libtommath/bn_mp_dr_reduce.c

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#include "tommath_private.h"
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#ifdef BN_MP_DR_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
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/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
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*
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* Based on algorithm from the paper
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*
* "Generating Efficient Primes for Discrete Log Cryptosystems"
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* Chae Hoon Lim, Pil Joong Lee,
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* POSTECH Information Research Laboratories
*
* The modulus must be of a special format [see manual]
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*
* Has been modified to use algorithm 7.10 from the LTM book instead
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*
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* Input x must be in the range 0 <= x <= (n-1)**2
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*/
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int mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
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{
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int err, i, m;
mp_word r;
mp_digit mu, *tmpx1, *tmpx2;
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/* m = digits in modulus */
m = n->used;
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/* ensure that "x" has at least 2m digits */
if (x->alloc < (m + m)) {
if ((err = mp_grow(x, m + m)) != MP_OKAY) {
return err;
}
}
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/* top of loop, this is where the code resumes if
* another reduction pass is required.
*/
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top:
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/* aliases for digits */
/* alias for lower half of x */
tmpx1 = x->dp;
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/* alias for upper half of x, or x/B**m */
tmpx2 = x->dp + m;
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/* set carry to zero */
mu = 0;
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/* compute (x mod B**m) + k * [x/B**m] inline and inplace */
for (i = 0; i < m; i++) {
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r = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu;
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*tmpx1++ = (mp_digit)(r & MP_MASK);
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mu = (mp_digit)(r >> ((mp_word)MP_DIGIT_BIT));
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}
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/* set final carry */
*tmpx1++ = mu;
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/* zero words above m */
for (i = m + 1; i < x->used; i++) {
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*tmpx1++ = 0;
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}
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/* clamp, sub and return */
mp_clamp(x);
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/* if x >= n then subtract and reduce again
* Each successive "recursion" makes the input smaller and smaller.
*/
if (mp_cmp_mag(x, n) != MP_LT) {
if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
return err;
}
goto top;
}
return MP_OKAY;
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}
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#endif