### view libtommath/bn_mp_montgomery_calc_normalization.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_MP_MONTGOMERY_CALC_NORMALIZATION_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/*
* shifts with subtractions when the result is greater than b.
*
* The method is slightly modified to shift B unconditionally upto just under
* the leading bit of b.  This saves alot of multiple precision shifting.
*/
mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
{
int    x, bits;
mp_err err;

/* how many bits of last digit does b use */
bits = mp_count_bits(b) % MP_DIGIT_BIT;

if (b->used > 1) {
if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
return err;
}
} else {
mp_set(a, 1uL);
bits = 1;
}

/* now compute C = A * B mod b */
for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
if ((err = mp_mul_2(a, a)) != MP_OKAY) {
return err;
}
if (mp_cmp_mag(a, b) != MP_LT) {
if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
return err;
}
}
}

return MP_OKAY;
}
#endif
```