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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is library that provides for multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library is designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
*/
#include <tommath.h>
/* computes xR^-1 == x (mod N) via Montgomery Reduction */
int
mp_montgomery_reduce (mp_int * a, mp_int * m, mp_digit mp)
{
int ix, res, digs;
mp_digit ui;
digs = m->used * 2 + 1;
if ((digs < 512)
&& digs < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
return fast_mp_montgomery_reduce (a, m, mp);
}
if (a->alloc < m->used * 2 + 1) {
if ((res = mp_grow (a, m->used * 2 + 1)) != MP_OKAY) {
return res;
}
}
a->used = m->used * 2 + 1;
for (ix = 0; ix < m->used; ix++) {
/* ui = ai * m' mod b */
ui = (a->dp[ix] * mp) & MP_MASK;
/* a = a + ui * m * b^i */
{
register int iy;
register mp_digit *tmpx, *tmpy, mu;
register mp_word r;
/* aliases */
tmpx = m->dp;
tmpy = a->dp + ix;
mu = 0;
for (iy = 0; iy < m->used; iy++) {
r = ((mp_word) ui) * ((mp_word) * tmpx++) + ((mp_word) mu) + ((mp_word) * tmpy);
mu = (r >> ((mp_word) DIGIT_BIT));
*tmpy++ = (r & ((mp_word) MP_MASK));
}
/* propagate carries */
while (mu) {
*tmpy += mu;
mu = (*tmpy >> DIGIT_BIT) & 1;
*tmpy++ &= MP_MASK;
}
}
}
/* A = A/b^n */
mp_rshd (a, m->used);
/* if A >= m then A = A - m */
if (mp_cmp_mag (a, m) != MP_LT) {
return s_mp_sub (a, m, a);
}
return MP_OKAY;
}
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