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#ifdef BN_MP_DR_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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* The library is free for all purposes without any express
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* Tom St Denis, tomstdenis@gmail.com, http://libtom.org
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/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
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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
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* 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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* Input x must be in the range 0 <= x <= (n-1)**2
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mp_dr_reduce (mp_int * x, mp_int * n, mp_digit k)
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mp_digit mu, *tmpx1, *tmpx2;
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/* m = digits in modulus */
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/* ensure that "x" has at least 2m digits */
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if (x->alloc < m + m) {
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if ((err = mp_grow (x, m + m)) != MP_OKAY) {
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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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/* aliases for digits */
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/* alias for lower half of x */
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/* alias for upper half of x, or x/B**m */
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/* set carry to zero */
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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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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)DIGIT_BIT));
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/* zero words above m */
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for (i = m + 1; i < x->used; i++) {
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/* clamp, sub and return */
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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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if (mp_cmp_mag (x, n) != MP_LT) {
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/* $Source: /cvs/libtom/libtommath/bn_mp_dr_reduce.c,v $ */
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/* $Revision: 1.4 $ */
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/* $Date: 2006/12/28 01:25:13 $ */