69 lines
1.8 KiB
C
69 lines
1.8 KiB
C
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#include "tommath_private.h"
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#ifdef MP_DR_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* 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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*
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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
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*
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* The modulus must be of a special format [see manual]
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*
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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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mp_err mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
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{
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mp_err err;
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/* m = digits in modulus */
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int m = n->used;
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/* ensure that "x" has at least 2m digits */
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if ((err = mp_grow(x, m + m)) != MP_OKAY) {
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return err;
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}
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/* top of loop, this is where the code resumes if
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* another reduction pass is required.
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*/
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for (;;) {
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int i;
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mp_digit mu = 0;
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/* compute (x mod B**m) + k * [x/B**m] inline and inplace */
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for (i = 0; i < m; i++) {
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mp_word r = ((mp_word)x->dp[i + m] * (mp_word)k) + x->dp[i] + mu;
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x->dp[i] = (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 */
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x->dp[i] = mu;
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/* zero words above m */
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s_mp_zero_digs(x->dp + m + 1, (x->used - m) - 1);
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/* clamp, sub and return */
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mp_clamp(x);
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/* if x >= n then subtract and reduce again
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* Each successive "recursion" makes the input smaller and smaller.
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*/
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if (mp_cmp_mag(x, n) == MP_LT) {
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break;
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}
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if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
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return err;
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}
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}
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return MP_OKAY;
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}
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#endif
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