60 lines
1.5 KiB
C
60 lines
1.5 KiB
C
#include <tommath.h>
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#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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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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*
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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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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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/*
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* shifts with subtractions when the result is greater than b.
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*
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* The method is slightly modified to shift B unconditionally upto just under
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* the leading bit of b. This saves alot of multiple precision shifting.
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*/
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int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
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{
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int x, bits, res;
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/* how many bits of last digit does b use */
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bits = mp_count_bits (b) % DIGIT_BIT;
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if (b->used > 1) {
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if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
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return res;
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}
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} else {
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mp_set(a, 1);
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bits = 1;
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}
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/* now compute C = A * B mod b */
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for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
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return res;
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}
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if (mp_cmp_mag (a, b) != MP_LT) {
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if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
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return res;
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}
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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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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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