt Johnston Fri, 26 Jun 2020 21:45:59 +0800 1051e4eea25a
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line source
```#include "tommath_private.h"
#ifdef BN_S_MP_KARATSUBA_SQR_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* Karatsuba squaring, computes b = a*a using three
* half size squarings
*
* See comments of karatsuba_mul for details.  It
* is essentially the same algorithm but merely
* tuned to perform recursive squarings.
*/
mp_err s_mp_karatsuba_sqr(const mp_int *a, mp_int *b)
{
mp_int  x0, x1, t1, t2, x0x0, x1x1;
int     B;
mp_err  err = MP_MEM;

/* min # of digits */
B = a->used;

/* now divide in two */
B = B >> 1;

/* init copy all the temps */
if (mp_init_size(&x0, B) != MP_OKAY)
goto LBL_ERR;
if (mp_init_size(&x1, a->used - B) != MP_OKAY)
goto X0;

/* init temps */
if (mp_init_size(&t1, a->used * 2) != MP_OKAY)
goto X1;
if (mp_init_size(&t2, a->used * 2) != MP_OKAY)
goto T1;
if (mp_init_size(&x0x0, B * 2) != MP_OKAY)
goto T2;
if (mp_init_size(&x1x1, (a->used - B) * 2) != MP_OKAY)
goto X0X0;

{
int x;
mp_digit *dst, *src;

src = a->dp;

/* now shift the digits */
dst = x0.dp;
for (x = 0; x < B; x++) {
*dst++ = *src++;
}

dst = x1.dp;
for (x = B; x < a->used; x++) {
*dst++ = *src++;
}
}

x0.used = B;
x1.used = a->used - B;

mp_clamp(&x0);

/* now calc the products x0*x0 and x1*x1 */
if (mp_sqr(&x0, &x0x0) != MP_OKAY)
goto X1X1;           /* x0x0 = x0*x0 */
if (mp_sqr(&x1, &x1x1) != MP_OKAY)
goto X1X1;           /* x1x1 = x1*x1 */

/* now calc (x1+x0)**2 */
if (s_mp_add(&x1, &x0, &t1) != MP_OKAY)
goto X1X1;           /* t1 = x1 - x0 */
if (mp_sqr(&t1, &t1) != MP_OKAY)
goto X1X1;           /* t1 = (x1 - x0) * (x1 - x0) */

if (s_mp_add(&x0x0, &x1x1, &t2) != MP_OKAY)
goto X1X1;           /* t2 = x0x0 + x1x1 */
if (s_mp_sub(&t1, &t2, &t1) != MP_OKAY)
goto X1X1;           /* t1 = (x1+x0)**2 - (x0x0 + x1x1) */

/* shift by B */
if (mp_lshd(&t1, B) != MP_OKAY)
goto X1X1;           /* t1 = (x0x0 + x1x1 - (x1-x0)*(x1-x0))<<B */
if (mp_lshd(&x1x1, B * 2) != MP_OKAY)
goto X1X1;           /* x1x1 = x1x1 << 2*B */

if (mp_add(&x0x0, &t1, &t1) != MP_OKAY)
goto X1X1;           /* t1 = x0x0 + t1 */
if (mp_add(&t1, &x1x1, b) != MP_OKAY)
goto X1X1;           /* t1 = x0x0 + t1 + x1x1 */

err = MP_OKAY;

X1X1:
mp_clear(&x1x1);
X0X0:
mp_clear(&x0x0);
T2:
mp_clear(&t2);
T1:
mp_clear(&t1);
X1:
mp_clear(&x1);
X0:
mp_clear(&x0);
LBL_ERR:
return err;
}
#endif
```