### view libtommath/bn_s_mp_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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```#include "tommath_private.h"
#ifdef BN_S_MP_SQR_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* low level squaring, b = a*a, HAC pp.596-597, Algorithm 14.16 */
mp_err s_mp_sqr(const mp_int *a, mp_int *b)
{
mp_int   t;
int      ix, iy, pa;
mp_err   err;
mp_word  r;
mp_digit u, tmpx, *tmpt;

pa = a->used;
if ((err = mp_init_size(&t, (2 * pa) + 1)) != MP_OKAY) {
return err;
}

/* default used is maximum possible size */
t.used = (2 * pa) + 1;

for (ix = 0; ix < pa; ix++) {
/* first calculate the digit at 2*ix */
/* calculate double precision result */
r = (mp_word)t.dp[2*ix] +
((mp_word)a->dp[ix] * (mp_word)a->dp[ix]);

/* store lower part in result */

/* get the carry */
u           = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);

/* left hand side of A[ix] * A[iy] */
tmpx        = a->dp[ix];

/* alias for where to store the results */
tmpt        = t.dp + ((2 * ix) + 1);

for (iy = ix + 1; iy < pa; iy++) {
/* first calculate the product */
r       = (mp_word)tmpx * (mp_word)a->dp[iy];

/* now calculate the double precision result, note we use
*/
r       = (mp_word)*tmpt + r + r + (mp_word)u;

/* store lower part */

/* get carry */
u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
}
/* propagate upwards */
while (u != 0uL) {
r       = (mp_word)*tmpt + (mp_word)u;
u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
}
}

mp_clamp(&t);
mp_exch(&t, b);
mp_clear(&t);
return MP_OKAY;
}
#endif
```